Model for the allosteric regulation of the Na+/Ca2+ exchanger NCX

Proteins. 2016 May;84(5):580-90. doi: 10.1002/prot.25003.

The Na⁺/Ca²⁺ exchanger provides a major Ca²⁺ extrusion pathway in excitable cells and plays a key role in 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 also it should 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 interdomain 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. Proteins 2016; 84:580–590. © 2016 Wiley Periodicals, Inc.