QLS Motif in Transmembrane Helix VII of the Glucose Transporter Family Interacts with the C-1 Position of d-Glucose and Is Involved in Substrate Selection at the Exofacial Binding Site^†

The liver-type (GLUT2) and brain-type (GLUT3) human facilitative glucose transporters exhibit distinct kinetics (K_m values for deoxyglucose transport of 11 mM and 1.5 mM, respectively) and patterns of substrate transport (GLUT2 is capable of d-fructose transport, while GLUT3 is not). Using a range of chimeric glucose transporters comprised of regions of GLUT2 and GLUT3 studied by expression in Xenopus oocytes after microinjection of cRNA, we have proposed that the seventh putative transmembrane helix is intimately involved in the selection of transported substrate and that this region plays an important role in determining the K_m for 2-deoxyglucose [Arbuckle, M. I., Kane, S., Porter, L. M., Seatter, M. J., and Gould, G. W. (1996) Biochemistry 35, 16519−16527]. Inspection of the predicted amino acid sequence of this region reveals that GLUTs 1, 3, and 4 (high-affinity glucose transporters) contain a conserved QLS motif in this helix (residues 277−279 in human GLUT3). In the glucose/fructose transporter (GLUT2) this motif is replaced by HVA. To study the role of the QLS motif in substrate selection, we have engineered substitutions in this region between GLUT2 and GLUT3. GLUT3 (QLS > HVA) exhibits a K_m for deoxyglucose transport identical to that of native GLUT3 but increased sensitivity for inhibition of deoxyglucose transport by d-fructose. However, unlike native GLUT3, this species is capable of transporting d-fructose. Compared to wild-type GLUT2, GLUT2 (HVA > QLS) exhibits a lower K_m for deoxyglucose transport (3 mM vs 11 mM), the ability to transport d-fructose is reduced, and d-fructose is a less efficient inhibitor of deoxyglucose transport. Analysis of the ability of a range of glucose epimers and analogues to inhibit transport by these species suggests that the QLS motif interacts with the incoming d-glucose at the C-1 position; this may be a key interaction in the high-affinity recognition of the transported substrate. We further argue that this interaction acts as a molecular filter that is involved in the selection of the transported substrate.