Question 1

Which of the following statements best describes the default pathway for newly synthesized proteins in a eukaryotic cell?

Accepted Answer

Proteins without targeting sequences remain in the cytoplasm.

Question 2

A protein destined for secretion from a cell is synthesized on ribosomes attached to which cellular structure?

Accepted Answer

Rough endoplasmic reticulum

Question 3

What is the role of the Signal Recognition Particle (SRP) during co-translational translocation of proteins into the ER?

Accepted Answer

SRP binds the signal sequence and ribosome, pausing translation until docking at the ER membrane.

Question 4

Which of the following sequences is necessary and sufficient to target a protein to the ER membrane?

Accepted Answer

A stretch of hydrophobic amino acids at the N-terminus (signal sequence)

Question 5

During co-translational translocation, what would happen if the signal sequence was not cleaved from a soluble ER protein?

Accepted Answer

The protein would remain anchored in the ER membrane.

Question 6

Which experimental approach demonstrates that proteins are protected from protease during co-translational import into the ER?

Accepted Answer

Adding protease during synthesis in the presence of microsomes results in intact protein.

Question 7

Which of the following best describes the function of oligosaccharyltransferase during protein import into the ER?

Accepted Answer

It transfers a preformed oligosaccharide to an asparagine residue in the growing polypeptide chain.

Question 8

A single-pass transmembrane protein with its N-terminus in the ER lumen and C-terminus in the cytoplasm is most likely to have which of the following features?

Accepted Answer

An N-terminal signal sequence and an internal stop transfer sequence

Question 9

Which of the following is true regarding glycosylation of proteins in the ER?

Accepted Answer

N-linked glycosylation occurs only in the ER lumen and not on cytoplasmic proteins.

Question 10

What is the fate of misfolded proteins in the ER that cannot be properly refolded?

Accepted Answer

They are polyubiquitinated and degraded by the proteasome in the cytoplasm.

Question 11

Which of the following best explains why the ER lumen is a more oxidizing environment than the cytoplasm?

Accepted Answer

It promotes the formation of disulfide bonds between cysteine residues.

Question 12

A multipass transmembrane protein is synthesized with alternating internal start and stop transfer sequences. What determines its final topology in the ER membrane?

Accepted Answer

The order and orientation of the topogenic sequences in the polypeptide chain.

Question 13

Which of the following equations correctly represents the process of N-linked glycosylation in the ER?

Accepted Answer

$$	ext{Protein} + 	ext{(Oligosaccharide)} \xrightarrow{	ext{Oligosaccharyltransferase}} 	ext{Glycoprotein}$$

Question 14

Which of the following is a function of protein disulfide isomerase (PDI) in the ER?

Accepted Answer

Catalyzes the breaking and reforming of disulfide bonds to aid in proper protein folding.

Question 15

A protein with the sequence Asn-X-Ser is being synthesized in the ER. What modification is most likely to occur at this site?

Accepted Answer

Addition of an N-linked oligosaccharide to the asparagine residue.

Question 16

Which of the following best describes the role of chaperone proteins in the ER lumen?

Accepted Answer

They bind unfolded or misfolded proteins to assist in proper folding.

Question 17

What would be the consequence of adding a stop transfer sequence to a soluble ER protein?

Accepted Answer

The protein would become anchored in the ER membrane as a transmembrane protein.

Question 18

Which of the following statements about GPI-anchored proteins is correct?

Accepted Answer

They are initially synthesized as transmembrane proteins and then cleaved and attached to a lipid anchor.

Question 19

Which of the following best describes the process by which misfolded proteins are returned to the cytosol for degradation?

Accepted Answer

Dislocation across the ER membrane, polyubiquitination, and degradation by the proteasome.

Protein Targeting to the Endoplasmic Reticulum (ER): Mechanisms and Pathways

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