The major pathways of N-terminal processing in eukaryotes. Two methionine aminopeptidases (MAP), Map1p and Map2p, cleave N-terminal methionine residues that have small side chains (glycine, alanine, serine, cysteine, threonine, proline, and valine), although methionine is retained on some proteins having penultimate residues of valine. Subsequently, NatA, NatB, and NatC acetylate specific sequences as shown in the figure and in Table 1. Acetylation occurs at least partially on all proteins with Met-Glu-, Met-Asp- and Met-Asn- termini, but only on subclasses of proteins with the other termini. For example, acetylation occurs at least partially on 43% of proteins in yeast and on 96% of proteins in humans with Ala- termini. In addition, Ac-Cys-, Ac-Val-, Ac-Met-Met-, and Ac-Met-Lys- termini occurs on some proteins from humans but not from yeast; it is unknown which NATs are responsible for Ac-Cys-, Ac-Met-Met-, and Ac-Met-Lys- acetylations.
Polevoda et al. BMC Proceedings 2009 3(Suppl 6):S2 doi:10.1186/1753-6561-3-S6-S2