Catalytic subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins (By similarity). Preferentially, cleaves after an arginine at position P2 (By similarity). 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 the alpha subunit is required for catalytic activity. Heterodimer of an alpha subunit and a beta subunit subunits, forming the mitochondrial processing protease (MPP) in which the alpha subunit is involved in substrate recognition and binding and the beta subunit is the catalytic subunit. Belongs to the peptidase M16 family.