Molecular aspects of mannosyltransferases in Candida albicans
It was of interest to clone key genes involved in O-glycosylation with a view to using reverse genetics to establish their function. The Candida homolog of the S. cerevisiae MNT1 gene (Hausler and Robbins, 1992) was cloned by heterologous probing of a genomic DNA library. The CaMNT1 gene was found to be regulated differentially in response to the environment and exhibited a transitory increase in the level of transcription during early germ tube formation. Low stringency Southern analysis of C. albicans genomic DNA identified several CaMNT1 homologs suggesting CaMNT1 is part of a multigene family whose members are presumed to be yeast Golgi mannosyltransferases. In order to demonstrate that specific glycosyl residues were actively involved in the host-fungus interaction, the CaMNT1 gene was disrupted in two strains using the ura-blaster technique. Disruption at the CaMNT1 locus led to a 90% reduction in -1,2-mannosyltransferase activity when -methyl mannoside was used as an acceptor, but had no obvious influence on viability, growth rate, germ tube formation or proteinase production. CaMnt1 appears to be involved in O-glycosylation since the Camnt1 null mutant strain accumulated intracellularly the O-glycosylated enzyme chitinase. Mannosyltransferase-deficient Camnt1 mutants were significantly reduced in their ability to adhere to human buccal epithelial cells in vitro and were attenuated in virulence in systemic models of candidosis. O-linked mannan may therefore be important for direct interactions with epithelial surfaces or for the stabilization and function of cell surface adhesins. The low virulence potential displayed by Camnt1 mutants clearly demonstrates the important role glycosylation plays in the virulence of C. albicans. Given that O-glycosylation differs significantly between yeast and man, this protein modification may constitute a novel target for antifungal agents.