Identification of an amino acid residue that lies between the exofacial vestibule and exofacial substrate-binding site of the Glut1 sugar permeation pathway.

A valine-to-isoleucine mutation at amino acid residue 197 of Glut2 or the equivalent residue 165 of Glut1 has been shown to impair glucose transport activity. This mutation was originally discovered in the Glut2 gene of a patient with type 2 diabetes. We investigated the mechanism of the effect of this mutation on transport activity via the analysis of Glut1 mutants expressed in Xenopus oocytes combined with cysteine substitution mutagenesis and the use of cysteine-reactive chemical probes. Aliphatic side chain substitutions at position 165 that were bulkier than the native valine residue inhibited glucose transport activity, whereas substitutions of less bulky side chains had little effect on transport, suggesting a role for steric hindrance. A cysteine residue was introduced at position 165 of a functional, cysteine-less Glut1 construct, and this mutant was then tested for inhibition of transport activity by a membrane-impermeant sulfhydryl-specific reagent (p-chloromercuribenzenesulfonate). p-Chloromercuribenzenesulfonate inhibited activity of the Cys165 mutant when it was added to the external buffer but not when it was injected directly into oocytes, indicating that this residue is accessible from the external solvent but not from the cytoplasm. Competition experiments indicated that Cys165 lies near the exofacial substrate-binding site or directly in the sugar permeation pathway. These data provide evidence that the side chain of Val165, which resides in the middle of transmembrane helix 5, juts into the aqueous permeation pathway of Glut1, probably between the exofacial substrate-binding site and the outer vestibule of the pathway.