Non-catalytic subunit of the NatB complex which catalyzes acetylation of the N-terminal methionine residues of peptides beginning with Met-Asp-Glu. May play a role in normal cell-cycle progression. Component of the N-terminal acetyltransferase B (NatB) complex which is composed of NAA20 and NAA25. Belongs to the MDM20/NAA25 family.