Identification of a galactose-specific flocculin essential for non-sexual flocculation and filamentous growth in Schizosaccharomyces pombe

Summary: Although various mutant strains of the fission yeast Schizosaccharomyces pombe exhibit non-sexual flocculation, little is known about the mechanistic basis for this phenomenon, nor have genes encoding the implicated flocculin been identified. In the budding yeast Saccharomyces cerevisiae, the transcription factor Flo8 controls expression of some of the genes involved in non-sexual flocculation. We have found that overexpression of S. cerevisiae FLO8 induced non-sexual flocculation in S. pombe. This non-sexual flocculation was Ca ²⁺-dependent, and was inhibited by addition of galactose, but not by mannose, glucose or sucrose. In the FLO8-overexpressing strain, a gene designated gsf2 ⁺ (galactose-specific flocculation) was specifically induced. The gsf2 ⁺ gene was also highly expressed in lkh1Δ, tup12Δ and gsf1 mutants, all of which exhibited non-sexual flocculation dependent on gsf2 ⁺. We show that the N-terminal region of Gsf2 recognizes galactose in mediating cell-cell interaction. Disruption of gsf2 ⁺ also abolished the adhesion phenotype and invasive growth of the wild-type strain cultured in low ammonium medium. The newly identified flocculin Gsf2 in fission yeast was not only required for non-sexual flocculation but was also required for adhesion and filamentous growth through recognition of galactose residues on cell surface glycoconjugates.