Identification and characterization of a gene required for α1,2-mannose extension in the O-linked glycan synthesis pathway in Schizosaccharomyces pombe

The KTR alpha 1,2-mannosyltransferase gene family of Saccharomyces cerevisiae is responsible not only for outer-chain modifications of N-linked oligosaccharides but also for elongation of O-linked mannose residues. To identify genes involved in the elongation step of O-linked oligosaccharide chains in Schizosaccharomyces pombe, we characterized six genes, omh1(+)-omh6(+), that share significant sequence similarity to the S. cerevisiae KTR family. Six deletion strains were constructed, each carrying a single disrupted omh allele. All strains were viable, indicating that none of the omh genes was essential. Heterologous expression of a chitinase from S. cerevisiae in the omh mutants revealed that O-glycosylation of chitinase had decreased in omh1 Delta cells, but not in the other mutants, indicating that the other omh genes do not appear to be required for O-glycan synthesis. Addition of the second alpha 1,2-linked mannose residue was blocked in omh1 Delta cells. An Omh1-GFP fusion protein was found to be localized in the Golgi apparatus. These results indicate that Omh1p plays a major role in extending alpha 1,2-linked mannose in the O-glycan pathway in S. pombe.