FACILITATIVE GLUCOSE TRANSPORTERS

Facilitative glucose transport is mediated by members of the Glut prot ein family that belong to a much larger superfamily of 12 transmembran e segment transporters. Six members of the Glut family have been descr ibed thus far. These proteins are expressed in a tissue- and cell-spec ific manner and exhibit distinct kinetic and regulatory properties tha t reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement . It also plays a special role in transporting glucose across epitheli al and endothelial barrier tissues. Glut2 is a high-K-m isoform expres sed in hepatocytes, pancreatic beta cells, and the basolateral membran es of intestinal and renal epithelial cells. It acts as a high-capacit y transport system to allow the uninhibited (non-rate-limiting) flux o f glucose into or out of these cell types. Glut3 is a low-K-m isoform responsible for glucose uptake into neurons. Glut4 is expressed exclus ively in the insulin-sensitive tissues, fat and muscle. It is responsi ble for increased glucose disposal in these tissues in the postprandia l state and is important in whole-body glucose homeostasis. Glut3 is a fructose transporter that is abundant in spermatozoa and the apical m embrane of intestinal cells. Glut7 is the transporter present in the e ndoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphat ase on glucose 6-phosphate. This review summarizes recent advances con cerning the structure, function, and regulation of the Glut proteins.