Extension of O-Glycans, Mediated by Ktr3 and Cap6, respectively, is Critical for Cell Wall Integrity Signaling and Virulence in Cryptococcus neoformans

The human fungal pathogen Cryptococcus neoformans assembles N-/O-linked glycans on its proteins in two types with and without xylose (1, 2). In this study, the CAP6 gene, encoding an α1,3-mannosyltransferase responsible for the second mannose addition to the minor O-glycans with xylose, was identified and functionally analyzed. The CAP6 deletion in the ktr3Δ strain, in which the α1,2-mannose addition to the major O-glycans is defected, resulted in almost complete blockage of O-glycan extension. Notably, the putative two cell surface sensor proteins, Wml (Wsc/Mid2-Like)1p and Wml2p, were shown to be subjected to minor and major O-mannosylation by Cap6 and Ktr3, respectively. The proteins levels of Wml1 and Wml2 were remarkably decreased in the ktr3Δ cap6Δ mutant, indicating that proper O-mannosylation is essential for their stability. The phosphorylation of Mpk1, induced by tunicamycin, was greatly decreased in the ktr3Δcap6Δ and the wml1Δwml2Δ, supporting an essential role of O-glycans on cell surface sensors in cell wall integrity signaling. As reflecting its defective growth under several stress conditions, the ktr3Δcap6Δ strain showed fully attenuated virulence in a mouse model of cryptococcosis. The delineation of the roles of protein glycosylation in fungal pathogenesis will not only provide a deep insight into the glycan-based fungal infection mechanism but also aid in the development of novel antifungal agents.