A Small-Angle X-ray Scattering Study of the Binding of Cyclosporin A to Cyclophilin

The small-angle X-ray scattering (SAXS) technique was used to investigate 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 used to analyse the experimental SAXS data. The measured radius of gyration, R_g, for cyclophilin is 16.3 (5) Å. This is equivalent to a compact sphere of radius 21.0 Å. There is qualitative agreement between the experimental SAXS profiles and the derived distance-distribution function, p(r), for cyclophilin, and similar profiles calculated from the crystallographic structure. The notable discrepancy is the difference of approximately 1.5 Å in the estimated radius of the equivalent sphere. On binding cyclosporin A, the main structure-related change in cyclophilin observed under these experimental conditions is an increased propensity to form oligomers. Meaningful estimates of R_g for the monomeric complex are not possible because of the presence 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.