Protein O mannosylation is initiated in the endoplasmic reticulum by protein O-mannosyltransferases (Pmt proteins) and plays an important role in the secretion, localization, and function of many proteins, as well as in cell wall integrity and morphogenesis in fungi. Three Pmt proteins, each belonging to one of the three respective Pmt subfamilies, are encoded in the genome of the human fungal pathogen Cryptococcus neoformans. Disruption of the C. neoformans PMT4 gene resulted in abnormal growth morphology and defective cell separation. Transmission electron microscopy revealed defective cell wall septum degradation during motherdaughter cell separation in the pmt4 mutant compared to wild-type cells. The pmt4 mutant also demonstrated sensitivity to elevated temperature, sodium dodecyl sulfate, and amphotericin B, suggesting cell wall defects. Further analysis of cell wall protein composition revealed a cell wall proteome defect in the pmt4 mutant, as well as a global decrease in protein mannosylation. Heterologous expression of C. neoformans PMT4 in a Saccharomyces cerevisiae pmt1pmt4 mutant strain functionally complemented the deficient Pmt activity. Furthermore, Pmt4 activity in C. neoformans was required for full virulence in two murine models of disseminated cryptococcal infection. Taken together, these results indicate a central role for Pmt4-mediated protein O mannosylation in growth, cell wall integrity, and virulence of C. neoformans.Protein O glycosylation is a fundamentally important protein modification in eukaryotes, in which glycosyl residues are covalently attached to secreted proteins. In fungi, O glycosylation is often referred to as O mannosylation. In the yeast Saccharomyces cerevisiae, protein O mannosylation is essential for cell viability, cell wall formation, cell integrity, morphology, and budding, and the secretion, localization, and function of many proteins (6,22,35,36). In the human fungal pathogen, Candida albicans, O mannosylation is involved in morphogenic transitions and virulence (16,23). Furthermore, decreased O mannosylation of proteins in these fungi also results in cell wall changes leading to increased sensitivity to certain antifungal compounds and cell wall destabilizing agents (25,32,38,39).Protein O mannosylation is initiated in the endoplasmic reticulum by protein O-mannosyltransferases (Pmts) that catalyze the transfer of mannose from the sugar donor, dolichol phosphate-mannose, to the hydroxyl groups of serine and threonine residues in the secreted acceptor protein. The Pmt enzymes have been extensively studied in S. cerevisiae, for which seven Pmt proteins have been described previously (37). Recently, five Pmts were described for C. albicans and three for the fission yeast Schizosaccharomyces pombe (32,46). Two PMT genes in S. pombe, OMA1 and OMA4, are not essential for viability. However, oma1⌬ and oma4⌬ mutants exhibit abnormal cell morphology, altered cell wall structure, and partial cell separation defects (46). The third S. pombe PMT gene,