Multiple hexose transporters of Schizosaccharomyces pombe

We have identified a family of six hexose transporter genes (Ght1 to Ght6) in the fission yeast Schizosaccharomyces pombe. Sequence homology to Saccha romyces cerevisiae and mammalian hexose transporters (Hxtp and GLUTp, respe ctively) and secondary-structure predictions of 12 transmembrane domains fo r each of the Ght proteins place them into the sugar porter subfamily withi n the major facilitator superfamily. Interestingly, among this sugar porter family, the emerging S. pombe hexose transporter family clusters are separ ate from monosaccharide transporters of other yeasts (S. cerevisiae, Kluyve romyces lactis, and Candida albicans) and of humans, suggesting that these proteins form a distinct structural family of hexose transporters. Expressi on of the Ght1, Ght2, Ght5, and Ght6 genes in the S. cerevisiae mutant RE70 0A may functionally complement its D-glucose uptake-deficient phenotype. No rthern blot analysis and reverse transcription-PCR showed that among all Gh t's of S. pombe, Ght5 is the most prominently expressed hexose transporter. Ght1p, Ght2p, and Ght5p displayed significantly higher specificities for D -glucose than for D-fructose. Analysis of the previously described S. pombe D-glucose transport-deficient mutant YGS-5 revealed that this strain is de fective in the Ght1, Ght5, and Ght6 genes. Based on an analysis of three S. pombe strains bearing single or double mutations in Ght3 and Ght4, we conc lude that the Ght3p function is required for D-gluconate transport in S. po mbe. The function of Ght4p remains to be clarified. Ght6p exhibited a sligh tly higher affinity to D-fructose than to D-glucose, and among the Ght's it is the transporter with the highest specificity for D-fructose.