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, Met-Gln or Met-Met. NatB acetylates TPM1 protein and regulates tropomyocin-actin interactions, it is presumed to N-acetylate 15% of all yeast proteins. acetyl-CoA + N-terminal L-methionyl-L-asparaginyl-[protein] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-methionyl-L-asparaginyl-[protein] acetyl-CoA + N-terminal L-methionyl-L-glutaminyl-[protein] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-methionyl-L-glutaminyl-[protein] acetyl-CoA + N-terminal L-methionyl-L-aspartyl-[protein] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-methionyl-L-aspartyl-[protein] acetyl-CoA + N-terminal L-methionyl-L-glutamyl-[protein] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-methionyl-L-glutamyl-[protein] Component of the N-terminal acetyltransferase B (NatB) complex, which is composed of NAT3 and MDM20. Present with 639 molecules/cell in log phase SD medium. Belongs to the acetyltransferase family. GNAT subfamily.