Contribution of glycosylation to the structure and properties of the Candida albicans cell wall
Adhesion of the opportunistic fungal pathogen, Candida albicans, to host surfaces is mediated through the mannoprotein-rich surface. This thesis examines the role of cell wall mannoproteins in the host-fungus interactions and the mechanism by which these proteins are tethered to the major structural polysaccharides of the wall. The mannosyl modifications of cell surface proteins are involved in host interaction. Periodate oxidation of fungal surface carbohydrates reduced adhesion to epithelial cells confirming that mannan is a component adhesion of C. albicans. Strains of C. albicans disrupted in MNT1, 2 and 3 genes encoding mannosyl transferases, were reduced in adhesion to both epithelial cells and Matrigel. Adhesion was also influenced by the Ura-status of C. albicans strains, which differs as a result of targeted gene disruption by the URA-blaster strategy. Ura-strains were less adhesive but this was not due to altered growth rate and could not be alleviated by adding excess uridine. Therefore current methods for gene deletion have to be questioned when considering adhesion as a virulence factor. Strains defective in glycosylation were altered in covalently associated cell wall proteins (CWPs) in terms of mobility during electrophoresis. The greatest alterations observed were in the CWPs of C. albicans strains with severe glycosylation defects, such as in Dochl/Doch1 and Dpmt1/Dpmt1 mutants, defective in N- and O-linked glycosylation, respectively. These strains also secreted more mannoprotein were altered in morphology or gross cell wall structure, and had elevated cell wall chitin. Therefore glycosylation is required for normal CWP incorporation and cell wall construction. For the first time C. albicans was shown to link a CWP, other than Pir-CWP, to the cell wall network via an alkali-sensitive linkage. The nature of this type of linkage remains unknown, however an O-mannan chain, whose synthesis is not initiated by Pmt1p, may be involved. Pir-CWP incorporation was increased in a Dpmt1/Dpmt1 null mutant and could be partially attributed to increased expression. Pir-CWP expression and incorporation was pH-dependent and may be regulated in response to the different pH at different host niches.