The cell wall architecture of Candida albicans wild-type cells and cell wall-defective mutants

In Candida albicans wild-type cells, the beta 1,6-glucanase-extractable glycosylphosphatidylinositol (GPI)dependent cell wall proteins (CWPs) account for about 88% of all covalently linked CWPs. Approximately 90% of these GPI-CWPs, including Als1p and Als3p, are attached via beta 1,6-glucan to beta 1,3-glucan. The remaining GPI-CWPs are linked through beta 1,6-glucan to chitin. The beta 1,6-glucanase-resistant protein fraction is small and consists of Pir-related CWPs, which are attached to beta 1,3-glucan through an alkalilabile linkage. Immunogold labelling and Western analysis, using an antiserum directed against Saccharomyces cerevisiae Pir2p/Hsp150, point to the localization of at least two differentially expressed Pir2 homologues in the cell wall of C. albicans. In mnn9 Delta and pmt1 Delta mutant strains, which are defective in N- and O-glycosylation of proteins respectively, we observed enhanced chitin levels together with an increased coupling of GPI-CWPs through beta 1,6-glucan to chitin. In these cells, the level of Pir-CWPs was slightly upregulated. A slightly increased incorporation of Pir proteins was also observed in a beta 1,6-glucan-deficient hemizygous kre6 Delta mutant. Taken together, these observations show that C. albicans follows the same basic rules as S. cerevisiae in constructing a cell wall and indicate that a cell wall salvage mechanism is activated when Candida cells are confronted with cell wall weakening.