HUMAN CYCLOPHILIN-C - PRIMARY STRUCTURE, TISSUE DISTRIBUTION, AND DETERMINATION OF BINDING-SPECIFICITY FOR CYCLOSPORINES

A complementary DNA (cDNA) for human cyclophilin C (Cyp-C) was isolate d from a human kidney cDNA library. Northern blot experiments with sev eral human tissues and cell lines revealed that Cyp-C is less abundant than Cyp-A. The amount of Cyp-C mRNA was 10-fold lower than that of C yp-A in kidney. Expression of human Cyp-C in the kidney is not signifi cantly elevated compared to pancreas, skeletal muscle, heart, lung, an d liver. This argues against a previously postulated specific role for Cyp-C in the nephrotoxic effects of CsA in humans, based on the studi es of its relative abundance in murine kidney. It is present in extrem ely low concentrations in brain and in the Jurkat T cell line. The bin ding of recombinant human Cyp-A, -B, and -C to cyclosporin A (CsA) was studied by immunochemical methods. The relative affinity of Cyp-C for CsA is lower by a factor of 2 than that of Cyp-A, which itself is 10- fold lower than that of Cyp-B. Cross-reactivity studies with a series of Cs derivatives showed that Cyp-C binds CsA with a fine specificity similar to that of Cyp-A and Cyp-B. Cs amino acid residues 1, 2, 10, a nd 11 seemed essential for the interaction with all three Cyp subtypes . However, Cyp-C tolerates a greater variety of structures on Cs at po sition 2 than Cyp-A does, suggesting that this residue of CsA might no t be in tight contact with Cyp-C. This was confirmed by modeling of hu man Cyp-C on the structure of the complex formed by Cyp-A and CsA. The knowledge of the fine specificity of human Cyps for CsA and of their expression levels may provide better insights into how CsA acts on its different target proteins in vivo.