N-terminal methylation of proteins: structure, function and specificity.

A common site for the posttranslational modification of proteins is at the N-terminal alpha-amino group. Here we consider the enzymatic addition of one or more methyl groups that has been found to occur in several proteins. Although the methylated proteins have different overall functions, they all appear to be involved in large macromolecular structures such as ribosomes, myofibrils, nucleosomes, pilins, or flagella. Structural features at the N-termini of these methylated proteins suggest that sequences in this region may serve as recognition sites for only a few different types of methylating enzymes. Thus, we propose that three enzymes could account for the N-methylated species so far identified in bacteria, the hypothetical MAK, QP, and pilin methyltransferases, and a single additional enzyme, the hypothetical PK methyltransferase, could account for all of the alpha-amino methylations observed in eukaryotic cells. Finally, we discuss criteria that could be used in conjunction with primary sequence data to predict proteins that might be subject to methylation at their amino termini.