X-RAY STRUCTURE OF A DECAMERIC CYCLOPHILIN CYCLOSPORINE CRYSTAL COMPLEX

HUMAN cyclophilin A (CypA), a ubiquitous intracellular protein of 165 amino acids, is the major receptor for the cyclic undecapeptide immunosuppressant drug cyclosporin A (CsA)1,2, which pre vents allograft rejection after transplant surgery3,4 and is efficacious in the field of autoimmune diseases5. CsA prevents T-cell proliferation by blocking the calcium-activated pathway leading to interleukin-2 transcription. Besides their ability to bind CsA, the cyclophilin isoforms6-8 also have peptidyl-prolyl isomerase activity9-11 and enhance the rate of protein folding12,13. The macrolide FK506 acts similarly to CsA and its cognate receptor FKBP also has peptidyl-prolyl isomerase activity14. Inhibition of this enzymatic activity alone is not sufficient to achieve immunosuppression15,16. A direct molecular interaction between the drug-immunophilin complex (CsA-CypA, or FK506-FKBP) and the phosphatase calcineurin, is responsible for modulating the T-cell receptor signal transduction pathway17,18. Here we describe the crystal structure of a decameric CypA-CsA complex. The crystallographic asymmetric unit is composed of a pentamer of 1 : 1 cyclophilin-cyclosporin complexes of rather exact non-crystallographic fivefold symmetry. The 2.8 angstrom electron density map is of high quality. The five independent cyclosporin molecules are clearly identifiable, providing an unambiguous picture of the detailed interactions between a peptide drug and its receptor. It broadly confirms the results of previous NMR, X-ray and modelling studies, but provides further important structural details which will be of use in the design of drugs that are analogues of CsA.