The phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) is a structurally and functionally complex system that mediates sugar uptake in bacteria. Besides several soluble subunits, the glucose-specific PTS includes the integral membrane protein IICB that couples the transmembrane transport of glucose to its phosphorylation. Here, we used electron crystallography of sugar-embedded tubular crystals of the glucose-specific IIC transport domain from Escherichia coli (ecIICglc) to visualize the structure of the transporter in the presence and absence of its substrate. Using an in vivo transport assay and binding competition experiments, we first established that, while it transports d-glucose, ecIICglc does not bind l-glucose. We then determined the projection structure of ecIICglc from tubular crystals embedded in d- and l-glucose and found a subtle conformational change. From comparison of the ecIICglc projection maps with crystal structures of other IIC transporters, we can deduce that the transporter adopts an inward-facing conformation, and that the maps in the presence and absence of the substrate reflect the transporter before and after release of the transported glucose into the cytoplasm. The transition associated with substrate release appears to require a subtle structural rearrangement in the region that includes hairpin 1.
Keywords: Electron crystallography; Glucose transporter; Membrane protein; Projection structure; Scintillation proximity assay; Two-dimensional crystal.
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