Substrate recognition and binding subunit of the essential mitochondrial processing protease (MPP), which cleaves the mitochondrial sequence off newly imported precursors proteins. This is a component of the ubiquinol-cytochrome c reductase complex (complex III or cytochrome b-c1 complex), which is part of the mitochondrial respiratory chain. Mediates formation of the complex between cytochromes c and c1. a quinol + 2 Fe(III)-[cytochrome c](out) = a quinone + 2 Fe(II)-[cytochrome c](out) + 2 H(+)(out) 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. Belongs to the peptidase M16 family. Does not seem to have a protease activity as it lack the zinc-binding site.