Core I protein of bovine ubiquinol–cytochrome‐c reductase; an additional member of the mitochondrial‐protein‐processing family

Abstract

Core I and core II proteins are the largest nuclear-encoded subunits of the mitochondrial ubiquinol-cytochrome-c reductase (bc1 complex) lacking redox prosthetic groups. cDNA clones of the two bovine core proteins have been isolated by the screening of lambda ZAP cDNA libraries either with an oligonucleotide probe based on the sequence of an internal peptide or with a polymerase-chain-reaction-amplified fragment. The core I precursor protein consists of 362 amino acids with a 34-amino-acid presequence typical for mitochondrial targeting signals. The mature protein migrates in SDS/polyacrylamide gels with an apparent molecular mass of 47 kDa, which does not correspond to the actual molecular mass of the protein of 35.8 kDa deduced from the cDNA sequence. The core II precursor protein is composed of 453 amino acids having a 14-amino-acid presequence as a targeting sequence. Comparison of the core I amino acid sequence with sequences of the newly discovered protein family [Schulte, U., Arretz, M., Schneider, H., Tropschug, M., Wachter E., Neupert, W. & Weiss, H. (1989) Nature 339, 147 - 149] comprising the processing enhancing protein (PEP), matrix processing peptidase (MPP), and core I and II proteins from Neurospora crassa and Saccharomyces cerevisiae, revealed a remarkable identity of 39% and a high similarity of 49% to N. crassa PEP, which in this fungus is identical to core I. Core II protein is only a distant relative of this protein family. Based on these sequence comparisons and data obtained by genomic Southern blots, we anticipate that the bovine core I subunit, like the N. crassa core I protein, is bifunctional, being responsible for the maintenance of electron transport and processing of proteins during their import into the mitochondrial matrix. The analysis of the primary structure of the two core proteins completes the set of primary structures of all subunits of bovine ubiquinol-cytochrome-c reductase.