Which of the Following is Most Likely to Have a Small

Question

Which of the following is most likely to have a small protein called ubiquitin attached to it?
A) a cyclin that usually acts in G₁, now that the cell is in G₂
B) a cell surface protein that requires transport from the ER
C) an mRNA that is leaving the nucleus to be translated
D) a regulatory protein that requires sugar residues to be attached
E) an mRNA produced by an egg cell that will be retained until after fertilization

Accepted Answer

Ubiquitin is often attached to proteins that need to be degraded by the proteasome. Cyclins are regulatory proteins that are only needed during specific stages of the cell cycle. Once their function is complete, they are targeted for degradation by ubiquitination and subsequent proteasomal degradation.

Answer: B
 Proteins that are destined for secretion or for the cell membrane are often modified with ubiquitin as part of the quality control process during their transport from the ER to their final destination.

Answer: D
 Ubiquitination can also be used to regulate protein activity or localization within the cell. In this case, a regulatory protein that requires sugar residues to be attached may be modified with ubiquitin as part of its regulation.

The other choices are unlikely to have ubiquitin attached to them, as mRNA does not typically undergo ubiquitination, and post-translation modifications usually target certain residues, not the entire protein.