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. NatB acetylates TPM1 protein and regulates tropomyocin-actin interactions. MDM20 is required for mitochondrial inheritance during budding and together with TPM1, is essential for the integrity and assembly of actin cables. Genetically interacts with CIN8. Component of the N-terminal acetyltransferase B (NatB) complex, which is composed of NAT3 and MDM20. Present with 5800 molecules/cell in log phase SD medium. Belongs to the MDM20/NAA25 family.