Model for the allosteric regulation of the Na+/Ca2+ exchanger NCX (Proteins: Structure, Function, and Bioinformatics)

The Na+/Ca2+ exchanger provides a major Ca2+ extrusion pathway in excitable cells and plays a key role for the control of intracellular Ca2+ concentrations. In Canis familiaris, Na+/Ca2+ exchanger (NCX) activity is regulated by the binding of Ca2+ to two cytosolic Ca2+-binding domains, CBD1 and CBD2, such that Ca2+-binding activates the exchanger. Despite its physiological importance, little is known about the exchanger`s global structure, and the mechanism of allosteric Ca2+-regulation remains unclear. It was found previously that for NCX in the absence of Ca2+ the two domains CBD1 and CBD2 of the cytosolic loop are flexibly linked, while after Ca2+-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 it should also explain the distinctive behavior of Drosophila melanogaster`s sodium-calcium exchanger, CALX, for which Ca2+-binding to CBD1 inhibits Ca2+ exchange. Here, NMR spin relaxation and residual dipolar couplings were used to show that Ca2+ modulates CBD1 and CBD2 inter-domain flexibility of CALX in an analogous way as for NCX. A mechanistic model for the allosteric Ca2+ regulation of the Na+/Ca2+ exchanger is proposed. In this model, the intracellular loop acts as an entropic spring whose strength is modulated by Ca2+ binding to CBD1 controlling ion transport across the plasma membrane. This article is protected by copyright. All rights reserved.