Binding of trifluoperazine to apocalmodulin revealed by a combination of small-angle X-ray scattering and nuclear magnetic resonance

Small-angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) studies were performed to investigate the binding of trifluoperazine (TFP) to Ca²⁺-free calmodulin (apoCaM) with N- and C-terminal globular domains connected by a linker. The SAXS and NMR measurements were taken throughout the titration of TFP. The SAXS analyses indicate that the binding of TFP induces structural changes from a dumbbell shape to a compact globular shape in solution. The formation of the complete globular structure requires 5.0 added equivalents of TFP. An analysis of NMR chemical-shift changes indicates that the C-terminal domain of apoCaM is involved in the binding of TFP. The SAXS and NMR data reflect the high structural flexibility of apoCaM.