The mitochondrial processing peptidase from potato: a self-processing enzyme encoded by two differentially expressed genes.

Cytochrome c reductase from potato is a bifunctional protein complex located in the inner mitochondrial membrane, which is involved in respiratory electron transport and processing of mitochondrial precursor proteins. The three largest subunits of the complex share the highest degree of sequence identity with the alpha- and beta-subunits of the soluble processing peptidase (MPP) from fungi and mammals. Evidence is provided that another substoichiometric polypeptide of the cytochrome c reductase complex resembles the alpha-subunit of MPP. A cDNA clone corresponding to the second alpha-MPP protein (alpha-II MPP) encodes a polypeptide of 504 amino acids which is 84% identical to alpha-I MPP. The two different alpha-MPP polypeptides have similar sizes on SDS-polyacrylamide gels but can be distinguished with an antibody raised against a decapeptide that is specific for alpha-II MPP. The presequences of both alpha-subunits of MPP are proteolytically removed by the integrated processing enzyme complex indicating that it acts on the targeting signals of its own precursor proteins. Gene-specific oligonucleotides reveal that the genes encoding alpha-subunit I and alpha-subunit II of MPP are differentially expressed in all tissues analysed but the transcript levels do not vary between tissues.