Abstract
Small angle X-ray scattering (SAXS) was applied to the binding of the immunosuppressant drug cyclasporin-A to the protein calmodulin. Guinier analysis of the SAXS profiles yielded a radius of gyration, Rg, of 19.7 ± 0.3 Å for the native protein and 16.9 ± 0.3 Å for the drug/protein complex. Maximum entropy (maxent) methods of data analysis were used to calculate the distance distribution function, p(r). From this analysis, the R g for the native protein is 20.9 ± 0.1 Å and that for the complex 16.7 ± 0.1 Å. The measured SAXS profiles and the derived p(r) for calmodulin agree with profiles calculated from the crystallographic structure of calmodulin. Major structural changes are induced in calmodulin on binding cyclosporin-A. A model consistent with the observed scattering profiles is an ellipsoid with major axes 55 and 36 Å. Molecular modeling of the calmodulin molecule suggests that bond rotation in the flexible α-helix linker region produces models consistent with the above observations.
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