The fungal pathogen Candida albicans has a multilayered cell wall composed of an outer layer of proteins glycosylated with N-or O-linked mannosyl residues and an inner skeletal layer of β-glucans and chitin. We demonstrate that cytokine production by human mononuclear cells or murine macrophages was markedly reduced when stimulated by C. albicans mutants defective in mannosylation. Recognition of mannosyl residues was mediated by mannose receptor binding to N-linked mannosyl residues and by TLR4 binding to O-linked mannosyl residues. Residual cytokine production was mediated by recognition of β-glucan by the dectin-1/ TLR2 receptor complex. C. albicans mutants with a cell wall defective in mannosyl residues were less virulent in experimental disseminated candidiasis and elicited reduced cytokine production in vivo. We concluded that recognition of C. albicans by monocytes/macrophages is mediated by 3 recognition systems of differing importance, each of which senses specific layers of the C. albicans cell wall.

The green fluorescent protein (GFP) of Aequorea victoria has been developed here as a reporter for gene expression and protein localization in Candida albicans. When wild-type (wt) GFP was expressed in C. albicans, it was not possible to detect fluorescence or a translation product for the wt protein.Since this was probably due in part to the presence of the non-canonical CTG serine codon in the Aequorea sequence, this codon was changed to the leucine codon TlG. C. albicans cells expressing this construct contained GFP mRNA but were non-fluorescent and contained no detectable translation product. Hence a codon-optimized GFP gene was constructed in which all of the 239 amino acids are encoded by optimal codons for C. albicans. In this gene were also incorporated two previously identified mutations in the chromophore that increase GFP fluorescence. C. albicans cells expressing this yeast-enhanced GFP gene (yEGFP3) are fluorescent and contain GFP protein. yEGFP3 can be used as a versatile reporter of gene expression in C. albicans and Saccharomyces cerevisiae and the optimized GFP described here should have broad applications in these and other fungal species.

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated mannoproteins that are the immediate point of contact with the host and strongly influence the host-fungal interaction. N-Glycans are the major form of mannoprotein modification and consist of a core structure, common to all eukaryotes, that is further elaborated in the Golgi to form the highly branched outer chain that is characteristic of fungi. In yeasts, outer chain branching is initiated by the action of the ␣1,6-mannosyltransferase Och1p; therefore, we disrupted the C. albicans OCH1 homolog to determine the importance of outer chain N-glycans on the host-fungal interaction. Loss of CaOCH1 resulted in a temperature-sensitive growth defect and cellular aggregation. Outer chain elongation of N-glycans was absent in the null mutant, demonstrated by the lack of the ␣1,6-linked polymannose backbone and the underglycosylation of N-acetylglucosaminidase. A null mutant lacking OCH1 was hypersensitive to a range of cell wall perturbing agents and had a constitutively activated cell wall integrity pathway. These mutants had near normal growth rates in vitro but were attenuated in virulence in a murine model of systemic infection. However, tissue burdens for the Caoch1⌬ null mutant were similar to control strains with normal N-glycosylation, suggesting the host-fungal interaction was altered such that high burdens were tolerated. This demonstrates the importance of N-glycan outer chain epitopes to the host-fungal interaction and virulence.Candida albicans is a commensal organism carried by a significant proportion of healthy individuals. It is the most common opportunistic fungal pathogen of humans causing superficial infections of the mucosa and in the immunocompromised host life-threatening systemic infections (1-4). The cell wall is the immediate point of contact between fungus and host and plays an important role in adherence, antigenicity, and the modulation of the host immune response (5-9). The outer layer of the cell wall is enriched in highly glycosylated mannoproteins (10), and both the protein and carbohydrate components have been implicated in the host-fungal interaction (5, 6, 11). The study of glycosylation in C. albicans therefore has its own relevance in identifying the carbohydrate epitopes involved in pathogenesis.Cell surface mannoproteins contain both O-and N-linked oligosaccharides. The O-linked oligosaccharides, attached to serine or threonine, consist of a linear chain of one to five ␣1,2-linked mannose residues (12)(13)(14) and are known to be required for full virulence (14). The process of N-glycosylation has been studied extensively in Saccharomyces cerevisiae. N-Linked glycosylation is initiated in the endoplasmic reticulum with the transfer of the Glc 3 Man 9 GlcNAc 2 oligosaccharide precursor to the protein target (15, 16). The oligosaccharide precursor is then processed by endoplasmic reticulum-resident glucosidases and a mannosidase to yield the mature triantennary Man 8 GlcNAc 2 core (17). Outer chain ...

Eukaryote ribosomal translation is terminated when release factor eRF1, in a complex with eRF3, binds to one of the three stop codons. The tertiary structure and dimensions of eRF1 are similar to that of a tRNA, supporting the hypothesis that release factors may act as molecular mimics of tRNAs. To identify the yeast eRF1 stop codon recognition domain (analogous to a tRNA anticodon), a genetic screen was performed to select for mutants with disabled recognition of only one of the three stop codons. Nine out of ten mutations isolated map to conserved residues within the eRF1 N-terminal domain 1. A subset of these mutants, although wild-type for ribosome and eRF3 interaction, differ in their respective abilities to recognize each of the three stop codons, indicating codon-specific discrimination defects. Five of six of these stop codon-specific mutants define yeast domain 1 residues (I32, M48, V68, L123, and H129) that locate at three pockets on the eRF1 domain 1 molecular surface into which a stop codon can be modeled. The genetic screen results and the mutant phenotypes are therefore consistent with a role for domain 1 in stop codon recognition; the topology of this eRF1 domain, together with eRF1-stop codon complex modeling further supports the proposal that this domain may represent the site of stop codon binding itself.