The mitochondrial processing enzyme consists of two components, the mitochondrial processing peptidase (MPP) and processing enhancing protein (PEP). MPP and PEP act cooperatively in proteolytic processing of mitochondrial precursor proteins. Most of the mitochondrial precursors possess aminoterminal presequences (also called "targeting sequences" or "signal sequences"), that do not display a common motif and that show only limited similarities of the cleavage sites. The mitochondrial processing peptidase is a metal-dependent endoprotease, sensitive to sulfhydryl-modifying reagents and appears to belong to a new class of proteases. MPP and PEP, together with the core 1 and core 2 proteins of the respiratory complex III, form a new protein family.