Question 1

Which of the following statements best describes the role of sorting signals in protein targeting within eukaryotic cells?

Accepted Answer

Sorting signals are necessary and sufficient to direct proteins to their correct cellular compartment.

Question 2

A protein is synthesized in the cytosol and lacks any sorting signal. What is its most likely fate?

Accepted Answer

It will remain as a permanent resident of the cytosol.

Question 3

Which physical property of a signal sequence is most critical for targeting a protein to the endoplasmic reticulum (ER)?

Accepted Answer

Hydrophobicity and placement of charge within the sequence.

Question 4

A researcher attaches a nuclear localization signal (NLS) to pyruvate kinase, a cytosolic protein. What is the expected result?

Accepted Answer

Pyruvate kinase will be imported into the nucleus.

Question 5

Which of the following best describes the mechanism by which most proteins enter the ER during synthesis?

Accepted Answer

Co-translational translocation, where import begins before synthesis is complete.

Question 6

What is the function of the Signal Recognition Particle (SRP) during ER protein import?

Accepted Answer

SRP binds the signal sequence and docks the ribosome to the ER membrane.

Question 7

Which of the following modifications does NOT typically occur in the endoplasmic reticulum?

Accepted Answer

Phosphorylation of proteins.

Question 8

A protein destined for the mitochondrial matrix typically contains which type of signal sequence?

Accepted Answer

An amphiphilic alpha-helix with positively charged residues on one side and hydrophobic residues on the other.

Question 9

Which of the following statements about the nuclear pore complex (NPC) is correct?

Accepted Answer

NPCs use FG-repeat domains to create a selective gel-like matrix for transport.

Question 10

Which process is primarily responsible for the movement of proteins between the ER and Golgi apparatus?

Accepted Answer

Vesicular transport using membrane-enclosed intermediates.

Question 11

A mutation in the PEX genes affects which organelle and leads to which human disorder?

Accepted Answer

Peroxisomes; Zellweger spectrum disorders

Question 12

Which of the following is TRUE regarding glycosylation in the ER?

Accepted Answer

N-linked glycosylation involves the transfer of an oligosaccharide en bloc to an asparagine residue.

Question 13

Which protein modification in the ER is essential for the correct folding and stability of many secreted proteins?

Accepted Answer

Formation of disulfide bonds by protein disulfide isomerase (PDI)

Question 14

What is the main energy source for post-translational translocation of proteins into the ER in yeast?

Accepted Answer

ATP hydrolysis by BiP

Question 15

Which of the following best explains why most proteins cannot simply diffuse across the cell membrane?

Accepted Answer

Proteins are hydrophilic and the membrane interior is hydrophobic.

Question 16

A transmembrane protein with an odd number of membrane-spanning domains will have its N- and C-termini located where?

Accepted Answer

On opposite sides of the membrane.

Question 17

Which of the following is a unique feature of C-tail anchored proteins compared to other ER-targeted proteins?

Accepted Answer

They use Get3 ATPase dimer for membrane insertion.

Question 18

Which of the following is a function of calnexin and calreticulin in the ER?

Accepted Answer

They bind oligosaccharide tags on incompletely folded proteins to retain them in the ER.

Question 19

Which of the following best describes the role of the translocon in ER protein import?

Accepted Answer

It is a membrane channel that opens during translation to allow polypeptide entry.

Chapter 13: Moving Proteins into Membranes and Organelles

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