Galactose transporters discriminate steric anomers at the cell surface in yeast |
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Authors: | Toshio Fukasawa Hiroshi Sakurai Yasuhisa Nogi & Enrico Baruffini |
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Institution: | Department of Microbiology and Immunology, Keio University School of Medicine, Tokyo, Japan;;Kanazawa University Graduate School of Medical Science, Ishikawa, Japan;;Department of Molecular Biology, Saitama Medical University, Saitama, Japan;and;Department of Genetics, Anthropology, and Evolution, University of Parma, Parma, Italy |
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Abstract: | Aldose-1-epimerase or mutarotase (EC 5.1.3.3) catalyzes interconversion of α/β-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 α-form of aldoses in the solid state spontaneously converts to the β-form in an aqueous solution until an equilibrium of α : β=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 β-specific galactose transporter, which excludes the α-anomer from the α/β-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. |
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Keywords: | GAL10 galactose metabolism epimerase mutarotase sugar transporter yeast |
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