Functional characterization of the alpha-glucoside transporter Sut1p from Schizosaccharomyces pombe, the first fungal homologue of plant sucrose transporters

Disaccharide transporters have not previously been identified in Schizosacc haromyces pombe. This is in contrast to Saccharomyces cerevisiae in which s everal maltose permeases belonging to the sugar porter (SP) family have bee n characterized. Here we report that a novel S. pombe gene, sut1(+), encode s a proton-coupled disaccharide uptake transporter in the glycoside-pentosi de-hexuronide (GPH):cation symporter family. Previously, members of the GPH family were restricted to bacteria and plants. The closest homologues of s ut1(+) are the sucrose uptake transporters (SUTs) from higher plants that t ransport sucrose with a higher affinity than maltose. The transport functio n of Sut1p was analysed by expression in S. cerevisiae. Sut1p was found to transport maltose with a K-m of 6.5 +/- 0.4 mM and sucrose with a K-m of 36 .3 +/- 9.7 mM. Therefore, the substrate specificity of Sut1p from S. pombe is different from that of its plant homologues. Glucose repression of sut1( +) at the transcriptional level is also consistent with a physiological fun ction for Sut1p in maltose uptake. These results indicate that, unlike S. c erevisiae, S. pombe utilizes maltose transporters derived from a common anc estor with the plant SUTs.