Ca2+-bound calmodulin forms a compact globular structure on binding four trifluoperazine molecules in solution

Small-angle X-ray scattering (SAXS), which determines the radius of gyration, R-g, 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) FEES 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 R-g than Ca2+-free CaM alone.