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

In Candida albicans wild-type cells, the β1,6-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 β1,6-glucan to β1,3-glucan. The remaining GPI-CWPs are linked through β1,6-glucan to chitin. The β1,6-glucanase-resistant protein fraction is small and consists of Pir-related CWPs, which are attached to β1,3-glucan through an alkali-labile 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 mnn9Δ and pmt1Δ 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 β1,6-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 β1,6-glucan-deficient hemizygous kre6Δ 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.