The fission yeast Schizosaccharomyces pombe is a promising host for production of heterologous proteins. However, the oligosaccharide structures of yeasts, including S. pombe, differ significantly from those of mammalian cells and humans. In S. pombe, galactose residues are transferred to oligosaccharide moieties of glycoproteins by galactosyltransferases in the lumen of the Golgi apparatus. Therefore, UDP-galactose, a substrate for galactosyltransferases, should be synthesized in the cytosol, and transported into the Golgi apparatus by a UDP-galactose transporter. Because S. pombe cannot use galactose as a carbon or energy source, little is known about galactose metabolism in this species. A galactose-assimilating mutant of S. pombe that was able to grow in minimal galactose medium was isolated. Through DNA microarray analysis of gene expression profiles in the wild type and the mutant, three gal genes (gal7 +, gal10 +, and gal1 +) involved in galactose utilization were found to be highly expressed in the mutant. Although galactose residues are not essential for growth of S. pombe, galactosylation of protein is required for maintenance of normal cell shape, tolerance toward various drugs, and nonsexual flocculation. We identified fission yeast gsf2 +, encoding a flocculin that binds galactose residues located on cell-surface glycoconjugates by DNA microarray analysis. S. pombe appears to have a unique galactose-specific recognition system in which Gsf2/flocculin plays an essential role in mediating cell-cell interactions.
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