In the fission yeast Schizosaccharomyces pombe, galactose residues are transferred to N- and O-linked oligosaccharides of glycoproteins by galactosyltransferases in the lumen of the Golgi apparatus. Although galactose residues are not essential for growth of S. pombe, the galactosylation of protein is required for maintenance of normal cell shape, sexual agglutination, and tolerance toward various drugs. Cell surface glycoproteins play a key role in flocculation and filamentous invasive growth in yeasts. We identified fission yeast gsf2+, encoding a flocculin that binds to galactose residues located on cell surface glycoconjugates. Flocculation and invasive growth of S. pombe is tightly controlled by gsf2+ expression. Furthermore, pyruvylation of galactose residues negatively regulates flocculation by capping galactose residues in N-linked galactomannan. S. pombe appears to have a unique galactose-specific recognition system in which Gsf2p/flocculin plays an essential role in mediating cell-cell interactions.
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