Non-catalytic subunit of the NatB complex which catalyzes acetylation of the N-terminal methionine residues of proteins beginning with Met-Asp or Met-Glu (By similarity). Has 2 roles in the larval immune response: required both for the phagocytic degradation of internalized bacteria and for the induction of Defensin in the fat body. Within the phagocytic blood cells, has a role in detection of infection and activation of the humoral immune response (By similarity). Component of the N-terminal acetyltransferase B (NatB) complex. Blood cell lysosomes. Belongs to the MDM20/NAA25 family.