Cation binding sites in a membrane loop domains of Na⁺/Ca²⁺ exchangers

Abstract

Na⁺/Ca²⁺ exchangers of ionic transporters expel the intracellular Ca²⁺ to the extracellular side by utilising the concentration gradient between the cell membrane of only Na⁺ in the cardiac-type K⁺-independent family (NCX1) or both Na⁺ and K⁺ in the cone/brain neuron-type K⁺-dependent one (NCKX2). The structural and functional properties on the bindings of the substrate and the other cations to the exchanger molecule requires to examine to clarify the physiological mechanisms of the ion transports of the exchangers. The MTSET susceptibility test demonstrated that the exchanger protein molecule is consisted of multiple transmembranal segments, two membrane α-loops and one cytoplasmic loop. A variety of extracellular cations affected the reverse-mode exchange activity in both families of the exchangers in a dose-dependent manner. Any cations were considered to bind to a single site of the exchanger molecule from the Hill analysis on the dose response curves. The results obtained from the site-directed mutagenesis of NCX1 and NCKX2 suggested that the cation binding sites of the both exchangers are located in the α-loop domains. [Jpn J Physiol 54 Suppl:S108 (2004)]