Protein methylation.

Proteins can be enzymatically modified in several ways by the addition of methyl groups from S-adenosylmethionine. Reactions forming methyl esters on carboxyl groups are potentially reversible and can modulate the activity of the target protein; in the past year, advances have been made in understanding the physiological roles of four distinct systems that modify normal and abnormal carboxyl groups on proteins. On the other hand, methylation reactions occurring on nitrogen atoms in N-terminal and side-chain positions are generally irreversible. These reactions create new types of amino acid residues and can expand the repertoire of chemistry that a protein can perform.