KRE5 Gene Null Mutant Strains of Candida albicans Are Avirulent and Have Altered Cell Wall Composition and Hypha Formation Properties

摘要

The UDP-glucose:glycoprotein glucosyltransferase (UGGT) is an endoplasmic reticulum sensor for quality control of glycoprotein folding. Saccharomyces cerevisiae is the only eukaryotic organism so far described lacking UGGT-mediated transient reglucosylation of N-linked oligosaccharides. The only gene in S. cerevisiae with similarity to those encoding UGGTs is KRE5. S. cerevisiae KRE5 deletion strains show severely reduced levels of cell wall β-1,6-glucan polymer, aberrant morphology, and extremely compromised growth or lethality, depending on the strain background. Deletion of both alleles of the Candida albicans KRE5 gene gives rise to viable cells that are larger than those of the wild type (WT), tend to aggregate, have enlarged vacuoles, and show major cell wall defects. C. albicans kre5/kre5 mutants have significantly reduced levels of β-1,6-glucan and more chitin and β-1,3-glucan and less mannoprotein than the WT. The remaining β-1,6-glucan, about 20% of WT levels, exhibits a β-1,6-endoglucanase digestion pattern, including a branch point-to-linear stretch ratio identical to that of WT strains, suggesting that Kre5p is not a β-1,6-glucan synthase. C. albicans KRE5 is a functional homologue of S. cerevisiae KRE5; it partially complements both the growth defect and reduced cell wall β-1,6-glucan content of S. cerevisiae kre5 viable mutants. C. albicans kre5/kre5 homozygous mutant strains are unable to form hyphae in several solid and liquid media, even in the presence of serum, a potent inducer of the dimorphic transition. Surprisingly the mutants do form hyphae in the presence of N-acetylglucosamine. Finally, C. albicans KRE5 homozygous mutant strains exhibit a 50% reduction in adhesion to human epithelial cells and are completely avirulent in a mouse model of systemic infection.