Interaction of membrane surface charges with the reconstituted ADP/ATP-carrier from mitochondria

Various modulating influences of negative and positive membrane charges on binding and transport properties of the reconstituted ADP/ATP carrier from mitochondria were investigated. The results are interpreted in terms of functional and structural asymmetries of the adenine nucleotide carrier embedded in the liposomal membrane. The surface potential of liposomes was measured directly either by potential-dependent adsorption of the fluorescent dye 2-p-toluidinylnaphthalene 6-sulfonate (TNS) or by the pK shift of the lipophilic pH indicator pentadecylumbelliferone. These results were correlated with the following observations. (1) Negative surface potentials increase the apparent dissociation constant, Kd, for binding of the negatively charged inhibitor carboxyatractylate to the reconstituted carrier protein. (2) Surface potentials modulate the apparent transport affinity, Km, of the reconstituted adenine nucleotide carrier for ADP and ATP. The interaction of surface charges with the transport function was investigated with carrier proteins oriented both right-side-out and inside-out. Thus the influence of the surface potential on the function of the ADP/ATP carrier could be determined for the internal and external active sites of the translocator on the outer side of the membrane. Large discrepancies were observed not only between the potentials measured directly (fluorescent dyes) and those measured indirectly (binding and transport affinities), but also between the different surface potentials determined from the influence on the alternatively oriented carrier proteins. The effect of surface charges was rather weak on the cytosolic side of the translocator, whereas there was a strong influence of surface charges on the active site at the matrix side. The most obvious explanation, i.e., screening of negative membrane charges by positively charged amino acid residues at the protein surface, could be ruled out. Besides the modulation of binding affinities for substrates and inhibitors, an additional side-specific effect of surface charges on the transport velocity was observed. Again, the influence on the internal active site of the ADP/ATP carrier was found to be much higher than that on the cytosolic site. The observed effects can be explained by a definite structural asymmetry of the carrier embedded in the liposomal membrane. That site which is physiologically exposed to the cytosol is located at a considerable distance from the plane of the membrane, whereas the opposite site seems to be in close proximity to the membrane surface. Moreover, a spatial equivalence of carboxyatractylate binding site and nucleotide binding site at the external side of the carrier protein was concluded.