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 (By similarity). Monomer. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase M3 family.