Catalytic subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins (Probable). Preferentially, cleaves after an arginine at position P2 (Probable). Release of N-terminal transit peptides from precursor proteins imported into the mitochondrion, typically with Arg in position P2. Binds 1 zinc ion per subunit. Binding to mppa-1 is required for catalytic activity (PubMed:16788047). Inhibited by metal chelator ethylenediaminetetraacetic acid (EDTA) (PubMed:16788047). Heterodimer of mppa-1 (alpha) and mppb-1 (beta) subunits, forming the mitochondrial processing protease (MPP) in which mppa-1 is involved in substrate recognition and binding and mppb-1 is the catalytic subunit. RNAi-mediated knockdown causes 54 percent embryonic lethality (PubMed:16788047). Embryonic lethality is further increased in simultaneous RNAi-mediated knockdown of mmpa-1 and mmpb-1 or ucr-1 and mppb-1 (PubMed:16788047). Belongs to the peptidase M16 family.