Cysteine-scanning mutagenesis of transmembrane segment 1 of glucose transporter GLUT1: extracellular accessibility of helix positions.

Transmembrane segment 1 of the cysteine-less GLUT1 glucose transporter was subjected to cysteine-scanning mutagenesis. The majority of single-cysteine mutants were functional transporters, as assessed by 2-deoxy-d-glucose uptake or 3-O-methyl-d-glucose transport. Substitution of cysteine for Leu-21, Gly-22, Ser-23, Gln-25, and Gly-27, however, led to uptake rates that were less than 10% of that of the nonmutated cysteine-less GLUT1. NEM, a membrane-permeable agent, was used to identify positions that are sensitive to transport alteration by sulfhydryl reagents, whereas uptake modification by the membrane-impermeant pCMBS indicated accessibility to water-soluble solutes from the external cell environment. Twelve of the 21 single-cysteine mutants were significantly (p < 0.01) affected by NEM, and on the basis of this sensitivity, four positions were identified by pCMBS to form a water-accessible surface within helix 1. The pCMBS-sensitive positions are localized at the exofacial C-terminal end along a circumference of the helix.