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) 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.