KINETIC CHARACTERIZATION OF INDIVIDUAL HEXOSE TRANSPORTERS OF SACCHAROMYCES-CEREVISIAE AND THEIR RELATION TO THE TRIGGERING MECHANISMS OF GLUCOSE REPRESSION

In Saccharomyces cerevisiae, there are a large number of genes (HXT1-H XT17/SNF3/RGT2) encoding putative hexose transporters which, together with a galactose permease gene (GAL2), belong to a superfamily of mono saccharide facilitator genes. We have performed a systematic analysis of the HXT1-7 and GAL2 genes and their function in hexose transport. G lucose uptake was below the detection level in the hxt1-7 null strain growing on maltose. Determination of the kinetic parameters of individ ual hexose transporter-related proteins (Hxtp) expressed in the hxt nu ll background revealed Hxt1p and Hxt3p as low-affinity transporters (K -m(glucose) = 50-100 mM), Hxt2p and Hxt3p as moderately low in affinit y (K-m/glucose about 10 mM), and Hxt6p, Hxt7p as well as Gal2p as high -affinity transporters (K-m/glucose = 1-2 mM). However, Hxt2p kinetics in cells grown on low glucose concentrations showed a high-affinity ( K-m = 1.5 mM) and a low-affinity component (K-m = 60 mM). Furthermore, we investigated the involvement of glucose transport in glucose signa lling. Glucose repression of MAL2, SUC2 and GAL1 was not dependent on a specific transporter but, instead, the strength of the repression si gnal was dependent on the level of expression, the properties of the i ndividual transporters and the kind of sugar transported. The strength of the glucose repression signal correlated with the glucose consumpt ion rates in the different strains, indicating that glucose transport limits the provision of a triggering signal rather then being directly involved in the triggering mechanism.