Perturbations of the yeast cell wall trigger a repair mechanism that reconfigures its molecular structure to preserve cell integrity. To investigate this mechanism, we compared the global gene expression in five mutant strains, each bearing a mutation (i.e. fks1, kre6, mnn9, gas1, and knr4 mutants) that affects in a different manner the cell wall construction. Altogether, 300 responsive genes were kept based on high stringency criteria during data processing. Functional classification of these differentially expressed genes showed a substantial subset of induced genes involved in cell wall construction and an enrichment of metabolic, energy generation, and cell defense categories, whereas families of genes belonging to transcription, protein synthesis, and cellular growth were underrepresented. Clustering methods isolated a single group of ϳ80 up-regulated genes that could be considered as the stereotypical transcriptional response of the cell wall compensatory mechanism. The in silico analysis of the DNA upstream region of these co-regulated genes revealed pairwise combinations of DNA-binding sites for transcriptional factors implicated in stress and heat shock responses (Msn2/4p and Hsf1p) with Rlm1p and Swi4p, two PKC1-regulated transcription factors involved in the activation genes related to cell wall biogenesis and G 1 /S transition. Moreover, this computational analysis also uncovered the 6-bp 5 -AGCCTC-3 CDRE (calcineurindependent response element) motif in 40% of the coregulated genes. This motif was recently shown to be the DNA binding site for Crz1p, the major effector of calcineurin-regulated gene expression in yeast. Taken altogether, the data presented here lead to the conclusion that the cell wall compensatory mechanism, as triggered by cell wall mutations, integrates three major regulatory systems: namely the PKC1-SLT2 mitogen-activated protein kinase-signaling module, the "global stress" response mediated by Msn2/4p, and the Ca 2؉ /calcineurindependent pathway. The relative importance of these regulatory systems in the cell wall compensatory mechanism is discussed.Yeast and fungi are surrounded by a cell wall that is a complex structure essential for maintenance of the cell shape, prevention of lysis, and protection against harmful environmental conditions. The yeast cell wall architecture has been determined in detail over the past decade (1). It is a layered structure that is composed of ␤-1,3-and ␤-1,6-glucan (50 -60% of the cell wall dry mass), mannoproteins (40 -50%), and chitin (2%). ␤-1,3-Glucan and chitin form a fibrillar network to which mannoproteins are anchored, mostly through ␤-1,6-glucan. Some cell wall proteins, like the PIR family, can be directly linked to ␤-1,3-glucan (reviewed in Ref.2). The cell wall is not a rigid structure, since it endures all of the changes that the cell undergoes during division, morphogenesis, and differentiation.To ensure continuous integrity of the wall in accordance with its plasticity, complex mechanisms must be operating, which need to be strictly ...

The fungal cell wall confers cell morphology and protection against environmental insults. For fungal pathogens, the cell wall is a key immunological modulator and an ideal therapeutic target. Yeast cell walls possess an inner matrix of interlinked β-glucan and chitin that is thought to provide tensile strength and rigidity. Yeast cells remodel their walls over time in response to environmental change, a process controlled by evolutionarily conserved stress (Hog1) and cell integrity (Mkc1, Cek1) signaling pathways. These mitogen-activated protein kinase (MAPK) pathways modulate cell wall gene expression, leading to the construction of a new, modified cell wall. We show that the cell wall is not rigid but elastic, displaying rapid structural realignments that impact survival following osmotic shock. Lactate-grown Candida albicans cells are more resistant to hyperosmotic shock than glucose-grown cells. We show that this elevated resistance is not dependent on Hog1 or Mkc1 signaling and that most cell death occurs within 10 min of osmotic shock. Sudden decreases in cell volume drive rapid increases in cell wall thickness. The elevated stress resistance of lactate-grown cells correlates with reduced cell wall elasticity, reflected in slower changes in cell volume following hyperosmotic shock. The cell wall elasticity of lactate-grown cells is increased by a triple mutation that inactivates the Crh family of cell wall cross-linking enzymes, leading to increased sensitivity to hyperosmotic shock. Overexpressing Crh family members in glucose-grown cells reduces cell wall elasticity, providing partial protection against hyperosmotic shock. These changes correlate with structural realignment of the cell wall and with the ability of cells to withstand osmotic shock.

Cell-wall damage caused by mutations of cell-wall-related genes triggers a compensatory mechanism which eventually results in hyperaccumulation of chitin reaching 20% of the cell-wall dry mass. We show that activation of chitin synthesis is accompanied by a rise, from 1.3-fold to 3.5-fold according to the gene mutation, in the expression of most of the genes encoding enzymes of the chitin metabolic pathways. Evidence that GFA1, which encodes glutamine-fructose-6-Phosphate amidotransferase (Gfa1p), the first committed enzyme of this pathway, plays a major role in this process was as follows. Activation of chitin synthesis in the cell-wall mutants correlated with activation of GFA1 and with a proportional increase in Gfa1p activity. Overexpression of GFA1 caused an approximately threefold increase in chitin in the transformed cells, whereas chitin content was barely affected by the joint overexpression of CHS3 and CHS7. Introduction of a gfa1-97 allele mutation in the cellwall-defective gas1D mutant or cultivation of this mutant in a hyperosmotic medium resulted in reduction in chitin synthesis that was proportional to the decrease in Gfa1p activity. Finally, the stimulation of chitin production was also accompanied by an increase in pools of fructose 6-Phosphate, a substrate of Gfa1p. In quantitative terms, we estimated the flux-coefficient control of Gfa1p to be in the range of 0.90, and found that regulation of the chitin metabolic pathway was mainly hierarchical, i.e. dominated by regulation of the amount of newly synthesized GFA1 protein. In the search for the mechanism by which GFA1 is activated in response to cell-wall perturbations, we could only show that neither MCM1 nor RLM1, which encode two transcriptional factors of the MADS box family that are required for expression of cell-cycle and cell-wall-related genes, was involved in this process.

Yeast cells are surrounded by a thick cell wall, the composition and structure of which have been characterized by biochemical and genetic methods. In this study, we used atomic force microscopy (AFM) to visualize the cell surface topography and to determine cell wall nanomechanical properties of yeast mutants defective in cell wall architecture. While all mutants investigated showed some alteration in cell surface topography, this alteration was particularly salient in mutants defective in β-glucan elongation (gas1), chitin synthesis (chs3) and cross-linkages between chitin and β-glucan (crh1crh2). In addition, these alterations in surface topology were accompanied by increased roughness of the cell. From force-indentation curves, the Young's modulus was determined, as it gives a measure of the elasticity of the cell wall. A value of ∼1.6 MPa was obtained for the cell walls of the wild-type strain in exponential and stationary phases of growth. The same value was measured in a mnn9 mutant defective in protein mannosylation, and was two-fold reduced in a mutant with reduced β-glucan (fks1 and knr4 ), only in the stationary phase of growth. In contrast, the elasticity was dramatically reduced in mutants defective in chitin synthesis (chs3 ), β-glucan elongation (gas1 ) and, even more remarkably, in a crh1 crh2 mutant defective in the enzymes that catalyse cross-linkages of chitin to β-glucan. Taken together, these results provide direct physical evidence that the nanomechanical properties of the yeast cell wall are mainly dependent on cross-links and cell wall remodelling, rather than on cell wall composition or thickness.

Saccharomyces cerevisiae and Candida albicans are model yeasts for biotechnology and human health, respectively. We used atomic force microscopy (AFM) to explore the effects of caspofungin, an antifungal drug used in hospitals, on these two species. Our nanoscale investigation revealed similar, but also different, behaviors of the two yeasts in response to treatment with the drug. While administration of caspofungin induced deep cell wall remodeling in both yeast species, as evidenced by a dramatic increase in chitin and decrease in ␤-glucan content, changes in cell wall composition were more pronounced with C. albicans cells. Notably, the increase of chitin was proportional to the increase in the caspofungin dose. In addition, the Young modulus of the cell was three times lower for C. albicans cells than for S. cerevisiae cells and increased proportionally with the increase of chitin, suggesting differences in the molecular organization of the cell wall between the two yeast species. Also, at a low dose of caspofungin (i.e., 0.5؋ MIC), the cell surface of C. albicans exhibited a morphology that was reminiscent of cells expressing adhesion proteins. Interestingly, this morphology was lost at high doses of the drug (i.e., 4؋ MIC). However, the treatment of S. cerevisiae cells with high doses of caspofungin resulted in impairment of cytokinesis. Altogether, the use of AFM for investigating the effects of antifungal drugs is relevant in nanomedicine, as it should help in understanding their mechanisms of action on fungal cells, as well as unraveling unexpected effects on cell division and fungal adhesion.T he yeast cell wall is composed of 50 to 60% ␤-glucans (glucose residues attached by 1,3-␤-and 1,6-␤-linkages), 40 to 50% mannoproteins (highly glycosylated polypeptides), and 1 to 3% chitin (1, 2). It is an essential dynamic structure playing roles in maintaining cell shape and integrity, sensing the surrounding environment, and interacting with surfaces and other cells (3). The cell wall represents 15 to 25% of the cell dry mass, the chemical composition of which is well established. Saccharomyces cerevisiae, also called baker's yeast, is the best-characterized eukaryotic model for scientific and biomedical research. Although the chemical composition of the yeast cell wall is well known, its molecular ultrastructure (organization or assembly) has not been extensively studied at nanoscale (4, 5), although there are a few reports on the nanomechanical and adhesive properties of the yeast cell wall under native conditions or under stress conditions (6-8). As for Candida albicans, it is by far the most common human-pathogenic fungal species. It can cause a range of pathogenic effects, including painful superficial infections, severe surface infections, and lifethreatening bloodstream infections (9). It is a major cause of morbidity and mortality in immunocompromised patients as a result of AIDS, cancer chemotherapy, or organ transplantation (10).Given its medical relevance, C. albicans has been the subject of extens...

Biofilm formation is an important virulence trait of the pathogenic yeast Candida albicans. We have combined gene overexpression, strain barcoding and microarray profiling to screen a library of 531 C. albicans conditional overexpression strains (∼10% of the genome) for genes affecting biofilm development in mixed-population experiments. The overexpression of 16 genes increased strain occupancy within a multi-strain biofilm, whereas overexpression of 4 genes decreased it. The set of 16 genes was significantly enriched for those encoding predicted glycosylphosphatidylinositol (GPI)-modified proteins, namely Ihd1/Pga36, Phr2, Pga15, Pga19, Pga22, Pga32, Pga37, Pga42 and Pga59; eight of which have been classified as pathogen-specific. Validation experiments using either individually- or competitively-grown overexpression strains revealed that the contribution of these genes to biofilm formation was variable and stage-specific. Deeper functional analysis of PGA59 and PGA22 at a single-cell resolution using atomic force microscopy showed that overexpression of either gene increased C. albicans ability to adhere to an abiotic substrate. However, unlike PGA59, PGA22 overexpression led to cell cluster formation that resulted in increased sensitivity to shear forces and decreased ability to form a single-strain biofilm. Within the multi-strain environment provided by the PGA22-non overexpressing cells, PGA22-overexpressing cells were protected from shear forces and fitter for biofilm development. Ultrastructural analysis, genome-wide transcript profiling and phenotypic analyses in a heterologous context suggested that PGA22 affects cell adherence through alteration of cell wall structure and/or function. Taken together, our findings reveal that several novel predicted GPI-modified proteins contribute to the cooperative behaviour between biofilm cells and are important participants during C. albicans biofilm formation. Moreover, they illustrate the power of using signature tagging in conjunction with gene overexpression for the identification of novel genes involved in processes pertaining to C. albicans virulence.

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SummaryIn budding yeast, PKC1 plays an essential role in cell integrity and proliferation through a linear MAP (Mitogen Activated Protein) kinase phosphorylation cascade, which ends up with the activation of the Slt2-MAP kinase by dual phosphorylation on two conserved threonine and tyrosine residues. In this phosphorylated form, Slt2p kinase activates by phosphorylation at least two known downstream targets: Rlm1p, which is implicated in the expression of cell wall-related genes, and SBF, required for transcription activation of cell cycle-regulated genes at the G1 to S transition. In this paper, we demonstrate by twohybrid, in vitro immunoprecipitation and tandem affinity purification (TAP) methods that Knr4p physically interacts with Slt2p. Moreover, we show that the absence of Knr4p alters proper signalling of Slt2p to its two known downstream targets. In a knr4 null mutant, the SLT2 -dependent activation of Rlm1p is strongly reduced and the transcriptional activity of Rlm1p is decreased, although the phosphorylated form of Slt2p is more abundant than in wild-type cells. On the contrary, SBF is abnormally activated in this mutant, as shown by a more abundant phosphorylated form of Swi6p, by higher b b b b -galactosidase levels from a SCB-lacZ gene fusion, and by deregulation of the cyclic behaviour of several cell cycle-regulated genes. These results, taken together with our recent finding that Bck2p requires Knr4p to activate additively with Cln3-Cdc28p SBF target genes, lead to a model in which Knr4p is involved in co-ordinating the Slt2p-mediated cell wall integrity pathway with progression of the cell cycle.