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

The KTR α1,2-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 Δ 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 α1,2-linked mannose residue was blocked in omh1 Δ 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 α1,2-linked mannose in the O -glycan pathway in S. pombe .