A SMALL-ANGLE X-RAY-SCATTERING STUDY OF THE BINDING OF CYCLOSPORINE-ATO CYCLOPHILIN

The small-angle X-ray scattering (SA;YS) technique was used to investi gate structural characteristics of the protein cyclophilin in solution and to attempt to detect major changes induced by the binding of the immunosuppressant drug cyclosporin A. Maximum-entropy methods were use d to analyse the experimental SAXS data. The measured radius of gyrati on, R(g), for cyclophilin is 16.3 (5) Angstrom. This is equivalent to a compact sphere of radius 21.0 Angstrom. There is qualitative agreeme nt between the experimental SAXS profiles and the derived distance-dis tribution function, p(r), for cyclophilin, and similar profiles calcul ated from the crystallographic structure. The notable discrepancy is t he difference of approximately 1.5 Angstrom in the estimated radius of the equivalent sphere. On binding cyclosporin A, the main structure-r elated change in cyclophilin observed under these experimental conditi ons is an increased propensity to form oligomers. Meaningful estimates of R(g) for the monomeric complex are not possible because of the pre sence of a significant population of aggregates. In a second series of experiments, both native cyclophilin and the cyclophilin/cyclosporin A complex readily formed aggregates under the prevailing experimental conditions.