Interactions of Cyclophilin with the Mitochondrial Inner Membrane and Regulation of the Permeability Transition Pore, a Cyclosporin A-sensitive Channel

Mammalian mitochondria possess an inner membrane channel, the permeability transition pore (MTP), which can be inhibited by nanomolar concentrations of cyclosporin (CS) A. The molecular basis for MTP inhibition by CSA remains unclear. Mitochondria also possess a matrix cyclophilin (CyP) with a unique N-terminal sequence (CyP-M). To test the hypothesis that it interacts with the MTP, we have studied the interactions of CyP-M with rat liver mitochondria by Western blotting with a specific antibody against its unique N terminus. Although sonication in isotonic sucrose at pH 7.4 releases a large proportion of CyP-M, a sizeable CyP-M fraction sediments with submitochondrial particles at 150,000 × g . We show that the interactions of this CyP-M pool with submitochondrial particles are disrupted (i) by the addition of CSA, which inhibits the pore, but not of CSH, which does not, and (ii) by acidic pH condition, which also leads to selective inhibition of the MTP; furthermore, we show that the effect of acidic pH on CyP-M binding is prevented by diethylpyrocarbonate, which fully prevents the inhibitory effect of H on the MTP (Nicolli, A., Petronilli, V., and Bernardi, P.(1993) Biochemistry 32, 4461-4465). These data suggest that CyP-M binding is involved in opening of the MTP and that pore inhibition by CSA and protons may be due to unbinding of CyP-M from its putative binding site on the MTP. A role for CyP-M in MTP regulation is also supported by a study with a series of CSA derivatives with graded affinity for CyP. We show that with each derivative the potency at inhibition of the peptidylprolyl cis-trans- isomerase activity of CyP-M purified to homogeneity is similar to that displayed at inhibition of MTP opening, relative to that displayed by CSA. Decreased binding to CyP-M (but not CyP-A) and decreased efficiency at MTP inhibition is obtained by substitutions in position 8 while a 4-substituted, nonimmunosuppressive derivative is as effective as the native CSA molecule, indicating that calcineurin is not involved in MTP inhibition by CSA.