Galactose transporters discriminate steric anomers at the cell surface in yeast

Aldose-1-epimerase or mutarotase (EC 5.1.3.3) catalyzes interconversion of alpha/beta-anomers of aldoses, such as glucose and galactose, and is distributed in a wide variety of organisms from bacteria to humans. Nevertheless, the physiological role of this enzyme has been elusive in most cases, because the alpha-form of aldoses in the solid state spontaneously converts to the beta-form in an aqueous solution until an equilibrium of alpha : beta=36.5 : 63.5 is reached. A gene named GAL10 encodes this enzyme in yeast. Here, we show that the GAL10-encoded mutarotase is necessary for utilization of galactose in the milk yeast Kluyveromyces lactis, and that this condition is presumably created by the presence of the beta-specific galactose transporter, which excludes the alpha-anomer from the alpha/beta-mixture in the medium at the cell surface. Thus, we found that a mutarotase-deficient mutant of K. lactis failed to grow on medium, in which galactose was the sole carbon source, but, surprisingly, that the growth failure is suppressed by concomitant expression of the Saccharomyces cerevisiae-derived galactose transporter Gal2p, but not by that of the K. lactis galactose transporter Hgt1p. We also suggest the existence of another mutarotase in K. lactis, whose physiological role remains unknown, however.