Model for the allosteric regulation of the N a + / C a 2+ exchanger NCX

The Na + /Ca 2+ exchanger provides a major Ca 2+ extrusion pathway in excitable cells and plays a key role in the control of intracellular Ca 2+ concentrations. In Canis familiaris , Na + /Ca 2+ exchanger (NCX) activity is regulated by the binding of Ca 2+ to two cytosolic Ca 2+ ‐binding domains, CBD1 and CBD2, such that Ca 2+ ‐binding activates the exchanger. Despite its physiological importance, little is known about the exchanger's global structure, and the mechanism of allosteric Ca 2+ ‐regulation remains unclear. It was found previously that for NCX in the absence of Ca 2+ the two domains CBD1 and CBD2 of the cytosolic loop are flexibly linked, while after Ca 2+ ‐binding they adopt a rigid arrangement that is slightly tilted. A realistic model for the mechanism of the exchanger's allosteric regulation should not only address this property, but also it should explain the distinctive behavior of Drosophila melanogaster 's sodium/calcium exchanger, CALX, for which Ca 2+ ‐binding to CBD1 inhibits Ca 2+ exchange. Here, NMR spin relaxation and residual dipolar couplings were used to show that Ca 2+ modulates CBD1 and CBD2 interdomain flexibility of CALX in an analogous way as for NCX. A mechanistic model for the allosteric Ca 2+ regulation of the Na + /Ca 2+ exchanger is proposed. In this model, the intracellular loop acts as an entropic spring whose strength is modulated by Ca 2+ ‐binding to CBD1 controlling ion transport across the plasma membrane. Proteins 2016; 84:580–590. © 2016 Wiley Periodicals, Inc.