Small-angle X-ray scattering (SAXS), which determines the radius of gyration, Rg, and the pair distance distribution function, was used to investigate the conformational changes of calmodulin (CaM) on binding to an antagonist, trifluoperazine (TFP), with or without Ca2+ in solution. We previously applied this SAXS method to CaM complexed with N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W-7) [Osawa, Kuwamoto, Izumi, Yap, Ikura, Shibanuma, Yokokura, Hidaka and Matsushima (1999) FEBS Lett. 442, 173-177] and found that the binding of two W-7 TFP molecules to one Ca2+-saturated CaM molecule induces structural changes from a ‘dumb-bell’ shape to a compact globular shape. We report here that the most compact globular shape whose size is consistent with that of the 1:2 Ca2+-saturated CaM-W-7 complex is formed by the binding of four TFP molecules to one Ca2+-saturated CaM molecule. Even in the absence of Ca2+, the conformational changes of CaM occur on TFP binding, giving a slightly smaller Rg than Ca2+-free CaM alone.

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