The two high molecular weight subunits of cytochrome c reductase from potato are immunologically related to the mitochondrial processing enhancing protein

A soluble heterodimeric enzyme localized in the matrix space of fungal and mammalian mitochondria removes the presequences of mitochondrial precursor proteins. It consists of two subunits, the matrix processing peptidase (MPP) and the so-called processing enhancing protein (PEP). We have used antibodies directed against the matrix processing peptidase from fungi to analyse the corresponding plant enzyme and its submitochondrial localization. Antibodies directed against PEP specifically recognize two polypeptides of 55 kDa and 53 kDa which are exclusively present in the membrane fraction of potato mitochondria. Also the processing activity removing the presequences of precursor proteins synthesized in vitro is found in this fraction suggesting that the immunopositive proteins may be involved in the processing reaction. As it has been shown that in potato MPP forms part of cytochrome c reductase, a protein complex of the respiratory chain, we analysed this complex with antibodies against PEP. The 55 kDa and 53 kDa subunits are recognized by antibodies directed against PEP from yeast and Neurospora. Incubation of potato cytochrome c reductase with plant mitochondrial precursor proteins synthesized in vitro shows that the purified complex itself has processing activity. Molecular and functional analysis of the two immuno-positive subunits may now reveal which of these represents the processing enhancing protein of the mitochondrial processing peptidase from potato.