Which post-translational modification is commonly involved in targeting proteins for degradation in the cell?

• A. Ubiquitination.
• B. Phosphorylation.
• C. Glycosylation.
• D. Acetylation.

Answer: (A)

Ubiquitination is the process by which proteins are tagged for destruction by the proteasome. A ubiquitin molecule is covalently attached to lysine residues on the target through a cascade of enzymes (E1, E2, and E3). When a protein accumulates a polyubiquitin chain—especially one linked through lysine-48—the proteasome recognizes it and degrades the protein into peptides, using energy from ATP to unfold and process it. Deubiquitinases can remove this tag, reversing the signal. Other modifications can influence stability or localization—phosphorylation can help create signals for degradation in some contexts, but it isn’t the universal tag for destruction; glycosylation and acetylation mainly affect folding, trafficking, or interactions rather than serving as the primary degradation tag.