N-alpha-acetyltransferase that acetylates the N-terminus of proteins that retain their initiating methionine . Has a broad substrate specificity: able to acetylate the initiator methionine of most peptides, except for those with a proline in second position . Also displays N-epsilon-acetyltransferase activity by mediating acetylation of the side chain of specific lysines on proteins . Autoacetylates in vivo . The relevance of N-epsilon-acetyltransferase activity is however unclear: able to acetylate H4 in vitro, but this result has not been confirmed in vivo . Component of a N-alpha-acetyltransferase complex containing NAA10 and NAA15, but NAA50 does not influence the acetyltransferase activity of NAA10: this multiprotein complex probably constitutes the major contributor for N-terminal acetylation at the ribosome exit tunnel, with NAA10 acetylating all amino termini that are devoid of methionine and NAA50 acetylating other peptides . Required for sister chromatid cohesion during mitosis by promoting binding of CDCA5/sororin to cohesin: may act by counteracting the function of NAA10 .