Layer et al. present a crystal structure of Naa20, the catalytic subunit of an N-terminal acetyltransferase NatB, in complex with its competitive inhibitor CoA-Ac-MDEL. They find that Naa20 alone can acetylate NatB in vitro while Naa25, the auxiliary subunit of Naa20, increases the substrate affinity of Naa20. This study provides insights into the development of NAT inhibitors.