Question 1

Which of the following post-translational modifications (PTMs) most commonly targets serine, threonine, or tyrosine residues and is crucial for regulating protein activity in signal transduction pathways?

Accepted Answer

Phosphorylation

Question 2

A kinase catalyzes the transfer of a phosphate group from ATP to a substrate protein. Which of the following equations best represents this reaction?

Accepted Answer

$$\mathrm{Protein{-}OH} + \mathrm{ATP} \rightarrow \mathrm{Protein{-}P} + \mathrm{ADP}$$

Question 3

Which of the following best describes the role of E3 ubiquitin ligase in the ubiquitylation process?

Accepted Answer

It transfers ubiquitin from E2 to the target protein, conferring substrate specificity.

Question 4

A protein is tagged with a polyubiquitin chain linked through lysine 48 (K48) of ubiquitin. What is the most likely fate of this protein?

Accepted Answer

It will be targeted for proteasomal degradation.

Question 5

Which of the following statements about the proteasome is TRUE?

Accepted Answer

It contains multiple protease active sites and degrades a wide variety of polypeptides.

Question 6

During the cell cycle, which complex is responsible for targeting cyclins for degradation, thereby allowing progression from metaphase to anaphase?

Accepted Answer

APC/C (Anaphase Promoting Complex/Cyclosome)

Question 7

Which of the following mechanisms can inactivate cyclin-dependent kinases (Cdks)?

Accepted Answer

Phosphorylation by inhibitory kinases

Question 8

A mutation causes the loss of function of the Cdc25 phosphatase in a cell. What is the most likely effect on M-Cdk activity?

Accepted Answer

M-Cdk will remain inactive due to persistent inhibitory phosphorylation.

Question 9

Which of the following best describes the function of the signal recognition particle (SRP) during protein targeting to the endoplasmic reticulum (ER)?

Accepted Answer

It binds the ER signal sequence and pauses translation until the ribosome docks at the ER membrane.

Question 10

A protein has an N-terminal ER signal sequence followed by a stop-transfer sequence. After insertion into the ER membrane, what will be the orientation of the protein?

Accepted Answer

N-terminus in the ER lumen, C-terminus in the cytosol

Question 11

Which of the following is a key function of ER-resident chaperones?

Accepted Answer

Facilitating the folding of newly translocated proteins

Question 12

Which of the following statements about translation is CORRECT?

Accepted Answer

tRNAs are charged with amino acids by aminoacyl-tRNA synthetases.

Question 13

In a ribosome profiling experiment, which of the following is measured to infer translation rates across the transcriptome?

Accepted Answer

The abundance of ribosome-protected mRNA fragments

Question 14

Which of the following is an example of a G protein-coupled receptor (GPCR) and its associated G protein?

Accepted Answer

Beta-adrenergic receptor and Gs

Question 15

Which of the following best defines a second messenger in cell signaling?

Accepted Answer

A small intracellular molecule that relays signals from receptors to target molecules

Question 16

Which of the following scenarios best illustrates negative feedback in a signaling pathway?

Accepted Answer

A kinase activates a phosphatase that dephosphorylates and inactivates the kinase.

Question 17

Which of the following is a molecular switch in cell signaling?

Accepted Answer

A GTPase that is active when bound to GTP and inactive when bound to GDP

Question 18

A mutation in the gene encoding the E1 enzyme of the ubiquitin-proteasome system would most likely result in which of the following cellular outcomes?

Accepted Answer

Accumulation of proteins normally targeted for degradation

Question 19

Which of the following best explains how specificity is achieved in cell signaling pathways?

Accepted Answer

Scaffold proteins organize signaling components into complexes.

Question 20

A researcher observes that inhibiting the SRP receptor prevents a newly synthesized protein from entering the ER. Which step in protein targeting is directly affected?

Accepted Answer

Docking of the ribosome-SRP complex to the ER membrane

Unit 2 Study Guide: Post-Translational Modifications, Proteasome, Cell Cycle, Translation, Protein Targeting, and Cell Signaling

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