The C-Terminal Random Coil Region Tunes the Ca(2+)-Binding Affinity of S100A4 through Conformational Activation

S100A4 interacts with many binding partners upon Ca(2+) activation and is strongly associated with increased metastasis formation. In order to understand the role of the C-terminal random coil for the protein function we examined how small angle X-ray scattering of the wild-type S100A4 and its C-terminal deletion mutant (residues 1–88, Δ13) changes upon Ca(2+) binding. We found that the scattering intensity of wild-type S100A4 changes substantially in the 0.15–0.25 Å(−1) q-range whereas a similar change is not visible in the C-terminus deleted mutant. Ensemble optimization SAXS modeling indicates that the entire C-terminus is extended when Ca(2+) is bound. Pulsed field gradient NMR measurements provide further support as the hydrodynamic radius in the wild-type protein increases upon Ca(2+) binding while the radius of Δ13 mutant does not change. Molecular dynamics simulations provide a rational explanation of the structural transition: the positively charged C-terminal residues associate with the negatively charged residues of the Ca(2+)-free EF-hands and these interactions loosen up considerably upon Ca(2+)-binding. As a consequence the Δ13 mutant has increased Ca(2+) affinity and is constantly loaded at Ca(2+) concentration ranges typically present in cells. The activation of the entire C-terminal random coil may play a role in mediating interaction with selected partner proteins of S100A4.