Roman Wieczorke

Concurrent knock‐out of at least 20 transporter genes is required to block uptake of hexoses in Saccharomyces cerevisiae

The hexose transporter family of Saccharomyces cerevisiae comprises 18 proteins (Hxt1–17, Gal2). Here, we demonstrate that all these proteins, except Hxt12, and additionally three members of the maltose transporter family (Agt1, Ydl247, Yjr160) are able to transport hexoses. In a yeast strain deleted for HXT1–17, GAL2, AGT1, YDL247w and YJR160c, glucose consumption and transport activity were completely abolished. However, as additional deletion of the glucose sensor gene SNF3 partially restored growth on hexoses, our data indicate the existence of even more proteins able to transport hexoses in yeast.

Characterisation of mammalian GLUT glucose transporters in a heterologous yeast expression system.

We have developed a new heterologous expression system for mammalian glucose transporters. The system is based on a Saccharomyces cerevisiae strain completely deleted for all its endogenous hexose transporters and unable to take up and to grow on hexoses. To target the heterologous glucose transporters into the yeast plasma membrane in a fully active form, additional mutations had to be introduced into the hexose transport-deficient strain. Although GLUT1 was localized at the cell surface already in the parent strain, it supported uptake of glucose only in an Δhxt fgy1-1 mutant strain. Moreover, various mutations within the first half of the second predicted transmembrane helix converted GLUT1 into a form able to support uptake of glucose into yeast cells. GLUT4 was trapped in intracellular structures but became functionally expressed in the plasma membrane in Δhxt fgy1-1 fgy4X mutant strains. Glucose transport kinetics were determined with intact yeast cells by zero-trans influx measurements with a Km of 3.2 mM for human GLUT1 and of 12.6 mM for human GLUT4. Cytochalasin B inhibited these activities. Growth tests revealed that both transporter proteins are able to mediate uptake of glucose, mannose and galactose, but not of fructose. The new heterologous expression system should be a valuable tool to develop cell based high-throughput screening assays for identifying pharmaceutical compounds influencing the transporters.

The HTR1 Gene Is a Dominant Negative Mutant Allele of MTH1 and Blocks Snf3- and Rgt2-Dependent Glucose Signaling in Yeast

Saccharomyces cerevisiae HTR1 mutants are severely impaired in the uptake of glucose. We have cloned dominant HTR1 mutant alleles and show that they encode mutant forms of the Mth1 protein. Mth1 is shown to be involved in carbon source-dependent regulation of its own, invertase and hexose transporter gene expression. The mutant forms block the transduction of the Snf3- and Rgt2-mediated glucose signals upstream of the Rgt1 transcriptional regulator.

Glucose-dependent and -independent signalling functions of the yeast glucose sensor Snf3

The yeast Snf3 protein has been described to function as a sensor for low concentrations of extracellular glucose. We have found that Snf3 is able to transduce a signal in the complete absence of extracellular glucose. High basal activity of the HXT7 promoter during growth on ethanol required Snf3 as well as other components of the signalling pathway activated by Snf3. Moreover, the C‐terminal domain of Snf3 was sufficient to complement the role of Snf3 in this regulation. As the C‐terminal tail of Snf3 interacted with other components at the plasma membrane independent of the carbon source, our data suggest that Snf3 is involved in signalling complexes which can be activated by other signals than extracellular glucose.