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 conformations for this carrier have been demonstrated on the basis of interactions with specific inhibitors, namely carboxyatractyloside (CATR) and bongkrekic acid (BA). The two conformations, referred to as CATR and BA conformations, are interconvertible, provided that ADP or ATP are present. The functional ADP/ATP carrier is probably organized as a tetramer. In the presence of CATR or BA the tetramer is split into two dimers combined with either of the two inhibitors. The amino acid sequence of the beef heart carrier monomer (297 residues) contains three repeats of about 100 residues each. Experimental results obtained through different approaches, including photolabeling, immunochemistry, and limited proteolysis, can be interpreted on the basis of a model with five or six transmembrane helices per carrier monomer. Two mobile regions involved in the binding of nucleotides and accessible to proteolytic enzymes have been identified. Each of them may be visualized as consisting of two pairs of short amphipathic helices, which can be juxtaposed to form hydrophilic channels facilitating the nucleotide transport. Mutagenesis in yeast is currently being used to detect strategic amino acids in ADP/ATP transport.