Question 1

Which experimental result best supports the hypothesis that ER signal sequences are both necessary and sufficient for protein targeting to the ER?

Accepted Answer

A cytosolic protein is targeted to the ER when an ER signal sequence is attached to it.

Question 2

A protein is synthesized with the following N-terminal sequence: $$	ext{-Met-Ser-Phe-Val-Ser-Leu-Leu-Leu-Val-Gly-Ile-Leu-Phe-Trp-Ala-Thr-}. $$Based on the sequence, where is this protein most likely to be targeted?

Accepted Answer

Endoplasmic reticulum

Question 3

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

Accepted Answer

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

Question 4

What would happen to a protein normally destined for the ER lumen if its signal sequence was mutated so it was no longer recognized by the SRP?

Accepted Answer

It would remain in the cytosol.

Question 5

Which of the following best describes the fate of a soluble protein with an ER signal sequence and no transmembrane domain?

Accepted Answer

It is translocated into the ER lumen and may be secreted or targeted to an organelle lumen.

Question 6

A protein contains the C-terminal sequence $$	ext{-Ser-Lys-Leu} ($$PTS1). Where is this protein most likely to be found?

Accepted Answer

Peroxisomal lumen

Question 7

Which of the following is a key difference between protein import into mitochondria and the ER?

Accepted Answer

Mitochondrial proteins are imported post-translationally, while most ER proteins are imported co-translationally.

Question 8

A mutation causes the loss of the signal peptidase enzyme in the ER. What is the most likely consequence for secretory proteins?

Accepted Answer

They will retain their signal sequence and may not fold or function properly.

Question 9

Which of the following best explains why the orientation of membrane proteins is preserved during vesicular transport?

Accepted Answer

The topology of the membrane is maintained from donor to target compartment.

Question 10

Which coat protein is primarily responsible for vesicle formation from the Golgi apparatus to the plasma membrane?

Accepted Answer

Clathrin

Question 11

A protein is found to have a large N-terminal domain in the ER lumen and a single transmembrane segment. Which process most likely determined its orientation?

Accepted Answer

The presence of a stop-transfer sequence following the signal sequence.

Question 12

Which of the following is true regarding the import of proteins into peroxisomes?

Accepted Answer

Folded proteins can be imported, and the signal sequence is usually retained.

Question 13

A mutation in the TOM complex would most directly affect which cellular process?

Accepted Answer

Import of proteins into the mitochondrial matrix

Question 14

Which of the following best describes the role of adaptor proteins in clathrin-coated vesicle formation?

Accepted Answer

They bind both clathrin and membrane-bound cargo receptors, selecting cargo for transport.

Question 15

A protein with the sequence $$	ext{-Lys-Asp-Glu-Leu} at $$its C-terminus is most likely to:

Accepted Answer

Be retained in the ER

Question 16

Which of the following is a consequence of swapping the ER signal sequence of an ER protein with that of a cytosolic protein?

Accepted Answer

The protein will remain in the cytosol.

Question 17

Which of the following best explains why the signal sequence is often cleaved off after a protein enters the ER lumen?

Accepted Answer

To allow the protein to fold properly and function in its final location.

Question 18

A protein is synthesized on free ribosomes in the cytosol and contains an N-terminal amphipathic helix with positively charged residues on one side. This protein is most likely destined for:

Accepted Answer

The mitochondrial matrix

Question 19

Which of the following is NOT a determinant of vesicular trafficking specificity?

Accepted Answer

The presence of ribosomes on the vesicle surface

18: Protein Sorting and Intracellular Membrane Trafficking

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