Protein O-glycosylation is an essential protein modification in eukaryotic cells. In Saccharomyces cerevisiae, O-mannosylation is initiated in the lumen of the endoplasmic reticulum by O-mannosyltransferase gene products (Pmt1p-7p). A search of the Schizosaccharomyces pombe genome database revealed a total of three O-glycoside mannosyltransferase homologs (ogm1(+), ogm2(+), and ogm4(+)), closely related to Saccharomyces cerevisiae PMT1, PMT2, and PMT4. Although individual ogm genes were not found to be essential, ogm1 Delta and ogm4 Delta mutants exhibited aberrant morphology and failed to agglutinate during mating. The phenotypes of the ogm4 Delta mutant were not complemented by overexpression of ogm1(+) or ogm2(+), suggesting that each of the Ogm proteins does not have overlapping functions. Heterologous expression of a chitinase from S. cerevisiae in the ogm mutants revealed that O-glycosylation of chitinase had decreased in ogm1 Delta cells. A GFP-tagged Fus1p from S. cerevisiae was specifically not glycosylated and accumulated in the Golgi in ogm4 Delta cells. These results indicate that O-glycosylation initiated by Ogm proteins plays crucial physiological roles and can serve as a sorting determinant for protein transport of membrane glycoproteins in S. pombe. (c) 2005 Elsevier Inc. All rights reserved.