CHEMICAL, IMMUNOLOGICAL, ENZYMATIC, AND GENETIC APPROACHES TO STUDYING THE ARRANGEMENT OF THE PEPTIDE-CHAIN OF THE ADP ATP CARRIER IN THE MITOCHONDRIAL-MEMBRANE/

In the process of oxidative phosphorylation, the exchange of cytosolic ADP(3-) against mitochondrial ATP(4-) across the inner mitochondrial membrane is mediated by a specific carrier protein. Two different conf ormations for this carrier have been demonstrated on the basis of inte ractions with specific inhibitors, namely carboxyatractyloside (CATR) and bongkrekic acid (BA). The two conformations, referred to as CATR a nd BA conformations, are interconvertible, provided that ADP or ATP ar e present. The functional ADP/ATP carrier is probably organized as a t etramer. In the presence of CATR or BA the tetramer is split into two dimers combined with either of the two inhibitors. The amino acid sequ ence of the beef heart carrier monomer (297 residues) contains three r epeats of about 100 residues each. Experimental results obtained throu gh different approaches, including photolabeling, immunochemistry, and limited proteolysis, can be interpreted on the basis of a model with five or six transmembrane alpha helices per carrier monomer. Two mobil e regions involved in the binding of nucleotides and accessible to pro teolytic enzymes have been identified. Each of them may be visualized as consisting of two pairs of short amphipathic ct helices, which can be juxtaposed to form hydrophilic channels facilitating the nucleotide transport. Mutagenesis in yeast is currently being used to detect str ategic amino acids in ADP/ATP transport.