Substrate recognition and binding subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins. Heterodimer of MAS2 (alpha) and MAS1 (beta) subunits, forming the mitochondrial processing protease (MPP) in which MAS2 is involved in substrate recognition and binding and MAS1 is the catalytic subunit. Present with 31400 molecules/cell in log phase SD medium. Belongs to the peptidase M16 family. Was originally thought to be the catalytic subunit (PubMed:3061797). The low processing activity which was previously observed with alpha-MPP which has been purified from a mitochondrial extract is most likely due to contamination by the beta-subunit. Does not seem to have a protease activity as it lack the zinc-binding site (PubMed:9299349).