Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter

摘要

Rat Glut4 glucose transporter was expressed in the yeast Saccharomyces cerevisiae, but was retained in an intracellular membranous compartment and did not contribute to glucose uptake by intact cells. A crude membrane fraction was prepared and reconstituted in liposome with the use of the freeze–thaw/sonication method. -glucose-specific, cytochalasin B inhibitable glucose transport activity was observed. Kinetic analysis of -glucose transport was performed by an integrated rate equation approach. The Km under zero-trans influx condition was 12±1 mM (mean±S.E., n=3) and that under equilibrium exchange condition was 22±3 mM (n=4). -glucose transport was inhibited by 2-deoxy--glucose or 3-O-methyl--glucose, but not by -allose, -fructose or -glucose. Cytochalasin B, phloretin and phlorizin inhibited -glucose transport, but neither p-chloromercuribenzoic acid (pCMB) (0–0.1 mM) nor p-chloromercuribenzene sulfonic acid (pCMBS) (0–1.0 mM) inhibited this activity. High concentrations of HgCl2 were required to inhibit -glucose transport (IC50, 370 μM). Comparing these properties to those of rat Glut1, we found two notable differences; (1) in Glut1, Km under zero-trans influx was significantly smaller than that under equilibrium exchange but in Glut4 less than two-fold difference was seen between these two Km values; and (2) Glut1 was inhibited with pCMB, pCMBS and low concentrations of HgCl2 (IC50, 3.5 μM), whereas Glut4 was almost insensitive to SH reagents. To examine the role of the exofacial cysteine, we replaced Met-455 of Glut4 (corresponding to Cys-429 of Glut1) with cysteine. The mutated Glut4 was inhibited by pCMB or pCMBS and the IC50 of HgCl2 decreased to 47 μM, whereas Km, substrate specificity and the sensitivity to cytochalasin B were not significantly changed, indicating that the existence of exofacial cysteine contributed only to increase SH sensitivity in Glut4.