THE MITOCHONDRIAL PROCESSING PEPTIDASE FROM POTATO - A SELF-PROCESSING ENZYME ENCODED BY 2 DIFFERENTIALLY EXPRESSED GENES

Cytochrome c reductase from potato is a bifunctional protein complex l ocated in the inner mitochondrial membrane, which is involved in respi ratory electron transport and processing of mitochondrial precursor pr oteins. The three largest subunits of the complex share the highest de gree of sequence identity with the alpha- and beta-subunits of the sol uble processing peptidase (MPP) from fungi and mammals. Evidence is pr ovided that another substoichiometric polypeptide of the cytochrome c reductase complex resembles the alpha-subunit of MPP. A cDNA clone cor responding to the second alpha-MPP protein (alpha-II MPP) encodes a po lypeptide of 504 amino acids which is 84% identical to alpha-I MPP. Th e two different alpha-MPP polypeptides have similar sizes on SDS-polya crylamide gels but can be distinguished with an antibody raised agains t a decapeptide that is specific for alpha-II MPP. The presequences of both alpha-subunits of MPP are proteolytically removed by the integra ted 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 o f MPP are differentially expressed in all tissues analysed but the tra nscript levels do not vary between tissues.