A proton NMR study of the mechanism of the erythrocyte glucose transporter.

A generalizable 1H NMR technique is developed and used to monitor beta-D-glucose binding to glucose transport sites on erythrocyte membranes. This technique provides resolution of beta-D-glucose binding sites on opposite sides of the membrane, thereby enabling study of recruitment of transport sites from one side of the membrane to the other. Cytochalasin B, which competitively and specifically inhibits glucose binding to the inward-facing glucose transport site, recruits all glucose transport sites on both sides of the membrane to the inward-facing conformation. This result strongly supports a one-site model in which a single transport site alternates between distinct inward- and outward-facing conformations. The rate-limiting step in the transport process is translocation of the transport site between the two conformations, since the beta-D-glucose binding and dissociation events at both the inward- and outward-facing transport sites are shown to be fast compared to the known turnover rate of the glucose transport cycle. A model is presented for the transport machinery in which the glucose molecule binds in a cleft between channel-forming transmembrane helices, and during the transport event a sliding barrier moves past the transport site, thereby exposing the site to the opposite solution compartment.