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

• Published in: PloS one 2014-05, Vol.9 (5)
• Main Authors: Duelli, Annette, Kiss, Bence, Lundholm, Ida, Bodor, Andrea, Petoukhov, Maxim V, Svergun, Dmitri I, Nyitray, László, Katona, Gergely
• Format: Article
• Language: English
• Subjects: Chemical properties; Protein binding; Proteins
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0097654

S100A4 interacts with many binding partners upon Ca.sup.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, [DELTA]13) changes upon Ca.sup.2+ binding. We found that the scattering intensity of wild-type S100A4 changes substantially in the 0.15-0.25 Å.sup.-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.sup.2+ is bound. Pulsed field gradient NMR measurements provide further support as the hydrodynamic radius in the wild-type protein increases upon Ca.sup.2+ binding while the radius of [DELTA]13 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.sup.2+ -free EF-hands and these interactions loosen up considerably upon Ca.sup.2+ -binding. As a consequence the [DELTA]13 mutant has increased Ca.sup.2+ affinity and is constantly loaded at Ca.sup.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.