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Fungal Hexose transporter subfamily of the Major Facilitator Superfamily of transporters and similar proteins The fungal hexose transporter (HXT) subfamily is comprised of functionally redundant proteins that function mainly in the transport of glucose, as well as other sugars such as galactose and fructose. Saccharomyces cerevisiae has 20 genes that encode proteins in this family (HXT1 to HXT17, GAL2, SNF3, and RGT2). Seven of these (HXT1-7) encode functional glucose transporters. Gal2p is a galactose transporter, while Rgt2p and Snf3p act as cell surface glucose receptors that initiate signal transduction in response to glucose, functioning in an induction pathway responsible for glucose uptake. Rgt2p is activated by high levels of glucose and stimulates expression of low affinity glucose transporters such as Hxt1p and Hxt3p, while Snf3p generates a glucose signal in response to low levels of glucose, stimulating the expression of high affinity glucose transporters such as Hxt2p and Hxt4p. Schizosaccharomyces pombe contains eight GHT genes (GHT1-8) belonging to this family. Ght1, Ght2, and Ght5 are high-affinity glucose transporters; Ght3 is a high-affinity gluconate transporter; and Ght6 high-affinity fructose transporter. The substrate specificities for Ght4, Ght7, and Ght8 remain undetermined. The HXT subfamily belongs to the Glucose transporter -like (GLUT-like) family of the Major Facilitator Superfamily (MFS) of membrane transport proteins. MFS proteins are thought to function through a single substrate binding site, alternating-access mechanism involving a rocker-switch type of movement. |
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