Fsy1, the sole hexose-proton transporter characterized in Saccharomyces yeasts, exhibits a variable fructose:H⁺ stoichiometry

In the model yeast Saccharomyces cerevisiae, hexose uptake is mediated exclusively by a family of facilitators (Hxt, hexose transporters). Some other Saccharomyces species (e.g. Saccharomyces bayanus and Saccharomyces pastorianus) possess, in addition, a specific fructose transporter (Fsy1, fructose symporter) that has been previously described to function as a proton symporter. In the present work, we compared growth of a yeast strain in which FSY1 occurs naturally in anaerobic, fructose- and glucose-limited chemostat cultures. Especially at low specific growth rates, fructose-proton symport was shown to have a strong impact on the biomass yield on sugar. We subsequently employed energized hybrid plasma membrane vesicles to confirm previous observations concerning the mode of operation and specificity of Fsy1 mediated transport. Surprisingly, these experiments suggested that the carrier exhibits an unusual fructose:H⁺ stoichiometry of 1:2. This energetically expensive mode of operation was also found consistently in vivo, in shake flask and in chemostat cultures, and both when Fsy1 is the sole transporter and when the Hxt carriers are present. However, it is observed only when Fsy1 is operating at higher glycolytic fluxes, a situation that is normally prevented by downregulation of the gene. Taken together, our results suggest the possibility that fructose symport with more than one proton may constitute an energetically unfavorable mode of operation of the Fsy1 transporter that, in growing cultures, is prevented by transcriptional regulation.