Distinct roles of the Na⁺ binding sites in the allosteric coupling mechanism of the glutamate transporter homolog, Glt_Ph

SignificanceGlutamate transporters harness ionic gradients present across the membrane for the rapid removal of glutamate from the synaptic space. Normal function of glutamate transporters is required for efficient synaptic transmission and preventing excitotoxicity. Central to the transport mechanism is the coupled binding of Na+ and the substrate. While structural studies have identified the Na+ and the substrate binding sites, the mechanism by which Na+ and substrate binding is coupled is not known. In this study, we developed assays to monitor Na+ binding and to track key conformational changes in GltPh, an archaeal homolog of glutamate transporters. We use these assays along with previously developed assays to describe the specific roles of the Na+ sites in the coupling mechanism.