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 (By similarity). Plays an essential role in mitochondrial biogenesis (PubMed:18603769). 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 alpha subunit is required for catalytic activity. Heterodimer of alpha and beta subunits, forming the mitochondrial processing protease (MPP) in which subunit alpha is involved in substrate recognition and binding and subunit beta is the catalytic subunit (By similarity). mppB is probably also part of the cytochrome bc1 complex as a core I protein in the mitochondrial inner membrane (Probable). The expression level of the transcript is highest in vegetative cells and in early development (5 hours), and decreases at 5 to 10 hours of development. Inn contrast the protein level does not significantly change during the life cycle, despite the marked reduction in the level of mRNA after 5 to 10 hours of development, suggesting that the protein is stable throughout the life cycle. Antisense RNA inhibition of the mppB gene induces gene expression of nuclear-encoded mitochondrial proteins (mppA and cxdA) and surprisingly also of the mppB gene itself. These results suggest that antisense RNA inhibition of mppB induces gene expression of mitochondrial proteins, presumably in a retrograde signaling manner. Up-regulated by Pseudomonas aeruginosa, PAO1 strain and PA14 strain infection. Disruption of the mppB is lethal. Belongs to the peptidase M16 family.