Cell surface anchorage and ligand-binding domains of the Saccharomyces cerevisiae cell adhesion protein alpha-agglutinin, a member of the immunoglobulin superfamily

The Candida albicans HYR1 gene, which is activated in response to hyphal development, belongs to a gene family encoding yeast cell wall proteins

A hyphally regulated gene (HYR1) from the dimorphic human pathogenic fungus Candida albicans was isolated and characterized. Northern (RNA) analyses showed that the HYR1 mRNA was induced specifically in response to hyphal development when morphogenesis was stimulated by serum addition and temperature elevation, increases in both culture pH and temperature, or N-acetylglucosamine addition. The HYR1 gene sequence revealed a 937-codon open reading frame capable of encoding a protein with an N-terminal signal sequence, a C-terminal glycosylphosphatidylinositol-anchoring domain, 17 potential N glycosylation sites, and a large domain rich in serine and threonine (51% of 230 residues). These features are observed in many yeast cell wall proteins, but no homologs are present in the databases. In addition, Hyr1p contained a second domain rich in glycine, serine, and asparagine (79% of 239 residues). The HYR1 locus in C. albicans CAI4 was disrupted by "Ura-blasting," but the resulting homozygous delta hyr1/delta hyr1 null mutant displayed no obvious morphological phenotype. The growth rates for yeast cells and hyphae and the kinetics of germ tube formation in the null mutant were unaffected. Aberrant expression of HYR1 in yeast cells, when an ADH1-HYR1 fusion was used, did not stimulate hyphal formation in C. albicans or pseudohyphal growth in Saccharomyces cerevisiae. HYR1 appears to encode a nonessential component of the hyphal cell wall.

Regulation by low temperatures and anaerobiosis of a yeast gene specifying a putative GPI-anchored plasma membrane protein [corrected]

Expression of the yeast Saccharomyces cerevisiae SRP1 (Serine-rich Protein) gene is shown here to be induced both by low temperature and anaerobic growth conditions. We show that anaerobic SRP1 expression is haem-dependent; however, haem influence does not operate through the action of the hypoxic-gene ROX1 repressor. The SRP1 promoter region displaying the stress-responsive elements is restricted to its first 551 bp, upstream of the initiation codon, although an upstream activation site contained in upstream sequences is required for full promoter activity. In addition, we demonstrate that the TIP1 gene, sharing similar nucleotide and polypeptide structure with SRP1, and previously reported to be a cold-shock-inducible gene, is also a hypoxic gene. Srp1 protein production is similarly induced by low temperature and anaerobic growth conditions. This protein, detected in the plasma membrane fraction, is shown to be exposed on the cell surface via a glycosyl-phosphatidylinositol membrane anchoring.

Developmental expression of a tandemly repeated, proline-and glutamine-rich amino acid motif on hyphal surfaces on Candida albicans

cDNA sequences encoding a cell wall protein have been isolated from the opportunistic pathogen, Candida albicans, an organism that can cause serious disease in immunocompromised patients such as those with AIDS. The cDNA encodes a peptide that is largely composed of an acidic, repeated motif 10 amino acids in length that is rich in proline and glutamine residues. The cDNA gene product was found to be present on hyphal surfaces by immunofluorescence assays using monospecific antisera raised to the recombinant protein produced in Pichia pastoris. The hyphae-specific surface location was also seen on organisms colonizing the gastrointestinal mucosa of mice, indicating that the antigen is produced and developmentally regulated during growth in host tissues. The cDNA clone hybridized to an abundant messenger RNA 2.3 kilobases in size that was present in hyphal but not yeast forms. These studies demonstrate that the bud-hypha transition is accompanied by the de novo synthesis of proteins that are targeted to hyphal surfaces. The primary sequence of the unique amino acid motif shares features with surface proteins of other lower eukaryotic microorganisms and with host acidic salivary proline-rich proteins.

Yeast Kre1p is a cell surface O-glycoprotein

The Saccharomyces cerevisiae KRE1 gene encodes a secretory protein required for the production of the cell wall polymer (1-->6)-beta-glucan. Here we report further characterization of the KRE1 gene product, Kre1p. A functional, epitope-tagged Kre1p is shown to be highly modified in a SEC53-dependent manner. Kre1p is O-glycosylated, but the basis for the majority of its post-translational modification is unknown. Fractionation of Kre1p reveals a cell wall-associated form and a less abundant membrane-associated species. Indirect immunoflurorescence demonstrates that Kre1p localizes to the cell surface, where it becomes concentrated at the surface of mother cells. Such a localization of Kre1p seems to parallel the CAL1/CSD2-dependent cell wall deposition of chitin found in S. cerevisiae, and is consistent with evidence from Schizophyllum commune that (1-->6)-beta-glucan accumulates during maturation of the subapical region of the wall distal to the hyphal tip.

Structure of Saccharomyces cerevisiae alpha-agglutinin. Evidence for a yeast cell wall protein with multiple immunoglobulin-like domains with atypical disulfides

alpha-Agglutinin of Saccharomyces cerevisiae is a cell wall-associated protein that mediates cell interaction in mating. Although the mature protein includes about 610 residues, the NH2-terminal half of the protein is sufficient for binding to its ligand a-agglutinin. alpha-Agglutinin20-351, a fully active fragment of the protein, has been purified and analyzed. Circular dichroism spectroscopy, together with sequence alignments, suggest that alpha-agglutinin20-351 consists of three immunoglobulin variable-like domains: domain I, residues 20-104; domain II, residues 105-199; and domain III, residues 200-326. Peptide sequencing data established the arrangement of the disulfide bonds in alpha-agglutinin20-351. Cys97 is disulfide-bonded to Cys114, forming an interdomain bond between domains I and II. Cys202 is bonded to Cys300, in an atypical intradomain disulfide bond between the A and F strands of domain III. Cys227 and Cys256 have free sulfhydryls. Sequencing also showed that at least two of three potential N-glycosylation sites with sequence Asn-Xaa-Thr are glycosylated. At least one of three Asn-Xaa-Ser sequences is not glycosylated. No residues NH2-terminal to Ser282 were O-glycosylated, whereas Ser282, and all hydroxy amino acid residues COOH-terminal to this position were modified. Therefore O-glycosylated Ser and Thr residues cluster in the COOH-terminal region of domain III, and the O-glycosylation continues into a Ser/Thr-rich sequence that extends from domain III to the COOH-terminal of the full-length protein.

Homology modeling of an immunoglobulin-like domain in the Saccharomyces cerevisiae adhesion protein alpha-agglutinin

The Saccharomyces cerevisiae adhesion protein alpha-agglutinin is expressed by cells of alpha mating type. On the basis of sequence similarities, alpha-agglutinin has been proposed to contain variable-type immunoglobulin-like (IgV) domains. The low level of sequence similarity to IgV domains of known structure made homology modeling using standard sequence-based alignment algorithms impossible. We have therefore developed a secondary structure-based method that allowed homology modeling of alpha-aggulutinin domain III, the domain most similar to IgV domains. The model was assessed and where necessary refined to accommodate information obtained by biochemical and molecular genetic approaches, including the positions of a disulfide bond, glycosylation sites, and proteolytic sites. The model successfully predicted surface exposure of glycosylation and proteolytic sites, as well as identifying residues essential for binding activity. One side of the domain was predicted to be covered by carbohydrate residues. Surface accessibility and volume packing analyses showed that the regions of the model that have greatest sequence dissimilarity from the IgV consensus sequence are poorly structured in the biophysical sense. Nonetheless, the utility of the model suggests that these alignment and testing techniques should be of general use for building and testing of models of proteins that share limited sequence similarity with known structures.

The yeast proprotein convertase encoded by YAP3 is a glycophosphatidylinositol-anchored protein that localizes to the plasma membrane

The yeast YAP3 gene encodes an aspartyl endoprotease that cleaves precursor proteins at selected pairs of basic amino acids and after single arginine residues. Biosynthetic studies of this proprotein processing enzyme indicate that Yap3 is predominantly cell-associated and migrates as a approximately 160-kDa protein on SDS-polyacrylamide gel electrophoresis. Nearly equal amounts of Yap3 are immunodetected in a-haploid, alpha-haploid, and a/alpha-diploid yeast, demonstrating that the expression of YAP3 is not mating type-specific. As shown by endoglycosidase H treatment, which drastically reduces both the estimated molecular mass and the heterogeneity of the protein on SDS-polyacrylamide gel electrophoresis (68 versus 160 kDa), the oligosaccharides N-linked to the protein are subjected to extensive outer chain mannosylation. Outer chain sugar mannosylation takes place in the Golgi apparatus and is commonly found on yeast secreted glycoproteins and/or cell wall mannoproteins. Treatment of the total yeast membranes with chemical agents known to disrupt protein-protein and protein-lipid interactions reveal that Yap3 is membrane-associated. Based upon the release of the membrane-bound form by bacterial phosphatidylinositol phospholipase C digestion and metabolic labeling of the protein with myo-[3H]inositol, Yap3 owes its association with the membrane to the addition of a glycophosphatidylinositol anchor. The cellular localization of Yap3 has been addressed by subcellular fractionation studies. In both differential centrifugation of intracellular organelles and sucrose density gradients, the bulk of Yap3 at steady state co-localizes with the plasma membrane azide-insensitive ATPase. Furthermore, consistent with the transport of Yap3 to the plasma membrane, the endoprotease sediments with secretory vesicles which accumulate at restrictive temperature in the late secretory mutant sec1-1. We therefore conclude that the endoprotease encoded by YAP3 is a glycophosphatidylinositol-anchored protein, which can process substrates both intracellularly and at the cell surface.

Genetics of a-agglutunin function in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cell adhesion protein a-agglutinin is composed of an anchorage subunit (Aga1p) and an adhesion subunit (Aga2p). Although functional a-agglutinin is expressed only by a cells, previous results indicated that AGA1 RNA is expressed in both a and alpha cells after pheromone induction. Expression of the Aga2p adhesion subunit in alpha cells allowed a-agglutinability, indicating that alpha cells express the a-agglutinin anchorage subunit, although no role for Aga1p in alpha cells has been identified. Most of the a-specific agglutination-defective mutants isolated previously were defective in AGA1; a single mutant (La199) was a candidate for an aga2 mutant. Expression of AGA2 under PGK control allowed secretion of active Aga2p from control strains but did not complement the La199 agglutination defect or allow secretion of Aga2p from La199, suggesting that the La199 mutation might identify a new gene required for a-agglutinin function. However, the La199 agglutination defect showed tight linkage to aga2::URA3 and did not complement aga2::URA3 in a/a diploids. The aga2 gene cloned from La199 was nonfunctional and contained an ochre mutation. The inability of pPGK-AGA2 to express functional Aga2p in La199 was shown to result from an additional mutation(s) that reduces expression of plasmid-borne genes. AGA2 was mapped to the left arm of chromosome VII approximately 28 cM from the centromere.

Identification of SPT14/CWH6 as the yeast homologue of hPIG-A, a gene involved in the biosynthesis of GPI anchors

Cwh6 is a temperature-sensitive cell wall mutant of Saccharomyces cerevisiae. CWH6 was found to be identical to SPT14, a gene that is highly homologous to both human PIG-A and to RFAK from Salmonella typhimurium. PIG-A and RFAK are involved in transferring N-acetylglucosamine to, respectively, a GPI anchor precursor and to lipopolysaccharides. Because cell walls of cwh6 are greatly reduced in mannose, and because some cell wall proteins are known to be incorporated into the cell wall through a GPI-anchor dependent mechanism, we propose that Spt14p/Cwh6p is involved in transferring N-acetylglucosamine to a precursor of GPI anchors. We further propose that the majority of cell wall proteins are incorporated into the cell wall through a GPI anchor.

Candida albicans ALS1: domains related to a Saccharomyces cerevisiae sexual agglutinin separated by a repeating motif

Transfer of budding Candida albicans yeast cells from the rich, complex medium YEPD to the defined tissue culture medium RPMI 1640 (RPMI) at 37 degrees C and 5% CO2 causes rapid onset of hyphal induction. Among the genes induced under these conditions are hyphal-specific genes as well as genes expressed in response to changes in temperature, CO2 and specific media components. A cDNA library constructed from cells incubated for 20 min in RPMI was differentially screened with yeast (YEPD)- and hyphal (RPMI)-specific probes resulting in identification of a gene expressed in response to culture conditions but not regulated by the yeast-hyphal transition. The deduced gene product displays significant identity to Saccharomyces cerevisiae alpha-agglutinin, encoded by AG alpha 1, an adhesion glycoprotein that mediates mating of haploid cells. The presence of this gene in C. albicans is curious since the organism has not been observed to undergo meiosis. We designate the C. albicans gene ALS1 (for agglutinin-like sequence). While the N- and C-termini of the predicted 1260-amino-acid ALS1 protein resemble those of the 650-amino-acid AG alpha 1, ALS1 contains a central domain of tandem repeats consisting of a highly conserved 36-amino-acid sequence not present in AG alpha 1. These repeats are also present on the nucleotide level as a highly conserved 108 bp motif. Southern and Northern blot analyses indicate a family of C. albicans genes that contain the tandem repeat motif; at least one gene in addition to ALS1 is expressed under conditions similar to those for ALS1 expression. Genomic Southern blots from several C. albicans isolates indicate that the number of copies of the tandem repeat element in ALS1 differs across strains and, in some cases, between ALS1 alleles in the same strain, suggesting a strain-dependent variability in ALS1 protein size. Potential roles for the ALS1 protein are discussed.

A new approach for isolating cell wall mutants in Saccharomyces cerevisiae by screening for hypersensitivity to calcofluor white

To study cell wall assembly, a simple screening method was devised for isolating cell wall mutants. Mutagenized cells were screened for hypersensitivity to Calcofluor White, which interferes with cell wall assembly. The rationale is that Calcofluor White amplifies the effect of cell wall mutations. As a result, the cells stop growing at lower concentrations of Calcofluor White than cells with normal cell wall. In this way, 63 Calcofluor White-hypersensitive (cwh), monogenic mutants were obtained, ordered into 53 complementation groups. The mannose/glucose ratios of the mutant cell walls varied from 0.15 to 3.95, while wild-type cell walls contained about equal amounts of mannose and glucose. This indicates that both low-mannose and low-glucose cell wall mutants had been obtained. Further characterization showed the presence of three low-mannose cell wall mutants with a mnn9-like phenotype, affected, however, in different genes. In addition, four new killer-resistant (kre) mutants were found, which are presumably affected in the synthesis of beta 1,6-glucan. Most low-glucose cell wall mutants were not killer resistant, indicating that they might be defective in the synthesis of beta 1,3-glucan. Eleven cwh mutants were found to be hypersensitive to papulacandin B, which is known to interfere with beta 1,3-glucan synthesis, and four cwh mutants were temperature-sensitive and lysed at the restrictive temperature. Finally, nine cwh mutants were hypersensitive to caffeine, suggesting that these were affected in signal transduction related to cell wall assembly.

Glucosylation of chimeric proteins in the cell wall of Saccharomyces cerevisiae

Extension of a reporter protein with the carboxyterminal thirty amino acids of the cell wall mannoprotein alpha-agglutinin of Saccharomyces cerevisiae resulted in incorporation of the chimeric protein in the cell wall. By Western analysis it was shown that the incorporated protein contained beta-1,6-glucan similar to endogenous cell wall proteins, whereas excreted reporter protein was not glucosylated. This suggests that beta-1,6-glucan is involved in anchoring mannoproteins in the cell wall.

Is there a role for GPIs in yeast cell-wall assembly?

Glycosylphosphatidylinositol (GPI) membrane anchors are essential for the integration of yeast cell adhesion proteins into the cell wall, but mature cell-wall proteins are unlikely to be attached directly to the membrane. We thus propose that GPI-anchored glycoprotein forms are intermediates in a process that crosslinks the major components of the cell wall by transglycosylation. This mechanism may be critical for both the biosynthesis and overall architecture of the cell wall.