The yeast glucose transporters Hxt1, Hxt2, Hxt3, Hxt4, Hxt6, Hxt7 and Gal2, individually expressed in an hxt1-7 null mutant strain, demonstrate the phenomenon of countertransport. Thus, these transporters, which are the most important glucose transporters in Saccharomyces cerevisiae, are facilitated diffusion transporters. Apparent K-m-values from high to low affinity, determined from countertransport and initial-uptake experiments, respectively, are: Hxt6 0.9 +/- 0.2 and 1.4 +/- 0.1 mM, Hxt7 1.3 +/- 0.3 and 1.9 +/- 0.1 mM, Gal2 1.5 and 1.6 +/- 0.1 mM, Hxt2 2.9 +/- 0.3 and 4.6 +/- 0.3 mM, Hxt4 6.2 +/- 0.5 and 6.2 +/- 0.3 mM, Hxt3 28.6 +/- 6.8 and 34.2 +/- 3.2 mM, and Hxt1 107 +/- 49 and 129 +/- 9 mM. From both independent methods, countertransport and initial uptake, the same range of apparent K-m-values was obtained for each transporter. In contrast to that in human erythrocytes, the facilitated diffusion transport mechanism of glucose in yeast was symmetric. Besides facilitated diffusion there existed in all single glucose transport mutants, except for the HXT1 strain, significant first-order behaviour. (C) 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
