Evidence for the Involvement of a Membrane-Associated Cyclosporin-A-Binding Protein in the Ca2+-Activated Inner Membrane Pore of Heart Mitochondria

Abstract

Heart and liver mitochondria contain a pore in the inner membrane that is activated by Ca2+ and oxidative stress and that has been implicated in cell injury. Pore opening is blocked by cyclosporin A (CSA). Following previous indications that the interaction of CSA with the pore is inhibited by Ca2+ and promoted by ADP, we have investigated how covalent labelling of heart mitochondria by a photoactive CSA derivative is influenced by these agents. In situ photolabelling of an 11-22-kDa (approximately) membrane fraction was selectively increased in the presence of ADP and decreased in the presence of Ca2+. This fraction also accounted for all the high affinity [3H]CSA-binding capacity and contained peptidylprolyl cis-trans isomerase activity (PPIase). The membrane PPIase was extracted using Chaps as detergent, and was purified to a 22-kDa protein (SDS/PAGE). The enzyme was inhibited by CSA (Ki 5 nM). The major component of the 11-22-kDa fraction, photolabelled in an ADP/Ca(2+)-sensitive manner, also migrated at 22 kDa on SDS/PAGE; a minor 11-kDa component was also detected. On the basis of these criteria, it is suggested that the membrane PPIase may be the target for CSA when it blocks the pore. The presence of a similar PPIase in the membrane fraction of liver mitochondria was also demonstrated. The implications of these findings are discussed.