X-ray structure of a decameric cyclophilin-cyclosporin crystal complex

Abstract

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 prevents allograft rejection after transplant surgery^3,4 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 isoforms^6–8 also have peptidyl–prolyl isomerase activity^9–11 and enhance the rate of protein folding^12,13. 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 immunosuppression^15,16. 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 pathway^17,18. 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 Å 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.