Chapter 8 Flashcards
What does the endomembrane system include?
Organelles such as ER, Golgi, endosomes, lysosomes, and vacuoles
How does one study the endomembrane system?
Using autoradiography and electron microscopy (kills cells)
- Green fluorescent protein labelling (living cells)
How does one use autoradiography to study the endomembrane system? What discovery was made through this method?
Incubate tissue in solution containing labeled amino acids. These amino acids are then taken up by cells and incorporated into proteins. When tissues are viewed, you see large black dots where proteins are localized. This is how it was discovered that proteins are synthesized in the ER
_____ of the endomembrane system are part of an integrated network in which materials are shuttled back and forth
organelles
How are materials shuttled between organelles?
In membrane bound transport vesicles
Name and describe the pathways through the cytoplasm
Biosynthetic pathway - Synthesis, modification and transport of proteins
Secretory pathway - When proteins are discharged (secreted) from the cell either in continuous fashion (constitutive secretion) or in response to a stimulus (regulated secretion)
Endocytic pathway - Move from outer surface of cell to compartments such as endosomes
_____ is a method used to visualize biochemical processes using radiolabeled materials exposed to photographic film
Autoradiography
Describe how green fluorescent protein is used to study cytomembranes
IT is a protein isolated from jellyfish which emits green fluorescent light. GFP needs to be added to the protein of interest. A GFP-DNA chimera enables observe protein synthesis.
_____ determine where proteins are targeted/where they will go
Sorting signals
If GFP shows that the protein is in the golgi, how does one prove that it IS, in fact, the golgi?
You can prove it by doing a second run with GFP tagged “marker proteins” (proteins known to be present in the Golgi. You can also do “z” section - series of sections with x,y,z axes
Does centrifugation break open cells?
NO!!
How are organelles separated from one another for study?
Homogenizing cells, then performing subcellular fractionation. A series of centrifuge cycles can separate out “microsomes”, which are endomembrane vesicles
How can one separate rough and smooth ER?
homogenize solution until microsomes have been separated out. Then undergo sucrose gradient centrifugation. smooth have a low density, rough have a high density.
What is sucrose gradient centrifugation?
Gradient of increasing sucrose concentration established in centrifuge tube with ER pellet on top. After centrifugation, smooth ER should be on top and rough on bottom
________ systems do not contain whole cells and have provided information about the roles of proteins involved in membrane trafficking
Cell-free systems
Why is cell-free system research so helpful?
One can develop mutants for endomembranes, observe cell-free synthesis of proteins
Why is the study of mutant cytomembranes so important?
They provide insights about the functions of normal gene products.
Isolation of proteins from _____ has led to identification of homologous proteins in mammals
yeast
Describe the findings from studying yeast mutant ER
Mutation in gene for vesicle formation (Sec12) prevented vesicles from forming, building up ER volume.
Mutation in gene for vesicle fusion (Sec17) prevents vesicle fusion, resulting in a bunch of unfused vesicles
______ is the process in which cells produce small RNAs that bind to specific mRNAs and inhibit the translation of these proteins
RNA interference
How do scientists figure out which genes are involved in transportation of proteins?
By identifying small interfering RNAs that interfere with those process
Describe the components of the endoplasmic reticulum
Rough ER (covered in ribosomes), smooth ER (devoid of ribosomes). The luminal/cisternal space is inside the ER membrane, and the cytosolic space is outside.
Describe the rough ER
Composed of a network of flattened sacs called cisternae. Continuous with outer membrane of nuclear envelope and has ribosomes on cytosolic surface. Mostly functions in protein synthesis. Polarity in RER sometimes reflects flow from synthesis to discharge of proteins
Describe the smooth ER
Not covered in ribosomes. Functions include synthesis of steroid hormones (in endocrine cells), detoxification in liver, sequestration of calcium ions in muscle cells
Are ratios of Smooth ER to rough ER always the same?
No, they vary based on cell type. ex. pancreas cells have a lot of RER for secretion, whereas SER is prominent in muscle, kidneys
Describe the difference between synthesis of free ribosomes in rough ER and membrane bound ribosomes
Polypeptides synthesized by membrane bound ribosomes (attached to RER) incude secreted proteins, integral membrane proteins, soluble organelle proteins.
Polypeptides synthesized on “free” ribosomes include 1/3 of those encoded by human genome, cytosolic proteins, peripheral membrane proteins, nuclear proteins, proteins in cloroplasts, mitochondria, peroxisomes
The site of synthesis of a protein is determined by a sequence of amino acids at ______. This is called a ______.
N-terminus. Called a “signal sequence”
When a protein being formed, how does it enter the ER?
Moves into cisternal space through protein lined pore, which can occur cotranslationally or posttranslationally
What is a signal recognition particle (SRP)?
A particle which allows secretory proteins that have been synthesized on membrane-bound organelles to have their signal sequence recognized.
Describe how SRPs work
SRPs must interact with SRP receptor. The ribosome binds to the SRP receptor, and the nascent polypeptide is inserted into the translocon (protein-lined channel), where peptidases cleave the signal sequence
The release of SRP requires _____ proteins
GTP-binding proteins (G proteins)
What happens to a newly synthesized protein when it enters the RER lumen?
Sequence is cleaved by a signal peptidase. Carbohydrates are added by the enzyme oligosaccharitransferase and chaperones assist in folding. Protein disulfide isomerase adds disulfide bonds to cysteines (to aid in folding)
How are trans membrane proteins synthesized in the RER when they must be at least partially hydrophobic?
The translocon helps with proper orientation, and arrangement is determined by the orientation of the first trans-membrane segment inserted.
How are membranes synthesized in the ER?
Membranes must arise from preexisting membranes. Lipids are inserted, and proteins and lipids are modified as it moves from one compartment to another.
How do membranes remain symmetrical in vesicles?
Vesicles maintain a particular membrane orientation (membrane facing cytoplasm stays the same even when ejecting things from the cell)
What happens to membrane lipids synthesized in the ER?
New phospholipids are inserted into half of bilayer facing cytosol, then flipped by “flippases” if they need to be in the opposite leaflet
Which membrane lipids are synthesized in the ER?
Most are, but glycolipids and sphingomyelin are not
______ is addition of sugars to molecules. IT is catalyzed by ______
Glycosylation. Glycosyltransferases
Describe, step by step and in detail, glycosylation in the ER
- The lipid carrier dolichol phosphate, embedded in the ER membrane, has five mannose sugars and two NAG residues added to the cytosolic side
- Dolichol phosphate is flipped across the ER membrane
- Four mannose sugars and 3 glucose sugars are added by attaching to dolichol phosphate molecules facing the cytosol, which flips across the membrane and gives the sugars to the growing oligosaccharide
- After assembly, the oligosaccharide chain is transferred to asparagine residue of the nascent polypeptide (the protein currently being produced on the rough ER)
- The dolichol-PP is flipped across the membrane again to restart the process
How are ER synthesized glycoproteins quality controlled?
- All newly synthesized glycoproteins are tagged with a terminal glucose molecule, which is recognized by chaperones.
2a. If the glycoprotein is properly folded, glucose will be removed by glucosidase II and glycoprotein will detach from the chaperone, free to go.
2b. Chaperones should recognize misfolded proteins because hydrophobic regions will be exposed. If the chaperone fails to recognize that the glycoprotein is folded wrong, conformation-sensing enzymes (UGGT) re-add the terminal glucose. - Chaperones then try to fold it again. This can go on forever, unless the proteasome is activated, bringing the glycoprotein to the cytosol for degradation
In order to make a glycoprotein, what must happen?
Glycosylation - production of an oligosaccharide which will be added to the protein. 2/3 terminal glucose molecules are removed (to indicate that the protein is properly synthesized). After this, protein folding must occur (via chaperones). Terminal glucose is then removed
When misfolded proteins must be degraded, it is called ______
ER- associated degradation (ERAD)
What disease is a result of misfolded proteins not being degraded?
Cystic Fibrosis
what happens in the ER if misfolded proteins accumulate?
Unfolded protein response - BiP chaperone molecules (usually bound to sensors) are needed to refold proteins, which means they dissociate from sensors, triggering destructive enzymes, chaperones to refold proteins, proteins to transport them out of the ER.
The golgi complex is made of a stack of flattened _____
Cisternae
The ____ face of the golgi faces the ER, the ____ face is the opposite side of the stack
Cis, trans
N-linked glycosylation takes place in ____. How is this process continued as the glycoprotein moves through the cell to the ____?
ER. Golgi.
Once in the cis and medial cisternae of the Golgi, most of the mannose residues on the glycoprotein are removed and other sugars are added by glycosyltransferases.
____linked oligosaccharide modification occurs exclusively in the Golgi complex
O-linked
Sequence of sugars in oligosaccharides produced in the Golgi are determined by ______
Glycosyltransferases
What is the function of the cis Golgi network?
Sorts proteins for return to ER (if misfolded) or next sorting station (trans Golgi network)
What is the function of the trans Golgi network?
Functions in sorting proteins either to the membrane or various intracellular destinations
Describe the Golgi transport model: cisternal maturation model
The idea that each cisternae themselves “mature” and move from cis to trans orientation
Describe the Golgi transport model: vesicular transport model
The idea that cargo is shuttled from cis golgi network to trans golgi network in vesicles - golgi cisternae are in fied position
What Golgi transport model is the currently accepted one? Why?
Cisternal maturation model, but with a clause allowing backwards movement using vesicles.
This is currently accepted because:
1. In this model, the existence of the golgi must be dependent on vesicle formation by the ER and ERGIC. Experiments have shown that this is the case.
2. Materials produced in the ER have been found to go straight through the golgi without ever entering transport vesicles
3. It is now known that transport vesicles exist for retrograde transmission
4. Studiesof yeast have shown that the Golgi cisternae change position over time
Transport vesicles in the golgi move in what fashion?
Both anterograde and retrograde.
Anterograde being ER to golgi or golgi to vesicles
Retrograde being golgi to ER or vesicle to golgi or cell membrane to vesicles
What are the functions of protein coats on vesicles?
Cause membrane to curve and form vesicle, enable selection the components to be carried
COPII-coated vesicles do what?
Move materials from ER “forward” to ERGIC and Golgi (anterograde)
COPI-coated vesicles do what?
Move materials from ERGIC and golgi “backward” to ER, or from trans Golgi to cis Golgi (retrograde)
Clathrin-coated vesicles do what?
Move materials from trans golgi network to enzymes, lysosomes, plant vacuoles
How do COPII coated vesicles form from the ER (Explain step by step)?
- Sar1-GDP is recruited to ER membrane and the GDP is converted to GTP, becoming Sar1-GTP
- Sar1 undergoes conformational change resulting in insertion into ER bilayer (binding to GEF), resulting in a bend in the bilayer.
- Recruitment of Sec23 and Sec24 is initiated, which bind to the COPII coat and curve it into a “banana shape”. Sec24 also acts as a cargo receptor
- Sec13 and Sec31 are recruited to provide an outer, curved structural cage to the vesicle
- After coat has been completed, vesicle buds off
How does a COPII coated vesicle fuse with a target membrane?
Must first disassemble the protein coat by hydrolyzying bound Sar1-GTP to Sar1-GDP, which has decreased affinity for the vesicle membrane and will dissociate, causing the other COPII subunits to disperse
________________, a G protein, is required for vesicle transfer between golgi cisternae (COPI-coated transport)
Adenosylation ribose factor 1 (ARF 1)
Name and describe the mechanisms by which proteins are maintained in the organelle
Retention: Resident molecules excluded from transport vesicles
Retrieval: “escaped” molecules brought back to compartment
A G-protein is active when ____?
It is in bound form with GTP
How are resident ER proteins recognized outside of the ER? What brings them back to the ER?
They contain a short amino acid sequence on the C-terminus serving as a retrieval signal. Specific receptors capture these proteins and bring them back to the ER in COPI coated vesicles
What is the sequence for ER resident soluble proteins?
What is the sequence for ER resident membrane proteins?
Resident: KDEL
Membrane: KKXX
How are lysosomal enzymes sorted and transported
Sorted in the trans golgi network. Tagged with phosphorylated mannose residues, recognized and captured by mannose 6-phosphate receptors (MPRs), which are integral membrane proteins on the trans golgi network.
Mannose sugars on lysosomal enzymes must be _____ before being released from the golgi
Phosphorylated
What is the composition of the lysosomes that lysosomal enzymes are transferred in?
Clathrin-coated outer shell. Inner shell of protein adapters
Lysosomal enzymes are escorted from TGN by adapted proteins called _____
GGAs
How does a vesicle containing lysosomal enzymes form at the TGN?
Clathrin-coated vesicle formation is kickstarted by the attachment of lysosomal enzymes to mannose-6-phosphate receptors. This recruits a GGA adapter containing multiple domains: one attaches to mannose-6-phosphate receptors, one to Arf1, and one to clathrin molecules. After budding, the clathrin coat and MPRs are lost
How do lysosomal enzyme-containing clathrin-coated vesicles differ from transport vesicles?
Have different G proteins
How do transport vesicles move?
Move along the cytoskeleton, with motor proteins that carry them
How do vesicles tether to the target compartment? What molecule starts this process?
Tethering proteins function as a bridge between transport vesicles and target membrane.
The small G-protein “Rab” on the target membrane and the vesicle membrane recruits the tethering proteins.
What are the two types of “SNARE” proteins and what do they do?
All SNAREs function to dock vesicles to the target compartment. Interactions between the two pull the vesicle into the compartment
v-SNAREs are incorporated into vesicles
t-SNAREs are located at the target
Interactions between these two snares draws the vesicle to the membrane (once activated by rise in Ca2+)
____ is the discharge of a secretory vesicle or granule after fusion with plasma membrane, and is triggered by an release of ____ from cytoplasmic stores
Exocytosis. In Ca2+
How do lysosomes function?
Contain acid hydrolases which lower the pH via a protein pump and allow digestion of biological molecules
How does autophagy occur?
When lysosome fuses with a double-membraned complex called an autophagosome to break down a selected organelle. Within this complex, the inner membrane of the autophagosome and the enclosed organelle are both degraded.
______ result from the absence of specific lysosomal enzymes thus allowing undigested material to accumulate. This includes the condition ____
Lysosomal storage disorders.
Includes Tay-Sachs disease
How can lysosomal storage diseases be treated?
Enzyme replacement therapy or substrate reduction therapy
A ______ is a membrane bound, fluid filled compartment. The membrane of which is called _____
vacuole. Called a tonoplast
______ is the uptake of cell surface receptors and bound extracellular ligands. Whereas _____ is the uptake of particulate matter.
Endocytosis, phagocytosis
What are the types of endocytosis?
Pinocytosis ro bulk-phase endocytosis - nonspecific uptake of cellular fluids
Receptor-mediated endocytosis - uptake of specific extracellular ligands following their binding to receptors on the plasma membrane
How do coated pits contribute to receptor-mediated endocytosis (RME)?
Clathrin-coated pits exist at slightly indented portions of the plasma membrane. Substances that need to be taken into the cell are concentrated on the extracellular surface and the pit invaginates into the cytoplasm.
How did scientists use the pulse-chase experiment to observe movement of molecules through the endomembrane system of the cell
Tagged specific proteins, allowing them to move throughout the cell. They observed cells under the microscope at different time intervals, seeing the movement of proteins from the ER to the golgi to the microsomes
Describe, step by step and in detail, how secretory, lysosomal, or plant vacuolar proteins are synthesized in the ER
- Free ribosome begins to synthesize a protein, beginning with a signal sequence
- Signal sequence is recognized by a signal recognition protein (SRP), enabling complex to bind to SRP receptor and translocon on the ER membrane surface
- SRP is released from its receptor and the ribosome fully attaches to the translocon, directing nascent polypeptide into the ER lumen
- Contact of the interior of the translocon with the signal sequence results in the plug coming out
- Once translation is finished, the ribosome is released from the ER and the plug is re-inserted
Describe, step by step and in detail, what happens once newly synthesized proteins enter the RER cisterna
- N-terminal portion of protein containing the signal peptide is removed by the enzyme signal peptidase
- Carbohydrates are added by the enzyme oligosaccharyltransferase
- Chaperones assist in folding and protein disulfide isomerase (PDI) add disulfide bonds to cysteines.
Describe, step by step and in detail, how integral membrane proteins are sythesized on the membranes of the ER
- As integral proteins are synthesized, transmembrane segments are shunted directly from the channel of the translocon into the lipid bilayer
- The “gate” of the translocon channel continually opens and closes to allow sections of the nascent polypeptide to partition itself according to solubility
- Hydrophobic segments of the polypeptide will “dissolve” into the membrane
- If the protein must have C terminus in the ER lumen and N terminus in the cytoplasm, it is reoriented within the translocon but still synthesized outside the ER
The part of a vesicle facing the ER lumen will face _____ when it reaches the cell membrane
the external environment
The composition of the membranes of different organelles varies widely. How do these changes occur when all membrane proteins come from the ER?
- Most membranous organelles contain enzymes that convert lipids already present in the membrane
- Some phospholipids will be preferentially included in budding vesicle
- Cells contain lipid-transfer proteins to bind and transport lipids through cytosol from one membrane to another
Describe the two pathways for unfolded protein response in the cell?
- BiP is removed from inactive sensors in response to unfolded proteins, sensors activate
2a. Translation factor is phosphorylated and further protein synthesis is prevented until the cell can deal with unfolded proteins
2b. Sensor moves into the golgi complex
3b. Cytosolic domain of the sensor diffuses through the cytosol and heads to the nucleus, where it activates gene expression for proteins that will alleviate unfolded protein stress on the ER (ex. quality control proteins)
How do proteins travel from the ER to the golgi?
Via a series of large (multiple fused together) transport vesicles forming an interconnected tube (called VTCs - vesicular-tubular carriers) called the ERGIC (endoplasmic reticulum golgi intermediate compartment).
COPII coated vesicles usually select proteins that do what?
- Are important for later stage biosynthetic pathways
- Membrane proteins involved in docking/fusion of vesicle
- Membrane proteins to bind soluble cargo
Describe, step by step and in detail, the process that occurs after a lysosomal enzyme is released from the ER into the golgi (from ER to final lysosome)
- Lysosomal enzyme synthesized in ER, transferred to cis golgi cisterna
- Mannose residues of lysosomal enzyme phosphorylated within cis golgi cisterna
- Phosphorylated enzyme attaches to mannose-6-phosphate receptors on trans golgi network membrane
- Clathrin-coated vesicle begins to form and adapter proteins called GGAs assist in enzymes leaving the TGN
- Lysosomal enzyme dissociates from mannose-6-phosphate receptor, which is recycled back to golgi
- Lysosome is formed
How does selective fusion of vesicles help ensure directed flow in the cell
Certain vesicles can only fuse with certain compartments. Ex. vesicles from ER can only fuse with ERGIC or cis golgi, not with trans golgi
Describe the two types of tethering proteins
Highly elongated fibrous proteins and multiprotein tethering complexes
Which protein causes dissociation of SNARE bundles?
NSF
How is autophagy thought to have evolved and why is it an imporatn cellular process
Probably evolved in response to nutrient deprivation, as autophagy is observed under starvation conditions.
What is the product of organelle autophagy and how is the affected by human aging?
Product is called a “residual body”, made of the leftover organelle parts. Either it leaves the cell by exocytosis or stays as a lipfuscin granule. These can build up over time and elderly people and can cause neurological symptoms
Why are lysosomal storage disorders? What’s an example of one? How can they be treated?
Disorders resulting from the absence of specific lysosomal enzymes, resulting in the buildup of undigested substances. One such disorder is Tay-Sachs disease, results from not being able to break down gangliosides.
Treated with enzyme replacement therapy or substrate reduction therapy
A plant cell vacuole serves what purpose?
Holds macromolecules such as ions, sugars, amino acids, proteins. By raising the concentration of these molecules the vacuole triggers water to rush into it. On top of this, vacuoles are the site for intracellular digestion (plants have no lysosomes and therefore need this).
What do clathrin-coated bits of the plasma membrane look when viewed from the cytoplasm? How is the structure of clathrin related to this?
Polygons resembling a honeycomb, caused by blocks of clathrin. Clathrin is composed of thre heavy chains and three light chains joined together in the center to create a triskelion (think 3-prong swastika). The clathrin molecules overlap in a such a way that each vertex contains the center of a clathrin moecule.
How are clathrin molecules in clathrin-coated vesicles (from the PLASMA MEMBRANE) secured to the vesicle?
Using an adapter protein (AP2 complex, not to be confused with the GGA used to attach clathrin to vesicles in the trans golgi network)
____ is the large G protein required for the release of a clathrin coated vesicle from the PLASMA membrane. How does this occur?
Dynamin. Dynamin forms a ring around the point at which the clathrin-coated endosome is starting to pinch off. Hydrolysis of GTP by dynamin results in a twisting motion of the dynamin helix, severing the coated vesicle
What happens to molecules that enter the cell via endocytosis?
They are routed through well-defined endocytic pathway
What are the two types of receptors present in endosomes and what are their functions?
“housekeeping” receptors - include things like transferrin (transfers iron), and Low density lipoprotein receptors (transfers cholesterol)
“Signalling receptors” - bind extracellular ligands such as hormones
___ endosomes are located near the periphery of the cell (just entered cell), and _____ endosomes, AKA _____ are near the nucleus, containing bound ligands and materials.
Early endosomes.
Late endosomes.
AKA multivesicular bodies (MVBs)
Describe, step by step, the process of cholesterol intake via endocytosis
- Cholesterol is transported in the blood as part of low density lipoprotein (LDL) complex.
- This binds to LDL receptors which are concentrated on clathrin coated pits of cell membrane, clathrin-sheathed endosome forms
- Early endosome forms. Clathrin sheath break down, LDL receptors are sent back to plasma membrane in recycling compartment while their ligands and signalling receptors stay
- Late endosomes receive lysosomal enzymes from the trans golgi network (brought by mannose-6-phosphate receptors).
5a. Lysosome forms from late endosome and ligands inside are broken down
5b. Late endosome experiences membrane receptor degradation
High LDL (“high cholesterol”) in blood results in
atherosclerosis - plaques forming in inner lining of arteries, reducing flow of blood through vessel, acting as sites for blood clots to develop
What is HDL? What are the costs/benefits of high/low HDL?
HDL is high density lipoproteins, which function to carry cholesterol from parts of the body where it is in excess, to the liver for processing (and eventual conversion to LDL). High levels of HDL are associated with decreased risk of heart disease. We do not yet know the consequences of artificially raising HDL levels.
What occurs (step by step) when a human macrophage phagocytizes a bacterial cell?
- Plasma membrane engulfs a particle and pinches around it - producing a vacuole called a phagosome
- Phagosome fuses with a lysosome and material is digested by the resultant phagolysosome
- Bacterium is killed by lysosomal enzymes, or oxygen free radicals
Which major pathogenic species of bacteria can survive after being phagocytized by macrophage and what part of the process do they inhibit?
Mycobacterium tuberculosis - inhibits fusion of phagosome and lysosome.
Listeria monocytogenes - Destroys the integrity of lysosomal membrane
Coxiella burnetii - Resists acidity and lysosomal enzymes
The process of engulfment in phagocytosis is driven by contractile activities of the _________ hat underlie the plasma membrane
actin-containing microfilaments
Proteins destined for peroxisomes possess a _______, which is divided into two groups: ____ and _____
Peroxisomal targeting signal: either peroxisomal matrix protein or peroxisomal membrane protein
What kind of a substances can enter peroxisomes. Why is this unique?
folded proteins. No one has any idea how they can accept folded proteins when mitochondria/chloroplasts need to have proteins unfolded first
What kind of targeting sequence directs mitochondrial proteins to the mitochondria? How must these proteins be presented?
Mitochondrial proteins have a removable targeting sequence called a “presequence” at the N-terminus of molecules. Usually includes positively charged residues on one face and hydrophobic residues on the opposite.
Proteins destined for the mitochondria must be in a relatively extended/unfolded state.
What does the outer mitochondrial membrane contain to direct proteins to it?
a protein-important complex called the TOM complex, which includes receptors for mitochondrial proteins and protein lined channels to pull unfolded proteins across outer membrane
What does the inner mitochondrial membrane contain to direct proteins to it?
Proteins coming to the IMM must pass through the inter-membrane space and engage with the TIM complexes. TIM22 binds integral proteins and inserts them in the lipid bilayer of the IMM. TIM23 recognizes proteins with an N-terminus, which is basically all the proteins of the matrix, taking them through the membrane to the matrix.
Where and how does translocation of proteins bound for the mitochondrial matrix occur
Occurs at sites where inner and outer mitochondrial membranes are extremely close to each other. Movement across them is powered by electric potential across inner membrane. Chaperones then re-fold the protein.
What do the outer and inner chloroplast envelope membranes contain to direct proteins?
Two complexes: TOC in outer, TIC in inner
Most proteins destined for the chloroplast are synthesized with removable N-terminus sequence called he ______, that is highly variable in length and sequence
Transit peptide
The mechanism of endocytosis was first proposed in the 1960s. What were scientists trying to explain and how did they discover it?
To explain the uptake of large, high molecular weight yolk proteins during oocyte growth. They found that during rapid oocyte growth, the number of pitlike depressions on the surface of the oocyte increased. They proposed that the pits were formed by invaginations of the plasma membrane and that they were covered by a bristly coat which was lost once they entered the cell
How were coated vesicles first observed? What did the scientists propose?
Via electron microscopic examination of guinea pig brains. The scientists proposed that the polygonal structure was a control mechanism for infolding of the plasma membrane
How was clathrin discovered?
Pig brains were centrifuged using sucrose density gradients until purified coated vesicles were obtained. SDS-PAGE was performed and the protein was isolated. The same was done with other animals. The protein was named clathrin
Cells from individuals with familial hypercholesterolemia (FH) are unable to regulate ______ biosynthesis in response to LDL
cholesterol
Brown and Goldstein’s findings on familial hypercholesterolemia resulted in what?
They demonstrated that FH results from a defect in the receptor for LDL, which is called RME.
