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M2M Cellular Physiology > Membrane Structure & Cytoskeleton > Flashcards

Membrane Structure & Cytoskeleton Flashcards

1
Q

Classes of SNARE Proteins

A

Syntaxin
SNAP-25 (synaptosome associated protein)
VAMP (vessicle-associated membrane proteins)

Syntaxin and SNAP are located in the target membrane; VAMP is located in the transport vessicle

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2
Q

SNARE Complex

A

Formed by association between the conserved, helical domains of Syntaxin, SNAP-25, and VAMP proteins; hydrophobic surfaces of each helix orient toward each other and form a stable complex; an “ionic bubble” is formed by 1 charged residue on each helix: 1 R residue (from VAMP) and 3 Q residues (from Syntaxin or SNAP-25)

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3
Q

NSF and alpha-SNAP

A

NSF is an ATP-ase that forms a hexamer that sits on the SNARE complex in association with alpha-SNAP; as NSF hydrolyzes ATP, the complex dis-assembles the SNARE complex

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4
Q

N-sec-1

A

Acts as a molecular chaperone which aids Syntaxin in re-folding to its active conformation after it is recycled from the SNARE complex

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5
Q

Viral fusion complex

A

Viral fusion proteins have 2 coil motifs, 1 TM domain embedded in the viral envelope, and 1 fusion peptide motif - a stretch of hydrophobic AAs that is buried inside the protein until activated to insert into the host cell membrane

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6
Q

Fusion of the Influenza Virus

A

Influenza is brought into the cell through the endosome and is activated to fuse with the endosomal membrane by acidification

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7
Q

Fusion of the HIV Virus

A

HIV fuses at the plasma membrane of CD4 cells; HIV FP has gp41 and gp120 subunits; gp120 subunit binds CD4 on the host membrane and changes conformation, which activates gp41 to insert into the host cell membrane

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8
Q

3 classes of lipids in a membrane

A
  1. Phospholipids
    Ex: PE, PC, PS, PI
  2. Sphingolipids
  3. Cholesterol
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9
Q

Cholesterol content of membranes

A

ER - 7%
Golgi - 13%
Plasma membrane - 26%

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10
Q

Distribution of lipids in the plasma membrane

A

Extracellular surface - more abundant in PC, sphingomyelin, and glycolipids

Cytosolic surface - more abundant in PS, PE, and PI

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11
Q

Cholesterol Regulation Pathway

A

Sterol regulatory element binding protein (SREBP) contains a bHLH TF domain that regulates LDLR and all 30 cholesterol synthesis proteins. SREBP is held in the ER and bound by Insig/SCAP when cholesterol is high. When cholesterol is low, Insig no longer binds SCAP and SCAP signaling is recognized by a vesicular coat protein that packages the SCAP/SREBP complex into the Golgi, where the TF is cleaved from SREBP by S1P and S2P

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12
Q

Voltage-gated channel structure

A

4 membrane-spanning domains, each containing 6 alpha helices (S1-S6). S4 helices have positively charged residues (Lys or Arg) at every 3rd position, which form the voltage sensor. S5 and S6 helices are connected by the P loop, which assemble to form the ion conducting pathway.

Kv = 4 separate polypeptides
Nav = 1 polypeptide
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13
Q

Channel Selectivity - K vs. Na

A

Kv is highly selective - only 1:10,000 ions permeating a Kv channel is not K+

Nav channels are less selective - 1:12 ions permeating Nav channels are not Na+

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14
Q

Factors affecting channel selectivity

A

Ion charge
Ion size
Dehydration of the ion, compensated by electronic stabilization within the channel
Multiple binding sites - enhances differences in selectivity

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15
Q

Vk Mechanism

A

Activation: Depolarization of the cell triggers causes repulsive electrostatic interaction with the voltage sensing positive charge (Lys or Arg on S4), causing rotation of the activation gate

Deactivation: Repolarization allows the activation gate to rotate back into its closed position

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16
Q

Nav Mechanism

A

Activation: Depolarization causes repulsive electrostatic interactions with the positive charge (Lys or Arg on S4), causing the activation gate to swing open

Inactivation: The inactivation gate is formed by the cytoplasmic loop which connects repeats III and IV; it exists as a “ball and chain” mechanism that swings up into a binding site on the inner portion of the channel, causing current to decay to 0

Removal of inactivation: The inactivation gate leaves its binding site, allowing deactivation to occur in which the activation gate swings shut and the channel is re-set

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17
Q

Channel “Sidedness”

A

Some channel-modifying agents only have access to their sites of action form one side of the membrane

Ex: Tetrodoxin (TTX) is a charged molecule that cannot cross the membrane; when added to the extracellular side, it binds within the entrance of the pore above the selectivity filter; it has no effect when added intracellularly

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18
Q

State Dependent Channel Blocking

A

Ex: Lidocaine, in its physiologically dominant protonated form, has no effect on Nav from the extracellular side; it can block the channel from the intracellular side only if it has access to the vestibule, which requires that the activation gate be open and that the inactivation gate not be closed.

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19
Q

Facilitated Diffusion

A

The mechanism by which large or polar/charged molecules are enabled to passively diffuse across lipid membranes through transporters acting like ion channels

Ex: Glucose transporter in muscle cells

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20
Q

Mechanism of Glucose Transport

A

Normally, glucose transporters are sequestered within the membranes of intracellular vesicles; High plasma glucose triggers the release of insulin from pancreatic beta cells, which signals the receptor-containing vesicles to fuse with the plasma membrane, exposing the glucose transporter to the ECF. Glucose is taken up by the cell via facilitated diffusion and immediately phosphorylated to G-6-P, which cannot diffuse back across the transporter.

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21
Q

Primary Active Transporters

A

Hydrolyze ATP to power the movement of solutes against their electrochemical gradients

Ex: Na/K Pump, H+ pump in the inner mitochondrial membrane, H+ pumps in lysosomes

22
Q

Secondary active transport

A

Energy is provided by the “downhill leak” of Na into the cell, which powers the transport of substances across the membrane against their electrochemical gradients

Secondary active transporters may be electrogenic (producing a net charge transfer across the membrane) or not

23
Q

Co-transporters

A

Secondary active transporters that move solute species in the same direction (usually into the cell, with the Na leak)

Ex: Amino acid uptake

24
Q

Exchangers

A

Secondary active transporters that move solute species in opposite directions (usually out of the cell, against the Na leak)

Ex: Na/Ca exchanger

25
Q

Calcium Transport

A

Ca++ experiences a strong electrochemical pull into the cell, caused by favorable concentration and electrical gradients; ECa = +111mV

The Na/Ca exchanger pumps calcium ions out of the cell, using the inward leak of sodium ions as it’s energy source.

26
Q

Na/Ca Exchanger in Cardiac Muscle

A

While the ventricles are relaxed during diastole (resting Vm < -60mV), the driving force on Na+ to leak into the cell powers the Na/Ca exchanger forward, pumping Ca2+ out of the cell

As the ventricles contract during systole (Vm depolarized to > -60mV) the driving force on Na+ isn’t strong enough to power transport; the exchanger switches direction so that the leak of Ca2+ into the cell down its stronger electrochemical gradient pumps Na+ out of the cell;

27
Q

Digitalis

A

Blocks the Na/K pump; increase of intracellular Na+ reduces the energy available to the Na/Ca exchanger, allowing Ca2+ to accumulate within the cell and increasing cardiac contractility

28
Q

Na+/H+ Exchanger

A

Secondary active transporter which uses the energy provided by Na+ leak into the cell to pump H+ out of the cell; because EH = -24mV, H+ must be pumped out of the cell even though this is down it’s concentration gradient

29
Q

Hypothetical H+/K+ Exchanger

A

Infusing K+ causes acidemia as the K+ is taken up by cells ‘in exchange’ for H+ and vice versa (infusing acid causes hyperkalemia)

Hyperkalemia is treated by giving base, which reduces extracellular H+ and creates a gradient that drives the re-uptake of K+ into the cell

30
Q

Main functions of the cytoskeleton (5)

A 
Provides cell shape
Provides mechanical strength
Enables cellular locomotion 
Supports the plasma membrane
Forms the scaffold for intracellular transport
31
Q

Microtubule structure

A

The basic subunit is a heterodimer of alpha and beta tubulin, which bind GTP and align in tandem to form a protofilament with a free beta subunit at the (+) end and a free alpha subunit at the (-) end; 13 protofilaments assemble to form a microtubule ~25nm in diameter

32
Q

“Treadmilling” of microtubules

A

Tubulin subunits are more stable when bound to GTP than GDP; gradually, GTP hydrolyzes to GDP and so microtubules tend to spontaneously depolymerize from the (-) end; this is balanced by constant addition of GTP-bound subunits at the (+) end, forming a stable GTP cap

33
Q

Microtubule Severing Proteins

A

I.e. spastin and fidgetin members of Katanin family, increase microtubule instability by exposing GDP-rich parts of microtubules; mutations in either gene may lead to spastic paraplesia

34
Q

Centrosome

A

A.K.A. microtubule organizing center (MTOC); most cells contain a single MTOC (the centrosome) containing a pair of centrioles located near the nucleus; microtubules grow from the pericentriolar material with their (-) ends anchored in the complex and their (+) ends growing into the cell

35
Q

Kinesin structure

A

Two isoforms, kinesin 1 and kinesin 2; both form homodimers consisting of 2 head groups, which bind microtubules, a coil-coil motif, and 2 tail groups, which bind an adapter molecule for a specific cargo

36
Q

Kinesin mechanism

A

Kinesin is an ATPase that moves cargo along microtubules toward the (+) end; kinesin head groups bind the microtubule when kinesin is bound to ATP, in an alternating fashion; ATP hydrolysis powers a conformational change allowing the head group to release the microtubule and swing forward 16nm along the microtubule

37
Q

Dynein

A

Dynein transports cargo toward the (+) end of microtubules; ex: it is responsible for retrograde transport of materials from axonal terminals to neuronal cell bodies; mutation leads to neuronal cell death because NGF survival signals from neuronal target cells never reach the neuronal cell body

38
Q

Role of centrosome during mitosis

A

During mitosis, the centrosome is duplicated and one moves to each pole of the dividing cell; astral microtubules radiate out from the centrosomes; (+) ends of kinetochore microtubules attach to centromere of chromosomes; (+) ends of overlap microtubules slide against each other, creating a force that pushes the poles apart while (-) directed motors separate daughter chromosomes and move them toward the centrosomes

39
Q

Intermediate filament structure & function

A

Basic subunit is composed of two globular protein domains linked by an alpha helical region; the subunit forms tetramers and 8 tetramers twist into a rope-like filament 10 nm in diameter

The main function of IFs is the mechanical stability of the cell

40
Q

Types of intermediate filaments

A

Two main types: cytoplasmic IFs and nuclear lamins

3 subtypes of cytoplasmic IFs: Keratins, vimentin, and neurofilaments

41
Q

Keratins & Disease

A

Mutations in keratins may cause liver problems (K8 and K18) or epidermolysis bullosa simplex, in which the epidermis is less strongly anchored to the dermis and is sensitive to mechanical stress, blistering easily

42
Q

Neurofilaments & Disease

A

Charcot-Marie-Tooth peripheral neuropathy

Amyotrophic Lateral Sclerosis (ALS)

43
Q

Lamins & Disease

A

Lamins are crucial to the integrity of the nuclear envelope; mutations may cause Progeria (premature aging) syndrome

44
Q

Actin microfilament structure

A

In the presnece of ATP, globular actin (G-actin) monomers assemble to form two-stranded, helical filaments (F-actin) 7 nm in diameter; actin filaments exhibit polarity and polymerization of ATP-actin monomers occurs at (+) end

45
Q

Mechanism of Actin nucleation

A

Nucleation requires the assembly of 3 actin monomers

FH2/Formin forms linear networks of microfilaments; FH2/Formin is activated by binding of GTP-Rho

Arp2/3 forms branched networks of microfilaments

46
Q

Actin role in epithelial cell polarity

A

Actin is important in anchoring proteins involved in tight junctions and adherens junctions between epithelial cells; decreased association of junction proteins (cadherins and catenins) with actin leads to loss of cell-cell adhesion, a prerequisite step for epithelial-to-mesenchimal (EMT) transition and cancer formation.

47
Q

4 main roles of actin in cell function

A

Epithelial cell polarity
Contraction
Motility
Division

48
Q

Role of actin in Microvillus

A

Linear actin microfilaments form the core of microvilli and anchor microvilli on the apical border of epithelial cells; actin filaments are cross-linked by villin and fimbrin; mutations in actin may cause weak villi that are ripped off of gut epithelial cells by peristalsis, causing microvillus inclusion disease; these children cannot absorb adequate nutrition and do not survive.

49
Q

Mechanism of muscle contraction

A

Myosin II forms “thick filaments” in skeletal muscle; myosin binds actin in it’s ADP-bound state; the release of ADP is coupled to the power-stroke, shortening the sarcomere; ATP then binds to myosin, allowing it to release actin; ATP hydrolysis powers the movement of myosin into the “cocked” formation and ADP-bound myosin again binds actin in a new location further along the (+) end

50
Q

Cell motility

A

Actin polymerization via FH2/Formin nucleation in the filopodia and via Arp2/3 in the lamellipodium occurs at (+) ends of microfilaments, extending the leading edge of the cell; this is accompanied by “treadmilling” via profilin, which participates in depolymerization at the (-) end; myosin II contraction draws in the lagging edge of the cell

51
Q

Actin role in cell division

A

Actomyosin ring formation drives the cleavage furrow between dividing cells; the site and timing of this ring formation determine symmetry of cleavage; asymmetric division occurs in division of erythroblasts, formation of platelets, and division of spermatids to form sperm

M2M Cellular Physiology (6 decks)

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