Isolation and composition of the constitutive agglutinins from haploid Saccharomyces cerevisiae cells

Delineation of functional regions within the subunits of the Saccharomyces cerevisiae cell adhesion molecule a-agglutinin

a-Agglutinin from Saccharomyces cerevisiae is a cell adhesion glycoprotein expressed on the surface of cells of a mating type and consists of an anchorage subunit Aga1p and a receptor binding subunit Aga2p. Cell wall attachment of Aga2p is mediated through two disulfide bonds to Aga1p (Cappellaro, C., Baldermann, C., Rachel, R., and Tanner, W. (1994) EMBO J. 13, 4737-4744). We report here that purified Aga2p was unstable and had low molar specific activity relative to its receptor alpha-agglutinin. Aga2p co-expressed with a 149-residue fragment of Aga1p formed a disulfide-linked complex with specific activity 43-fold higher than Aga2p expressed alone. Circular dichroism of the complex revealed a mixed alpha/beta structure, whereas Aga2p alone had no periodic secondary structure. A 30-residue Cys-rich Aga1p fragment was partially active in stabilization of Aga2p activity. Mutation of either or both Aga2p cysteine residues eliminated stabilization of Aga2p. Thus the roles of Aga1p include both cell wall anchorage and cysteine-dependent conformational restriction of the binding subunit Aga2p. Mutagenesis of AGA2 identified only C-terminal residues of Aga2p as being essential for binding activity. Aga2p residues 45-72 are similar to sequences in soybean Nod genes, and include residues implicated in interactions with both Aga1p (including Cys(68)) and alpha-agglutinin.

Saccharomyces cerevisiae mannoproteins that protect wine from protein haze: their release during fermentation and lees contact and a proposal for their mechanism of action

A fraction containing the mannoproteins released during fermentation from the winemaking strain of Saccharomyces cerevisiae, Maurivin PDM, was able to reduce the visible protein haze in white wine. This fraction of haze protective mannoprotein material (HPM) could be recovered by either ultrafiltration or ethanol precipitation. The kinetics of the release of both mannose- and glucose-containing polymers during the growth cycle of PDM were determined as a guide to the release of HPM. Active HPM was first detected in the culture supernatant when the cells were exponentially growing. HPM was also released into the medium under an environment simulating winemaking conditions by PDM cells during fermentation as well as during storage on yeast lees. Since the amounts of HPM released during fermentation are greater than those subsequently extracted from the cell wall, fermentation would be a more viable procedure than extraction from yeast cells for the commercial production of HPM. Yeast invertase, a mannoprotein with haze protective activity, was used as a model substrate to investigate the mechanism of haze protection. Invertase was found to reduce visible turbidity but not prevent protein precipitation. Invertase itself did not precipitate but remained soluble in the wine. On the basis of these observations, we propose that the mechanism of haze protection may be one of competition between HPM and wine proteins for unknown wine component(s), the latter being required for the formation of large insoluble aggregates of denatured protein. As the available concentration of these components decreases, due to the presence of HPM, the particle size of the haze decreases and thus visible turbidity declines.

Isolation and characterization of an antifreeze protein with ice nucleation activity from the plant growth promoting rhizobacterium Pseudomonas putida GR12-2

An antifreeze protein secreted to the growth medium by the plant growth promoting rhizobacterium Pseudomonas putida GR12-2 was purified to apparent homogeneity. The purified protein has a molecular mass of 164 ± 15 kDa and an isoelectric point of 5.3, contains both carbohydrate and lipid moieties, and is relatively rich in glycine and alanine. The properties of the purified antifreeze protein are similar to the properties previously reported for bacterial ice-nucleation proteins. In fact, the purified antifreeze protein also displays a low level of ice-nucleation activity. Removal of approximately 92 kDa of carbohydrate from the 164-kDa antifreeze glycoprotein did not noticeably alter the antifreeze activity of the molecule, although it did diminish the ice-nucleation activity. This is the first report of an antifreeze protein that also is active as an ice-nucleation protein.Key words: antifreeze protein, plant growth promoting rhizobacteria, freezing tolerance, ice-nucleation protein.

Identification of a ligand-binding site in an immunoglobulin fold domain of the Saccharomyces cerevisiae adhesion protein alpha-agglutinin

The Saccharomyces cerevisiae adhesion protein alpha-agglutinin (Ag alpha 1p) is expressed by alpha cells and binds to the complementary a-agglutinin expressed by a cells. The N-terminal half of alpha-agglutinin is sufficient for ligand binding and has been proposed to contain an immunoglobulin (Ig) fold domain. Based on a structural homology model for this domain and a previously identified critical residue (His292), we made Ag alpha 1p mutations in three discontinuous patches of the domain that are predicted to be in close proximity to His292 in the model. Residues in each of the three patches were identified that are important for activity and therefore define a putative ligand binding site, whereas mutations in distant loops had no effect on activity. This putative binding site is on a different surface of the Ig fold than the defined binding sites of immunoglobulins and other members of the Ig superfamily. Comparison of protein interaction sites by structural and mutational analysis has indicated that the area of surface contact is larger than the functional binding site identified by mutagenesis. The putative alpha-agglutinin binding site is therefore likely to identify residues that contribute to the functional binding site within a larger area that contacts a-agglutinin.

A glycosylation pattern that is unique to fimbriae from the taxon Microbotryales

Eukaryotic fimbriae are extracellular structures that have been shown to play a key role in mating and pathogenesis. Previous work has established that the major component of the fimbriae of the anther smut fungus Microbotryum violaceum (Pers.) G. Deml & Oberw. (Ustilago violacea (Pers.) Rouss.) is a 74-kDa glycoprotein subunit. This study analyzes the carbohydrate moiety in the fimbrial subunit and reveals that it is composed of mannose and possibly some acetylated sugars that are anchored to an asparagine residue of the protein via two N-acetylglucosamine residues. The carbohydrate component comprises 35% of the fimbrial subunit and the aglycone has an apparent molecular mass of 47 kDa. Using polyclonal antibodies, we have examined the conservation of the two components of the fimbrial subunits (protein and carbohydrate) in four divisions of fungi. Antibodies raised against native intact fimbriae of M. violaceum detected only the unique glycosylation pattern of these fimbriae, whilst antibodies raised against denatured fimbrial subunits were able to detect protein epitopes. Our results indicate that the fimbrial subunits of M. violaceum are composed of a protein moiety that is highly conserved amongst fungal species and a carbohydrate moiety that has a pattern that is unique to members of the dicot-infecting smut fungi. The possible implications of a conserved glycosylation pattern and the limited host range of this phylogenetic line of smut fungi are discussed in relation to current attempts to reclassify these species into the taxon Microbotryales.Key words: fimbriae, Ustilago, Microbotryum, glycoprotein, antibodies, systematics, basidiomycetes.

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.

A pathway for cell wall anchorage of Saccharomyces cerevisiae alpha-agglutinin

Saccharomyces cerevisiae alpha-agglutinin is a cell wall-anchored adhesion glycoprotein. The previously identified 140-kDa form, which contains a glycosyl-phosphatidylinositol (GPI) anchor (D. Wojciechowicz, C.-F. Lu, J. Kurjan, and P. N. Lipke, Mol. Cell. Biol. 13:2554-2563, 1993), and additional forms of 80, 150, 250 to 300, and > 300 kDa had the properties of intermediates in a transport and cell wall anchorage pathway. N glycosylation and additional modifications resulted in successive increases in size during transport. The 150- and 250- to 300-kDa forms were membrane associated and are likely to be intermediates between the 140-kDa form and a cell surface GPI-anchored form of > 300 kDa. A soluble form of > 300 kDa that lacked the GPI anchor had properties of a periplasmic intermediate between the plasma membrane form and the > 300-kDa cell wall-anchored form. These results constitute experimental support for the hypothesis that GPI anchors act to localize alpha-agglutinin to the plasma membrane and that cell wall anchorage involves release from the GPI anchor to produce a periplasmic intermediate followed by linkage to the cell wall.

A pathway for cell wall anchorage of Saccharomyces cerevisiae alpha-agglutinin

Saccharomyces cerevisiae alpha-agglutinin is a cell wall-anchored adhesion glycoprotein. The previously identified 140-kDa form, which contains a glycosyl-phosphatidylinositol (GPI) anchor (D. Wojciechowicz, C.-F. Lu, J. Kurjan, and P. N. Lipke, Mol. Cell. Biol. 13:2554-2563, 1993), and additional forms of 80, 150, 250 to 300, and > 300 kDa had the properties of intermediates in a transport and cell wall anchorage pathway. N glycosylation and additional modifications resulted in successive increases in size during transport. The 150- and 250- to 300-kDa forms were membrane associated and are likely to be intermediates between the 140-kDa form and a cell surface GPI-anchored form of > 300 kDa. A soluble form of > 300 kDa that lacked the GPI anchor had properties of a periplasmic intermediate between the plasma membrane form and the > 300-kDa cell wall-anchored form. These results constitute experimental support for the hypothesis that GPI anchors act to localize alpha-agglutinin to the plasma membrane and that cell wall anchorage involves release from the GPI anchor to produce a periplasmic intermediate followed by linkage to the cell wall.

Sexual agglutination in budding yeasts: structure, function, and regulation of adhesion glycoproteins

The sexual agglutinins of the budding yeasts are cell adhesion proteins that promote aggregation of cells during mating. In each yeast species, complementary agglutinins are expressed by cells of opposite mating type that interact to mediate aggregation. Saccharomyces cerevisiae alpha-agglutinin and its analogs from other yeasts are single-subunit glycoproteins that contain N-linked and O-linked oligosaccharides. The N-glycosidase-sensitive carbohydrate is not necessary for activity. The proposed binding domain of alpha-agglutinin has features characteristic of the immunoglobulin fold structures of cell adhesion proteins of higher eukaryotes. The C-terminal region of alpha-agglutinin plays a role in anchoring the glycoprotein to the cell surface. The S. cerevisiae alpha-agglutinin and its analogs from other species contain multiple subunits; one or more binding subunits, which interact with the opposite agglutinin, are disulfide bonded to a core subunit, which mediates cell wall anchorage. The core subunits are composed of 80 to 95% O-linked carbohydrate. The binding subunits have less carbohydrate, and both carbohydrate and peptide play roles in binding. The alpha-agglutinin and alpha-agglutinin genes from S. cerevisiae have been cloned and shown to be regulated by the mating-type locus, MAT, and by pheromone induction. The agglutinins are necessary for mating under conditions that do not promote cell-cell contact. The role of the agglutinins therefore is to promote close interactions between cells of opposite mating type and possibly to facilitate the response to phermone, thus increasing the efficiency of mating. We speculate that they mediate enhanced response to sex pheromones by providing a synapse at the point of cell-cell contact, at which both pheromone secretion and cell fusion occur.

The glucanase-soluble mannoproteins limit cell wall porosity in Saccharomyces cerevisiae

The cell wall porosity of batch-grown Saccharomyces cerevisiae was maximal in the early exponential phase and fell off rapidly to lower levels in later growth phases. Treatment of stationary-phase cells with alpha-mannosidase restored wall porosity to the level of cells in early exponential phase. When cells in the early exponential phase were treated with alpha-mannosidase, or tunicamycin, an inhibitor of N-glycosylation, even higher porosities were obtained. Mutants with truncated mannan side-chains in their wall proteins also had very porous walls. The importance of the mannan side-chains for wall porosity was also seen during sexual induction. Treatment with alpha pheromone, which leads to the formation of wall proteins with shorter mannan side-chains, enhanced wall porosity. Disulphide bridges also affect cell wall porosity. They were predominantly found in the glucanase-soluble wall proteins. Because the main part of the mannan side-chains is also found in this family of wall proteins, our results demonstrate that the glucanase-soluble mannoproteins limit cell wall porosity in yeast.

An alpha-specific gene, SAG1 is required for sexual agglutination in Saccharomyces cerevisiae

Seven alpha-specific mutants specifically defective in sexual agglutinability were isolated. The other alpha mating functions exhibited by these mutants, designated sag mutants, such as the production of alpha pheromone and response to a mating pheromone, were normal. While the MAT alpha sag1 cells did not agglutinate with wild-type a cells, the MATa sag1 cells did, indicating that the SAG1 gene is expressed only in alpha cells. The mutations were semi-dominant and fell into a single complementation group, SAG1, which was mapped near met3 on chromosome X. Complementation analysis showed that sag1 and ag alpha 1, the latter being a previously reported alpha-specific mutation, were mutations in the same gene.