cell biology 4 Flashcards

Question

Electrochemical gradients

Answer 1

K+ “wants” to exit the cell. If the membrane were permeable to K+ only: Vm = Ek ≈ -90mV. Na+ “wants” to enter the cell. If the membrane were permeable to Na+ only: Vm = ENa ≈ +60mV

Answer 2

Permeability is determined by ion channels in the membrane- are they open or closed?

Answer 3

Vm is determined by Ek, ENa, and the relative membrane permeability to K+ and Na+. Since the membrane is much more permeable to K+ than Na +, Vm is much closer to Ek: Vm=-70mV

Answer 4

Initial depolarization: Synaptic transmission. Typical cells have both excititory (drive closer to threshhold) and inhibitory (away). The cell integrates this in the trigger zone (hillock),

Answer 5

Voltage-gated Na+ channels inactivate. Na+ stops entering the cell. Voltage-gated K+ channels open. Channel opening is delayed from when the threshold is reached. K+ exits the cell. Therefore, Vm is pulled back towards Ek

Answer 6

1)Voltage-gated channels close. Overshoot / hyperpolarization / after hyperpolarization. 2) Relative permeability to Na+ and K+ returns to resting baseline. 3) Role of Na+/K+ pump

Answer 7

Cardiac muscle bears some similarities to skeletal muscle, as well as important differences. Like skeletal myocytes (and axons for that matter), in the resting state a given cardiac myocyte has a negative membrane potential. Within the cell, K+ is the principal cation, and phosphate and the conjugate bases of organic acids are the dominant anions. Outside the cell, Na+ and Cl- predominate. A notable difference between skeletal and cardiac myocytes is how each elevates the myoplasmic Ca2+ to induce contraction. When skeletal muscle is stimulated by somatic motor axons, influx of Na+ quickly depolarizes the skeletal myocyte and triggers calcium release from the sarcoplasmic reticulum. However, in cardiac myocytes the release of Ca2+ from the sarcoplasmic reticulum is induced by Ca2+ influx into the cell through voltage-gated calcium channels on the sarcolemma. This phenomenon is called calcium-induced calcium release and increases the myoplasmic free Ca2+ concentration causing muscle contraction. In both muscle types, after a delay (the absolute refractory period), potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization. The voltage-gated calcium channels in the cardiac sarcolemma are generally triggered by an influx in sodium during the "0" phase of the action potential (see below). Cardiac muscle is a syncytium in which the cardiac muscle cells are so tightly bound that when one of these cells is excited the action potential spreads to all of them. This "plateau" phase of the cardiac action potential (absent in pacemaker cells), is sustained by a balance between inward movement of Ca2+

Answer 8

The distance that current will propagate depends on: Ri = internal resistance. Higher Ri -> smaller spread of current. Rm = membrane resistance. Higher Rm -> larger spread of current. Cm = membrane capacitance. Higher Cm -> slower spread of current. Axons are poor “cables”. Rm is low, Ri is high, and Cm is high. Therefore, for a typical axon, λ (Length constant of axon) is small: ~ 1mm. However, axons are well-built for active propagation. The membrane resistance is a function of the number of open ion channels, and the axial resistance is generally a function of the diameter of the axon.

Answer 9

Depends on voltage-gated Na+ channels

Answer 10

Ranges from 0.5 – 120 m/s. Depends on: Myelination and Axon width. Axons carrying the most time-sensitive information are larger and more myelinated

Answer 11

Multiple sclerosis- Damages myelin in central nervous system. Guillain-Barré syndrome- Immune response against myelin in axons innervating muscles. Myelin sheaths are damaged, slowing and/or blocking conduction (and therefore nervous system function)

Answer 12

Action potential generation and conduction depends on voltage-gated Na+ channels. If these channels are blocked, current can only be propagated passively

Answer 13

Extracellular blockers of voltage-gated Na+ channels – abolish action potentials: Tetrodotoxin (e.g., from pufferfish), Saxitoxin (e.g., from clams). Modulators of voltage-gated Na+ channel function – may shift voltage dependence; alter ion selectivity: Batrochotoxin (e.g., from frogs)

Answer 14

(λ) is a mathematical constant used to quantify the distance that a graded electric potential will travel along a neurite via passive electrical conduction. The greater the value of the length constant, the further the potential will travel. A large length constant can contribute to spatial summation—the electrical addition of one potential with potentials from adjacent areas of the cell. In the neuron it is the distance by which voltage has dropped to 37% of its initial value

Answer 15

are the gaps (approximately 1 micrometer in length) formed between the myelin sheaths generated by different cells. A myelin sheath is a many-layered coating, largely composed of a fatty substance called myelin, that wraps around the axon of a neuron and very efficiently insulates it. At nodes of Ranvier, the axonal membrane is uninsulated and therefore capable of generating electrical activity. Demyelination may play a role in chronic pain. Nodes of Ranvier are disrupted. Isolated clusters of voltage-gated Na+ channels. This could contribute to the perception that there is pain present.

Answer 16

Myelin decreases capacitance and increases electrical resistance across the cell membrane (the axolemma). Thus, myelination helps prevent the electrical current from leaving the axon. It has been suggested that myelin permits larger body size by maintaining agile communication between distant body parts.

Answer 17

A nerve cell (or indeed any cell) is surrounded by a plasma membrane, made of phospholipid. The cell can be seen as two electrically-conducting regions (the cytoplasm and the extracellular fluid, both electrolyte solutions), separated by a thin layer of insulator (the plasma membrane). The membrane therefore acts as a capacitor! For this reason, we portray charge as lined up on either side of the membrane, just as charge builds up on each plate of a capacitor. Because of the capacitance of the membrane, any change in voltage across the membrane, as for example the depolarization associated with an action potential, will take time to occur: the membrane has a time constant, τ. This will affect the velocity at which the action potential is propagated. If you think of myelin as effectively increasing the thickness of the cell membrane, you would expect this to decrease its capacitance (thicker insulating layer). Although this is a gross oversimplification of what is really happening, it turns out that myelination does have the effect of reducing membrane capacitance. However, myelination also increases the membrane resistance, so overall the time constant (= RC) might not actually change much. Myelination increases conduction velocity mainly because the increased membrane resistance increases the length constant, λ.

Answer 18

usually smaller and myelinated, therefore propagation speed is slower

Answer 19

slightly larger and myelinated, therefore slighlty faster propagation speed

Answer 20

Ion Channels are Membrane Proteins that open and close (“gate”). When open, they only allow certain substances to pass (“selectivity”). Ion channels are present both in the plasma membrane and in membranes of intracellular organelles. Their gating is controlled by a vast array of stimuli including: temperature (hot/cold), mechanical deformation, membrane potential, Extracellular chemicals (taste, olfaction, neurotransmitters), Intracellular second messengers (ATP, cAMP, Ca2+). Some channels respond to multiple stimuli (e.g., both hot and capsaicin; cold and menthol, voltage and Ca2+)

Answer 21

There is no consistent method for naming channels. Thus, some are named according to their gating stimuli: e.g., nicotinic acetylcholine receptors (AChRs), GABAA receptors. Some are named for their selectivity (sodium channels, potassium channels, aquaporins), and others given names that reflect neither gating nor selectivity (TransientReceptorPotential channels, Orai, ryanodine receptors).

Answer 22

Ion channels are essential for function of diverse cell types including muscle (triggering contraction/relaxation), neurons (sensory transduction, signal propagation, neurotransmitter release, postsynaptic responses, plasticity), T lymphocytes (activation), and pancreatic β cells (insulin release).

Answer 23

Ion channels are important targets of natural products that are used for predation (cone snails, spiders, kraits, cobras, scorpions) and defense (puffer fish, bees, plant alkaloids). Ion channels are important therapeutic targets (pain, arrhythmia, hypertension).

Answer 24

Mutations of ion channels are responsible for diverse human diseases (severe chronic pain, complete lack of pain, periodic paralysis, familial hemiplegic migraine, arrhythmias, diabetes mellitus, immunodeficiency, congenital cataracts and nephrogenic diabetes insipidus)

Answer 25

Ion channels have diverse structures. Voltage gated channels of the KV, NaV and CaV families have four membrane-spanning domains, each of which contains six alpha helices (S1 through S6). In KV channels, each domain is a separate polypeptide, and four of these assemble to form the channel. In NaV and CaV the four domains (I, II, III, and IV) are linked into a single polypeptide. For both, the S4 helices have positively charged residues (lys or arg) at every third position and are the structures that “sense” voltage. Also for both, the S5 and S6 helices and the “P loop”, which connects them, assemble to form the ion conducting pathway. The predominant selectivity of these channels is indicated by their names. These channels are essential for the function of nerve and muscle cells.

Answer 26

are either directly coupled to ion channels (i.e. the receptor and channel are part of the same protein), or activate second messenger pathways which can affect physically separate ion channels. The former are called ionotropic receptors, and the latter are called metabotropic (and are not a topic of this lecture). A given neurotransmitter typically is able to activate both ionotropic and metabotropic receptors.

Answer 27

Within the ionotropic category are the pentameric ligand gated channels, also called the Cys-loop family of neurotransmitter receptor channels, including the GABAARs, GlyRs, nAChRs and 5-HT3Rs (all of which are named after the ligand most important for controlling their gating). These channels are heteropentamers. Each subunit has four transmembrane alpha helices (M1 through M4), with M2 assembling around the central, ion-conducting pathway. These channels are either selective for the permeation of chloride (inhibitor receptor), or allow permeation of both sodium and potassium (with a slight preference for sodium) (excititory receptor).

Answer 28

The ability of synapses to modify their synaptic strength in response to activity is a fundamental property of the nervous system and may be an essential component of learning and memory. There are four classes of ionotropic glutamate receptors, namely NMDA receptor, AMPA receptor, Delta receptor and kainate receptors. They are believed to play critical roles in synaptic plasticity. At many synapses in the brain, transient activation of NMDA receptors leads to a persistent modification in the strength of synaptic transmission mediated by AMPA receptors and kainate receptors can act as the induction trigger for long-term changes in synaptic transmission. Ionotropic glutamate receptors are tetrameric ligand gated channels. In the case of the kind of glutamate receptors called NMDA receptors, two of the four subunits bind glutamate (causes channel to open or close) and the other two bind glycine (constitutively bound).

Answer 29

Chloride channels of the CLC family are dimers in which each subunit has an ion permeation pathway. Each permeation pathway can gate open and closed independently of the other, although there is also another gate which controls both pathways simultaneously. Some members of the CLC family are H+-Cl- exchangers. The CLC chloride channels are important for stabilizing the resting membrane potential

Answer 30

are tetramers in which each of the subunits contains a permeation pathway for water molecules. Strictly speaking, these are “anti-ion” channels since they exclude all ions including protons. Aquaporins are expressed in cells/tissues where rapid movement of water is important, such as the kidney. In addition to the four water channels, the assemblage of the four subunits also produces a central pore which may allow ion permeation and be gated between open and closed.

Answer 31

Selective permeation depends on a number of factors. Selectivity varies. Some channels are highly selective. For example, only about 1:10,000 ions permeating a KV channel is not K+, and CaV channels select for Ca2+ over Na+ by ~3000:1. Others are moderately selective (1 ion in 12 permeating NaV channels is not sodium). Others display little selectivity: nicotinic AChRs show only a slight preference for Na+ over K+ (~1.3 fold).

Answer 32

an important determinant of what substances will pass through a channel. Channels that are cation selective do not allow permeation of anions, and those that are anion selective do not allow permeation of cations. Ionic valence is also important.

Answer 33

ions larger than the narrowest part of the pore will be rejected. However, size is by no means the only determinant: the crystal ionic radius of sodium is smaller than that of potassium, yet KV channels are highly selective (only about 1 ion In 10,000 is not K+).

Answer 34

Ions are energetically stabilized in solution by waters of hydration, which make the ions effectively larger in size. Furthermore, the waters of hydration essentially mask small differences in the size of the ions. Thus, ions must be substantially de-hydrated before they pass through the channel pore, To compensate for this dehydration, which is energetically unfavorable, the ion is stabilized within the pore by energetic interactions with the amino acids forming the pore (but not too much, or the ion would stay “stuck” in the pore). The energetic interactions of the ion with the pore can occur with amino acid side chains (positive/basic residues: lysine or arginine; negative/acidic residues glutamate or aspartate), with backbone carbonyls (negative) or alpha helix dipoles (N-terminal: positive; C-teminal: negative). These and other interactions depend on the sign, valence and size of the ion.

Answer 35

Multiple binding sites can increase selectivity: If an ion interacts with multiple sites while traversing the channel pore, even relatively slight differences in the strength of interaction between preferred and non-preferred ions at each site can result in a significant enhancement of overall selectivity for the preferred ion.

Answer 36

This section discusses the gating of the channels important for generating the electrical impulse (action potential) in nerve and skeletal muscle, which will be discussed in detail in subsequent lectures. For both KV and NaV, gating is controlled by membrane potential (Vm). If one experimentally controls the voltage and steps it from a steady negative potential where the channels are in their resting state to a more positive steady voltage, the opening of the channels produces a current whose time course is a reflection of how many channels are open. A typical K+ current (IK) in response to a step from -60 mV to +20 mV is shown above, together with a cartoon summarizing the underlying mechanism. Specifically, the channel has an “activation gate”, which can rotate around a center pivot point. This rotation is controlled by a “voltage-sensing” charge, indicated by the “+” sign on the activation gate. When the inside of the cell has a negative potential with respect to the outside, the gate is held in its closed position and the current is zero. When the inside is made positive, the gate rotates to its open position and K+ ions flow out of the cell (upward deflection in the current). This process is called activation. When the inside of the cell is made negative the gate rotates back to the closed position and the current decays away. This process is the reverse of activation and is called deactivation. The gating of NaV channels is more complicated because they have both an activation gate and an inactivation gate. The operation of the NaV activation gate is similar to that of the KV activation gate. At negative potentials it is closed and making the inside of the cell positive causes the NaV activation gate to swing open (“activate”) and sodium ions to flow into the cell (INa). The second gate, the inactivation gate, is open at the resting potential because the activation gate occludes access to a site within the inner end of the pore at which the inactivation gate can bind. However, after the activation gate opens, the inactivation gate closes, a process called “inactivation,” causing the current to decay to zero during a maintained depolarization. Note that inactivation is not the same process as deactivation. Also, the reversal of inactivation is called “removal of inactivation.

Answer 37

Actual KV and NaV structures important for selectivity and gating. Selectivity occurs within a central, ion conducting pathway formed by the four KV subunits or four repeats of NaV, where this central pathway is surrounded by S5 and S6 helices and connecting P loop contributed by the each of the four subunits or repeats (see above for the structures of NaV and KV). Voltage sensing (the + sign in the cartoon activation gate) is accomplished by the S4 helices. These helices contain positively-charged Lys or Arg residues at every third position and translocate in response to changes in voltage across the membrane. It is not known precisely how the translocation of the S4 helices (4 per channel) cause activation, although the movement corresponding to the opening of the activation gate likely corresponds to a hinge-like motion of the S6 segments around a conserved glycine. This hinge-like re-orientation can be seen by comparing the crystal structures of the closed and open states of two bacterial K+ channels which are related to, but simpler than, KV channels (the bacterial channels have four subunits, each with only two transmembrane helices).Note that when the activation gates are closed, they occlude access to an enlarged space (“vestibule”) through which the ions pass before/after transiting a narrower constriction (the “selectivity filter” nearer to the extracellular side. The inactivation gate of NaV channels is formed by the cytoplasmic loop which connects repeats III and IV. When this cytoplasmic III-IV linker folds over the inner end of the central ion-conducting pathway, the channels is in a closed/inactivated state.

Answer 38

The location of the selectivity filter near the extracellular side, and of the vestibule nearer the intracellular side of the KV/NaV channels, together with the location of the activation/inactivation gates near the intracellular side, has the result that many channel modifying reagents have access to their sites of action only from one side of the membrane, and that this access may require that the channel be open. For example, tetrodoxin (TTX) is a charged molecule that cannot cross the membrane. When it is added to the extracellular side, it binds within the entrance of the pore, just above the selectivity filter of NaV. The binding of TTX is essentially independent of the position of the activation/inactivation gates; TTX has no effect when added intracellularly. The binding of the commonly used local anesthetic, lidocaine, is much different. Lidocaine is a tertiary amine and equilibrates between de-protonated (neutral) and protonated (positively charged) forms. The protonated form is dominant at physiological pH and cannot cross the membrane, whereas the de-protonated form can. Protonated lidocaine has no effect on NaV from the extracellular side but can block the channel from the intracellular side (thereby producing local anesthesia). The block from the intracellular side can only occur if the protonated lidocaine can access the vestibule, which requires that the activation gate be open and that the inactivation gate not be closed. That is, the lidocaine block is “state-dependent.” Note that if lidocaine is at its binding site within the vestibule, it can be trapped there if the activation or inactivation gates are closed.

Answer 39

a group of ion channels located mostly on the plasma membrane of numerous human and animal cell types. There are about 28 TRP channels that share some structural similarity to each other. TRP ion channels are widely expressed in many different tissues and cell types, where they are involved in diverse physiological processes, such as sensation of different stimuli or ion homeostasis. Most TRPs are non-selective cation channels, only few are highly Ca2+ selective, some are even permeable for highly hydrated Mg2+ ions. This channel family shows a variety of gating mechanisms, with modes of activation ranging from ligand binding, voltage and changes in temperature to covalent modifications of nucleophilic residues. Activated TRP channels cause depolarization of the cellular membrane, which in turn activates voltage-dependent ion channels, resulting in a change of intracellular Ca2+ concentration; they serve as gatekeeper for transcellular transport of several cations (such as Ca2+ and Mg2+), and are required for the function of intracellular organelles (such as endosomes and lysosomes). Because of their function as intracellular Ca2+ release channels, they have an important regulatory role in cellular organelles.

Answer 40

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances release calcium stores from the sarcoplasmic reticulum and may promote entry of calcium from the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience: acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some affected individuals will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high. Variations of the CACNA1S and RYR1 genes increase the risk of developing malignant hyperthermia. Malignant hyperthermia susceptibility is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to increase the risk of a severe reaction to certain drugs used during surgery. In most cases, an affected person inherits the altered gene from a parent who is also at risk for the condition.

Answer 41

neuron receptor proteins that signal for muscular contraction upon a chemical stimulus. They are cholinergic receptors that form ligand-gated ion channels in the plasma membranes of certain neurons and on the presynaptic and postsynaptic sides of the neuromuscular junction. As ionotropic receptors, nAChRs are directly linked to ion channels and do not use second messengers (as metabotropic receptors do). Nicotinic acetylcholine receptors are the best-studied of the ionotropic receptors.

Answer 42

are multiprotein aqueous channels that penetrate the nuclear envelope connecting the nucleus and the cytoplasm. NPCs consist of multiple copies of roughly 30 different proteins known as nucleoporins (NUPs). Due to their essential role in controlling nucleocytoplasmic transport, NPCs have traditionally been considered as structures of ubiquitous composition. The overall structure of the NPC is indeed conserved in all cells, but new evidence suggests that the protein composition of NPCs varies among cell types and tissues. Moreover, mutations in various nucleoporins result in tissue-specific diseases. These findings point towards a heterogeneity in NPC composition and function. This unexpected heterogeneity suggests that cells use a combination of different nucleoporins to assemble NPCs with distinct properties and specialized functions. Although a recent report suggests that large ribonucleoprotein (RNP) complexes can be exported from the nucleus by budding of the nuclear membrane2, the vast majority of nucleocytoplasmic exchange occurs through NPCs

Answer 43

NUPs contain a very limited set of domains, including transmembrane domains (which are found in only four mammalian and yeast NUPs), α-helices, β-propellers, WD domains and FG repeats. FG repeats are the most common domains found in NUPs. NUPs carrying 4 to 48 FG repeats fill the central channel of the NPC, thereby forming a meshwork that is responsible for controlling nucleocytoplasmic transport and that determines the pore permeability limit. Most NUPs associate in biochemically stable subcom- plexes that, due to the eightfold rotational symmetry of the NPC, are present in eight or multiples of eight copies. nuclear pores are highly dynamic and plastic molecular assemblies that can adapt in response to cellular changes.

Answer 44

a group of proteins involved in transporting molecules between the cytoplasm and the nucleus of a eukaryotic cell. The inside of the nucleus is called the karyoplasm (or nucleoplasm). Generally, karyopherin-mediated transport occurs through the nuclear pore, which acts as a gateway into and out of the nucleus. Most proteins require karyopherins to traverse the nuclear pore.

Answer 45

Ran is a small G protein that is essential for the translocation of RNA and proteins through the nuclear pore complex. During nuclear import, cargo release occurs when the transport receptor interacts with the small RAN GTPase bound to GTP (RAN·GTP) inside the nucleus. On the other hand, in nuclear export RAN·GTP is required for the assembly of the receptor–cargo complex, and cargo release takes place upon GTP hydrolysis.

Answer 46

NPCs show an eightfold rotational symmetry, and they consist of a membrane-embedded scaffold built around a central transport channel, a cytoplasmic ring, a nuclear ring and eight filaments attached to each ring. The nuclear filaments are connected to a distal nuclear ring to form the ‘nuclear basket’ of the NPC. most NPC scaffold members have residence times that are much longer than a cell cycle. These components are only exchanged when nuclear pores disassemble during mitosis and, remarkably, they do not turn over in post-mitotic cells. The extreme longevity of scaffold components was recently confirmed in rat brains using a pulse-chase approach. These findings indicate that the NPC scaffold is likely the most stable cellular structure. At the molecular level, NPCs consist of multiple copies of ~30 different NUPs

Answer 47

Ions and small molecules can freely diffuse through the pore, but molecules larger than 40–60 kDa need to be actively shuttled by nuclear transport receptors. In a very simplistic way, nuclear transport of most proteins and RNAs starts when an import or export receptor binds to its cargo in the cytoplasm or the nucleus, respectively. The complexes then move through the diffusion barrier of the pore by interacting with the FG repeats of NUPs to finally reach their destination compartment where cargoes are released. transport of mRNAs and proteins would take place through dif- ferent NPC regions. If different transport paths indeed exist within NPCs, the FG-rich meshwork of the central transport channel might not be as uniform as previously assumed.

Answer 48

Export complexes are dissociated at the cytoplasmic filaments of the NPC by the action of RAN-binding protein 1 (RANBP1) or RANBP2 and RAN GTPase-activating protein (RANGAP). Together they induce RAN to hydro- lyse bound GTP and render export complex dissociation irreversible. Therefore, a RAN·GTP gradient between the nucleus and the cytoplasm is what confers nucleocytoplasmic transport directionality

Answer 49

Regulation of genome architecture: nuclear envelope is tightly associated with heterochromatin, whereas NPCs are surrounded by decondensed chro- matin regions. Regulation of gene expression: It is clear that NPCs can indirectly control gene expression by regulating the transport of RNAs and gene regulatory factors through the nuclear envelope. Moreover, NPCs and NUPs have a direct role in transcriptional control. It is clear that NPCs can indirectly control gene expression by regulating the transport of RNAs and gene regulatory factors through the nuclear envelope. Moreover, NPCs and NUPs have a direct role in transcriptional control. Furthermore, NPCs have been shown to modulate gene expression by promoting the assembly and/or maintenance of gene loops. Gene looping seems to be a general phenomenon of Pol II transcribed genes.

Answer 50

Selective transport of specific transcription factors and mRNA molecules has a crucial role in differentiation and development. nuclear transport receptors also seem to have differential affinities for various FG domains, and individual NUPs have been shown to regulate specific transport processes. a change in NPC composition (by the addition of the transmembrane NUP210 (also known as GP210) is required for myo- genesis and neuronal differentiation in mice122. NUP210 expression is induced during differentiation, and the incorporation of this nucleoporin into the NPC does not affect its nuclear transport properties but changes the activity of genes important for myogenesis. A similar role in myogenesis has been described for the pore filament protein NUP358. The increased expression of this NUP is required for myoblast differenti- ation, but the mechanisms through which NUP358 regu- lates myogenesis are still unknown.

Answer 51

since the discovery of the first tissue-specific NPC component, NUP210, at least five additional NUPs, including NUP45, NUP50, NUP133, NUP155 and Aladin, exhibit differential expression levels among different cell types or tissues, suggesting an heterogeneity of NPCs. since the discovery of the first tissue-specific NPC com- ponent, NUP210, at least five additional NUPs, including NUP45, NUP50, NUP133, NUP155 and Aladin, exhibit differential expression levels among different cell types or tissues, suggesting an heterogeneity of NPCs. Loss-of-function mutations in several genes encoding NUPs are also associated with tissue-specific phenotypes. These findings expose an unanticipated diversity in the expression and function of NUPs and support a model for NPC heterogeneity, in which nuclear pores with different compositions will have distinct properties in nucleo- cytoplasmic transport, genome organization and/or gene expression regulation. The idea of an ‘NPC code’ is very interesting and prob- ably not unexpected. Increasing evidence has shown that organisms use the specialization of cellular structures to achieve cell type- and tissue-specific functions. The finding that myoblasts and ES cells have NPCs of different composi- tion than myotubes and neuroprogenitors and that the change in pore configuration is required for the differen- tiation of both cell types strongly supports the specialized function of nuclear pores, and so does the role of the tissue-specific NUP133 in neuronal differentiation and of NUP154 in gametogenesis.

Answer 52

any protease enzyme whose catalytic mechanism involves a metal. An example of this would be meltrin which plays a significant role in the fusion of muscle cells during embryo development, in a process known as myogenesis.

Answer 53

zinc-dependent endopeptidases; other family members are adamalysins, serralysins, and astacins. The MMPs belong to a larger family of proteases known as the metzincin superfamily. Collectively, they are capable of degrading all kinds of extracellular matrix proteins, but also can process a number of bioactive molecules. They are known to be involved in the cleavage of cell surface receptors, the release of apoptotic ligands (such as the FAS ligand), and chemokine/cytokine inactivation. MMPs are also thought to play a major role on cell behaviors such as cell proliferation, migration (adhesion/dispersion), differentiation, angiogenesis, apoptosis, and host defense.

Answer 54

a family of peptidase proteins. It is also known as the adamalysin family or MDC family(metalloproteinase-like, Disintegrin-like, cysteine rich). The ADAMs family has been implicated in the control of membrane fusion, cytokine and growth factor shedding, and cell migration, as well as processes such as muscle development, fertilization, and cell fate determination. The functional ADAM metalloproteinases are involved in "ectodomain shedding" of diverse growth factors, cytokines, receptors and adhesion molecules. The ADAM family are thus fundamental to many control processes in development and homeostasis, and unsurprisingly they are also linked to pathological states when their functions are dysregulated, including cancer, cardiovascular disease, asthma, Alzheimer's disease. In combination with low doses of herceptin, selective ADAM10 inhibitors decrease proliferation in HER2 over-expressing cell lines while inhibitors, that do not inhibit ADAM10, have no impact. These results are consistent with ADAM10 being a major determinant of HER2 shedding, the inhibition of which, may provide a novel therapeutic approach for treating breast cancer and a variety of other cancers with active HER2 signaling. The presence of the product of this gene in neuronal synapses in conjunction with protein AP2 has been seen in increased amounts in the hippocampal neurons of Alzheimer's disease patients

Answer 55

Proteins move between compartments using three fundamental mechanisms: (1) gated transport between the cytosol and the nucleus (nuclear transport), (2) transmembrane transport across a membrane from the cytosol into an organelle through translocators (e.g., protein synthesis and mitochondrial import), and (3) vesicular transport in which membrane bound transport intermediates move proteins and lipids from one compartment to another.

Answer 56

In a liver cell (hepatocyte) the cytosol is the largest compartment (54%) followed by mitochondria (22%). The compartments that are the focus of this topic occupy about 15% (rough ER: 9% and smooth ER plus Golgi: 6%). The nucleus is about 6% of the cell volume and lysosomes, endosomes and peroxisomes are about 1% each. Despite the fact that most diagrams of cells give you the impression that there is substantial empty space in the cell, this is misleading; even the cytosol is packed with vesicles, sugars, and macromolecular complexes for synthesizing and degrading proteins.

Answer 57

Cells need to select and move molecules (proteins and lipids) from their site of synthesis to their site of function. The major organelles of exocytosis are the endoplasmic reticulum (both rough ER and smooth ER), the Golgi complex, the plasma membrane, and the vesicles and tubules that serve as transport intermediates.

Answer 58

There are at least six major functions of the ER: (1) synthesis of lipids (phospholipid, ceramide, and cholesterol; primarily in the smooth ER), (2) control of cholesterol homeostasis (cholesterol sensor and synthesis), (3) storage of Ca+2 (rapid uptake and release), (4) synthesis of proteins on membrane bound ribosomes (rough ER), (5) co-translational folding of proteins and early posttranslational modifications, and (6) quality control.

Answer 59

It needs to make some proteins that will be secreted and other proteins that have domains that reside in membranes. Add to this the need to send each protein to the right place to be secreted, and membrane proteins need to go to the right destination in a cell (e.g., the correct organelle or the right place in the plasma membrane). A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are transported into the ER. The ER signal sequence on a newly formed polypeptide chain “directs” the engaged ribosome to the ER membrane. The signal sequence is recognized by a signal recognition particle (SRP), which is a complex of six proteins bound to one RNA molecule.

Answer 60

an abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes. The binding pocket of the SRP that recognizes the signal sequence is flexible, allowing it to bind a variety of signal sequences. The SRP is multifunctional – it binds the nascent polypeptide chain, the ribosome, and a receptor on the ER membrane. When the SRP binds the ribosome and nascent chain, it induces a pause in translation until it binds also binds the SRP receptor. Once the ribosome is attached to the translocon, which is a protein channel allowing the polypeptide chain to enter the ER, the SRP detaches and can bind another ribosome and nascent polypeptide. The signal sequence is typically cleaved shortly after the polypeptide chain enters the ER lumen. The mRNA molecule remains permanently bound to the ER as part of a polyribosome, while the ribosomes that move along it are recycled; at the end of each round of protein synthesis, the ribosomal subunits are released and rejoin the common pool in the cytosol. It is made up of 1 RNA and 6 proteins. The signal sequence binding pocket is lined by methionines and is flexible

Answer 61

the complex that transports nascent polypeptides with a targeting signal sequence into the interior (cisternal or lumenal) space of the endoplasmic reticulum (ER) from the cytosol. This translocation process requires the protein to cross a hydrophobic lipid bilayer. The same complex is also used to integrate nascent proteins into the membrane itself (membrane proteins). On binding an ER signal sequence (which acts as a start-transfer signal), the translocator (also called the translocon) opens its pore, allowing the transfer of the polypeptide chain across the lipid bilayer as a loop. The translocon provides an aqueous pore through the ER membrane; this pore is always closed unless a protein is being made. Opening of the pore and movement of the growing polypeptide are regulated by the ribosome at the cytosolic face and by BiP (chaperone) at the lumenal face. The translocon may also be bidirectional; there is evidence that it may be the exit route for proteins that are not folded properly and have to leave the ER to be degraded. After the protein has been completely translocated, the pore closes, but the translocator now opens laterally within the lipid bilayer, allowing the hydrophobic signal sequence to diffuse into the bilayer, where it is rapidly degraded.

Answer 62

The translocon is regulated by the ribosome at the cytoplasmic face and by BiP, a chaperone protein, at the luminal face. BiP binds the protein as it is entering the ER and helps it to fold and to interact with a protein disulfide isomerase to create disulfide bonds, which also are important for proper folding of the protein.

Answer 63

For membrane proteins with a single TMD and with the amino terminal in the ER lumen (designated a Type I membrane protein), the mRNA contains a sequence recognized by the translocon as a “stop transfer” signal. This stop transfer domain is released by the translocon and the remainder of the protein, the C-terminal end, is synthesized on the cytosolic face. A single TMD transmembrane protein can also have the opposite orientation (designated a Type II membrane protein).

Answer 64

These proteins have internal stop and start sequences. Some transporters and channels have many transmembrane domains. Most amino acid transporters have 12 TMDs, the CFTR (cystic fibrosis transmembrane conductance regulator) has 12 TMDs, and the voltage-gated sodium channel responsible for the neuronal and muscle action potential has 24 TMDs.

Answer 65

Many membrane proteins have a preformed carbohydrate complex added to an asparagine in the ER lumen. The single letter abbreviation for asparagine is N and thus, this is referred to as N-linked glycosylation. The cluster of carbohydrates has two important purposes. It helps to keep proteins from aggregating when hydrophobic domains are exposed. In addition, glucose residues act as tags to monitor unfolded proteins. Some of the sugars are removed in the ER and then complex glycosylation occurs in the Golgi complex.

Answer 66

packages proteins inside the cell before they are sent to their destination; it is particularly important in the processing of proteins for secretion. A fundamental feature is that the ER and Golgi never fuse – the lumens of these organelles have different membrane proteins and different lumenal environments. The movement of cargo and membrane proteins occurs by budding of vesicles, fusion of some of these vesicles into tubules, and fusion of these vesicles/tubules with the next compartment. This process requires the formation of coated vesicles and adapter proteins that recognize (1) the cargo, (2) the coat, and (3) the membrane proteins/lipids destined to move.

Answer 67

a type of organelle in the cells of eukaryotic organisms that forms an interconnected network of flattened, membrane-enclosed sacs or tubes known as cisternae. The membranes of the ER are continuous with the outer membrane of the nuclear envelope. Endoplasmic reticulum occurs in most types of eukaryotic cells, including the most primitive Giardia, but is absent from red blood cells and spermatozoa. There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis. The membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, and continuous with, the outer layer of the nuclear envelope. Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane-bound vesicles shuttle proteins between these two compartments.

Answer 68

The coat formation helps in physical deformation of the planar membrane. Coats are assembled from soluble cytoplasmic proteins and lipids at sites of vesicle formation. If you recall what you learned about cholesterol sensing, it will be sensible that some proteins that move from the ER to the Golgi should be reutilized by sending them back to the ER (the SCAP protein that bound SREBP is one such protein). Thus, in addition to vesicles moving “forward” from the ER to the Golgi and from the Golgi to the plasma membrane or other compartments, like the lysosome, there must be “backward” moving vesicles for retrieval. Not surprisingly, there are different coat proteins (plus different adaptors) for these different vesicular pathways. The three well-studied coat structures (see Fig. 7) are COPII (ER to Golgi: forward), COPI (Golgi to ER: backward), and clathrin (Golgi to plasma membrane). There are specific amino acid motifs on the cytosolic side of the membrane proteins that are recognized by the adaptor proteins for recruitment into the vesicles (again, remember the SCAP protein in cholesterol sensing).

Answer 69

a protein complex that coats vesicles transporting proteins from the cis end of the Golgi complex back to the rough endoplasmic reticulum (ER), where they were originally synthesized and between golgi compartments. This type of transport is termed as retrograde transport, in contrast to the anterograde transport associated with the COPII protein.

Answer 70

a protein that plays a major role in the formation of coated vesicles. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskelia interact they form a polyhedral lattice that surrounds the vesicle. it performs critical roles in shaping rounded vesicles in the cytoplasm for intracellular trafficking. Clathrin-coated vesicles (CCV) selectively sort cargo at the cell membrane, trans-Golgi network, and endosomal compartments for multiple membrane traffic pathways. After a vesicle buds into the cytoplasm, the coat rapidly disassembles, allowing the clathrin to recycle while the vesicle gets transported to a variety of locations. adapter molecules (adaptin) is responsible or self-assembly and recruitment. this leads to budding. The assembly of the coat is thought to introduce curvature into the membrane, which leads in turn to the formation of uniformly sized coated buds. The adaptins bind both clathrin triskelions and membrane-bound cargo receptors, thereby mediating the selective recruitment of both membrane and cargo molecules into the vesicle. The pinching-off of the bud to form a vesicle involves membrane fusion; this is helped by the GTP-binding protein dynamin, which assembles around the neck of the bud. The coat of clathrin-coated vesicles is rapidly removed shortly after the vesicle forms.

Answer 71

a type of vesicle coat protein that transports proteins from the rough endoplasmic reticulum to the Golgi apparatus. This process is termed anterograde transport, in contrast to the retrograde transport associated with the COPI protein.

Answer 72

There are several major functions of the Golgi Complex: (1) Synthesis of complex sphingolipids from the ceramide backbone, (2) Additional post-translational modifications of proteins and lipids, (3) Proteolytic processing, (4) Sorting of proteins and lipids for post-Golgi compartments. There are multiple compartments, usually referred to as “stacks” (like a stack of pancakes), that include cis, medial, trans Golgi and finally the trans Golgi network (TGN) with the cis Golgi closest to the ER. There is also a gradient across the Golgi stacks of enzymes performing different functions; for example, sulfation occurs primarily in the trans Golgi and TGN. Most of the glycosylation of lipids and proteins occurs in the Golgi. Vesicles bud from the TGN and are sorted to their correct destination. Those vesicles that are going to be secreted move into two general pathways: constitutive (secreted right away) and regulated (secreted only when the proper signal is received).

Answer 73

a specialized part of the cell body (or soma) of a neuron that connects to the axon. The axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon. For many years, it had been believed that the axon hillock was the usual site of action potential initiation. It is now thought that the earliest site of action potential initiation is found just adjacent, in the initial (unmyelinated) segment of the axon. However, the positive point, at which the action potential starts, varies between cells. It can also be altered by hormonal stimulation of the neuron, or by second messenger effects of neurotransmitters.

Answer 74

a type of neuroglia. Their main functions are to provide support and insulation to axons in the central nervous system of some vertebrates, equivalent to the function performed by Schwann cells in the peripheral nervous system. Oligodendrocytes do this by creating the myelin sheath, which is 80% lipid and 20% protein.

Answer 75

the principle glia of the peripheral nervous system (PNS). Glial cells function to support neurons and, in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle. There are two types of Schwann cell, myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath.

Answer 76

a medical condition in which there is a rapid-onset weakness of the limbs as a result of an acute polyneuropathy, a disorder affecting the peripheral nervous system. Ascending paralysis, weakness beginning in the feet and hands and migrating towards the trunk, is the most typical symptom, and some subtypes cause change in sensation or pain. During the acute phase, the disorder can be life-threatening with about a quarter requiring admission to intensive care admission for mechanical ventilation. Some are affected fluctuations in the function of the autonomic nervous system with dangerous abnormalities in heart rate and blood pressure. The disease is usually triggered by an infection, which provokes immune-mediated nerve dysfunction. All forms of Guillain–Barré syndrome are autoimmune diseases, due to an immune response to foreign antigens (such as infectious agents) that mistargets host nerve tissues through a mechanism known as molecular mimicry. The targets of such immune attack are thought to be gangliosides, compounds naturally present in large quantities in human peripheral nerve tissues. The most well-described antecedent infection is the bacterium Campylobacter jejuni. In addition, cytomegalovirus has a known association with GBS. In many cases, identification of a specific cause is impossible. Some cases may be triggered by the influenza virus,. The end result of this autoimmune process is an attack on the peripheral nerves and damage to myelin, the fatty insulating layer of the nerve, and a nerve-conduction block leading to muscle paralysis that may be accompanied by sensory or autonomic disturbances. In mild cases, nerve axon (the long slender conducting portion of a nerve) function remains intact and recovery can be rapid if remyelination occurs. In severe cases, axonal damage occurs, and recovery depends on the regeneration of this important tissue. About 80% of patients have myelin loss; in the remaining 20%, the pathological hallmark is axon loss.

Answer 77

places closer to north pole have higher incidence of MS. This may be due to over hygination. Places like africa with poor sanitation challenges their immune syste earlier on in life. There is also a social economical gradiant with MS

Answer 78

It is more common in males than females (maybe because women seek more health care). 50% require cane or more support for ambulation within 10 years of onset 30% will become wheelchair or bed bound. Average Life Span Decreased by <5 years. MS is leading cause of disability in young women and second leading cause of disability in young men in USA. MS is a common CNS disease causing disability and multiple neurological problems in patients. Inflammatory demyelination. Decreased speed of conduction in the nerves. Multiple lesions on the CNS (brain and spine), leading to multiple symptoms and problems. There is no cure but there are multiple treatments for MS. Some MS therapies can also increase the speed of conduction on the nerves.

Answer 79

First complaint is usualy mental fatigue (this is because pt needs to recruit more neurons to complete the same task. They use more of their brain). This is the first demyelinating event. During the relapsing-remitting phase, the disease is characterized by frequent inflammation, demyelination, axonal transection, and remyelination. Relapses (onset of deficiet that come and go) are more frequent and complete recovery from disability generally occurs. During the secondary-progressive phase of the disease, inflammation and relapses occur infrequently, axonal loss is increased and disability progresses. MRI-defined plaque burden and clinical impairment accumulate over time. During the transitional phase, their disability slowly gets worst. The most advanced stage is secondary progressive, where ppl with MS experience a double-whammy of attacks during remission periods and symptoms that steadily get worse.

Answer 80

A clinically isolated syndrome (CIS) is an individual's first neurological episode, caused by inflammation or demyelination of nerve tissue. An episode may be monofocal, in which symptoms present at a single site in the central nervous system, or multifocal, in which multiple sites exhibit symptoms. Brain lesions associated with a clinically isolated syndrome may be indicative of multiple sclerosis (MS). In order for such a diagnosis, multiple sites in the central nervous system must present lesions, typically over multiple episodes, and for which no other diagnosis is likely. A clinically definitive diagnosis of MS is made once an MRI detects lesions in the brain, consistent with those typical of MS. Other diagnostics include cerebrospinal fluid analysis and evoked response testing.

Answer 81

Several factors are believed to be triggers for MS, but the exact cause is unknown. The reasons for variations in the prevalence and incidence of MS worldwide are not understood. Environmental factors and genetic predisposition have been suggested as possible reasons for these variations. Infection also may play a role in the development of MS. Several viruses that cause demyelinating encephalomyelitis have been observed in humans. Demyelination can be induced in research animals infected with viruses. An individual in the general population (from northern Europe/northern North America) has a lifetime risk of MS of approximately 0.1%–0.2%. Risk in an individual with an affected family member is increased, roughly in proportion to the degree of shared genetic information. In an identical twin, the relative risk approaches 200 times that in the general population (~ 25%). Multiple genes must be involved in susceptibility. Environmental factors must also be involved in disease pathogenesis

Answer 82

Immunopathogenesis of MS is the result of 3 components of the disease: Inflammatory activity, Possible remyelination or regeneration, Axonal destruction. The circle represents the current view that the optimal time for intervention is early, when inflammatory processes predominate, to limit damage and later degeneration. Even if we don’t intervene at disease onset, it is vitally important to act somewhere along this path before axonal degeneration occurs. We now know that axonal degeneration is a correlate of disability. We also have discovered that axonal degeneration and brain atrophy emerge early in the disease and it is essential to act early and aggressively to limit the destructive phase of the disease. In this presentation, we will show how the treatment of patients with MS is undergoing a paradigm shift as we better understand the course and immunopathogenesis of MS.

Answer 83

During the relapsing-remitting phase, the disease is characterized by frequent inflammation, demyelination, axonal transection, and remyelination. Relapses are more frequent and complete recovery from disability generally occurs. During the secondary-progressive (SP) phase of the disease, inflammation and relapses occur infrequently, axonal loss is increased, and disability progresses. MRI-defined plaque burden and clinical impairment accumulate over time. Up to 80% of patients present with a relapsing form of MS, and studies suggest that 50% of these cases develop SPMS within 10 years. Inflammatory events detectable by magnetic resonance imaging (MRI) occur early in the disease, probably before the first clinical manifestation. MRI activity is 8 to 10 times more frequent than clinical relapses and signals an ongoing pathology that does not pause. The measures we have are not specific but represent temporary features such as inflammation and edema, and more permanent features such as damage to myelin and axonal transection. We know that axonal loss occurs as a result of inflammatory attacks and that signs of progressive degeneration appear early as signaled by accumulation of lesion load, increased black hole volume, and atrophy. Cognitive and physical disability can occur as sequelae from relapses and with time due to progressive deterioration with or without a relapse. Treatment early in the disease may change this representation of the MS disease process and improve the course and long-term outcome.

Answer 84

Inflammatory processes occurring early in MS lead to demyelination and axonal loss . Early on, there is also some regeneration. MS is characterized by acute inflammation, demyelination, axonal damage, oligodendrocyte loss, and neuronal loss. Although neuronal loss in MS has been recognized since the early descriptions of MS by Charcot, the disease has classically been viewed as a disorder of oligodendrocyte loss and demyelination, with relative sparing of axons. It is now clear that there is extensive neuronal loss in the disease. The axonal injury and loss occurs early in the disease process and is rapidly progressive. Indeed, the loss of axons beyond a critical threshold is thought to result in a loss of compensatory mechanisms, thereby triggering the conversion of relapsing-remitting MS (RRMS) to secondary progressive MS (SPMS)

Answer 85

a virus of the herpes family, and is one of the most common viruses in humans. It is best known as the cause of infectious mononucleosis (glandular fever). It is also associated with particular forms of cancer, such as Hodgkin's lymphoma, Burkitt's lymphoma, nasopharyngeal carcinoma, and conditions associated with human immunodeficiency virus (HIV), such as hairy leukoplakia and central nervous system lymphomas. There is evidence that infection with the virus is associated with a higher risk of certain autoimmune diseases, especially dermatomyositis, systemic lupus erythematosus, rheumatoid arthritis, Sjögren's syndrome, and multiple sclerosis.

Answer 86

Within the CNS, T cells activate microglia cells/macrophages to enhance phagocytic activity, production of cytokines, such as TNF-α, release of toxic mediators, such as nitric oxide, propagating demyelination, and axonal loss. Antibodies crossing the BBB or locally produced by B cells contribute to this process. The upregulation of Na and Ca channels on the axon as well as mitochondrial dysfunction and loss of trophic support contribute to axonal dysfunction resulting in axonal disintegration and degeneration. The inflammatory response is regulated by anti-inflammatory cytokines, such as IL-10 or TGF-β, as well as IL-2 inducing programmed cell death (apoptosis) in immunoreactive T lymphocytes.

Answer 87

Inflammatory CNS demyelination in MS is believed to be orchestrated by T cells, which migrate from the periphery across the BBB and are activated to become effector T cells (orange). Under physiologic conditions, potentially autoreactive encephalitogenic effector T cells are under the control of regulatory T cells (blue). It is assumed that the mechanisms that maintain the balance between encephalitogenic and regulatory function are disrupted in MS. Dendritic cells are thought to have an important role in this interplay through maintenance of the frequency and function of regulatory T cells.

Answer 88

APCs (dendritic cells) are central players in innate immune responses and are involved in the maintenance of peripheral tolerance by means of promotion of suppressor regulatory T-cell and anti-inflammatory Th2-cell responses. Abnormally activated (mature) antigen-presenting dendritic cells can be found in patients with MS. This activation results in increased production of pro-inflammatory cytokines that lead to aberrant activation of Th1 and Th17 pro-inflammatory responses. Activated encephalitogenic adaptive immune effectors (such as Th1 cells, Th17 cells, CD8+ cells, and B cells) express surface molecules that allow them to penetrate the BBB and to enter the CNS. The presence of autoreactive immune effectors, together with abnormally activated CNS astrocytes and microglia, lead to increased production of reactive species, excitotoxicity, autoantibody production, and direct cytotoxicity, which are all involved in demyelination, axonal and neuronal damage that is present in patients with MS.

Answer 89

B cells may have a role in MS pathogenesis not only through production of self-reactive antibodies, but also through their antigen-presenting and cytokine secreting role, which can lead to abnormal T-cell and macrophage activation, thereby perpetuating CNS inflammation and damage. Depletion of these autoreactive B cells is therefore a therapeutic strategy in MS. The implication of B cells in MS pathogenesis goes beyond the simplistic view of these cells as mere autoantibody producers. It is now well established that B cells also act as efficient antigen-presenting cells and cytokine producers, and are involved in key steps of effector T-cell activation and induction of regulatory T cells. B-cell–depleting agents should be expected to alter all aspects of B-cell participation in the immune response, and T-cell and B-cell cross-talk in particular, therefore potentially broadening their therapeutic effects in MS.

Answer 90

The name multiple sclerosis refers to the scars (sclerae – better known as plaques or lesions) that form in the nervous system. These lesions most commonly affect the white matter in the optic nerve, brain stem, basal ganglia, and spinal cord, or white matter tracts close to the lateral ventricles. The function of white matter cells is to carry signals between grey matter areas, where the processing is done, and the rest of the body. The peripheral nervous system is rarely involved. To be specific, MS involves the loss of oligodendrocytes, the cells responsible for creating and maintaining a fatty layer—known as the myelin sheath—which helps the neurons carry electrical signals (action potentials). This results in a thinning or complete loss of myelin and, as the disease advances, the breakdown of the axons of neurons. When the myelin is lost, a neuron can no longer effectively conduct electrical signals. A repair process, called remyelination, takes place in early phases of the disease, but the oligodendrocytes are unable to completely rebuild the cell's myelin sheath. Repeated attacks lead to successively less effective remyelinations, until a scar-like plaque is built up around the damaged axons. These scars are the origin of the symptoms and during an attack magnetic resonance imaging (MRI) often shows more than ten new plaques. This could indicate that there are a number of lesions below which the brain is capable of repairing itself without producing noticeable consequences. Another process involved in the creation of lesions is an abnormal increase in the number of astrocytes due to the destruction of nearby neurons. A number of lesion patterns have been described.

Answer 91

a radiographic feature depicting demyelinating plaques through the corpus callosum, arranged at right angles along medullary veins (callososeptal location). They are a relatively specific sign for multiple sclerosis (MS)

Answer 92

The open ring sign is a relatively specific sign for demyelination, most commonly multiple sclerosis (MS), and is helpful in distinguishing between ring enhancing lesions. Interestingly open rign enhancement is not seen in neuromyelitis optica (NMO). The enhancing component is thought to represent advancing front of demyelination and thus favours the white matter side of the lesion. The open part of the ring will therefore usually point towards the grey matter. If contrast shows in brain their is a disruption in the blood brain barrier. THis allows passage of inflammatory cells in to the CNS-> 5-10x more frewuent than relapses.

Answer 93

Fatigue, Walking impairment, Spasticity, Cognitive impairment, Bladder dysfunction, Pain, Mood instability, Sexual dysfunction. Gait description: Ataxic, Spastic, Paretic, Foot drop. Disabling impact: Negatively impacts work productivity, employability, and income, Impairs activities of daily living, ie, driving, Significantly affects QOL.

Answer 94

Prolonged Conduction Velocity = Demyelination. Can measure conduction multiple ways: Visual stimulation, Brainstem stimulation, auditory pathways, Somatosensory system stimulation. Main use is to define a lesion as “demyelinating”, or identify a “second lesion” if MRI is negative

Answer 95

The visual evoked potential is a gross electrical signal recorded from the occipital cortex in response a systematic change in some visual event such as a flashing light or an alternating chequered pattern. The recording electrode is placed over the occipital cortex and the amplitude and latency of the waveform generated can be measured.

Answer 96

Demyelination Produces Proliferation of Sodium Channels Along the Axon. With Increased Sodium Entry into the Cell, There is Slowing of Nerve Conduction. Ultimately there may be Reversal of Sodium-Calcium Exchanger > Calcium Influx, Producing Calcium-Mediated Nerve Injury. Sodium Channel Blockers Phenytoin and Flecainide Preserve Axons in EAE. Ultimately slowing down conduction.

Answer 97

Targets potassium channels in naked axons, which helps to bring the resting potential back to normal and enhancing conduction. Indication: to improve walking speed in patients with MS. This is not a disease-modifying therapy; it is symptom management. Mechanism: K+ channel blockade. Enhances conduction of action potentials in demyelinated axons through inhibition of K+ channels. Appropriate candidates. Contraindicated in patients with history of seizures or moderate to severe renal impairment. Monitor patients with history of multiple urinary tract infections.

Answer 98

A nuclear membrane, also known as the nuclear envelope, is the double lipid bilayer membrane which surrounds the genetic material and nucleolus in eukaryotic cells. The nuclear membrane consists of two lipid bilayers—the inner nuclear membrane, and the outer nuclear membrane. The space between the membranes is called the perinuclear space, a region contiguous with the lumen (inside) of the endoplasmic reticulum.

Answer 99

a dense (~30 to 100 nm thick) fibrillar network inside the nucleus of most cells. It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope.

Answer 100

imported: RNA polymerases, DNA polymerases Protein kinases/phosphatases Histones, RNA binding proteins, snRNAs, Transcription factors. Exported: 40s ribosomal subunits, 60s ribosomal subunits tRNAs, mRNAs, RNA binding proteins snRNAs Transcription factors

Answer 101

Beta-catenin is a proto-oncogene. Mutations of this gene are commonly found in a variety of cancers: in primary hepatocellular carcinoma, colorectal cancer, ovarial carcinoma, breast cancer, lung cancer and glioblastoma. It has been estimated that approximately 10% of all tissue samples sequenced from all cancers display mutations in the CTNNB1 gene. Most of these mutations cluster on a tiny area of the N-terminal segment of β-catenin: the β-TrCP binding motif. Loss-of-function mutations of this motif essentially make ubiquitinylation and degradation of β-catenin impossible. It will cause β-catenin to translocate to the nucleus without any external stimulus and continuously drive transcription of its target genes. Increased nuclear β-catenin levels have also been noted in basal cell carcinoma (BCC), head and neck squamous cell carcinoma (HNSCC), prostate cancer (CaP), pilomatrixoma (PTR) and medulloblastoma (MDB) These observations may or may not implicate a mutation in the β-catenin gene: other Wnt pathway components can also be faulty. Similar mutations are also frequently seen in the β-catenin recruiting motifs of APC. Hereditary loss-of-function mutations of APC cause a condition known as Familial Adenomatous Polyposis. Affected individuals develop hundreds of polyps in their large intestine. Most of these polyps are benign in nature, but they have the potential to transform into deadly cancer as time progresses. Somatic mutations of APC in colorectal cancer are also not uncommon. Beta-catenin and APC are among the key genes (together with others, like K-Ras and SMAD4) involved in colorectal cancer development. The potential of β-catenin to change the previously epithelial phenotype of affected cells into an invasive, mesenchyme-like type contributes greatly to metastasis formation.

Answer 102

about 30% of Nups contain phenylalanine (f) glycine (g) repeats. have either cohesive or non cohersive FG repeats that are hydrophobic with interspanning hydrophillic regions. Is very dynamic allowing the movement of protein complexes, this vibrational energy drives the movement of proteins. The FG repeats are typically clustered in specific region allowing other regions to have other domains (coiled- coil) to ancher the nups. these fg repeats can also interact with hydrophobic regions on other proteins. FG-repeats on nuclear pore complex proteins in the central channel have been proposed to interact with FGs on transport receptors to facilitate trans- port of soluble proteins. the cen- tral channel of the NPC contains FG- rich nucleoporins that interact to form a hydrophobic network. This network acts as a mesh to exclusively allow the transport of small molecules. For molecules of larger size the transport is al- lowed only when they are linked to transport receptors. These cargo-receptors complexes may then recognize the FG motifs in nucleoporins and subsequently cause the local dis- ruption of the network.

Answer 103

1) size-filtering diffusion (large hydrophilic molecules cant get through). 2) spontaneous migration (amphiphilic molecules that expose hydrophobic region can get through. Occurs due to change of surface hedrophobicity). 3) facilitated transport(transport occurs agains concentration gradient and is energy depedent).

Answer 104

cargo interacts with carrier and is transported across NPC with energy coupled dissociation

Answer 105

has cargo selective binding. he formation of import or export complexes is dependent on the interaction of β-karyopherins with small peptide motifs present in protein cargos. These motifs are generally called nuclear localization signals (NLS) or nuclear export signals (NES). The molecular mechanisms underlying RNA transport are usually more complex since they include the additional participation of adaptor proteins that interact with the transport receptor and the RNA. Besides being responsible for the nuclear import of proteins bearing a classical NLS, importin-β additionally participates in other import pathways that are independent of importin-α. These include the import of UsnRNPs and replication protein A (RPA). Importin-β is the only transport receptor of the karyopherin-β family that uses adaptor proteins to interact with the respective substrates. However, as the other members of the karyopherin-β family, importin-β also participates in nuclear import pathways that do not require adaptor proteins to interact with the substrates.

Answer 106

Here, importin α recognizes and binds cargo in the cytoplasm, linking it to the β-karyopherin, importin β (32). Importin β then mediates interaction of the trimeric complex with the nuclear pore as it translocates into the nucleus. Once the import complex reaches the nucleus, it is dissociated by RanGTP. Binding of RanGTP to importin β causes a conformational change that results in the release of the importin α-cargo complex (33). An autoinhibitory region on the importin β-binding (IBB) domain of importin α (34, 35), the nucleoporin Nup2 (Nup50 or Npap60 in vertebrates) (36–38), and the export receptor for importin α, Cse1/RanGTP (CAS/RanGTP in vertebrates) (39) then work together to deliver the cargo into the nucleus. Finally, Cse1/RanGTP recycles importin α back to the cytoplasm in preparation for another round of import

Answer 107

Some molecules or macromolecular complexes need to be exported from the nucleus to the cytoplasm, as do ribosome subunits and messenger RNAs. Thus there is an export mechanism similar to the import mechanism. In the classical export scheme, proteins with a nuclear export sequence (NES) can bind in the nucleus to form a heterotrimeric complex with an exportin and RanGTP (for example the exportin CRM1). The complex can then diffuse to the cytoplasm where GTP is hydrolysed and the NES-protein is released. CRM1-RanGDP diffuses back to the nucleus where GDP is exchanged to GTP by RanGEFs. This process is also energy dependent as it consumes one GTP. Export with the exportin CRM1 can be inhibited by Leptomycin B. There are different export pathways through the NPC for each RNA class that exists. RNA export is also signal mediated (NES); the NES is in RNA-binding proteins (except for tRNA which has no adapter). It is notable that all viral RNAs and cellular RNAs (tRNA, rRNA, U snRNA, microRNA) except mRNA are dependent on RanGTP. Conserved mRNA export factors are necessary for mRNA nuclear export. Export factors are Mex67/Tap (large subunit) and Mtr2/p15 (small subunit). In higher eukaryotes, mRNA export is thought to be dependent on splicing which in turn recruits a protein complex, TREX, to spliced messages. TREX functions as an adapter for TAP, which is a very poor RNA binding protein. However, there are alternative mRNA export pathways that do not rely on splicing for specialized messages such as histones. Recent work also suggest an interplay between splicing-dependent export and one of these alternative mRNA export pathways for secretory and mitochondrial transcripts.

Answer 108

Like all GTPases, Ran exists in two different states: either GTP-bound or GDP-bound. Depending on the nucleotide bound, Ran interacts with several different proteins, most of which have well defined roles in nucleocytoplasmic transport. To change its nucleotide state, Ran needs to interact with regulatory proteins, the GTPase-activating protein RanGAP1 and the guanine nucleotide exchange factor RCC1. RanGTP binds with high affinity to all receptors of the importin β family, irrespective of whether they are involved in import or export. Association of RanGTP with import receptors dis- rupts their interactions with import cargo, thus releasing the cargo into the nucleoplasm. In contrast, association of RanGTP with export receptors enhances their affinity for export cargo and is required for export. The distinct intra- cellular localization of RCC1 and RanGAP1 to the nuclear and cytoplasmic compartment, respectively, ensures that formation and disassembly of import and export com- plexes take place only in the desired cellular compartment . In interphase, a complete GTPase cycle requires shuttling of Ran into and out of the nucleus. After nuclear envelope breakdown, nucleotide exchange is believed to take place predominantly at condensed chromosomes, while hydroly- sis takes place throughout the cell.

Answer 109

Nuclear import of Ran does not rely on simple diffusion as Ran's small size would permit, but instead is stimulated by soluble transport factors. This facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier. NTF2 binds RanGDP initially to NPCs and probably also mediates translocation of the NTF2-RanGDP complex to the nuclear side of the NPCs. A direct NTF2-RanGDP interaction is crucial for this process, since point mutations that disturb the RanGDP-NTF2 interaction also interfere with Ran import. The subsequent nuclear accumulation of Ran also requires GTP, but not GTP hydrolysis. The release of Ran from NTF2 into the nucleus, and thus the directionality of Ran import, probably involves nucleotide exchange to generate RanGTP, for which NTF2 has no detectable affinity, followed by binding of the RanGTP to an importin beta family transport receptor.

Answer 110

a nuclear protein that binds to Ran-GDP promoting GDP dissociation and subsequent bind- ing of GTP in the active centre of Ran

Answer 111

an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different nuclear localized proteins may share the same NLS. An NLS has the opposite function of a nuclear export signal, which targets proteins out of the nucleus. A protein translated with a NLS will bind strongly to importin (aka karyopherin), and, together, the complex will move through the nuclear pore. At this point, Ran-GTP will bind to the importin-protein complex, and its binding will cause the importin to lose affinity for the protein. The protein is released, and now the Ran-GTP/importin complex will move back out of the nucleus through the nuclear pore. A GTPase-activating protein (GAP) in the cytoplasm hydrolyzes the Ran-GTP to GDP, and this causes a conformational change in Ran, ultimately reducing its affinity for importin. Importin is released and Ran-GDP is recycled back to the nucleus where a Guanine nucleotide exchange factor (GEF) exchanges its GDP back for GTP.

Answer 112

The protein encoded by this gene is located in the nuclear envelope. It has protein similarity to nuclear transport factor 2. This protein functions as a nuclear export factor in both RAN (Ras-related nuclear protein)- and CRM1 (required for chromosome region maintenance)-dependent pathways. It is found to stimulate the export of U1 snRNA in RAN- and CRM1-dependent pathways and the export of tRNA and mRNA in a CRM1-independent pathway. The encoded protein heterodimerizes with Tap protein and may regulate the ability of Tap protein to mediate nuclear mRNA export. The use of alternate polyadenylation sites has been found for this gene.

Answer 113

This gene is one member of a family of nuclear RNA export factor genes. Common domain features of this family are a noncanonical RNP-type RNA-binding domain (RBD), 4 leucine-rich repeats (LRRs), a nuclear transport factor 2 (NTF2)-like domain that allows heterodimerization with NTF2-related export protein-1 (NXT1), and a ubiquitin-associated domain that mediates interactions with nucleoporins. Alternative splicing results in transcript variants. The LRRs and NTF2-like domains are required for export activity. The encoded protein of this gene shuttles between the nucleus and the cytoplasm and binds in vivo to poly(A)+ RNA. It is the vertebrate homologue of the yeast protein Mex67p.

Answer 114

a short amino acid sequence of 4 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex using nuclear transport. It has the opposite effect of a nuclear localization signal, which targets a protein located in the cytoplasm for import to the nucleus. The NES is recognized and bound by exportins. In silico analysis of known NESs found the most common spacing of the hydrophobic residues to be LxxxLxxLxL, where "L" is a hydrophobic residue (often leucine) and "x" is any other amino acid; the spacing of these hydrophobic residues may be explained by examination of known structures that contain an NES, as the critical residues usually lie in the same face of adjacent secondary structures within a protein, which allows them to interact with the exportin. Ribonucleic acid (RNA) is composed of nucleotides, and thus, lacks the nuclear export signal to move out of the nucleus. As a result, most forms of RNA will bind to a protein molecule to form a ribonucleoprotein complex to be exported from the nucleus. Nuclear export first begins with the binding of Ran-GTP (a G-protein) to exportin. This causes a shape change in exportin, increasing its affinity for the export cargo. Once the cargo is bound, the Ran-exportin-cargo complex moves out of the nucleus through the nuclear pore. GTPase activating proteins (GAPs) then hydrolyze the Ran-GTP to Ran-GDP, and this causes a shape change and subsequent exportin release. Once no longer bound to Ran, the exportin molecule loses affinity for the nuclear cargo as well, and the complex falls apart. Exportin and Ran-GDP are recycled to the nucleus separately, and guanine exchange factor (GEF) in the nucleus switches the GDP for GTP on Ran.

Answer 115

The best understood system for the transport of macromolecules between the cytoplasm and the nucleus is the classical nuclear import pathway. In this pathway, a protein containing a classical basic nuclear localization signal (NLS) is imported by a heterodimeric import receptor consisting of the β-karyopherin importin β, which mediates interactions with the nuclear pore complex, and the adaptor protein importin α, which directly binds the classical NLS.

Answer 116

Many transport receptors are members of the importin β superfamily and are called collectively “β-karyopherins.” Cargo proteins can bind directly to β-karyopherins; however, in classical nuclear import, the interaction between the β-karyopherin and the cargo is mediated by the adaptor molecule importin α.

Answer 117

is recruited to messenger ribonucleoprotein (mRNP) complexes generated by splicing. In contrast, Aly does not associate with mRNPs assembled on identical mRNAs that already have no introns or with heterogenous nuclear RNP (hnRNP) complexes. Aly is recruited during spliceosome assembly, and then becomes tightly associated with the spliced mRNP. Aly shuttles between the nucleus and cytoplasm, and excess recombinant Aly increases both the rate and efficiency of mRNA export in vivo. Consistent with its splicing-dependent recruitment, Aly co-localizes with splicing factors in the nucleus. We conclude that splicing is required for efficient mRNA export as a result of coupling between the splicing and the mRNA export machineries.

Answer 118

The trafficking of most cargos that move between the nucleus and the cytoplasm involves karyopherin-mediated receptors, and transport directionality is determined by a gradient of the GTP-bound state of the small GTPase Ran. Bulk mRNA is exported via the non-karyopherin heterodimer of Nxf1 and Nxt1. The Nxf1-Nxt1 heterodimer is recruited to the mRNP via ALY. correct nuclear processing and recruitment of an export factor targets an mRNA for export from the nucleus. However, if a transcript is not properly processed, it can be recognized by the nuclear surveillance machinery, retained in the nucleus and degraded by the nuclear exosome. Following completion of proper nuclear processing and the recruitment of an export receptor, an mRNP is considered to be export competent. This export-competent mRNP is specifically targeted to the NPC via its export receptor. For some transcribed genes, the positioning of the respective chromatin region near the NPC might facilitate export by physically linking the processes. Binding of the export receptor to the FG- Nups is required for NPC docking and translocation of the mRNP. The final step of mRNP translocation through the NPC involves directional release into the cytoplasm.

Answer 119

ATP-dependent RNA helicase DDX19B is an enzyme that in humans is encoded by the DDX19B gene. DEAD box proteins, characterized by the conserved motif Asp-Glu-Ala-Asp (DEAD), are putative RNA helicases. They are implicated in a number of cellular processes involving alteration of RNA secondary structure such as translation initiation, nuclear and mitochondrial splicing, and ribosome and spliceosome assembly. Based on their distribution patterns, some members of this family are believed to be involved in embryogenesis, spermatogenesis, and cellular growth and division. This gene encodes a DEAD box protein, which exhibits RNA-dependent ATPase and ATP-dependent RNA-unwinding activities. This protein is recruited to the cytoplasmic fibrils of the nuclear pore complex, where it participates in the export of mRNA from the nucleus. Multiple alternatively spliced transcript variants encoding different isoforms have been found for this gene.

Answer 120

Regulation at NPC: pore permeability and protein expression and stability. Transport receptor: expression and sequestration. Cargo: posttranslational modification, porttranscriptional modification, and intermolecular or intramolecular interactions

Answer 121

1) Sequestration through binding to cytoskeleton/ chromatin, 2) affinity enhancement through phosphorylation and conformation change,3) intra molecular masking through inhibition, 4) intermolecular masking

Answer 122

The intracellular location and regulation of proteins within each cell is critically important and is typically deregulated in disease especially cancer. The clinical hypothesis for inhibiting the nucleo-cytoplasmic transport is based on the dependence of certain key proteins within malignant cells. This includes a host of well-characterized tumor suppressor and oncoproteins that require specific localization for their function. This aberrant localization of tumour suppressors and oncoproteins results in their their respective inactivation or over-activation. This incorrect localization occurs actively via the nuclear pore complex that spans the nuclear envelope and is mediated by transport receptors. Accordingly, given the significant need for novel, specific disease treatments, the nuclear envelope and the nuclear transport machinery have emerged as a rational therapeutic target in oncology to restore physiological nucleus/cytoplasmic homeostasis.

Answer 123

The protein encoded by this gene is a member of the RAD51 protein family which assist in repair of DNA double strand breaks. RAD51 family members are homologous to the bacterial RecA and yeast Rad51. The protein is highly conserved in most eukaryotes, from yeast to humans.

Answer 124

in many tumors, there is upregulation of factors related to export allowing TF (p53) to be mislocalized into the cytoplasm. If you can block specefic cargos to be exported into the cytoplasm

Answer 125

In principle, cycles of membrane budding and fusion permit the lumen of any of these organelles to communicate with any other and with the cell exterior by means of transport vesicles. there is an extensive network of outbound and inbound traffic routes. Some organelles, most notably mitochondria and (in plant cells) plastids do not take part in this communication and are isolated from the traffic between organelles.

Answer 126

The translocation channel is a proteinaceous pore lined by natively unfolded proteins that use hydrophobic interactions to form a selective filter. The signals are typically exposed linear sequences of amino acids, often containing multiple basic and/or hydrophobic residues. They are not cleaved following transport. With the notable exceptions of those for most mRNAs, and that for Ran.GDP, cargo transporters are generally members of the karyopherin family. Transport is regulated by the entropy barrier created by the Nups, the presence of the Ran.GDP/GTP gradient, the interaction of cargo transporters with nucleoporins, and cargo modifications/interactions that impact association with receptors. Diseases occur when proteins are mislocalized, either because the selectivity barrier is compromised, or mutations perturb key interactions.

Answer 127

A common pool of ribosomes is used to synthesize the proteins that stay in the cytosol and those that are transported into the ER. The ER signal sequence on a newly formed polypeptide chain directs the engaged ribosome to the ER membrane. The mRNA molecule remains permanently bound to the ER as part of a polyribosome, while the ribosomes that move along it are recycled; at the end of each round of protein synthesis, the ribosomal subunits are released and rejoin the common pool in the cytosol.

Answer 128

Two ways in which a sorting signal can be built into a protein. (A) The signal resides in a single discrete stretch of amino acid sequence, called a signal sequence, that is exposed in the folded protein. Signal sequences often occur at the end of the polypeptide chain (as shown), but they can also be located internally. (B) A signal patch can be formed by the juxtaposition of amino acids from regions that are physically separated before the protein folds. Alternatively, separate patches on the surface of the folded protein that are spaced a fixed distance apart can form the signal.

Answer 129

The SRP and its receptor are thought to act in concert. The SRP binds to both the exposed ER signal sequence and the ribosome, thereby inducing a pause in translation. The SRP receptor in the ER membrane, which is composed of two different polypeptide chains, binds the SRP-ribosome complex and directs it to the translocator. In a poorly understood reaction, the SRP and SRP receptor are then released, leaving the ribosome bound to the translocator in the ER membrane. The translocator then inserts the polypeptide chain into the membrane and transfers it across the lipid bilayer. Because one of the SRP proteins and both chains of the SRP receptor contain GTP-binding domains, it is thought that conformational changes that occur during cycles of GTP binding and hydrolysis ensure that SRP release occurs only after the ribosome has become properly engaged with the translocator in the ER membrane. The translocator is closed (indicated schematically by the ER-lumenal plug) until the ribosome has bound, so that the permeability barrier of the ER membrane is maintained at all times.

Answer 130

On binding an ER signal sequence (which acts as a start-transfer signal), the translocator opens its pore, allowing the transfer of the polypeptide chain across the lipid bilayer as a loop. After the protein has been completely translocated, the pore closes, but the translocator now opens laterally within the lipid bilayer, allowing the hydrophobic signal sequence to diffuse into the bilayer, where it is rapidly degraded.

Answer 131

The amino acid chain of transmembrane proteins, which often are transmembrane receptors, passes through a membrane one or several times. They are inserted into the membrane by translocation, until the process is interrupted by a stop-transfer sequence, also called a membrane anchor sequence. These complex membrane proteins are at the moment mostly understood using the same model of targeting that has been developed for secretory proteins. However, many complex multi-transmembrane proteins contain structural aspects that do not fit the model. Seven transmembrane G-protein coupled receptors (which represent about 5% of the genes in humans) mostly do not have an amino-terminal signal sequence. In contrast to secretory proteins, the first transmembrane domain acts as the first signal sequence, which targets them to the ER membrane. This also results in the translocation of the amino terminus of the protein into the ER membrane lumen. This would seem to break the rule of "co-translational" translocation which has always held for mammalian proteins targeted to the ER. Type 1 (and none others) have the singlaling sequence, the others have sequences directly recognoized by translocon. Topogenic sequences determine orientation of ER membrane proteins. The difference in the orientation of these proteins depends largely on whether there is a high density of positively charged amino acids (+++) on the N-terminal side of the single internal signal-anchor (SA) sequence. The positive aa will be on the cytosol side

Answer 132

the part of SRP that recognizes signal sequence, contains methionine residues, it is flexible allowing it to recognize different sequences

Answer 133

Almost as soon as a polypeptide chain enters the ER lumen, it is glycosylated on target asparagine amino acids (Asn-X-Ser/Thr, X can be any amino acid except proline). The precursor oligosaccharide is transferred to the asparagine as an intact unit in a reaction catalyzed by a membrane-bound oligosaccharyl transferase enzyme. As with signal peptidase, one copy of this enzyme is associated with each protein translocator in the ER membrane.

Answer 134

Dolichols play a role in the co-translational modification of proteins known as N-glycosylation in the form of dolichol phosphate. Dolichols function as a membrane anchor for the formation of the oligosaccharide Glc3-Man9-GlcNAc2 (where Glc is glucose, Man is mannose, and GlcNAc is N-acetylglucosamine). This oligosaccharide is transferred from the dolichol donor onto certain asparagine residues (onto a specific sequence that is "Asn-X-Ser/Thr") of newly forming polypeptide chains.

Answer 135

The processing pathway is highly ordered, so that each step shown is dependent on the previous one. Processing begins in the ER with the removal of the glucoses from the oligosaccharide initially transferred to the protein. Then a mannosidase in the ER membrane removes a specific mannose. The remaining steps occur in the Golgi stack, where Golgi mannosidase I first removes three more mannoses and N-acetyl glucosamine transferase I then adds an N-acetyl glucosamine, which enables mannosidase II to remove two additional mannoses. This yields the final core of three mannoses that is present in a complex oligosaccharide. At this stage, the bond between the two N-acetylglucosamines in the core becomes resistant to attack by a highly specific endoglycosidase (Endo H). Since all later structures in the pathway are also Endo H-resistant, treatment with this enzyme is widely used to distinguish complex from high-mannose oligosaccharides. Finally, additional N-acetylglucosamines, galactoses, and sialic acids are added. These final steps in the synthesis of a complex oligosaccharide occur in the cisternal compartments of the Golgi apparatus. Three types of glycosyl transferase enzymes act sequentially, using sugar substrates that have been activated by linkage to the indicated nucleotide. The membranes of the Golgi cisternae contain specific carrier proteins that allow each sugar nucleotide to enter in exchange for the nucleoside phosphates that are released after the sugar is attached to the protein on the lumenal face.

Answer 136

Transport vesicles bud from one compartment (donor) and fuse with another (target) compartment. In the process, soluble components are transferred from lumen to lumen. Note that membrane is also transferred, and that the original orientation of both proteins and lipids in the donor-compartment membrane is preserved in the target-compartment membrane. Thus, membrane proteins retain their asymmetric orientation, with the same domains always facing the cytosol.

Answer 137

The trans-Golgi network (TGN) is a major secretory pathway sorting station that directs newly synthesized proteins to different subcellular destinations. The TGN also receives extracellular materials and recycled molecules from endocytic compartments. Vesicles from the endoplasmic reticulum (via the vesicular-tubular clusters) fuse with the network and subsequently progress through the stack to the trans-Golgi network, where they are packaged and sent to their destination.

Answer 138

a motor protein (also called molecular motor or motor molecule) in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell center. Thus, they are called "minus-end directed motors." This form of transport is known as retrograde transport. In contrast, kinesins, which are motor proteins that move toward the microtubules' plus end, are called plus-end directed motors.

Answer 139

One stack of Golgi contains multiple cisternae from cis to trans. The cis, medial and trans cisternae are defined functionally by the enzymes specific to each type and thus the different functions carried out. Each of the cis, medial and trans cisternae can have multiple cisternae. Specific enzymes for posttranslational modifications are enriched in different regions of the Golgi. Early acting enzymes are in the cis Golgi and late acting enzymes are in the trans Golgi and TGN.

Answer 140

Those ER resident proteins that escape from the ER are returned to the ER by vesicular transport. (A) The KDEL receptor present in vesicular tubular clusters and the Golgi apparatus, captures the soluble ER resident proteins and carries them in COPI-coated transport vesicles back to the ER. Upon binding its ligands in this low-pH environment, the KDEL receptor may change conformation, so as to facilitate its recruitment into budding COPI-coated vesicles. (B) The retrieval of ER proteins begins in vesicular tubular clusters and continues from all parts of the Golgi apparatus. In the neutral-pH environment of the ER, the ER proteins dissociate from the KDEL receptor, which is then returned to the Golgi for reuse.

Answer 141

a sequence in the amino acid structure of a protein which keeps it from secreting from the endoplasmic reticulum (ER). It also targets proteins from other locations (such as the cytoplasm) to the ER. Proteins can only leave the ER after this sequence has been cleaved off. The KDEL sequence is responsible for retrieval of ER lumenal proteins from the Golgi apparatus. K—Lysine, D—Aspartic acid, E—Glutamic acid, L—Leucine

Answer 142

Mannose 6-phosphate is a sorting signal for lysosomal proteins. It binds to a receptor that is targeted to vesicles that fuse with the late endosome. In the late endosome pH is low and the receptor and M6P tagged protein dissociate. The endosome delivers the protein to the lysosome and the receptor is recycled. The M6P residues that direct proteins to lysosomes are generated in the cis-Golgi by two Golgi-resident enzymes. Step 1: An N-acetylglucosamine (GlcNAc) phosphotransferase transfers a phosphorylated GlcNAc group to carbon atom 6 of one or more mannose residues. Because only lysosomal enzymes contain sequences (red) that are recognized and bound by this enzyme, phosphorylated GlcNAc groups are added specifically to lysosomal enzymes. Step 2: After release of a modified protein from the phosphotransferase, a phosphodiesterase removes the GlcNAc group, leaving a phosphorylated mannose residue on the lysosomal enzyme.

Answer 143

one of several rare inborn errors of metabolism in which glycosylation of a variety of tissue proteins and/or lipids is deficient or defective. Congenital disorders of glycosylation are sometimes known as CDG syndromes. They often cause serious, sometimes fatal, malfunction of several different organ systems (especially the nervous system, muscles, and intestines) in affected infants. The most common subtype is CDG-Ia (also referred to as PMM2-CDG) where the genetic defect leads to the loss of phosphomannomutase 2, the enzyme responsible for the conversion of mannose-6-phosphate into mannose-1-phosphate. Mutations in the PMM2 gene cause CDG-Ia. This gene provides instructions for making an enzyme called phosphomannomutase (PMM). The PMM enzyme is involved in a process called glycosylation, which attaches groups of sugar molecules (oligosaccharides) to proteins. Glycosylation modifies proteins so they can perform a wider variety of functions. Mutations in the PMM2 gene lead to the production of an abnormal PMM enzyme with reduced activity. Without a properly functioning PMM enzyme, glycosylation cannot proceed normally. As a result, incorrect oligosaccharides are produced and attached to proteins. The wide variety of signs and symptoms in CDG-Ia are likely due to the production of abnormally glycosylated proteins in many organs and tissues.

Answer 144

Clinically characterized by progressive spasticity and weakness of the lower limbs. Affects corticospinal tract: axons of the corticospinal neurons are affected. Half the mutated genes that cause this disease are membrane trafficking genes. What are the mutated genes that are non membrane trafficking? They are important for mitochondrial function, myelination, lipid metabolism, and DNA repair.

Answer 145

Small particles (< ~40 kDa) are able to pass through the nuclear pore complex by passive diffusion. Larger particles are also able to pass through the large diameter of the pore but at almost negligible rates. Efficient passage through the complex requires several protein factors. Karyopherins, which may act as importins or exportins are part of the Importin-β super-family which all share a similar three-dimensional structure. Three models have been suggested to explain the translocation mechanism: Affinity gradients along the central plug, Brownian affinity gating, Selective phase

Answer 146

For the majority of macromolecules, the nucleocytoplas- mic transport through the NPCs is an energy-dependent process mediated by soluble transport receptors that gener- ally belong to a family of proteins designated -karyopherins. - karyopherins also mediate the transport of non-coding cellu- lar RNAs [10]. The formation of import or export complexes is dependent on the interaction of -karyopherins with small peptide motifs present in protein cargos. These motifs are generally called nuclear localization signals (NLS) or nuclear export signals (NES).

Answer 147

are characterized by the ability to directly interact with both the Ran GTPase and the FG domains of nucleoporins. the interaction of beta-karyopherins with the respective cargo substrates is regulated by the Ran-GTP gradient

Answer 148

Ran can be found in two distinct forms: GTP- bound and GDP-bound. The two forms of Ran are asymmet- rically distributed between the nucleus and the cytoplasm. Ran-GTP localizes predominantly in the nucleus while Ran- GDP is observed mainly in the cytoplasm. the intrinsic GTPase activ- ity of Ran, promoting the hydrolysis of GTP into GDP, is slow. However, this activity can be accelerated by two cyto- plasmic proteins: Ran-GAP and RanBP1. The reverse reaction, conversion of Ran-GDP into Ran-GTP is stimulated by the nuclear protein RCC1. The combined action of Ran regulatory proteins creates and maintains a Ran-GTP gradient across the NE. This gradient is a key element in establishing the direction of nucleocytoplasmic transport (high RAN GTP in nucleus, low in cytoplasm)

Answer 149

The import complexes cross the nuclear pore by a mechanism involving the interaction between importin-beta and the FG domains of nucleoporins. Once in the inner face of the NPC, the import complexes are dissociated due the presence of Ran-GTP in the nucleus. As a consequence, the cargos are released and importins are exported back to the cytoplasm. The dissociation of import com- plexes starts with the binding of Ran-GTP to three different regions of importin- beta. This interaction results in changes of the conformation of importin- that ultimately hampers the interaction with the IBB domain of importin- alpha

Answer 150

moves other protein molecules into the nucleus by binding to a specific recognition sequence, called the nuclear localization signal (NLS). Importin is classified as a karyopherin. Importin has two subunits, importin α and importin β. Members of the importin-beta family can bind and transport cargo by themselves, or can form heterodimers with importin-alpha. As part of a heterodimer, importin-beta mediates interactions with the pore complex, while importin-alpha acts as an adaptor protein to bind the nuclear localisation signal (NLS) on the cargo through the classical NLS import of proteins. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus. Ran GTPase helps to control the unidirectional transfer of cargo. The cytoplasm contains primarily RanGDP and the nucleus RanGTP through the actions of RanGAP and RanGEF, respectively. In the nucleus, RanGTP binds to importin-beta within the importin/cargo complex, causing a conformational change in importin-beta that releases it from importin-alpha-bound cargo. The N-terminal importin-beta-binding (IBB) domain of importin-alpha contains an auto-regulatory region that mimics the NLS motif. The release of importin-beta frees the auto-regulatory region on importin-alpha to loop back and bind to the major NLS-binding site, causing the cargo to be released