The predicted ATP-binding domains in the hexose transporter GLUT1 critically affect transporter activity

The glucose transporter GLUT1 has three short amino acid sequences (domains I-III) with homology to typical ATP-binding domains. GLUT1 is a facilitative transporter, however, and transports its substrates down a concentration gradient without a specific requirement for energy or hydrolysis of ATP. Therefore, we assessed the functional role of the predicted ATP-binding domains in GLUT1 by site-directed mutagenesis and expression in Xenopus oocytes. For each mutant, we determined the level of protein expression and the kinetics of transport under zero-trans influx, zero-trans efflux, and equilibrium exchange conditions. Although all five mutants were expressed at levels similar to that of the wild-type GLUT1, each single amino acid change in domains I or III profoundly affected GLUT1 function. The mutants Gly116-->Ala in domain I and Gly332-->Ala in domain III exhibited only 10-20% of the transport activity of the wild-type GLUT1. The mutants Gly111-->Ala in domain I and Leu336-->Ala in domain III showed altered kinetic properties; neither the apparent Km nor the Vmax for 3-methylglucose transport were increased under equilibrium exchange conditions, and they did not show the expected level of countertransport acceleration. The mutant Lys117-->Arg in domain I showed a marked increase in the apparent Km for 3-methylglucose transport under zero-trans efflux and equilibrium exchange conditions while maintaining countertransport acceleration. These results indicate that the predicted ATP-binding domains I and III in GLUT1 are important components of the region in GLUT1 involved in transport of the substrate and that their integrity is critical for maintaining the activity and kinetic properties of the transporter.