Cell biology

Which of the proteins below is(are) not made on the membrane-bound ribosomes of the RER?

) peripheral proteins of the inner surface of the plasma membrane

What GTP-binding protein is associated with the formation of the COPI coat on COPI-coated vesicles?

ARF1 (adenosylation ribose factor)

Sec23 and Sec24 bind together to form a "banana-shaped" dimer. What is the purpose of this dimer

Because of its curved shape, the dimer puts pressure on the membrane surface to help it further bend into a curved bud.

Why does the cell use proteasomes to destroy misfolded proteins?

Destruction of misfolded proteins assures that aberrant proteins are not sent to other parts of the cell.

What does the conformation-sensing enzyme GT do if it binds to a misfolded or incompletely folded glycoprotein?

It adds a single glucose back to one of the mannose residues at the exposed end of the recently trimmed oligosaccharide

The interaction between the membranes of vesicles and their target compartment is mediated by which proteins below?

SNAREs

What happens if the UPR is unsuccessful in relieving the stressful conditions in the cell?

The cell-death pathway is triggered and the cell is destroyed.

What kind(s) of modifications are made in proteins as they move through the Golgi complex?

The protein's carbohydrates are modified by a series of stepwise enzymatic reactions.

What is thought to dissociate the 4-stranded SNARE complex by attaching to the SNARE bundle and, using energy from ATP hydrolysis, twisting it apart?

a doughnut-shaped, cytosolic protein called NSF

What always serves as the donor of a sugar to the growing oligosaccharide chain of a glycoprotein

a nucleotide sugar

What do studies suggest governs the "decision" to destroy a defective protein that has been unable to fold correctly and has been in the ER for an extended period of time?

a slow-acting ER enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein

What circumstantial evidence supports the proposed role of the Rabs in recruiting cytosolic tethering proteins to specific membrane surfaces?

a) With over 60 Rab genes identified in humans, Rabs constitute the most diverse group of proteins involved in membrane trafficking. b) Rabs have the potential of giving each cell compartment a unique surface identity. c) Different Rabs have been found to be associated with different membrane compartments. d) The preferential localization of Rabs allows them to recruit the proteins involved in targeting specificity.

The coat of vesicles that transport materials around the cell interior ___________.

a) is composed of two distinct protein layers b) possesses an outer cage or scaffolding that forms the framework for the coat c) possesses adaptors that are able to select specific cargo molecules d) possesses an inner layer of adaptors that serves primarily to bind the vesicle's cargo

When electron micrographs were first taken of the cell interior, what kinds of membranous structures were seen?

a) membrane-bound vesicles of varying diameter, containing material of different electron density b) long channels bounded by membranes and radiating through the cytoplasm c) an interconnected network of canals d) stacks of flattened, membrane-bound sacs called cisternae

What components below are selected for transport by vesicles originating in the Golgi complex?

a) secretory proteins b) lysosomal proteins c) proteins required to dock the vesicle to an acceptor membrane d) proteins required to target the vesicle to an acceptor membrane

Which of the following is a function associated with the smooth endoplasmic reticulum in at least some cells?

a) synthesis of steroid hormones b) detoxification of many organic compounds, like barbiturates and ethanol c) release of glucose into the bloodstream d) sequestration of calcium Ca2+ ions within the cisternal space

What is the general name for a molecule that physically links two different types of materials?

adaptors

To what residue of a polypeptide are N-linked oligosaccharide chains attached as that poypeptide enters the RER lumen through the translocon?

asparagine

What is the name for a brief incubation of a tissue with radioactivity during which labeled amino acids are incorporated into protein?

b) pulse

The SRP and SRP receptor are thought to bind GTP ______ interacting with each other.

before

Where in the Golgi complex does most protein sorting occur?

the TGN

What are the two functional categories of SNAREs?

v-SNAREs and t-SNAREs

How do Rabs appear to associate with membranes?

via a lipid anchor

What allows the interface between the Sec13-Sec31 subunits to form cages of varying diameter, thus accommodating vesicles of varying size?

) a degree of flexibility built into the interface between the Sec13-Sec31 subunits

Why is the ER so well-suited and ideally constructed for its role as a port of entry for secretory proteins? 1) It has a large surface area allowing the attachment of many ribosomes. 2) The ER cisternae lumen favors unfolding and disassembly of proteins. 3) The RER can segregate secretory, lysosomal and cytoplasmic proteins from other newly made proteins, allowing their modification, and sends them to their final destination.

1

In addition to their key role in vesicle targeting by recruiting specific cytosolic tethering proteins to specific membrane surfaces, Rabs also play a key role in ________. 1) regulating the activities of numerous proteins involved in other aspects of membrane trafficking 2) regulating the aspects of motor proteins that move membranous vesicles through the cytoplasm 3) regulating metabolic processes

1 and 2

What is the fate of a misfolded or incompletely folded protein in the ER once one or more of its mannose residues has been removed from its oligosaccharide chain(s)? 1) The protein can no longer be recycled. 2) The protein is recycled. 3) The protein is sentenced to degradation. 4) The protein continues to be refolded.

1 and 3

Which of the following are enzymes that are involved in detoxification of organic compounds in the SER of liver cells? 1) oxygen-transferring enzymes 2) oxygenases 3) members of the cytochrome P450 family 4) oxidases

1, 2 and 3

Which pH below would be most likely to favor the operation of a lysosomal enzyme?

4.5

Which GTP-binding protein is associated with clathrin-coated vesicles and helps to initiate the formation of the coat?

ARF1 (adenosylation ribose factor)

Which coated vesicles move materials in a retrograde direction from the ERGIC and Golgi stack backwards toward the ER?

COPI-coated vesicles

How is the orientation of membrane proteins in the membrane thought to be accomplished?

During synthesis, the translocon inner lining orients the nascent polypeptide so the more positive end faces the cytosol.

What mediates the interaction between integral membrane proteins to be transported in COPII-coated vesicles and the COPII-coat?

ER export signals in the cytosolic tails of integral ER membrane proteins

How and where is the asymmetry of the phospholipid bilayers initially established?

It is initially established in the ER during lipid and protein synthesis.

What happens to the clathrin coat once the vesicle has budded from the Golgi body?

It is lost

What is usually the retrieval signal for ER integral membrane proteins, like the SRP receptor?

KKXX at the C-terminus of the protein

What are the recognition signals for lysosomal enzymes that allow them to be localized correctly in lysosomes?

Lysosomal enzymes possess phosphorylated mannose residues on N-linked carbohydrate chains.

What would happen if the enzyme that adds phosphate groups to the appropriate mannose residues on the carbohydrate chains of lysosomal enzymes were defective?

Lysosomal enzymes would continue through the Golgi complex to secretory vesicles and would eventually be secreted

What is the effect of CDG1b on cell physiology and what is the treatment that has shown some promise of being effective?

Mannose is unavailable for incorporation into oligosaccharides; oral supplements of mannose are the treatment.

What effect does the binding of the SRP to the growing polypeptide chain and the ribosome have on protein synthesis?

Protein synthesis ceases temporarily.

How do misfolded proteins get to the cytoplasm to be destroyed?

Proteins are transported back to the cytosol through the translocon that brought them into the ER lumen or through a separate dislocation channel of uncertain identity.

What GTP-binding protein plays a regulatory role by initiating vesicle formation and by regulating the assembly of the vesicle's COPII coat?

Sar1

What is the primary adaptor protein of the COPII coat that interacts specifically with the ER export signals in the cytosolic tails of membrane proteins that are destined to traffic on to the Golgi complex?

Sec24

What evidence suggests that the translocon, by itself, can properly orient transmembrane segments?

Studies performed with purified components in cell-free systems show that the translocon, by itself, is capable of properly orienting transmembrane segments.

How does GT recognize incompletely folded or misfolded proteins that have been recently synthesized?

Such proteins display exposed hydrophobic residues that are absent from properly folded proteins.

How are integral membrane proteins thought to enter the lipid bilayer?

The aqueous translocon channel seems to have a gate that continuously opens and closes, giving each nascent polypeptide segment a chance to partition itself into the lipid bilayer's hydrophobic core.

How do protein coats select the cargo molecules to be carried by the vesicles they help to form?

The protein coats have a specific affinity for the cytosolic tails of integral membrane proteins that reside in the donor membrane.

Phospholipids are made by integral ER membrane enzymes whose active sites face the cytosol and they are inserted into the outer (cytoplasmic) leaflet of the ER membrane. How then do lipids destined for the luminal leaflet of the ER membrane get there?

There are enzymes called flippases that flip these lipids later into the opposite leaflet.

What are the differences between ribosomes that make secretory proteins and those that make proteins intended for the cytosol?

There are no differences between them.

What happens if misfolded proteins are generated in the ER at a faster rate than they can be exported to the cytoplasm?

They accumulate in the ER.

What happened to COPI-coated vesicles within the cell when the cell was treated with GTP analogues that could not be hydrolyzed?

They accumulated in the cytoplasm

What happens to the breakdown products of materials brought into many single-celled organisms from the extracellular environment?

They are used as nutrients and are released into the cytoplasm.

What is one problem created by the detoxifying enzymes of the SER?

They can cause a compound to be converted into a carcinogen.

What appears to be the purpose of molecular chaperones like BiP?

They recognize and bind to unfolded or misfolded proteins and help them attain their native structure.

When Rabs have bound to GTP, what do they do?

They recruit specific cytosolic tethering proteins to specific membrane surfaces.

What happens to yeast cells that cannot transport proteins into the ER lumen cotranslationally?

They survive, but grow more slowly than normal yeast cells.

What would happen to the movement of vesicles toward their eventual target if a microtubule inhibitor like colchicine were added to the cells?

Vesicle movement would slow or stop.

What happens to a newly synthesized glycoprotein after the binding of calnexin or calreticulin to help the protein correctly complete its folding?

When the glycoprotein's folding is correctly completed, the remaining glucose on its oligosaccharide chain is eventually removed enzymatically and the glycoprotein is released from the chaperone.

Based on what is known about the involvement of calcium ions in exocytosis, what should happen if Ca2+ ions are injected into a cell?

Wholesale exocytosis of secretory product occurs.

Which protein(s) below is(are) recruited to the COPII coat by Sar1-GTP?

both Sec23 and Sec24

What subunit(s) of the COPII coat bind(s) to the vesicle membrane to form the outer structural cage of the protein coat?

both Sec31and Sec13

What is thought to shield lysosomal membranes against attack by their enclosed enzymes?

carbohydrate chains attached to integral membrane proteins

) Lysosomal enzymes are transported from the TGN in vesicles coated with what protein?

clathrin

What are the two sites within a cell at which protein synthesis is generally thought to occur?

cytosolic surface of RER and free ribosomes

Most vesicles budding from the Golgi body have a fuzzy, electron-dense coat on their ______ surface. The coat appears to be made of _______.

cytosolic, protein

What is the name sometimes given to a single Golgi stack in a plant cell?

dictyosome

) What is responsible for recognizing lysosomal enzymes and localizing them to the lysosomes? 1) mannose 6-phosphate receptors 2) MPRs 3) integral membrane proteins that span the TGN membranes 4) intraGolgi receptors that reside in the TGN lumen

e) 1, 2, and 3

The process of membrane fusion and subsequent content discharge is called ______ and is usually triggered by a release of ______.

exocytosis, Ca2+ ions

Which of the following carbohydrates is not synthesized in the Golgi complex?

glycogen

What enzymes are responsible for determining the sequence of sugars added to growing oligosaccharide chains of membrane proteins or secretory proteins as they travel through the Golgi complex?

glycosyltransferase

Which of the following enzymes are typically found in lysosomes?

hydrolytic enzymes (acid hydrolases)

Where are misfolded secretory proteins eventually destroyed?

in the cytosol (cytoplasm)

SNAREs vary in structure quite a bit, but all of them contain a common domain. Where is this domain located, of what is it composed and what is it called?

in the cytosol, 60 - 70 amino acids that form a complex with another SNARE motif

Where are v-SNAREs and t-SNARES found, respectively?

incorporated into transport vesicle membranes during budding, in target compartment membranes

What determines the specificity of vesicle fusion to a target membrane?

interactions between specific combinations of interacting proteins, including tethering proteins, Rabs and SNAREs assembled at that site in the cell

What determines the function of a cell's smooth endoplasmic reticulum?

its protein content

What sugar is usually removed from the N-linked core oligosaccharide chains on proteins in the Golgi complex as opposed to the glucose residues trimmed off in the ER

mannose

What is responsible for adding sugars to dolichol phosphate?

membrane-bound glycosyltransferases

The movement of vesicular-tubular carriers (VTCs) farther away from the ER and toward the Golgi complex occurs along tracks composed of what material?

microtubules

What is thought to direct the movement of vesicles through the cytoplasm to their final destination?

microtubules

TB 8.066 The ER reportedly contains sensors that monitor the concentration of unfolded or misfolded proteins in the lumen. One proposal suggests that the sensors are normally kept in an inactive state by ______, particularly ______.

molecular chaperones, BiP

What is the arrangement of organelles in a secretory cell from the basal end to the apical end, an arrangement that reflects the flow of secretory products from synthesis to discharge?

nucleus and RER - SER - Golgi complex - secretory vesicles

What enzyme transfers a block of sugars to asparagine residues of a polypeptide as it enters the RER?

oligosaccharyltransferase

CDG1b results from a deficiency in what enzyme?

phosphomannose isomerase

What is responsible for degrading misfolded proteins in the cytoplasm?

proteasomes

Vesicles that move through the Golgi complex from a trans-donor to a cis-acceptor membrane are said to move in a(n) __________ direction

retrograde

What specific cellular responses are known to be triggered by the regulated release of Ca2+ ions from the SER?

skeletal muscle cell contraction

Which type of cells below is not known for its extensively developed SER?

skin cells

Synaptic vesicle fusion to the presynaptic membrane in a neuron is regulated by what calcium-binding protein found in the membrane of the synaptic vesicle?

synaptotagmin

What appears to be an early step in the process of vesicle fusion to its target compartment?

tethering of vesicles to the target compartment by extended, fibrous proteins

Which part of the Golgi complex is thought to function primarily as a sorting station that distinguishes between proteins to be shipped back to the ER and those that are allowed to proceed to the next Golgi station?

the CGN

TB 8.034 Blöbel, Sabatini and Dobberstein proposed that the site of protein synthesis is determined by information contained in the N-terminal portion of the protein, the first part to emerge from the ribosome. What did they call their proposal?

the Signal Hypothesis

Which of the models below suggests that the Golgi cisternae are transient structures that form at the cis face of the stack by fusion of membranous carriers from the ER and ERGIC and that each cisterna travels through the Golgi complex from the cis to the trans end of the stack, changing in composition as it progresses?

the cisternal maturation model

) To what site does Sar1 bind after it binds to GTP?

the cytosolic leaflet of the ER bilayer

With what structure is the RER often seen to be continuous, as seen by its association with ribosomes?

the outer membrane of the nuclear envelope

) What determines the sequence of sugar addition to glycoproteins traveling through the Golgi complex?

the spatial arrangement of specific glycosyltransferases that contact proteins as they pass through the Golgi complex

The accumulation of misfolded proteins in the ER is a potentially lethal situation and thus causes the triggering of what process?

the unfolded protein response (UPR)

Which model of Golgi complex formation suggests that the cisternae of a Golgi stack remain in place as stable compartments held together by a protein scaffold, while the cargo is shuttled through the Golgi via vesicles that bud from one compartment and fuse with a neighboring one?

the vesicular transport model

What allows smooth and rough vesicles (microsomes) to be readily separated by density gradient centrifugation?

their differences in density

The oligosaccharide block that is added to secretory proteins after they enter the ER lumen goes through a number of modifications after its attachment. What is the first modification that occurs?

trimming of some sugars from the oligosaccharide block