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

Question

Quizplus · Accepted Answer

Ubiquitin is commonly added to proteins for targeting them for degradation by the proteasome. Cyclin is a class of regulatory proteins that controls the progression of the cell cycle. A cyclin that is no longer needed due to completion of its function is degraded by the proteasome after ubiquitin is attached to it. A cyclin in G2 phase that is no longer needed would be a good candidate for ubiquitination and degradation.

Other answer choices are unlikely to have ubiquitin attached to them. For example, a cell surface protein that requires transport from the ER would be modified by other types of molecules, like signal peptides or chaperones, to reach its destination. mRNA leaving the nucleus and regulatory protein requiring sugar residues do not have ubiquitin attached to them. The production and retention of mRNA in the egg cell after fertilization is not related to ubiquitin modification.

Which of the Following Is Most Likely to Have a Small