Substitution of a Single Residue, Asp⁵⁷⁵, Renders the NCKX2 K⁺-dependent Na⁺/Ca²⁺Exchanger Independent of K⁺

Abstract

Retinal rod and cone photoreceptors express two distinct Na⁺/Ca²⁺-K⁺ exchanger (NCKX) gene products. Both the rod NCKX1 and cone NCKX2 are polytopic membrane proteins that use both the inward Na⁺ gradient and the outward K⁺ gradient to drive Ca²⁺ extrusion across the plasma membrane. Here, we have identified a single residue (out of >100 tested) for which substitution removed the K⁺ dependence of NCKX-mediated Ca²⁺ transport. Charge-removing substitution of Asp⁵⁷⁵ by either asparagine or cysteine rendered the mutant NCKX2 proteins independent of K⁺, while the charge-conservative replacement of Asp⁵⁷⁵ to glutamate resulted in a nonfunctional mutant NCKX2 protein, accentuating the critical role for the K⁺ dependence of NCKX2. Asp⁵⁷⁵ is conserved in the NCKX1-5 genes, while an asparagine is found in this position in the three NCX genes, coding for the K⁺-independent Na⁺/Ca²⁺ exchanger. [Jpn J Physiol 55 Suppl:S85 (2005)]