Non-catalytic subunit of the NatB complex which catalyzes acetylation of the N-terminal methionine residues of peptides beginning with Met-Asp, Met-Glu, Met-Asn and Met-Gln. May play a role in normal cell-cycle progression. . This is the function of ENSG00000111300 (NAA25, N-alpha-acetyltransferase 25, NatB auxiliary subunit).