Question 1

Which of the following best explains why a protein synthesized in the presence of microsomes is protected from protease digestion unless detergent is added?

Accepted Answer

The protein is translocated into the ER lumen, where it is shielded from protease.

Question 2

In the experiment shown in the first image, why does the protein in lane 5 migrate more slowly than in lane 1?

Accepted Answer

The protein in lane 5 is glycosylated, increasing its molecular weight.

Question 3

What is the role of endoglycosidase H (endo H) in the analysis of protein translocation and processing?

Accepted Answer

It removes N-linked oligosaccharides from glycoproteins.

Question 4

A protein is only protected from protease when microsomes are present and detergent is absent. What does this indicate about the protein's location?

Accepted Answer

It is translocated into the ER lumen.

Question 5

In the cotranslational import experiment, what is the expected outcome for translation and pore status when wild-type SRP and non-hydrolyzable GTP analog (GppNp) are used?

Accepted Answer

Translation is arrested, import does not occur, pore is closed.

Question 6

Which of the following best describes the fate of a protein with both an ER import signal and a mitochondrial targeting sequence (see image 3, protein 1)?

Accepted Answer

It is imported into the ER, but the mitochondrial tag is not recognized, so it stays in the secretory pathway.

Question 7

A protein with an RXR motif (see image 3, protein 2) is most likely to be found in which cellular compartment?

Accepted Answer

Endoplasmic reticulum

Question 8

Which mutant cell line from the N-linked glycosylation pathway (see image 4) is likely defective in the enzyme that removes glucose residues?

Accepted Answer

Mutant D

Question 9

If a protein is glycosylated in the ER but not further modified in the Golgi, which mutant cell line does this correspond to?

Accepted Answer

Mutant C

Question 10

Which of the following best explains why a protein with an HDEL sequence (see image 3, protein 4) is retained in the ER?

Accepted Answer

HDEL is recognized by a receptor in the ER that retrieves escaped proteins from the Golgi.

Question 11

Which step in the N-linked glycosylation pathway is defective in a mutant that accumulates high-mannose oligosaccharides but lacks galactose and NANA residues?

Accepted Answer

GlcNAc transferase II

Question 12

A protein is synthesized in vitro in the presence of microsomes, treated with protease, and then with detergent. What is the expected outcome for the protein band on SDS-PAGE?

Accepted Answer

The protein band disappears due to complete degradation.

Question 13

Which of the following is a direct consequence of a mutation in the SRP that prevents GTP binding?

Accepted Answer

Translation is arrested and the pore remains closed.

Question 14

Which of the following best describes the order of processing for N-linked oligosaccharides in the Golgi, based on the mutants provided?

Accepted Answer

Mutant D → Mutant H → Mutant G → Mutant C → Mutant E → Mutant I → Mutant B → Mutant F → Wild type

Question 15

Which of the following statements about the signal recognition particle (SRP) is correct?

Accepted Answer

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

Question 16

A protein is synthesized in the absence of microsomes and is sensitive to protease. What does this indicate about the protein's location?

Accepted Answer

It is in the cytosol.

Study Guidance: The Endomembrane System and Protein Targeting

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