What are common post-translational modifications and their purposes?

• A. Phosphorylation, glycosylation, acetylation, ubiquitination; they modify activity, localization, stability, and interactions.
• B. Methylation of DNA; changes transcription timing.
• C. RNA editing; changes coding sequence.
• D. Proteolytic cleavage only; activates proteins.

Answer: (A)

Post-translational modifications are chemical changes added to a protein after it’s made, and they provide rapid, reversible ways to control function. The four common modifications listed—phosphorylation, glycosylation, acetylation, and ubiquitination—illustrate how this tuning works. Phosphorylation adds a phosphate group, acting like a switch that can turn enzyme activity on or off and propagate signaling. Glycosylation attaches sugar groups that help proteins fold correctly, traffic to the right cellular location, and interact with other molecules. Acetylation changes charge on proteins, influencing their interactions and activity, including regulating gene expression when applied to histones and affecting non-histone proteins as well. Ubiquitination marks proteins for degradation by the proteasome or can alter their localization and interactions, thereby controlling protein levels and function.

Other choices describe modifications that are not post-translational changes to proteins (DNA methylation) or occur at the RNA level (RNA editing). Proteolytic cleavage is a recognized type of PTM, but it’s not the only one, and describing it as the sole modification misses the broader range of regulatory edits that proteins can undergo.