Unraveling Unique Structure and Biosynthesis Pathway of N-Linked Glycans in Human Fungal Pathogen Cryptococcus neoformans by Glycomics Analysis

초록

The encapsulated fungal pathogen Cryptococcus neoformans causes cryptococcosis in immunocompromised individuals. Although cell surface mannoproteins have been implicated in C. neoformans pathogenicity, the structure of N-linked glycans assembled on mannoproteins has not yet been elucidated. By analyzing oligosaccharide profiles combined with exoglycosi-dase treatment, we report here that C. neoformans has serotype-specific high mannose-type N-glycans with or without a beta 1,2-xylose residue, which is attached to the trimannosyl core of N-glycans. Interestingly, the neutral N-glycans of serotypes A and D were shown to contain a xylose residue, whereas those of serotype B appeared to be much shorter and devoid of a xylose residue. Moreover, analysis of the C. neoformans uxs1 Delta mutant demonstrated that UDP-xylose is utilized as a donor sugar in N-glycan biosynthesis. We also constructed and analyzed a set of C. neoformans mutant strains lacking genes putatively assigned to the reconstructed N-glycan biosynthesis pathway. It was shown that the outer chain of N-glycan is initiated by CnOch1p with addition of an alpha 1,6-mannose residue and then subsequently extended by CnMnn2p with multiple additions of alpha 1,2-mannose residues. Finally, comparative analysis of acidic N-glycans from wild-type, Cnoch1 Delta, Cnmnn2 Delta, and Cnuxs1 Delta strains strongly indicated the presence of xylose phosphate attached to mannose residues in the core and outer region of N-glycans. Our data present the first report on the unique structure and biosynthesis pathway of N-glycans in C. neoformans.

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