Identification of a New Family of Genes Involved in β-1,2-Mannosylation of Glycans in Pichia pastoris and Candida albicans

Structural studies of cell wall components of the pathogenic yeast Candida albicans have demonstrated the presence of β-1,2-linked oligomannosides in phosphopeptidomannan and phospholipomannan. During C. albicans infection, β-1,2-oligomannosides play an important role in host/pathogen interactions by acting as adhesins and by interfering with the host immune response. Despite the importance of β-1,2-oligomannosides, the genes responsible for their synthesis have not been identified. The main reason is that the reference species Saccharomyces cerevisiae does not synthesize β-linked mannoses. On the other hand, the presence of β-1,2-oligomannosides has been reported in the cell wall of the more genetically tractable C. albicans relative, P. pastoris. Here we present the identification, cloning, and characterization of a novel family of fungal genes involved in β-mannose transfer. Employing in silico analysis, we identified a family of four related new genes in P. pastoris and subsequently nine homologs in C. albicans. Biochemical, immunological, and structural analyses following deletion of four genes in P. pastoris and deletion of four genes acting specifically on C. albicans mannan demonstrated the involvement of these new genes in β-1,2-oligomannoside synthesis. Phenotypic characterization of the strains deleted in β-mannosyltransferase genes (BMTs) allowed us to describe the stepwise activity of Bmtps and acceptor specificity. For C. albicans, despite structural similarities between mannan and phospholipomannan, phospholipomannan β-mannosylation was not affected by any of the CaBMT1-4 deletions. Surprisingly, depletion in mannan major β-1,2-oligomannoside epitopes had little impact on cell wall surface β-1,2-oligomannoside antigenic expression.