Substrate recognition and binding subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins. Heterodimer of mpp (alpha) and pep (beta) subunits, forming the mitochondrial processing protease (MPP) in which mpp is involved in substrate recognition and binding and pep is the catalytic subunit. Appears to contain two domains of approximately equal size which are separated by a loop-like sequence. Belongs to the peptidase M16 family. Was originally thought to be the catalytic subunit (PubMed:2967109). The low processing activity which was previously observed with alpha-MPP which has been immunoprecipitated from a mitochondrial extract is most likely due to contamination by the beta-subunit (PubMed:8106471). Does not seem to have a protease activity as it lack the zinc-binding site.