Identification of three mannoproteins in the cell wall of Saccharomyces cerevisiae

Characterization of phiA, a gene essential for phialide development in Aspergillus nidulans

We have previously identified genes and proteins involved in the fungal response to the Streptomyces-produced antibiotics, bafilomycin B1 and concanamycin A, known inhibitors of V-ATPases. Using mRNA differential display we identified an Aspergillus nidulans gene with 30-fold up-regulated expression in the presence of bafilomycin. This gene, here denoted phiA, and its gene product, were further characterized by targeted gene disruption and immunohistochemistry. Phenotypically, the phiA mutation resulted in reduced growth and severely reduced sporulation. The abnormality could be traced to the phialides, which divided several times instead of forming a single flask-shaped cell. The importance of phiA for phialide and conidium development was supported by immunohistochemistry experiments that showed the protein to be mainly present in these two cell types. Attempts to relate phiA to inhibition of V-ATPases did not result in unambiguous conclusions, but suggest the possibility that changed expression of phiA is correlated with growth arrest caused by inhibited V-ATPases.

Characterization of a Candida albicans gene encoding a putative transcriptional factor required for cell wall integrity

After screening a Candida albicans genome database the product of an open reading frame (ORF) (CA2880) with 49% homology to the product of Saccharomyces cerevisiae YPL133c, a putative transcriptional factor, was identified. The disruption of the C. albicans gene leads to a major sensitivity to calcofluor white and Congo red, a minor sensitivity to sodium dodecyl sulfate, a major resistance to zymolyase, and an alteration of the chemical composition of the cell wall. For these reasons we called it CaCWT1 (for C. albicans cell wall transcription factor). CaCwt1p contains a putative Zn(II) Cys(6) DNA binding domain characteristic of some transcriptional factors and a PAS domain. The CaCWT1 gene is more expressed in stationary phase cells than in cells growing exponentially. To our knowledge, this is the first Zn(II) Cys(6) transcriptional factor-encoding gene implicated in the cell wall architecture.

DmAMP1, an antifungal plant defensin from dahlia (Dahlia merckii), interacts with sphingolipids from Saccharomyces cerevisiae

DmAMP1, an antifungal plant defensin from Dahlia merckii, was shown previously to require the presence of sphingolipids for fungicidal action against Saccharomyces cerevisiae. Sphingolipids may stabilize glycosylphosphatidylinositol (GPI)-anchored proteins, which interact with DmAMP1, or they may directly serve as DmAMP1 binding sites. In the present study, we demonstrate that S. cerevisiae disruptants in GPI-anchored proteins showed small or no increased resistance towards DmAMP1 indicating no involvement of these proteins in DmAMP1 action. Further, studies using an enzyme-linked immunosorbent assay (ELISA)-based binding assay revealed that DmAMP1 interacts directly with sphingolipids isolated from S. cerevisiae and that this interaction is enhanced in the presence of equimolar concentrations of ergosterol. Therefore, DmAMP1 antifungal action involving membrane interaction with sphingolipids and ergosterol is proposed.

Identification and study of a Candida albicans protein homologous to Saccharomyces cerevisiae Ssr1p, an internal cell-wall protein

After screening of a Candida albicans genome database, the product of an ORF (IPF 3054) that has 62 % homology with Saccharomyces cerevisiae Ssr1p, an internal cell-wall protein, was identified and named CaSsr1p. The deduced amino acid sequence shows that CaSsr1p contains an N-terminal hydrophobic signal peptide, is rich in Ser and Thr amino acids and has a potential glycosylphosphatidylinositol-attachment signal. CaSsr1p is released following degradation of isolated cell walls by zymolyase (mainly a 1,3-beta-glucanase) and therefore seems to be covalently linked to the beta-glucan of the cell walls. Both disruption and overexpression of the CaSSR1 gene caused an increased sensitivity to calcofluor white, Congo red and zymolyase digestion. These results suggest that CaSsr1p has a structural role associated with the cell-wall beta-glucan.

Mutations that are synthetically lethal with a gas1Delta allele cause defects in the cell wall of Saccharomyces cerevisiae

The GAS1-related genes of fungi encode GPI-anchored proteins with beta-1,3-glucanosyltransferase activity. Loss of this activity results in defects in the assembly of the cell wall. We isolated mutants that show a synthetic defect when combined with a gas1Delta allele in Saccharomyces cerevisiae, and identified nine wild-type genes that rescue this defect. The indispensability of BIG1 and KRE6 for the viability of gas1Delta cells confirmed the important role of beta-1,6-glucan in cells that are defective in the processing of beta-1,3-glucan. The identification of the Wsc1p hypo-osmotic stress sensor and components of the PKC signal transduction pathway in our screen also confirmed that the cell wall integrity response attenuates the otherwise lethal gas1Delta defect. Unexpectedly, we found that the KEX2 gene is also required for the viability of the gas1Delta mutant. Kex2p is a Golgi/endosome-membrane-anchored protease that processes secretory preproteins. A cell wall defect was also found in the kex2Delta mutant, which was suppressible by multiple copies of the MKC7 or YAP3 gene, both of which encode other GPI-anchored proteases. Therefore, normal cell wall assembly requires proteolytic processing of secretory preproteins. Furthermore, the genes CSG2 and IPT1 were found to be required for normal growth of gas1Delta cells in the presence of 1 M sorbitol. This finding suggests that complex sphingolipids play a role in the hyper-osmotic response.

Identification of cell surface determinants in Candida albicans reveals Tsa1p, a protein differentially localized in the cell

To identify cell surface proteins of Candida albicans, the predominant fungal pathogen in humans, we have established an approach using a membrane impermeable biotin derivative in combination with affinity purification. We were able to identify 29 different proteins under two distinct conditions. Among mannoproteins, heat shock proteins and glycolytic enzymes we found thiol-specific antioxidant-like protein 1 (Tsa1p) to be differentially localized depending on the conditions applied. Only in hyphally grown cells Tsa1p was localized to the cell surface whereas in blastospores no surface but mainly nuclear localization was found. This indicates that cell surface expression of at least some proteins is mediated by differential translocation.

Isolation of the MNN9 gene of Yarrowia lipolytica (YlMNN9) and phenotype analysis of a mutant ylmnn9 Delta strain

In this work we describe the isolation of the Yarrowia lipolytica homologue of Saccharomyces cerevisiae MNN9 gene, which we have named YlMNN9, and the phenotype analysis of a Y. lipolytica strain containing the disrupted YlMNN9 allele. YlMNN9 was cloned using degenerate consensus oligonucleotides to generate specific probes that were in turn used to screen mini-gene libraries. The gene is defined by a 1014 bp ORF predicted to encode a protein 337 amino acids long that shares significant homology with the Mnn9ps of S. cerevisiae, Candida albicans and Hansenula polymorpha, including a putative N-terminal transmembrane domain. Disruption of YlMNN9 leads to phenotypes such as resistance to sodium orthovanadate and sensitivity to hygromycin B, compatible with a glycosylation defect, and hypersensitivity to Calcofluor white, Congo red or zymolyase, characteristic of cell wall defects. Analysis of cell wall proteins present in beta-mercaptoethanol and zymolyase extracts showed significant differences between the parental and the ylmnn9 Delta strain. These results suggest that, as has been the case with the mnn9 strain of S. cerevisiae, the ylmnn9 Delta strain we present in this work, could be used to study the cell wall proteins of Y. lipolytica and how they are organized into the cell wall.

Expression of the mnpA gene that encodes the mannoprotein of Aspergillus nidulans is dependent on fadA and flbA as well as veA

The single copy mnpA gene that encodes a mannoprotein of Aspergillus nidulans and its cDNA were isolated from the genomic and cDNA libraries, respectively. The determined nucleotide sequences of the genomic DNA and its cDNA revealed that the gene has an open-reading frame of 261 amino acids without introns. The deduced amino acid sequence showed a 60% identity to that of Aspegillus fumigatus galactomannoprotein MP1. The mnpA gene was expressed more abundantly in the wild-type than in the veA-null mutant. It was expressed at a lower level in fadA-null mutants, veA(+) or veA1 (regardless of their genetic background), than in the fadA(+) strain. However, the expression level was slightly higher in the veA(+) DeltafadA strain than in the veA1 DeltafadA strain. Furthermore, the amount of the mnpA transcript was higher in the flbA(+) strain than in the flbA-null mutant. These results indicate that the fadA and flbA genes in addition to the veA gene are necessary for the mnpA expression. The mnpA gene was expressed highly in vegetative mycelia and at a reduced level in sexual structures, but not in conidia. Its expression was almost constitutive during asexual development up to 18h after the transfer of mycelial balls onto a solid medium, and decreased thereafter. During sexual development, its expression reached its maximum 0-20h after the induction of sexual development, and then decreased thereafter. The mnpA-null mutant, that was still viable, showed no phenotypic difference in development, growth rate, protein secretion, and germination of both the ascospores and conidia from the wild-type. This suggests that the mannoprotein that is encoded by the mnpA gene is dispensable.

RHO1 (YlRHO1) is a non-essential gene in Yarrowia lipolytica and complements rho1Delta lethality in Saccharomyces cerevisiae

The synthesis of beta-1,3-glucan, the structural component of the yeast cell wall that gives shape to the cell, occurs at the plasma membrane and is the result of the activity of at least a two-component complex. Fks1p is the catalytic subunit directly responsible for the synthesis of beta-1,3-glucan, whilst the second subunit, Rho1p, has a GTP-dependent regulatory role (Yamochi et al., 1994). RHO1 has been characterized in Saccharomyces cerevisiae (Yamochi et al., 1994), and in several other fungal species. In this work, we have used degenerate oligonucleotides derived from the conserved regions of Rho1ps to isolate the RHO1 gene of Yarrowia lipolytica. The gene isolated in this way, which we have named YlRHO1, encodes a 204 amino acid protein that shows a high degree of homology with other Rho1ps. However, unlike S. cerevisiae, the ylrho1Delta disruptant strain in Y. lipolytica is viable, although it exhibits an increased sensitivity to Calcofluor white and Congo red. Also, YlRHO1 complements rho1 lethality in S. cerevisiae at both 28 degrees C and 37 degrees C. The complete sequence of YlRHO1 can be obtained from GenBank under Accession No. AF279915.

AFLMP1 encodes an antigenic cel wall protein in Aspergillus flavus

We have previously reported the cloning and characterization of the MP1 gene in Penicillium marneffei and the AFMP1 gene in Aspergillus fumigatus and their use for serodiagnosis of penicilliosis and aspergilloma and invasive aspergillosis, respectively. In this study, we describe the cloning of the AFLMP1 gene, which encodes the homologous antigenic cell wall protein in Aspergillus flavus, the most common Aspergillus species associated with human disease in our locality and in other Asian countries and the second most common Aspergillus species associated with human disease in Western countries. AFLMP1 codes for a protein, Aflmp1p, of 273 amino acid residues, with a few sequence features that are present in Mp1p and Afmp1p, the homologous antigenic cell wall proteins in P. marneffei and A. fumigatus, respectively, as well as several other cell wall proteins of Saccharomyces cerevisiae and Candida albicans. It contains a serine- and threonine-rich region for O glycosylation, a signal peptide, and a putative glycosylphosphatidylinositol attachment signal sequence. Specific anti-Aflmp1p antibody was generated with recombinant Aflmp1p protein purified from Escherichia coli to allow further characterization of Aflmp1p. Indirect immunofluorescence analysis indicated that Aflmp1p is present on the surface of the hyphae of A. flavus. Finally, it was observed that patients with aspergilloma and invasive aspergillosis due to A. flavus develop a specific antibody response against Aflmp1p. This suggested that the recombinant protein and its antibody may be useful for serodiagnosis in patients with aspergilloma or invasive aspergillosis, and the protein may represent a good cell surface target for host humoral immunity.

The transcription factor Rim101p governs ion tolerance and cell differentiation by direct repression of the regulatory genes NRG1 and SMP1 in Saccharomyces cerevisiae

Environmental pH changes have broad consequences for growth and differentiation. The best-understood eukaryotic pH response pathway acts through the zinc-finger transcription factor PacC of Aspergillus nidulans, which activates alkaline pH-induced genes directly. We show here that Saccharomyces cerevisiae Rim101p, the pH response regulator homologous to PacC, functions as a repressor in vivo. Chromatin immunoprecipitation assays show that Rim101p is associated in vivo with the promoters of seven Rim101p-repressed genes. A reporter gene containing deduced Rim101p binding sites is negatively regulated by Rim101p and is associated with Rim101p in vivo. Deletion mutations of the Rim101p repression targets NRG1 and SMP1 suppress rim101Delta mutant defects in ion tolerance, haploid invasive growth, and sporulation. Therefore, transcriptional repression is the main biological function of Rim101p. The Rim101p repression target Nrg1p is in turn required for repression of two alkaline pH-inducible genes, including the Na+ pump gene ENA1, which is required for ion tolerance. Thus, Nrg1p, a known transcriptional repressor, functions as an inhibitor of alkaline pH responses. Our findings stand in contrast to the well-characterized function of PacC as a direct activator of alkaline pH-induced genes yet explain many aspects of Rim101p and PacC function in other organisms.

The protein kinase Kic1 affects 1,6-beta-glucan levels in the cell wall of Saccharomyces cerevisiae

KIC1 encodes a PAK kinase that is involved in morphogenesis and cell integrity. Both over- and underexpressing conditions of KIC1 affected cell wall composition. Kic1-deficient cells were hypersensitive to the cell wall perturbing agent calcofluor white and had less 1,6-beta-glucan. When Kic1-deficient cells were crossed with various kre mutants, which also have less 1,6-beta-glucan in their wall, the double mutants displayed synthetic growth defects. However, when crossed with the 1,3-beta-glucan-deficient strain fks1delta, no synthetic growth defect was observed, supporting a specific role for KIC1 in regulating 1,6-beta-glucan levels. Kic1-deficient cells also became highly resistant to the cell wall-degrading enzyme mixture Zymolyase, and exhibited higher transcript levels of the cell wall protein-encoding genes CWP2 and SED1. Conversely, overexpression of KIC1 resulted in increased sensitivity to Zymolyase and in a higher level of 1,6-beta-glucan. Multicopy suppressor analysis of a Kic1-deficient strain identified RHO3. Consistent with this, expression levels of RHO3 correlated with 1,6-beta-glucan levels in the cell wall. Interestingly, expression levels of KIC1 and the MAP kinase kinase PBS2 had opposite effects on Zymolyase sensitivity of the cells and on cell wall 1,6-beta-glucan levels in the wall. It is proposed that Kic1 affects cell wall construction in multiple ways and in particular in regulating 1,6-beta-glucan levels in the wall.

Cloning and characterization of WMSU1, a Williopsis saturnus var. mrakii gene encoding a new yeast SUN protein involved in the cell wall structure

SUN proteins of Saccharomyces cerevisiae have been defined on the basis of high homologies in their C-terminal domain. Recently, two of these four proteins were shown to be involved in cell wall morphogenesis (Mouassite et al., 2000a). In the present study, we have isolated WMSU1 (Accession No. AF418983), a new SUN-related gene, from W. saturnus var. mrakii MUCL 41968. Sequencing of the gene revealed an open reading frame coding for 402 amino acids. The predicted amino acid sequence of WMSU1 is closely related to the S. cerevisiae SUN proteins and to other yeast proteins involved in cell wall metabolism. WMSU1 is proposed to encode a cell wall protein since its predicted product contains a signal sequence, a Kex2p cleavage site and a serine/threonine-rich N-terminal domain. Southern blot analysis of the W. saturnus var. mrakii MUCL 41968 genome using the highly conserved domain of WMSU1 as a probe suggested that the isolated gene belongs to a multigenic family. Expression of WMSU1 in E. coli led to a 45 kDa protein, which appeared to be toxic to this host. Scanning electron microscopy analysis of a recombinant S. cerevisiae producing Wmsu1p showed that this strain exhibited an altered cell wall, thus pointing to a probable role of this protein in the cell wall structure.

A cell surface display system using novel GPI-anchored proteins in Hansenula polymorpha

A cell surface display system was developed in yeast Hansenula polymorpha. The four genes HpSED1, HpGAS1, HpTIP1and HpCWP1, encoding glycosylphosphatidyl-inositol (GPI)-anchored cell surface proteins from H. polymorpha, were cloned, characterized and evaluated for their efficacies as cell surface display motifs of reporter proteins. Sequence analysis of these genes revealed that each encodes a typical GPI-anchored protein that is structurally similar to a counterpart gene in S. cerevisiae. The genes showed a high content of serine-threonine (alanine) and harboured a putative secretion signal in the N-terminus and the GPI-attachment signal in the C-terminus. The surface anchoring efficiency of these putative cell surface proteins was tested by fusion to the C-terminal of carboxymethylcellulase (CMCase) from Bacillus subtilis. In all cases, high CMCase activities were detected in intact cell fraction, indicating anchoring of CMCase to the cell surface. HpCwp1p, HpGas1p and the 40 C-terminal amino acids of HpTip1p from H. polymorpha exhibited a comparatively high CMCase surface anchoring efficiency. When these proteins were used as anchoring motifs for surface display of the glucose oxidase (GOD) from Aspergillus niger, most enzyme activity was detected at the cell surface. Fluorescence activated cell sorter (FACS) analysis of cells displaying GOD on the cell surface demonstrated that GOD was well exposed on the cell surface. HpCwp1p showed the highest anchoring efficiency among others.

Characterization of a cell-wall acid phosphatase (PhoAp) in Aspergillus fumigatus

In the filamentous fungus Aspergillus fumigatus, the vast majority of the cell-wall-associated proteins are secreted proteins that are in transit in the cell wall. These proteins can be solubilized by detergents and reducing agents. Incubation of a SDS/beta-mercaptoethanol-treated cell-wall extract with various recombinant enzymes that hydrolyse cell-wall polysaccharides resulted in the release of a unique protein in minute amounts only after incubation of the cell wall in the presence of 1,3-beta-glucanase. Sequence analysis and biochemical studies showed that this glycoprotein, with an apparent molecular mass of 80 kDa, was an acid phosphatase (PhoAp) that was active on both phosphate monoesters and phosphate diesters. PhoAp is a glycosylphosphatidylinositol-anchored protein that was recovered in the culture filtrate and cell-wall fraction of A. fumigatus after cleavage of its anchor. It is also a phosphate-repressible acid phosphatase. The absence of PhoAp from a phosphate-rich medium was not associated with a reduction in fungal growth, indicating that this cell-wall-associated protein does not play a role in the morphogenesis of A. fumigatus.

Dynamics of cell wall structure in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae is an elastic structure that provides osmotic and physical protection and determines the shape of the cell. The inner layer of the wall is largely responsible for the mechanical strength of the wall and also provides the attachment sites for the proteins that form the outer layer of the wall. Here we find among others the sexual agglutinins and the flocculins. The outer protein layer also limits the permeability of the cell wall, thus shielding the plasma membrane from attack by foreign enzymes and membrane-perturbing compounds. The main features of the molecular organization of the yeast cell wall are now known. Importantly, the molecular composition and organization of the cell wall may vary considerably. For example, the incorporation of many cell wall proteins is temporally and spatially controlled and depends strongly on environmental conditions. Similarly, the formation of specific cell wall protein-polysaccharide complexes is strongly affected by external conditions. This points to a tight regulation of cell wall construction. Indeed, all five mitogen-activated protein kinase pathways in bakers' yeast affect the cell wall, and additional cell wall-related signaling routes have been identified. Finally, some potential targets for new antifungal compounds related to cell wall construction are discussed.

Transglutaminase activity is involved in Saccharomyces cerevisiae wall construction

Transglutaminase activity, which forms the interpeptidic cross-link N(epsilon)-(gamma-glutamyl)-lysine, was demonstrated in cell-free extracts of Saccharomyces cerevisiae by incorporation of [(14)C]lysine into an exogenous acceptor, N,N'-dimethylcasein. Higher levels of the activity were present in the cell wall, which also contained endogenous acceptors. The enzyme activity in the wall was inhibited by cystamine, a known inhibitor of transglutaminase, and by EDTA, indicating a cation-dependent activity. After the endogenous wall acceptors were labelled radioactively by transglutaminase, extraction with SDS solubilized about 50% of the total radioactivity, while Zymolyase and chitinase each released a further 3%. The proteins solubilized by SDS had molecular masses less than 50 kDa, whereas the material released by Zymolyase or chitinase had molecular masses greater than 180 kDa, suggesting a precursor-product relationship. Cystamine inhibited the growth of several strains of S. cerevisiae. Treated cells showed increased sensitivity to Zymolyase and appeared as protoplasts, indicating gross alterations in the cell wall. These data suggest that transglutaminase may be involved in the formation of covalent cross-links between wall proteins during wall construction.

The Awa1 gene is required for the foam-forming phenotype and cell surface hydrophobicity of sake yeast

Sake, a traditional alcoholic beverage in Japan, is brewed with sake yeasts, which are classified as Saccharomyces cerevisiae. Almost all sake yeasts form a thick foam layer on sake mash during the fermentation process because of their cell surface hydrophobicity, which increases the cells' affinity for bubbles. To reduce the amount of foam, nonfoaming mutants were bred from foaming sake yeasts. Nonfoaming mutants have hydrophilic cell surfaces and no affinity for bubbles. We have cloned a gene from a foam-forming sake yeast that confers foaming ability to a nonfoaming mutant. This gene was named AWA1 and structures of the gene and its product were analyzed. The N- and C-terminal regions of Awa1p have the characteristic sequences of a glycosylphosphatidylinositol anchor protein. The entire protein is rich in serine and threonine residues and has a lot of repetitive sequences. These results suggest that Awa1p is localized in the cell wall. This was confirmed by immunofluorescence microscopy and Western blotting analysis using hemagglutinin-tagged Awa1p. Moreover, an awa1 disruptant of sake yeast was hydrophilic and showed a nonfoaming phenotype in sake mash. We conclude that Awa1p is a cell wall protein and is required for the foam-forming phenotype and the cell surface hydrophobicity of sake yeast.

Meu10 is required for spore wall maturation in Schizosaccharomyces pombe

BACKGROUND

Many genes are meiosis and/or sporulation-specifically transcribed during this process. Isolation and analysis of these genes might help us to understand how meiosis and sporulation are regulated. For this purpose, we have isolated a large number of cDNA clones from Schizosaccharomyces pombe whose expression is up-regulated during meiosis.

RESULTS

We have isolated meu10+ gene, which encodes 416 amino acids and bears homology to SPS2 of Saccharomyces cerevisiae. A strain whose meu10+ gene has been deleted forms no viable spores. Thin-section electron micrographs showed that the meu10Delta strain has abnormally formed spore walls, and then they disrupt, allowing cytoplasmic material to escape. The Meu10-GFP fusion protein is localized to the spore periphery, thereafter returned to the cytoplasm after sporulation. Meu10-GFP localization to the spore wall was almost normal in the bgs2Delta or chs1Delta mutants that lack 1,3-beta-glucan or chitin, respectively. In contrast, 1,3-beta-glucan is abnormally localized in meu10Delta cells. Meu10 has an N-terminal domain with homology to the mammalian insulin receptor and a C-terminal domain with a transmembrane motif. Mutants whose N-terminal or C-terminal domain was truncated were severely defective for sporulation.

CONCLUSIONS

Meu10 is a spore wall component and plays a pivotal role in the formation of the mature spore wall structure.

Characterization of AFMP1: a novel target for serodiagnosis of aspergillosis

We cloned the AFMP1 gene, which encodes the first antigenic cell wall galactomannoprotein in Aspergillus fumigatus. AFMP1 codes for a protein, Afmp1p, of 284 amino acid residues, with a few sequence features that are present in Mp1p, the antigenic cell wall mannoprotein in Penicillium marneffei that we described previously, as well as several other cell wall proteins of Saccharomyces cerevisiae and Candida albicans. It contains a serine- and threonine-rich region for O glycosylation, a signal peptide, and a putative glycosylphosphatidyl inositol attachment signal sequence. Specific anti-Afmp1p antibody was generated with recombinant Afmp1p protein purified from Escherichia coli to allow further characterization of Afmp1p. Afmp1p has a high affinity for Galanthus nivalis agglutinin, a characteristic indicative of a mannoprotein. Furthermore, it was recognized by a rat monoclonal antibody against the galactofuran side chain of galactomannan, indicating that it is a galactomannoprotein. Ultrastructural analysis by immunogold staining indicated that Afmp1p is present in the cell walls of the hyphae and conidia of A. fumigatus. Finally, it was observed that patients with aspergilloma and invasive aspergillosis due to A. fumigatus develop a specific antibody response against Afmp1p. This suggested that the recombinant protein and its antibody may be useful for serodiagnosis in patients with aspergilloma or invasive aspergillosis, and the protein may represent a good cell surface target for host humoral immunity.

Proteome analysis of Aspergillus fumigatus identifies glycosylphosphatidylinositol-anchored proteins associated to the cell wall biosynthesis

Previous studies in Aspergillus fumigatus (Mouyna I., Fontaine T., Vai M., Monod M., Fonzi W. A., Diaquin M., Popolo L., Hartland R. P., Latgé J.-P, J. Biol. Chem. 2000, 275, 14882-14889) have shown that a glucanosyltransferase playing an important role in fungal cell wall biosynthesis is glycosylphosphatidylinositol (GPI) anchored to the membrane. To identify other GPI-anchored proteins putatively involved in cell wall biogenesis, a proteomic analysis has been undertaken in A. fumigatus and the protein data were matched with the yeast genomic data. GPI-anchored proteins of A. fumigatus were released from membrane preparation by an endogenous GPI-phospholipase C, purified by liquid chromatography and separated by two-dimensional electrophoresis. They were characterized by their peptide mass fingerprint through matrix-assisted laser desorption/ionization-time of flight-(MALDI-TOF)-mass spectrometry and by internal amino acid sequencing. Nine GPI-anchored proteins were identified in A. fumigatus. Five of them were homologs of putatively GPI-anchored yeast proteins (Csa1p, Crh1p, Crh2p, Ecm33p, Gas1p) of unknown function but shown by gene disruption analysis to play a role in cell wall morphogenesis. In addition, a comparative study performed with chitin synthase and glucanosyl transferase mutants of A. fumigatus showed that a modification of the growth phenotype seen in these mutants was associated to an alteration of the pattern of GPI-anchored proteins. These results suggest that GPI-anchored proteins identified in this study are involved in A. fumigatus cell wall organization.

Reciprocal regulation of anaerobic and aerobic cell wall mannoprotein gene expression in Saccharomyces cerevisiae

The DAN/TIR genes encode nine cell wall mannoproteins in Saccharomyces cerevisiae which are expressed during anaerobiosis (DAN1, DAN2, DAN3, DAN4, TIR1, TIR2, TIR3, TIR4, and TIP1). Most are expressed within an hour of an anaerobic shift, but DAN2 and DAN3 are expressed after about 3 h. At the same time, CWP1 and CWP2, the genes encoding the major mannoproteins, are down-regulated, suggesting that there is a programmed remodeling of the cell wall in which Cwp1 and Cwp2 are replaced by nine anaerobic counterparts. TIP1, TIR1, TIR2, and TIR4 are also induced during cold shock. Correspondingly, CWP1 is down-regulated during cold shock. As reported elsewhere, Mox4 is a heme-inhibited activator, and Mot3 is a heme-induced repressor of the DAN/TIR genes (but not of TIP1). We show that CWP2 (but not CWP1) is controlled by the same factors, but in reverse fashion-primarily by Mot3 (which can function as either an activator or repressor) but also by Mox4, accounting for the reciprocal regulation of the two groups of genes. Disruptions of TIR1, TIR3, or TIR4 prevent anaerobic growth, indicating that each protein is essential for anaerobic adaptation. The Dan/Tir and Cwp proteins are homologous, with the greatest similarities shown within three subgroups: the Dan proteins, the Tip and Tir proteins, and, more distantly, the Cwp proteins. The clustering of homology corresponds to differences in expression: the Tip and Tir proteins are expressed during hypoxia and cold shock, the Dan proteins are more stringently repressed by oxygen and insensitive to cold shock, and the Cwp proteins are oppositely regulated by oxygen and temperature.

Membrane and cell wall targets in Aspergillus fumigatus

Antifungal drugs directed against the human opportunistic fungal pathogen Aspergillus fumigatus are limited in number and ergosterol-targeted: the polyenes bind to the membrane ergosterol and the azoles block the ergosterol biosynthesis pathway. The efficacy of the drugs currently available for clinical use (amphotericin B and itraconazole) is limited and the frequent occurrence of therapeutic failures in the treatment of invasive aspergillosis emphasizes the need for the development of new agents. Cell wall biosynthetic pathways have been recognized for a long time as essential and unique specific drug targets. Recent studies of the chemical organization of the cell wall of A. fumigatus together with comparative analysis of yeast cell wall data have shown that beta 1-3 glucan branching and chitin-beta 1-3 glucan binding are essential exocellular enzymatic steps in cell wall biosynthesis. The enzymes involved in the biosynthesis and remodeling of cell wall polysaccharides especially in A. fumigatus are reviewed.

A Rox1-independent hypoxic pathway in yeast. Antagonistic action of the repressor Ord1 and activator Yap1 for hypoxic expression of the SRP1/TIR1 gene

Hypoxic SRP1/TIR1 gene expression depends on the absence of haem but is independent of Rox1-mediated repression. We have found a new hypoxic pathway involving an antagonistic interaction between the Ixr1/Ord1 repressor and the Yap1 factor, a transcriptional activator involved in oxidative stress response. Here, we show that Ord1 repressed SRP1 gene expression under normoxia and hypoxia, whereas Yap1 activated it. Ord1 and Yap1 have been shown to bind the SRP1 promoter in a region extending from -299 to -156 bp upstream of the start codon. A typical AP-1 responsive element lying from -247 to -240 bp allows Yap1 binding. Internal deletion of sequences within the SRP1 promoter were introduced. Two regions were characterized at positions -299/-251 and -218/-156 that, once removed, resulted in a constitutive expression of SRP1 in a wild-type strain under normoxic conditions. Deletion of both these two sequences allowed the bypass of YAP1 requirement in a Deltayap1 strain, whereas these two internal deletions did not yield increased expression in a Deltaord1 strain compared with the full-length promoter. Both a single Deltaord1 mutant and a doubly disrupted Deltayap1 Deltaord1 strain yielded normoxic constitutive SRP1 expression and increased hypoxic SRP1 induction, thereby demonstrating that ord1 is epistatic to yap1. Thus, Yap1 is not directly involved in SRP1 induction by hypoxia, but is necessary to counteract the Ord1 effect.

A proteomic approach for the study of Saccharomyces cerevisiae cell wall biogenesis

In fungi, cell shape is determined by the presence of a rigid cell wall which separates the cell from the extracellular medium. This highly dynamic structure is essential for the maintenance of cell integrity and is involved in several phenomena such as flocculation, adherence and pathogenicity. The composition of the fungal cell wall is well known, but issues such as the assembly and remodeling of its components remain poorly understood. In an attempt to study the de novo construction of the yeast cell wall, we have undertaken a large-scale proteomic approach to analyze the proteins secreted by regenerating protoplasts. Upon incubation of protoplasts in regenerating conditions, numerous proteins are secreted into the culture medium. These presumably include proteins destined for the cell wall, comprising both structural proteins as well as enzymes involved in cell wall biogenesis. This work reports the establishment of a reference map of proteins secreted by regenerating protoplasts by means of two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) and their identification by mass spectrometry. Thirty-two different proteins have been identified, including known cell wall proteins, glycolytic enzymes, heat shock proteins, and proteins involved in several other processes. Using this approach, novel proteins possibly involved in cell wall construction have also been identified. This reference map will allow comparative analyses to be carried out on a selected collection of mutants affected in the cell wall.

Molecular organization of the alkali-insoluble fraction of Aspergillus fumigatus cell wall

Physical and biological properties of the fungal cell wall are determined by the composition and arrangement of the structural polysaccharides. Cell wall polymers of fungi are classically divided into two groups depending on their solubility in hot alkali. We have analyzed the alkali-insoluble fraction of the Aspergillus fumigatus cell wall, which is the fraction believed to be responsible for fungal cell wall rigidity. Using enzymatic digestions with recombinant endo-beta-1,3-glucanase and chitinase, fractionation by gel filtration, affinity chromatography with immobilized lectins, and high performance liquid chromatography, several fractions that contained specific interpolysaccharide covalent linkages were isolated. Unique features of the A. fumigatus cell wall are (i) the absence of beta-1,6-glucan and (ii) the presence of a linear beta-1, 3/1,4-glucan, never previously described in fungi. Galactomannan, chitin, and beta-1,3-glucan were also found in the alkali-insoluble fraction. The beta-1,3-glucan is a branched polymer with 4% of beta-1,6 branch points. Chitin, galactomannan, and the linear beta-1, 3/1,4-glucan were covalently linked to the nonreducing end of beta-1, 3-glucan side chains. As in Saccharomyces cerevisiae, chitin was linked via a beta-1,4 linkage to beta-1,3-glucan. The data obtained suggested that the branching of beta-1,3-glucan is an early event in the construction of the cell wall, resulting in an increase of potential acceptor sites for chitin, galactomannan, and the linear beta-1,3/1,4-glucan.

YLL031c belongs to a novel family of membrane proteins involved in the transfer of ethanolaminephosphate onto the core structure of glycosylphosphatidylinositol anchors in yeast

MCD4 and GPI7 are important for the addition of glycosylphosphatidylinositol (GPI) anchors to proteins in the yeast Saccharomyces cerevisiae. Mutations in these genes lead to a reduction of GPI anchoring and cell wall fragility. Gpi7 mutants accumulate a GPI lipid intermediate of the structure Manalpha1-2[NH(2)-(CH(2))(2)-PO(4)-->]Manalpha1-2Manalpha 1-6[NH(2)-(C H(2))(2)-PO(4)-->]Manalpha1-4GlcNalpha1-6[acyl-->]inositol-P O(4)-lipi d, which, in comparison with the complete GPI precursor lipid CP2, lacks an HF-sensitive side chain on the alpha1-6-linked mannose. In contrast, mcd4-174 accumulates only minor amounts of abnormal GPI intermediates. Here we investigate whether YLL031c, an open reading frame predicting a further homologue of GPI7 and MCD4, plays any role in GPI anchoring. YLL031c is an essential gene. Its depletion results in a reduction of GPI anchor addition to GPI proteins as well as to cell wall fragility. YLL031c-depleted cells accumulate GPI intermediates with the structures Manalpha1-2Manalpha1-2Manalpha1-6[NH(2)-(CH(2))(2)-PO( 4)-->]Manalpha1 -4GlcNalpha1-6[acyl-->]inositol-PO(4)-lipid and Manalpha1-2Manalpha1-2Manalpha1-6Manalpha1-4G lcNalpha1-6[acyl-->]inos itol-PO(4)-lipid. Subcellular localization studies of a tagged version of YLL031c suggest that this protein is mainly in the ER, in contrast to Gpi7p, which is found at the cell surface. The data are compatible with the idea that YLL031c transfers the ethanolaminephosphate to the inner alpha1-2-linked mannose, i.e. the group that links the GPI lipid anchor to proteins, whereas Mcd4p and Gpi7p transfer ethanolaminephosphate onto the alpha1-4- and alpha1-6-linked mannoses of the GPI anchor, respectively.

Fungal cell wall phosphomannans facilitate the toxic activity of a plant PR-5 protein

Osmotin is a plant PR-5 protein. It has a broad spectrum of antifungal activity, yet also exhibits specificity for certain fungal targets. The structural bases for this specificity remain unknown. We show here that full sensitivity of Saccharomyces cerevisiae cells to the PR-5 protein osmotin is dependent on the function of MNN2, MNN4 and MNN6. MNN2 is an alpha-1, 2-mannosyltransferase catalyzing the addition of the first mannose to the branches on the poly l,6-mannose backbone of the outer chain of cell wall N-linked mannans. MNN4 and MNN6 are required for the transfer of mannosylphosphate to cell wall mannans. Null mnn2, mnn4 or mnn6 mutants lack phosphomannans and are defective in binding osmotin to the fungal cell wall. Both antimannoprotein antibody and the cationic dye alcian blue protect cells against osmotin cytotoxicity. MNN1 is an alpha-1,3-mannosyltransferase that adds the terminal mannose to the outer chain branches of N-linked mannan, masking mannosylphosphate. Null mnn1 cells exhibit enhanced osmotin binding and sensitivity. Several cell wall mannoproteins can bind to immobilized osmotin, suggesting that their polysaccharide constituent determines osmotin binding. Our results demonstrating a causal relationship between cell surface phosphomannan and the susceptibility of a yeast strain to osmotin suggest that cell surface polysaccharides of invading pathogens control target specificity of plant PR-5 proteins.

Glycosylation deficiency phenotypes resulting from depletion of GDP-mannose pyrophosphorylase in two yeast species

The genes encoding GDP-mannose pyrophosphorylase from Saccharomyces cerevisiae (SRB1/PSA1) and Candida albicans (CaSRB1) were expressed under the control of the tightly regulated promoters of MET3 and CaMET3 respectively. Northern analysis showed that the addition of methionine effectively blocks the transcription of pMET3-SRB1/PSA1 and pCaMET3CaSRB1 expression cassettes, which had been integrated into the genomes of appropriate mutants. Methionine-mediated repression of CaSRB1 caused loss of viability in C. albicans, demonstrating that, as in S. cerevisiae, the gene is essential for growth. Depletion of GDP-mannose pyrophosphorylase had a highly pleiotropic effect in the two yeasts. The major phenotypes observed were lysis, failure of cell separation and/or cytokinesis, impaired bud growth and bud's site selection, clumping and flocculation, as well as increased sensitivity to a wide range of antifungal drugs and cell wall inhibitors, and impaired hyphal switching ability. These phenotypes resulted from defects in glycosylation, as demonstrated by reduced affinity for Alcian blue and sensitivity to hygromycin B. Our results provide new information about the roles of protein glycosylation in yeast and, in particular, the steps that require GDP-mannose in the fungal pathogen C. albicans.

Glycosylphosphatidylinositol-anchored glucanosyltransferases play an active role in the biosynthesis of the fungal cell wall

A novel 1,3-beta-glucanosyltransferase isolated from the cell wall of Aspergillus fumigatus was recently characterized. This enzyme splits internally a 1,3-beta-glucan molecule and transfers the newly generated reducing end to the non-reducing end of another 1, 3-beta-glucan molecule forming a 1,3-beta linkage, resulting in the elongation of 1,3-beta-glucan chains. The GEL1 gene encoding this enzyme was cloned and sequenced. The predicted amino acid sequence of Gel1p was homologous to several yeast protein families encoded by GAS of Saccharomyces cerevisiae, PHR of Candida albicans, and EPD of Candida maltosa. Although the expression of these genes is required for correct morphogenesis in yeast, the biochemical function of the encoded proteins was unknown. The biochemical assays performed on purified recombinant Gas1p, Phr1p, and Phr2p showed that these proteins have a 1,3-beta-glucanosyltransferase activity similar to that of Gel1p. Biochemical data and sequence analysis have shown that Gel1p is attached to the membrane through a glycosylphosphatidylinositol in a similar manner as the yeast homologous proteins. The activity has been also detected in membrane preparations, showing that this 1,3-beta-glucanosyltransferase is indeed active in vivo. Our results show that transglycosidases anchored to the plasma membrane via glycosylphosphatidylinositols can play an active role in fungal cell wall synthesis.

Genetic immobilization of proteins on the yeast cell surface

A genetic system has been exploited to immobilize proteins in their active and functional forms on the cell surface of yeast, Saccharomyces cerevisiae. DNAs encoding proteins with a secretion signal peptide were fused with the genes encoding yeast agglutinins, a- and alpha-type proteins involved in mating. The fusion gene was introduced into S. cerevisiae and expressed under the control of several promoters. Appearance of the fused proteins expressed on the cell surface was demonstrated biochemically and by immunofluorescence and immunoelectron microscopy techniques. Alpha-galactosidase from Cyamopsis tetragonoloba seeds, peptide libraries including scFv and variable regions of the T cell receptor from mammalian cells have been successfully immobilized on the yeast cell wall in the active form. Recently, surface-engineered yeasts have been constructed by immobilizing the enzymes and a functional protein, for example, green fluorescent protein (GFP) from Aequorea victoria. The yeasts were termed 'arming yeasts' with biocatalysts or functional proteins. Such arming cells displaying glucoamylase from Rhizopus oryzae and alpha-amylase from Bacillus stearothermophilus, or carboxymethylcellulase and beta-glucosidase from Aspergillus acleatus, could assimilate starch or cellooligosaccharides as the sole carbon source, although S. cerevisiae cannot intrinsically assimilate these substrates. GFP-arming cells can emit green fluorescence from the cell surface in response to the environmental conditions. The approach described in this review will enable us to endow living cells, including yeast cells, with novel additional abilities and to open new dimensions in the field of biotechnology.

Active site determination of Gpi8p, a caspase-related enzyme required for glycosylphosphatidylinositol anchor addition to proteins

Glycosylphosphatidylinositol (GPI) anchors are attached to newly synthesized proteins in the ER by a transamidation reaction during which a C-terminal GPI attachment signal is replaced by a preformed GPI precursor lipid. This reaction depends on GAA1 and GPI8, the latter belonging to a novel cysteine protease family. Homologies between this family and other Cys proteinases, such as caspases, pointed to Cys199 and His157 as potential active site residues. Indeed, gpi8 alleles mutated at Cys199 or His157 are nonfunctional, i.e., they are unable to suppress the lethality of Deltagpi8 mutants. The overexpression of these nonfunctional alleles in wild-type cells leads to the accumulation of the free GPI precursor lipid CP2, delays the maturation of the GPI protein Gas1p, and arrests cell growth. The dominant negative effect of the Cys199 mutant cannot be overcome by the simultaneous overexpression of Gaa1p. Most GPI8 alleles mutated in other conserved regions of the protein can complement the growth defect of Deltagpi8, but nevertheless accumulate CP2. CP2 accumulation, a delay in Gas1p maturation and a slowing of cell growth can also be observed when Gpi8p is depleted to 50% of its normal level in wild-type cells. The dominant negative effect of nonfunctional and partially functional mutant alleles can best be explained by assuming that Gpi8p works as part of a homo- or heteropolymeric complex.

Sbe2p and sbe22p, two homologous Golgi proteins involved in yeast cell wall formation

The cell wall of fungal cells is important for cell integrity and cell morphogenesis and protects against harmful environmental conditions. The yeast cell wall is a complex structure consisting mainly of mannoproteins, glucan, and chitin. The molecular mechanisms by which the cell wall components are synthesized and transported to the cell surface are poorly understood. We have identified and characterized two homologous yeast proteins, Sbe2p and Sbe22p, through their suppression of a chs5 spa2 mutant strain defective in chitin synthesis and cell morphogenesis. Although sbe2 and sbe22 null mutants are viable, sbe2 sbe22 cells display several phenotypes indicative of defects in cell integrity and cell wall structure. First, sbe2 sbe22 cells display a sorbitol-remediable lysis defect at 37 degrees C and are hypersensitive to SDS and calcofluor. Second, electron microscopic analysis reveals that sbe2 sbe22 cells have an aberrant cell wall structure with a reduced mannoprotein layer. Finally, immunofluorescence experiments reveal that in small-budded cells, sbe2 sbe22 mutants mislocalize Chs3p, a protein involved in chitin synthesis. In addition, sbe2 sbe22 diploids have a bud-site selection defect, displaying a random budding pattern. A Sbe2p-GFP fusion protein localizes to cytoplasmic patches, and Sbe2p cofractionates with Golgi proteins. Deletion of CHS5, which encodes a Golgi protein involved in the transport of Chs3p to the cell periphery, is lethal in combination with disruption of SBE2 and SBE22. Thus, we suggest a model in which Sbe2p and Sbe22p are involved in the transport of cell wall components from the Golgi apparatus to the cell surface periphery in a pathway independent of Chs5p.

beta-1,6-Glucan synthesis in Saccharomyces cerevisiae

beta-1,6-Glucan is an essential fungal-specific component of the Saccharomyces cerevisiae cell wall that interconnects all other wall components into a lattice. Considerable biochemical and genetic effort has been directed at the identification and characterization of the steps involved in its biosynthesis. Structural studies show that the polymer plays a central role in wall structure, attaching mannoproteins via their glycosylphosphatidylinositol (GPI) glycan remnant to beta-1,3-glucan and chitin. Genetic approaches have identified genes that upon disruption result in beta-1,6-glucan defects of varying severity, often with reduced growth or lethality. These gene products have been localized throughout the secretory pathway and at the cell surface, suggesting a possible biosynthetic route. Current structural and genetic data have therefore allowed the development of models to predict biosynthetic events. Based on knowledge of beta-1,3-glucan and chitin synthesis, it is likely that the bulk of beta-1,6-glucan polymer synthesis occurs at the cell surface, but requires key prior intracellular events. However, the activity of most of the identified gene products remain unknown, making it unclear to what extent and how directly they contribute to the synthesis of this polymer. With the recent availability of new tools, reagents and methods (including genomics), the field is poised for a convergence of biochemical and genetic methods to identify and characterize the biochemical steps in the synthesis of this polymer.

At acidic pH, the diminished hypoxic expression of the SRP1/TIR1 yeast gene depends on the GPA2-cAMP and HOG pathways

The hypoxic SRP1/TIR1 gene encodes a stress-response cell wall mannoprotein, which is shown to be necessary for yeast growth at acidic pH in the presence of sodium dodecyl sulfate. However, the hypoxic expression of SRP1 is shown to be downregulated at acidic pH. The stress-responsive HOG pathway appeared necessary to maintain hypoxic SRP1 expression, but only at acidic pH. However, unlike known HOG pathway-dependent genes, SRP1 was under positive cAMP control and was positively modulated by protein kinase A at neutral and acidic pH. In addition, the HOG-independent hypoxic HEM13 gene was also positively regulated by cAMP levels. Therefore, the positive cAMP control of the hypoxic SRP1 and HEM13 genes was uncoupled from the HOG pathway. Surprisingly, this positive cAMP control was found to be mediated by GPA2 but not by RAS2, so the Gpa2p requirement appears critical at acidic pH. Although RAS2 is not involved in the regulation of SRP1 expression, the guanine nucleotide exchange factor Cdc25, which is known to control the GTP/GDP ratio on the Ras proteins, was nevertheless required for hypoxic SRP1 expression. Furthermore, the Ras proteins did not compensate for Gpa2p requirement in a delta gpa2 mutated strain. These results suggest that the Cdc25 factor might also control Gpa2p.

Molecular analysis of the Candida albicans homolog of Saccharomyces cerevisiae MNN9, required for glycosylation of cell wall mannoproteins

The fungal cell wall has generated interest as a potential target for developing antifungal drugs, and the genes encoding glucan and chitin in fungal pathogens have been studied to this end. Mannoproteins, the third major component of the cell wall, contain mannose in either O- or N-glycosidic linkages. Here we describe the molecular analysis of the Candida albicans homolog of Saccharomyces cerevisiae MNN9, a gene required for the synthesis of N-linked outer-chain mannan in yeast, and the phenotypes associated with its disruption. CaMNN9 has significant homology with S. cerevisiae MNN9, including a putative N-terminal transmembrane domain, and represents a member of a similar gene family in Candida. CaMNN9 resides on chromosome 3 and is expressed at similar levels in both yeast and hyphal cells. Disruption of both copies of CaMNN9 leads to phenotypic effects characteristic of cell wall defects including poor growth in liquid media and on solid media, formation of aggregates in liquid culture, osmotic sensitivity, aberrant hyphal formation, and increased sensitivity to lysis after treatment with beta-1,3-glucanase. Like all members of the S. cerevisiae MNN9 gene family the Camnn9Delta strain is resistant to sodium orthovanadate and sensitive to hygromycin B. Analysis of cell wall-associated carbohydrates showed the Camnn9Delta strain to contain half the amount of mannan present in cell walls derived from the wild-type parent strain. Reverse transcription-PCR and Northern analysis of the expression of MNN9 gene family members CaVAN1 and CaANP1 in the Camnn9Delta strain showed that transcription of those genes is not affected in the absence of CaMNN9 transcription. Our results suggest that, while the role MNN9 plays in glycosylation in both Candida and Saccharomyces is conserved, loss of MNN9 function in C. albicans leads to phenotypes that are inconsistent with the pathogenicity of the organism and thus identify CaMnn9p as a potential drug target.

PHR1 and PHR2 of Candida albicans encode putative glycosidases required for proper cross-linking of beta-1,3- and beta-1,6-glucans

PHR1 and PHR2 encode putative glycosylphosphatidylinositol-anchored cell surface proteins of the opportunistic fungal pathogen Candida albicans. These proteins are functionally related, and their expression is modulated in relation to the pH of the ambient environment in vitro and in vivo. Deletion of either gene results in a pH-conditional defect in cell morphology and virulence. Multiple sequence alignments demonstrated a distant relationship between the Phr proteins and beta-galactosidases. Based on this alignment, site-directed mutagenesis of the putative active-site residues of Phr1p and Phr2p was conducted and two conserved glutamate residues were shown to be essential for activity. By taking advantage of the pH-conditional expression of the genes, a temporal analysis of cell wall changes was performed following a shift of the mutants from permissive to nonpermissive pH. The mutations did not grossly affect the amount of polysaccharides in the wall but did alter their distribution. The most immediate alteration to occur was a fivefold increase in the rate of cross-linking between beta-1,6-glycosylated mannoproteins and chitin. This increase was followed shortly thereafter by a decline in beta-1,3-glucan-associated beta-1, 6-glucans and, within several generations, a fivefold increase in the chitin content of the walls. The increased accumulation of chitin-linked glucans was not due to a block in subsequent processing as determined by pulse-chase analysis. Rather, the results suggest that the glucans are diverted to chitin linkage due to the inability of the mutants to establish cross-links between beta-1,6- and beta-1,3-glucans. Based on these and previously published results, it is suggested that the Phr proteins process beta-1,3-glucans and make available acceptor sites for the attachment of beta-1,6-glucans.

A constitutive role for GPI anchors in Saccharomyces cerevisiae: cell wall targeting

GPI anchors are widely represented among organisms and have several cellular functions. It has been proposed that in yeast there are two groups of GPI proteins: plasma membrane-resident proteins, such as Gas1p or Yap3p, and cell wall-targeted proteins, such as Tir1p or alpha-agglutinin. A model has been proposed for the plasma membrane retention of proteins from the first group because of a dibasic motif located just upstream of the GPI-anchoring signal. The results we report here are not in agreement with such a model as we show that constructs containing the C-terminal parts of Gas1p and Yap3p are also targeted to the cell wall. We also detect the genuine Gas1p after cell wall treatment with Quantazyme or Glucanex glycanases. In addition, we show that the GPI-anchoring signal from the human placental alkaline phosphatase (PLAP) is not compatible with the yeast machinery unless the human transamidase hGpi8p is co-expressed. In this condition, this human signal is able to target a protein to the cell wall. Moreover, TIR1 proved to be a multicopy suppressor of Deltagas1 mutation. The present findings suggest a constitutive role for GPI anchors in yeast: the cell wall targeting of proteins.

Identification of two mannoproteins released from cell walls of a Saccharomyces cerevisiae mnn1 mnn9 double mutant by reducing agents

In this report, we present the identification of the main polypeptides that are extracted from purified cell walls of a Saccharomyces cerevisiae mnn1 mnn9 strain by reducing agents. Treatment of the purified cell walls of this strain with beta-mercaptoethanol releases several mannoproteins, of which three, with apparent sizes of 120, 45, and 40 kDa, are the most abundant. Analysis of the amino-terminal sequences revealed that the 120-kDa mannoprotein is Bar1p, the protease involved in the so-called barrier activity in yeast cells, and that the 45- and 40-kDa mannoproteins are the Kex2-unprocessed and Kex2-processed forms of the gene product of open reading frame (ORF) YJL158c, an ORF that belongs to the PIR (protein with internal repeats) family of genes, composed thus far of PIR1, PIR2/HSP150, and PIR3. Accordingly we have named this gene PIR4, and Pir4 denotes the 40-kDa Kex2-processed form of the mannoprotein. We have characterized Pir4 and have shown the feasibility of using it as a fusion partner for the targeting of recombinant proteins to the cell wall.

Amino acid residues in the omega-minus region participate in cellular localization of yeast glycosylphosphatidylinositol-attached proteins

The final destination of glycosylphosphatidylinositol (GPI)-attached proteins in Saccharomyces cerevisiae is the plasma membrane or the cell wall. Two kinds of signals have been proposed for their cellular localization: (i) the specific amino acid residues V, I, or L at the site 4 or 5 amino acids upstream of the GPI attachment site (the omega site) and Y or N at the site 2 amino acids upstream of the omega site for cell wall localization and (ii) dibasic residues in the region upstream of the omega site (the omega-minus region) for plasma membrane localization. The relationships between these amino acid residues and efficiencies of cell wall incorporation were examined by constructing fusion reporter proteins from open reading frames encoding putative GPI-attached proteins. The levels of incorporation were high in the constructs containing the specific amino acid residues and quite low in those containing two basic amino acid residues in the omega-minus region. With constructs that contained neither specific residues nor two basic residues, levels of incorporation were moderate. These correlations clearly suggest that GPI-attached proteins have two different signals which act positively or negatively in cell wall incorporation for their cellular localization.

Deletion of new covalently linked cell wall glycoproteins alters the electrophoretic mobility of phosphorylated wall components of Saccharomyces cerevisiae

The incorporation of radioactive orthophosphate into the cell walls of Saccharomyces cerevisiae was studied. 33P-labeled cell walls were extensively extracted with hot sodium dodecyl sulfate (SDS). Of the remaining insoluble radioactivity more than 90% could be released by laminarinase. This radioactive material stayed in the stacking gel during SDS-polyacrylamide gel electrophoresis but entered the separating gel upon treatment with N-glycosidase F, indicating that phosphate was linked directly or indirectly to N-mannosylated glycoproteins. The phosphate was bound to covalently linked cell wall proteins as mannose-6-phosphate, the same type of linkage shown previously for soluble mannoproteins (L. Ballou, L. M. Hernandez, E. Alvarado, and C. E. Ballou, Proc. Natl. Acad. Sci. USA 87:3368-3372, 1990). From the phosphate-labeled glycoprotein fraction released by laminarinase, three cell wall mannoproteins, Ccw12p, Ccw13p and Ccw14p, were isolated and identified by N-terminal sequencing. For Ccw13p (encoded by DAN1 [also called TIR3]) and Ccw12p the association with the cell wall has not been described before; Ccw14p is identical with cell wall protein Icwp (I. Moukadiri, J. Armero, A. Abad, R. Sentandreu, and J. Zueco, J. Bacteriol. 179:2154-2162, 1997). In ccw12, ccw13, or ccw14 single or double mutants neither the amount of radioactive phosphate incorporated into cell wall proteins nor its position in the stacking gel was changed. However, the triple mutant brought about a shift of the 33P-labeled glycoprotein components from the stacking gel into the separating gel. The disruption of CCW12 results in a pronounced sensitivity of the cells to calcofluor white and Congo red. In addition, the ccw12 mutant shows a decrease in mating efficiency and a defect in agglutination.

Detection of cell wall mannoprotein Mp1p in culture supernatants of Penicillium marneffei and in sera of penicilliosis patients

Mannoproteins are important and abundant structural components of fungal cell walls. The MP1 gene encodes a cell wall mannoprotein of the pathogenic fungus Penicillium marneffei. In the present study, we show that Mp1p is secreted into the cell culture supernatant at a level that can be detected by Western blotting. A sensitive enzyme-linked immunosorbent assay (ELISA) developed with antibodies against Mp1p was capable of detecting this protein from the cell culture supernatant of P. marneffei at 10(4) cells/ml. The anti-Mp1p antibody is specific since it fails to react with any protein-form lysates of Candida albicans, Histoplasma capsulatum, or Cryptococcus neoformans by Western blotting. In addition, this Mp1p antigen-based ELISA is also specific for P. marneffei since the cell culture supernatants of the other three fungi gave negative results. Finally, a clinical evaluation of sera from penicilliosis patients indicates that 17 of 26 (65%) patients are Mp1p antigen test positive. Furthermore, a Mp1p antibody test was performed with these serum specimens. The combined antibody and antigen tests for P. marneffei carry a sensitive of 88% (23 of 26), with a positive predictive value of 100% and a negative predictive value of 96%. The specificities of the tests are high since none of the 85 control sera was positive by either test.

Yarrowia lipolytica cell wall architecture: interaction of Ywp1, a mycelial protein, with other wall components and the effect of its depletion

Linkages of Ywp1 to other components of the Yarrowia lipolytica mycelial cell wall were studied by extraction with beta-mercaptoethanol and zymolyase (a beta-glucanase complex) and by the use of rabbit polyclonal antibody preparation raised against Ywp1. Ywp1 complexed with an N-glycosylated cell wall protein(s) to form supramolecular complexes through disulphide bridges (extractable with beta-mercaptoethanol) or bonded to beta-1,3-glucan (extractable with zymolyase). The lack of a specific morphological phenotype when YWP1 was knocked out by gene disruption might indicate that other proteins present in the cell wall of Y. lipolytica compensated for its loss. In this mutant, the electrophoretic pattern of proteins, detected with polyclonal antibodies against the entire cell wall, was different from that obtained with the parental strain, but sensitivity to calcofluor white, zymolyase and chitinase did not change. Quantitative analysis of fluorescence emitted by cells in the presence of fluorescent wheat germ agglutinin (FITC-WGA) indicated that chitin was organized in the cell wall of the mutant cells in a form different from that in the parental strain.

MCD4 encodes a conserved endoplasmic reticulum membrane protein essential for glycosylphosphatidylinositol anchor synthesis in yeast

Glycosylphosphatidylinositol (GPI)-anchored proteins are cell surface-localized proteins that serve many important cellular functions. The pathway mediating synthesis and attachment of the GPI anchor to these proteins in eukaryotic cells is complex, highly conserved, and plays a critical role in the proper targeting, transport, and function of all GPI-anchored protein family members. In this article, we demonstrate that MCD4, an essential gene that was initially identified in a genetic screen to isolate Saccharomyces cerevisiae mutants defective for bud emergence, encodes a previously unidentified component of the GPI anchor synthesis pathway. Mcd4p is a multimembrane-spanning protein that localizes to the endoplasmic reticulum (ER) and contains a large NH2-terminal ER lumenal domain. We have also cloned the human MCD4 gene and found that Mcd4p is both highly conserved throughout eukaryotes and has two yeast homologues. Mcd4p's lumenal domain contains three conserved motifs found in mammalian phosphodiesterases and nucleotide pyrophosphases; notably, the temperature-conditional MCD4 allele used for our studies (mcd4-174) harbors a single amino acid change in motif 2. The mcd4-174 mutant (1) is defective in ER-to-Golgi transport of GPI-anchored proteins (i.e., Gas1p) while other proteins (i.e., CPY) are unaffected; (2) secretes and releases (potentially up-regulated cell wall) proteins into the medium, suggesting a defect in cell wall integrity; and (3) exhibits marked morphological defects, most notably the accumulation of distorted, ER- and vesicle-like membranes. mcd4-174 cells synthesize all classes of inositolphosphoceramides, indicating that the GPI protein transport block is not due to deficient ceramide synthesis. However, mcd4-174 cells have a severe defect in incorporation of [3H]inositol into proteins and accumulate several previously uncharacterized [3H]inositol-labeled lipids whose properties are consistent with their being GPI precursors. Together, these studies demonstrate that MCD4 encodes a new, conserved component of the GPI anchor synthesis pathway and highlight the intimate connections between GPI anchoring, bud emergence, cell wall function, and feedback mechanisms likely to be involved in regulating each of these essential processes. A putative role for Mcd4p as participating in the modification of GPI anchors with side chain phosphoethanolamine is also discussed.

The contribution of the O-glycosylated protein Pir2p/Hsp150 to the construction of the yeast cell wall in wild-type cells and beta 1,6-glucan-deficient mutants

The cell wall of yeast contains a major structural unit, consisting of a cell wall protein (CWP) attached via a glycosylphosphatidylinositol (GPI)-derived structure to beta 1,6-glucan, which is linked in turn to beta 1, 3-glucan. When isolated cells walls were digested with beta 1,6-glucanase, 16% of all CWPs remained insoluble, suggesting an alternative linkage between CWPs and structural cell wall components that does not involve beta 1,6-glucan. The beta 1,6-glucanase-resistant protein fraction contained the recently identified GPI-lacking, O-glycosylated Pir-CWPs, including Pir2p/Hsp150. Evidence is presented that Pir2p/Hsp150 is attached to beta 1,3-glucan through an alkali-sensitive linkage, without beta 1,6-glucan as an interconnecting moiety. In beta 1,6-glucan-deficient mutants, the beta 1,6-glucanase-resistant protein fraction increased from 16% to over 80%. This was accompanied by increased incorporation of Pir2p/Hsp150. It is argued that this is part of a more general compensatory mechanism in response to cell wall weakening caused by low levels of beta 1,6-glucan.