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 Saccharomyces cerevisiae and mammalian hexose transporters (Hxtp and GLUTp, respectively) and secondary-structure predictions of 12 transmembrane domains for each of the Ght proteins place them into the sugar porter subfamily within the major facilitator superfamily. Interestingly, among this sugar porter family, the emerging S. pombe hexose transporter family clusters are separate from monosaccharide transporters of other yeasts (S. cerevisiae, Kluyveromyces lactis, and Candida albicans) and of humans, suggesting that these proteins form a distinct structural family of hexose transporters. Expression of the Ght1, Ght2, Ght5, and Ght6 genes in the S. cerevisiae mutant RE700A may functionally complement its d-glucose uptake-deficient phenotype. Northern blot analysis and reverse transcription-PCR showed that among all Ght'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 defective 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 conclude that the Ght3p function is required for d-gluconate transport in S. pombe. The function of Ght4p remains to be clarified. Ght6p exhibited a slightly 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.

本研究在粟酒裂殖酵母（Schizosaccharomyces pombe）中鉴定得到一个包含6个己糖转运蛋白基因的家族（Ght1至Ght6）。该家族成员与酿酒酵母（Saccharomyces cerevisiae）及哺乳动物的己糖转运蛋白（分别对应Hxtp与GLUTp）具有序列同源性，且二级结构预测显示所有Ght蛋白均含有12个跨膜结构域，因此将其归入主要促进物超家族（major facilitator superfamily）下的糖转运蛋白亚家族。值得注意的是，在该糖转运蛋白家族中，粟酒裂殖酵母己糖转运蛋白家族的聚类分支与其他酵母（酿酒酵母、乳酸克鲁维酵母（Kluyveromyces lactis）及白色念珠菌（Candida albicans））以及人类的单糖转运蛋白均相互独立，这提示此类蛋白构成了一个独特的己糖转运蛋白结构家族。将Ght1、Ght2、Ght5与Ght6基因在酿酒酵母突变株RE700A中异源表达，可在功能上互补其D-葡萄糖摄取缺陷表型。Northern印迹分析（Northern blot analysis）与逆转录PCR（reverse transcription-PCR）结果显示，在粟酒裂殖酵母的所有Ght基因中，Ght5的表达水平最为显著。Ght1p、Ght2p与Ght5p对D-葡萄糖的底物特异性显著高于其对D-果糖的特异性。对此前已报道的粟酒裂殖酵母D-葡萄糖转运缺陷突变株YGS-5的分析表明，该菌株的Ght1、Ght5及Ght6基因存在功能缺失。通过对3株分别携带Ght3与Ght4单突变或双突变的粟酒裂殖酵母菌株进行分析，我们证实Ght3p的功能是粟酒裂殖酵母完成D-葡萄糖酸转运所必需的，而Ght4p的功能尚待阐明。Ght6p对D-果糖的亲和力略高于其对D-葡萄糖的亲和力，且在所有Ght蛋白中，其对D-果糖的特异性最高。