Model for the allosteric regulation of the Na super(+)/Ca super(2+) exchanger NCX

The Na super(+)/Ca super(2+) exchanger provides a major Ca super(2+) extrusion pathway in excitable cells and plays a key role in the control of intracellular Ca super(2+) concentrations. In Canis familiaris, Na super(+)/Ca super(2+) exchanger (NCX) activity is regulated by the binding of Ca super(2+) to two cytosolic Ca super(2+)-binding domains, CBD1 and CBD2, such that Ca super(2+)-binding activates the exchanger. Despite its physiological importance, little is known about the exchanger's global structure, and the mechanism of allosteric Ca super(2+)-regulation remains unclear. It was found previously that for NCX in the absence of Ca super(2+) the two domains CBD1 and CBD2 of the cytosolic loop are flexibly linked, while after Ca super(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 super(2+)-binding to CBD1 inhibits Ca super(2+) exchange. Here, NMR spin relaxation and residual dipolar couplings were used to show that Ca super(2+) modulates CBD1 and CBD2 interdomain flexibility of CALX in an analogous way as for NCX. A mechanistic model for the allosteric Ca super(2+) regulation of the Na super(+)/Ca super(2+) exchanger is proposed. In this model, the intracellular loop acts as an entropic spring whose strength is modulated by Ca super(2+)-binding to CBD1 controlling ion transport across the plasma membrane. Proteins 2016; 84:580-590.