Question 1

Which of the following best describes the primary function of the rough endoplasmic reticulum (RER) in the endomembrane system?

Accepted Answer

Initial folding, quality control, and early post-translational modification of proteins

Question 2

A protein destined for secretion from the cell is synthesized with an N-terminal signal sequence. What is the fate of this signal sequence during cotranslational translocation into the ER?

Accepted Answer

It is cleaved off by signal peptidase once the protein enters the ER lumen.

Question 3

Which of the following amino acid sequences is most likely to function as an ER signal sequence?

Accepted Answer

A stretch of hydrophobic amino acids near the N-terminus

Question 4

A newly synthesized protein contains the sequence Asn-X-Ser (where X is not proline). Which post-translational modification is most likely to occur at this site in the ER?

Accepted Answer

N-linked glycosylation

Question 5

Which enzyme is responsible for catalyzing the formation and rearrangement of disulfide bonds in proteins within the ER lumen?

Accepted Answer

Protein disulfide isomerase (PDI)

Question 6

A mutation prevents the function of the signal recognition particle (SRP) in a cell. Which of the following is the most likely consequence?

Accepted Answer

Proteins with ER signal sequences will be synthesized in the cytosol and fail to enter the ER.

Question 7

Which of the following best explains why the lumens of organelles in the endomembrane system are chemically similar to the extracellular environment?

Accepted Answer

They are topologically equivalent to the extracellular space due to vesicular transport.

Question 8

A protein is found to be misfolded in the ER and is recognized by BiP and calnexin. What is the most likely fate of this protein if it cannot be properly refolded?

Accepted Answer

It will be exported to the cytosol, ubiquitinated, and degraded by the proteasome.

Question 9

Which of the following is a function of N-linked glycosylation in the ER?

Accepted Answer

It assists in protein folding and quality control.

Question 10

A cell is exposed to a drug that inhibits the function of the ER exit sites (ERES). Which of the following processes would be most directly affected?

Accepted Answer

Transport of properly folded proteins to the Golgi apparatus

Question 11

Which checkpoint in the ER quality control system is responsible for sensing the accumulation of misfolded proteins and increasing the folding capacity of the ER?

Accepted Answer

Unfolded Protein Response (UPR)

Question 12

A patient has a genetic defect in oligosaccharyl transferase. Which of the following would most likely be observed in their cells?

Accepted Answer

Proteins lacking N-linked glycosylation

Question 13

Which of the following best describes the role of the signal recognition particle (SRP) during protein synthesis?

Accepted Answer

It binds to the ER signal sequence and pauses translation until the ribosome docks on the ER membrane.

Question 14

A protein is synthesized with an internal hydrophobic signal-anchor sequence. What is the most likely fate of this sequence?

Accepted Answer

It will remain embedded in the membrane as a transmembrane domain.

Question 15

Which of the following is a direct consequence of the formation of disulfide bonds in proteins within the ER?

Accepted Answer

Increased resistance to proteases and stabilization of protein structure

Question 16

A researcher observes that a certain protein is present in the ER, Golgi, and lysosome, but not in the cytosol. Which of the following best explains this observation?

Accepted Answer

The protein is transported through the endomembrane system via vesicular trafficking.

Question 17

Which of the following best describes the fate of irreparably misfolded proteins in the ER?

Accepted Answer

They are exported to the cytosol, ubiquitinated, and degraded by the proteasome.

Question 18

Which of the following equations best represents the formation of a disulfide bond between two cysteine residues?

Accepted Answer

$$2\mathrm{RSH} \rightarrow \mathrm{RS{-}SR} + 2\mathrm{H}^+$$

Question 19

A protein is found to be heavily glycosylated and forms part of the glycocalyx on the cell surface. Which of the following is a likely function of this glycosylation?

Accepted Answer

Protection from harsh extracellular conditions and cell recognition

Intracellular Compartments and Protein Sorting: The Endomembrane System and ER Protein Trafficking

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