Non-catalytic subunit of the NatB N-terminal acetyltransferase, which catalyzes acetylation of the amino-terminal methionine residues of all proteins beginning with Met-Asp or Met-Glu and of some proteins beginning with Met-Asn or Met-Met. Component of the N-terminal acetyltransferase B (NatB) complex. Belongs to the MDM20/NAA25 family.