Question 1

A newly synthesized protein contains a C-terminal tripeptide sequence, Ser-Lys-Leu (SKL). Where is this protein most likely to be localized, and what is the function of this targeting signal?

Accepted Answer

It will be imported into the peroxisome; SKL is a peroxisomal targeting signal 1 (PTS1).

Question 2

During co-translational import into the ER, which of the following sequences of events is correct?

Accepted Answer

SRP binds the ER signal sequence, the ribosome docks to the translocon, GTP hydrolysis releases SRP, and the polypeptide is translocated into the ER lumen.

Question 3

A protein normally destined for secretion is experimentally modified to contain a KDEL sequence at its C-terminus. What is the most likely fate of this protein?

Accepted Answer

It will be retained in the endoplasmic reticulum due to the KDEL retrieval signal.

Question 4

Which of the following best describes the difference between COPI and COPII vesicle coats in the endomembrane system?

Accepted Answer

COPI mediates retrograde transport from the Golgi to the ER, while COPII mediates anterograde transport from the ER to the Golgi.

Question 5

A patient has a genetic defect that prevents the addition of mannose-6-phosphate (M6P) to lysosomal enzymes. Which of the following is the most likely cellular consequence?

Accepted Answer

Lysosomal enzymes will be secreted from the cell instead of being delivered to lysosomes.

Question 6

Which of the following is NOT a difference between lysosomes and peroxisomes?

Accepted Answer

Both lysosomes and peroxisomes are formed by budding from the Golgi apparatus.

Question 7

A cytosolic protein is found to contain a peroxisomal targeting signal 2 (PTS2) near its N-terminus. What is the most likely mechanism for its import into the peroxisome?

Accepted Answer

The protein is recognized by a cytosolic receptor, imported post-translationally into the peroxisome, and the PTS2 is cleaved after import.

Question 8

Which of the following best explains why GTP hydrolysis is essential during co-translational import of proteins into the ER?

Accepted Answer

GTP hydrolysis by SRP and its receptor provides the energy for the release of SRP from the ribosome and translocon, allowing translation to resume.

Question 9

A protein is engineered to contain both an ER signal sequence and a peroxisomal targeting signal (PTS1). Where is the protein most likely to be localized?

Accepted Answer

It will be imported into the ER because the ER signal sequence is recognized co-translationally, preventing cytosolic localization required for peroxisomal import.

Question 10

Which of the following scenarios would most likely result in a protein being secreted from the cell?

Accepted Answer

The protein contains an N-terminal ER signal sequence but lacks any retention or retrieval signals.

Question 11

A researcher observes that a certain protein is found in the Golgi apparatus and is being transported to the plasma membrane. Which vesicle coat is most likely involved in this transport?

Accepted Answer

Clathrin or no coat (as transport from the trans-Golgi to the plasma membrane often uses clathrin-coated or uncoated vesicles)

Question 12

Which of the following best describes the process of peroxisome biogenesis?

Accepted Answer

Peroxisomes can form by growth and division of pre-existing peroxisomes and by de novo formation from the ER.

Question 13

A protein is synthesized in the cytosol and contains no recognizable targeting signals. Where will this protein most likely be found?

Accepted Answer

In the cytosol, as it lacks any signal for import into organelles.

Question 14

Which of the following is a function unique to peroxisomes and not shared by lysosomes?

Accepted Answer

Beta-oxidation of very long-chain fatty acids.

Question 15

A mutation in the gene encoding the SRP receptor would most likely result in which of the following cellular defects?

Accepted Answer

Failure of ribosome-nascent chain complexes to dock at the ER membrane, leading to accumulation of secretory proteins in the cytosol.

Question 16

Which of the following best explains why proteins destined for the lysosome must be tagged with mannose-6-phosphate (M6P)?

Accepted Answer

M6P is recognized by specific receptors in the trans-Golgi, directing the proteins into clathrin-coated vesicles for delivery to lysosomes.

Question 17

A protein is found to be mislocalized to the cytosol instead of the peroxisome in a patient. Which of the following is the most likely explanation?

Accepted Answer

The protein lacks a functional peroxisomal targeting signal (PTS1 or PTS2).

Question 18

Which of the following best describes the directionality of vesicular transport mediated by COPII and COPI coats?

Accepted Answer

COPII mediates anterograde transport from the ER to the Golgi, while COPI mediates retrograde transport from the Golgi to the ER.

Question 19

A protein is engineered to lack its ER signal sequence but still contains a KDEL sequence. What is the most likely fate of this protein?

Accepted Answer

It will remain in the cytosol, as it cannot enter the ER without the signal sequence.

Cell Biology Study Guide: Endomembrane Trafficking & Organelle Targeting

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