Active transport of substrates across cytoplasmic membranes is of great physiological, medical and pharmaceutical importance. The glycerol-3-phosphate (G3P) transporter (GlpT) of the E. coli inner membrane is a secondary active antiporter from the ubiquitous major facilitator superfamily that couples the import of G3P to the efflux of inorganic phosphate (Pi) down its concentration gradient. Integrating information from a novel combination of structural, molecular dynamics simulations and biochemical studies, we identify the residues involved directly in binding of substrate to the inward-facing conformation of GlpT, thus defining the structural basis for the substrate-specificity of this transporter. The substrate binding mechanism involves protonation of a histidine residue at the binding site. Furthermore, our data suggest that the formation and breaking of inter- and intradomain salt bridges control the conformational change of the transporter that accompanies substrate translocation across the membrane. The mechanism we propose may be a paradigm for organophosphate:phosphate antiporters.
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Law, C., Almqvist, J., Bernstein, A., Goetz, R., Huang, Y., Soudant, C., Laaksonen, A., Hovmoller, S., & Wang, D-N. (2008). Salt-bridge dynamics control substrate-induced conformational change in the membrane transporter GlpT. Journal of Molecular Biology, 378(4), 828-839. https://doi.org/10.1016/j.jmb.2008.03.029