Question 1

What initially happens if misfolded proteins are generated in the ER at a faster rate than they can be exported to the cytoplasm?
A)They are degraded in the ER.
B)They are inserted into the ER membrane.
C)They are resynthesized in the ER.
D)They accumulate in the ER.
E)They accumulate in the Golgi complex.

Accepted Answer

If misfolded proteins are generated in the ER at a faster rate than they can be exported to the cytoplasm, they will accumulate in the ER. This can cause ER stress, which activates a signaling pathway called the unfolded protein response (UPR) to try and restore protein folding homeostasis. If the UPR is unsuccessful, the misfolded proteins can aggregate and lead to cell death.

Question 2

Which correctly describes the effects and treatment of CDG1b?
A)Mannose is unavailable for incorporation into oligosaccharides; oral supplements of mannose are the treatment.
B)Mannose is overproduced for incorporation into oligosaccharides; oral supplements of mannose are the treatment.
C)Mannose is unavailable for incorporation into oligosaccharides; a diet free of mannose is the treatment.
D)Mannose is overproduced for incorporation into oligosaccharides; a diet free of mannose is the treatment.
E)Fructose is unavailable for incorporation into oligosaccharides; oral supplements of fructose are the treatment.

Accepted Answer

CDG1b is caused by a deficiency of phosphomannomutase, which results in mannose being unavailable for incorporation into oligosaccharides. Therefore, oral supplements of mannose are given as treatment.

Question 3

What happens to a newly synthesized glycoprotein after the binding of calnexin or calreticulin
A)When the glycoprotein's folding is correctly completed, the remaining glucose on its oligosaccharide chain is eventually reduced and the glycoprotein is released from the chaperone.
B)The oligosaccharide and amino acid chain are totally degraded.
C)Correct folding of the glycoprotein is prevented until the chaperone is removed.
D)When the glycoprotein's folding is correctly completed, the remaining glucose on its oligosaccharide chain is removed enzymatically and the glycoprotein is released from the chaperone.

Accepted Answer

After the binding of calnexin or calreticulin, the glycoprotein undergoes correct folding assisted by the chaperones. Once the folding is completed, the remaining glucose on the oligosaccharide chain is removed enzymatically by the glucosidase I and II enzymes. This leads to the release of the correctly folded glycoprotein from the chaperones.

Question 4

What is responsible for adding sugars to dolichol phosphate?
A)membrane-bound glycosyltransferases
B)membrane-bound oligosaccharyltransferase
C)membrane-bound gangliosidase
D)glycosylsynthetase
E)peptidyltransferase

Accepted Answer

Membrane-bound glycosyltransferases are responsible for adding sugars to dolichol phosphate, as they transfer sugar molecules from nucleotide sugars to the dolichol phosphate. Oligosaccharyltransferase is involved in attaching the complete oligosaccharide to the protein during N-linked glycosylation, gangliosidase is involved in breaking down complex gangliosides, glycosylsynthetase is not a known enzyme, and peptidyltransferase is involved in peptide bond formation during protein synthesis.

Question 5

To what residue of a polypeptide are N-linked oligosaccharide chains attached as that poypeptide enters the RER lumen through the translocon?
A)arginine
B)asparagine
C)serine
D)threonine
E)ninhydrin

Accepted Answer

N-linked oligosaccharide chains are attached to the asparagine residue in the polypeptide as it enters the RER lumen through the translocon. This process is catalyzed by the enzyme oligosaccharyltransferase.

Question 6

Which of the following carbohydrates is NOT synthesized in the Golgi complex?
A)glycosaminoglycans in the animal extracellular matrix
B)plant cell wall polysaccharides like pectin and hemicellulose
C)the carbohydrates of glycolipids
D)the carbohydrates of glycoproteins
E)glycogen

Accepted Answer

Glycogen is synthesized in the cytoplasm by the enzyme glycogen synthase, not in the Golgi complex.

Question 7

What happens if the UPR is unsuccessful in relieving the stressful conditions in the cell?
A)The cell grows.
B)The cell divides.
C)The cell-death pathway is triggered and the cell is destroyed.
D)The cell shrinks.
E)The cell's temperature is raised.

Accepted Answer

If the UPR is unsuccessful in relieving the stressful conditions in the cell, the cell-death pathway is triggered and the cell is destroyed. This is known as apoptosis, a process that eliminates damaged or unnecessary cells in the body.

Question 8

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)a fast-acting ER enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein
B)a slow-acting ER enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein
C)a fast-acting cytoplasmic enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein
D)a slow-acting nuclear enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein
E)a slow-acting cytoplasmic enzyme that trims a mannose residue from an exposed end of the oligosaccharide of a protein

Accepted Answer

Studies suggest that a slow-acting ER enzyme trims a mannose residue from an exposed end of the oligosaccharide of a protein, which is a signal for the recognition and degradation of the protein by the ER-associated degradation (ERAD) pathway. This is part of the quality control system in the ER, which helps to ensure that only properly folded proteins are transported to their final destination.

Question 9

Where are misfolded secretory proteins eventually destroyed?
A)in the RER
B)in the SER
C)in the Golgi complex
D)in the cytosol (cytoplasm)
E)in the nucleus

Accepted Answer

Misfolded secretory proteins are recognized and removed by quality control mechanisms in the endoplasmic reticulum (ER). If they cannot be properly folded or repaired, they are retrotranslocated to the cytosol where they are polyubiquitinated and targeted for degradation by the proteasome.

Question 10

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?
A)addition of more sugars
B)addition of glucose
C)trimming of some sugars from the oligosaccharide block
D)chemical modification of the sugars on the oligosaccharide chain
E)both addition of more sugars and addition of glucose

Accepted Answer

The first modification that occurs to the oligosaccharide block is trimming of some sugars from the block. This is done by enzymes in the ER and is important for proper folding of the protein.

Question 11

The accumulation of misfolded proteins in the ER is a potentially lethal situation and thus causes the triggering of what process?
A)the unfolded protein response (UPR)
B)the posttranscriptional response
C)the polysomal response
D)the proteasomal response
E)the intracellular protein response

Accepted Answer

The accumulation of misfolded proteins in the ER triggers the unfolded protein response (UPR), which is a protective stress response that aims to restore proper folding and function of proteins in the ER. Failure to restore ER homeostasis through the UPR can lead to cell death. The other options are not relevant to this specific situation.

Question 12

How does UGGT recognize incompletely folded or misfolded proteins that have been recently synthesized?
A)Such proteins display exposed hydrophilic residues that are absent from properly folded proteins.
B)Five histidine residues are exposed on the protein's surface when it is improperly folded.
C)Such proteins display exposed hydrophobic residues that are absent from properly folded proteins.
D)Six arginine residues are exposed on the protein's surface when it is improperly folded.
E)Such proteins display numerous carboxyl groups on their surfaces, which decreases their solubility.

Accepted Answer

Incompletely folded or misfolded proteins display exposed hydrophobic residues on their surfaces that are absent from properly folded proteins. UGGT recognizes these exposed hydrophobic residues and adds a glucose molecule to the N-linked glycan, marking the protein for further chaperone-assisted folding or degradation.

Question 13

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 ______.
A)molecular chaperones, ribosomes
B)proteasomes, BiP
C)molecular chaperones, BiP
D)enzymes, ER
E)molecular chaperones, Rubisco

Accepted Answer

Molecular chaperones, especially BiP, are known to keep ER sensors inactive under normal conditions by binding and stabilizing their conformation. This helps in preventing the sensors from detecting normal levels of unfolded or misfolded proteins, which could trigger the unfolded protein response pathway.

Question 14

Which part of the Golgi complex distinguishes between proteins to be shipped back to the ER and those that are allowed to proceed to the next Golgi station?
A)the cis cisternae
B)the cis-Golgi network (CGN)
C)the medial cisternae
D)the trans cisternae
E)the trans-Golgi network (TGN)

Accepted Answer

The cis-Golgi network (CGN) functions as a sorting station where proteins are distinguished between those that need to be sent back to the Endoplasmic Reticulum (ER) for further processing or correction and those that are correctly folded and ready to proceed to the next station within the Golgi apparatus.

Question 15

CDG1b results from a deficiency in what enzyme?
A)phosphomannose phosphatase
B)phosphotungstate isomerase
C)phosphomannose isomerase
D)phosphatase
E)phosphoenol pyruvate carboxylase

Accepted Answer

CDG1b results from a deficiency in the enzyme phosphomannose isomerase, which is responsible for converting mannose-6-phosphate to mannose-1-phosphate.

Question 16

What kind(s)of modifications are made in proteins as they move through the Golgi complex?
A)The protein's carbohydrates are modified by a series of stepwise enzymatic reactions.
B)Amino acids can be added to either end of the polypeptide chain.
C)Amino acids in the proteins may be chemically altered into nucleic acids.
D)Small segments of amino acids can be added into the center of an existing protein.
E)All of these are correct.

Accepted Answer

As proteins move through the Golgi complex, their carbohydrates may be modified by a series of enzymatic reactions. This can include the addition or removal of sugars or other carbohydrate molecules. Not all of the answer choices are correct - amino acids cannot be chemically altered into nucleic acids, and small segments of amino acids cannot be added into the center of an existing protein.

Question 17

Why does the cell use proteasomes to destroy misfolded proteins?
A)Destruction of misfolded proteins assures that aberrant proteins are not sent to other parts of the cell.
B)These proteins can be degraded into components that can be used to make polynucleotides.
C)These proteins are degraded into components that can be used to make polysaccharides.
D)These proteins are degraded into components that are used to make lipids.
E)Destruction of misfolded proteins prevents the dissolution of the plasma membrane.

Accepted Answer

Destruction of misfolded proteins assures that aberrant proteins are not sent to other parts of the cell, which could cause cellular damage or malfunction. Proteasomes break down these misfolded proteins into smaller peptides that can then be recycled or disposed of.

Question 18

The movement of vesicular-tubular carriers (VTCs)farther away from the ER and toward the Golgi complex occurs along tracks composed of what material?
A)RNA
B)DNA
C)microtubules
D)microfilaments
E)intermediate filaments

Accepted Answer

Microtubules serve as tracks for the movement of VTCs from the ER to the Golgi complex.

Question 19

What is responsible for destroying misfolded proteins in the cytoplasm?
A)polysomes
B)polyribosomes
C)peroxisomes
D)proteasomes
E)spliceosome

Accepted Answer

Proteasomes are responsible for destroying misfolded proteins in the cytoplasm. They are large protein complexes that recognize and degrade proteins that have been marked for destruction by the addition of ubiquitin molecules. Polysomes (A) and polyribosomes (B) are both terms used to describe groups of ribosomes working together to translate mRNA into protein. Peroxisomes (C) are organelles that are involved in a variety of metabolic pathways, but are not specifically responsible for degrading misfolded proteins. The spliceosome (E) is a complex involved in RNA splicing, which is a process that removes introns from pre-mRNA molecules before they are translated into protein.

Question 20

What does the conformation-sensing enzyme UGGT do if it binds to a misfolded or incompletely folded glycoprotein?
A)It degrades the oligosaccharide chain.
B)It adds a single mannose back to one of the glucose residues at the exposed end of the recently trimmed oligosaccharide.
C)It adds a single glucose back to one of the mannose residues at the exposed end of the recently trimmed oligosaccharide.
D)It degrades the protein.
E)It refolds the protein on its own.

Accepted Answer

UGGT adds a single glucose back to one of the mannose residues at the exposed end of the recently trimmed oligosaccharide on misfolded or incompletely folded glycoproteins, which signals that the protein needs more time in the ER for proper folding.

Quiz 8: Cytoplasmic Membrane Systems: Structure, Function, and Membrane Trafficking

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

Which correctly describes the effects and treatment of CDG1b?

What happens to a newly synthesized glycoprotein after the binding of calnexin or calreticulin

What is responsible for adding sugars to dolichol phosphate?

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

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

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

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?

Where are misfolded secretory proteins eventually destroyed?

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?

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

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

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 .

Which part of the Golgi complex distinguishes between proteins to be shipped back to the ER and those that are allowed to proceed to the next Golgi station?

CDG1b results from a deficiency in what enzyme?

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

Why does the cell use proteasomes to destroy misfolded proteins?

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

What is responsible for destroying misfolded proteins in the cytoplasm?

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