Cleaves proteins, imported into the mitochondrion, to their mature size. Release of an N-terminal octapeptide as second stage of processing of some proteins imported into the mitochondrion. Binds 1 zinc ion. Activity is divalent cation-dependent. It is stimulated by manganese, magnesium or calcium ions and reversibly inhibited by zinc, cobalt and iron. Monomer. Belongs to the peptidase M3 family.