Sexual agglutination in Saccharomyces cerevisiae

Single-cell fluidic force microscopy reveals stress-dependent molecular interactions in yeast mating

Sexual agglutinins of the budding yeast Saccharomyces cerevisiae are proteins mediating cell aggregation during mating. Complementary agglutinins expressed by cells of opposite mating types "a" and "α" bind together to promote agglutination and facilitate fusion of haploid cells. By means of an innovative single-cell manipulation assay combining fluidic force microscopy with force spectroscopy, we unravel the strength of single specific bonds between a- and α-agglutinins (~100 pN) which require pheromone induction. Prolonged cell-cell contact strongly increases adhesion between mating cells, likely resulting from an increased expression of agglutinins. In addition, we highlight the critical role of disulfide bonds of the a-agglutinin and of histidine residue H₂₇₃ of α-agglutinin. Most interestingly, we find that mechanical tension enhances the interaction strength, pointing to a model where physical stress induces conformational changes in the agglutinins, from a weak-binding folded state, to a strong-binding extended state. Our single-cell technology shows promises for understanding and controlling the complex mechanism of yeast sexuality.

Cell aggregations in yeasts and their applications

Yeasts can display four types of cellular aggregation: sexual, flocculation, biofilm formation, and filamentous growth. These cell aggregations arise, in some yeast strains, as a response to environmental or physiological changes. Sexual aggregation is part of the yeast mating process, representing the first step of meiotic recombination. The flocculation phenomenon is a calcium-dependent asexual reversible cellular aggregation that allows the yeast to withstand adverse conditions. Biofilm formation consists of multicellular aggregates that adhere to solid surfaces and are embedded in a protein matrix; this gives the yeast strain either the ability to colonize new environments or to survive harsh environmental conditions. Finally, the filamentous growth is the ability of some yeast strains to grow in filament forms. Filamentous growth can be attained by two different means, with the formation of either hyphae or pseudohyphae. Both hyphae and pseudohyphae arise when the yeast strain is under nutrient starvation conditions and they represent a means for the microbial strain to spread over a wide area to survey for food sources, without increasing its biomass. Additionally, this filamentous growth is also responsible for the invasive growth of some yeast.

Conserved WCPL and CX4C domains mediate several mating adhesin interactions in Saccharomyces cerevisiae

Several adhesins are induced by pheromones during mating in Saccharomyces cerevisiae, including Aga1p, Aga2p, Sag1p (Agalpha1p), and Fig2p. These four proteins all participate in or influence a well-studied agglutinin interaction mediated by Aga1p-Aga2p complexes and Sag1p; however, they also play redundant and essential roles in mating via an unknown mechanism. Aga1p and Fig2p both contain repeated, conserved WCPL and CX(4)C domains. This study was directed toward understanding the mechanism underlying the collective requirement of agglutinins and Fig2p for mating. Apart from the well-known agglutinin interaction between Aga2p and Sag1p, three more pairs of interactions in cells of opposite mating type were revealed by this study, including bilateral heterotypic interactions between Aga1p and Fig2p and a homotypic interaction between Fig2p and Fig2p. These four pairs of adhesin interactions are collectively required for maximum mating efficiency and normal zygote morphogenesis. GPI-less, epitope-tagged forms of Aga1p and Fig2p can be co-immunoprecipitated from the culture medium of mating cells in a manner dependent on the WCPL and CX(4)C domains in the R1 repeat of Aga1p. Using site-directed mutagenesis, the conserved residues in Aga1p that interact with Fig2p were identified. Aga1p is involved in two distinct adhesive functions that are independent of each other, which raises the possibility for combinatorial interactions of this protein with its different adhesion receptors, Sag1 and Fig2p, a property of many higher eukaryotic adhesins.

The exchange factor Cdc24 is required for cell fusion during yeast mating

During Saccharomyces cerevisiae mating, chemotropic growth and cell fusion are critical for zygote formation. Cdc24p, the guanine nucleotide exchange factor for the Cdc42 G protein, is necessary for oriented growth along a pheromone gradient during mating. To understand the functions of this critical Cdc42p activator, we identified additional cdc24 mating mutants. Two mating-specific mutants, the cdc24-m5 and cdc24-m6 mutants, each were isolated with a mutated residue in the conserved catalytic domain. The cdc24-m6 mutant responds normally to pheromone and orients its growth towards a mating partner yet accumulates prezygotes during mating. cdc24-m6 prezygotes have two apposed intact cell walls and do not correctly localize proteins required for cell fusion, despite normal exocytosis. Our results indicate that the exchange factor Cdc24p is necessary for maintaining or restricting specific proteins required for cell fusion to the cell contact region during mating.

Posttranslational modifications required for cell surface localization and function of the fungal adhesin Aga1p

Adherence of fungal cells to host substrates and each other affects their access to nutrients, sexual conjugation, and survival in hosts. Adhesins are cell surface proteins that mediate these different cell adhesion interactions. In this study, we examine the in vivo functional requirements for specific posttranslational modifications to these proteins, including glycophosphatidylinositol (GPI) anchor addition and O-linked glycosylation. The processing of some fungal GPI anchors, creating links to cell wall beta-1,6 glucans, is postulated to facilitate postsecretory traffic of proteins to cell wall domains conducive to their functions. By studying the yeast sexual adhesin subunit Aga1p, we found that deletion of its signal sequence for GPI addition eliminated its activity, while deletions of different internal domains had various effects on function. Substitution of the Aga1p GPI signal domain with those of other GPI-anchored proteins, a single transmembrane domain, or a cysteine capable of forming a disulfide all produced functional adhesins. A portion of the cellular pool of Aga1p was determined to be cell wall resident. Aga1p and the alpha-agglutinin Agalpha1p were shown to be under glycosylated in cells lacking the protein mannosyltransferase genes PMT1 and PMT2, with phenotypes manifested only in MATalpha cells for single mutants but in both cell types when both genes are absent. We conclude that posttranslational modifications to Aga1p are necessary for its biogenesis and activity. Our studies also suggest that in addition to GPI-glucan linkages, other cell surface anchorage mechanisms, such as transmembrane domains or disulfides, may be employed by fungal species to localize adhesins.

Role of Fig2p in agglutination in Saccharomyces cerevisiae

In W303-derived strains, disruption of FIG2 increased agglutinability of alpha cells, but not a cells, and did not alter expression of alpha-agglutinin, binding of 125I-labeled alpha-agglutinin, or mating efficiency. Fig2p overexpression led to alpha-cell-specfic suppression of agglutinability. These results imply that Fig2p is an indirect masker of the active sites in alpha-agglutinin.

Maintenance of mating cell integrity requires the adhesin Fig2p

Fungal adhesins represent a large family of serine/threonine-rich secreted glycoproteins. Adhesins have been shown to play roles in heterotypic and homotypic cell-cell adhesion processes, morphogenetic pathways and invasive/pseudohyphal growth, frequently in response to differentiation cues. Here we address the role of the Saccharomyces cerevisiae mating-specific adhesin Fig2p. Cells lacking FIG2 possess a variety of mating defects that relate to processes involving the cell wall, including morphogenetic defects, cell fusion defects, and alterations in agglutination activities. We found that mating-specific morphogenetic defects caused by the absence of FIG2 are suppressible by increased external osmolarity and that, during mating, fig2delta cells display reduced viability relative to wild-type cells. These defects result from alterations in signaling activated by the mating and cell integrity pathways. Finally, we show that fig2delta zygotes also have defects in zygotic spindle positioning that are osmoremedial, whereas the requirements for FIG2 in normal cell-cell agglutination and cell fusion during mating are insensitive to changes in the extracellular osmotic environment. We conclude that FIG2 performs distinct functions in the mating cell wall that are separable with respect to their ability to be suppressed by changes in external osmolarity and that a fundamental role of FIG2 in mating cells is the maintenance of cell integrity.

Environmentally induced reversible conformational switching in the yeast cell adhesion protein alpha-agglutinin

The yeast cell adhesion protein alpha-agglutinin is expressed on the surface of a free-living organism and is subjected to a variety of environmental conditions. Circular dichroism (CD) spectroscopy shows that the binding region of alpha-agglutinin has a beta-sheet-rich structure, with only approximately 2% alpha-helix under native conditions (15-40 degrees C at pH 5.5). This region is predicted to fold into three immunoglobulin-like domains, and models are consistent with the CD spectra as well as with peptide mapping and site-specific mutagenesis. However, secondary structure prediction algorithms show that segments comprising approximately 17% of the residues have high alpha-helical and low beta-sheet potential. Two model peptides of such segments had helical tendencies, and one of these peptides showed pH-dependent conformational switching. Similarly, CD spectroscopy of the binding region of alpha-agglutinin showed reversible conversion from beta-rich to mixed alpha/beta structure at elevated temperatures or when the pH was changed. The reversibility of these changes implied that there is a small energy difference between the all-beta and the alpha/beta states. Similar changes followed cleavage of peptide or disulfide bonds. Together, these observations imply that short sequences of high helical propensity are constrained to a beta-rich state by covalent and local charge interactions under native conditions, but form helices under non-native conditions.

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.

Interaction of alpha-agglutinin and a-agglutinin, Saccharomyces cerevisiae sexual cell adhesion molecules

alpha-Agglutinin and a-agglutinin are complementary cell adhesion glycoproteins active during mating in the yeast Saccharomyces cerevisiae. They bind with high affinity and high specificity: cells of opposite mating types are irreversibly bound by a few pairs of agglutinins. Equilibrium and surface plasmon resonance kinetic analyses showed that the purified binding region of alpha-agglutinin interacted similarly with purified a-agglutinin and with a-agglutinin expressed on cell surfaces. At 20 degrees C, the K(D) for the interaction was 2 x 10(-9) to 5 x 10(-9) M. This high affinity was a result of a very low dissociation rate ( approximately 2.6 x 10(-4) s(-1)) coupled with a low association rate (= 5 x 10(4) M(-1) s(-1)). Circular-dichroism spectroscopy showed that binding of the proteins was accompanied by measurable changes in secondary structure. Furthermore, when binding was assessed at 10 degrees C, the association kinetics were sigmoidal, with a very low initial rate. An induced-fit model of binding with substantial apposition of hydrophobic surfaces on the two ligands can explain the observed affinity, kinetics, and specificity and the conformational effects of the binding reaction.

Genetically controlled self-aggregation of cell-surface-engineered yeast responding to glucose concentration

We constructed an arming (cell-surface-engineered) yeast displaying two types of agglutinin (modified a-agglutinin and alpha-agglutinin) on the cell surface, with agglutination being independent of both mating type and pheromones. The modified a-agglutinin was artificially prepared by the fusion of the genes encoding Aga1p and Aga2p. The modified a-agglutinin could induce agglutination of cells displaying Agalpha1p (alpha-agglutinin). The upstream region of the isocitrate lyase gene of Candida tropicalis (UPR-ICL), active at a low glucose concentration, was used as the promoter to express the modified a-agglutinin- and alpha-agglutinin-encoding genes. The arming yeast displaying both agglutinins agglutinated and sedimented in response to decreased glucose concentration. When the glucose concentration was high, the arming yeast grew normally. In the late log phase, when the glucose concentration became very low, agglutination occurred suddenly and drastically and yeast cells sedimented completely. Sedimentation was confirmed by weighing the aggregated cells after filtration of the broth. Strains in which aggregation can be genetically controlled can be used in industrial processes in which the separation of yeast cells from the supernatant is necessary.

Establishment of a simple system to analyse the molecular interaction in the agglutination of Saccharomyces cerevisiae

Saccharomyces cerevisiae a-agglutinin, which is involved in mating and covalently anchoring to the cell wall, consists of two components, Aga1p and Aga2p, whose syntheses are individually regulated. To facilitate the analysis of the protein-protein interaction on agglutination between a- and alpha-agglutinins, the construction of a yeast strain (MATa) with the functional protein prepared by genetic fusion of Aga1p- and Aga2p-encoding genes and by the expression system using the UPR-ICL promoter derived from the n-alkane-assimilating yeast, Candida tropicalis, which is functional under the condition of lower glucose concentration was tried and the agglutination ability of the constructed strain was evaluated with a yeast strain (MATa) which expressed AGalpha1 encoding alpha-agglutinin under the control of the same promoter. The genes were integrated into the yeast chromosomes. Cell agglutination between both (MATa) strains was observed microscopically when these two strains were mix-cultured to a glucose-decreased concentration. The agglutination was further confirmed by the sedimentation test and by the quantification using a filter. These results proved that the constructed Aga1p-Aga2p fusion protein was enoughly functional for the interaction with the Agalpha1 protein, and that this phenomenon occurred dependent on glucose concentration, but independent of the peptide pheromones secreted by the cells of the opposite mating types. Using this system, the role of two disulphide linkages between Aga1p and Aga2p on the binding activity between Aga2p and Aga1p was first evaluated. Under the treatment by the SH-compound (dithiothreitol), in which Agalpha2p is easily released into the medium from the intact cell surface, the Aga1p and Aga2p fusion protein was a good tool to make clear the role of the disulphide linkages. As a result, the linkages had a significant effect on not only the assembly but also the binding activity. The novel and simple system described here may further facilitate the study of molecular interaction in agglutination.

Inhibitory effects of nucleoside 5'-alkylphosphates on sexual agglutination in Saccharomyces cerevisiae

Among various nucleoside 5'-alkylphosphates, uridine 5'-hexadecylphosphate (UMPC16) and adenosine 5'-hexadecylphosphate (AMPC16) inhibited the sexual agglutination between a and alpha haploid cells of Saccharomyces cerevisiae. The effect of AMPC16 accompanied severe growth inhibition of the yeast cells but it was not observed with UMPC16. Sexual agglutination was not inhibited by the presence of UMPC16 or AMPC16 when the yeast cells had been pretreated with the mating pheromone. UMPC16 was characterized as a specific inhibitor of sexual agglutination without direct influence on the agglutinin function, being distinguishable from any of those ever known.

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.

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.

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.

Glycosyl phosphatidylinositol-dependent cross-linking of alpha-agglutinin and beta 1,6-glucan in the Saccharomyces cerevisiae cell wall

The cell adhesion protein alpha-agglutinin is bound to the outer surface of the Saccharomyces cerevisiae cell wall and mediates cell-cell contact in mating. alpha-Agglutinin is modified by addition of a glycosyl phosphatidylinositol (GPI) anchor as it traverses the secretory pathway. The presence of a GPI anchor is essential for cross-linking into the wall, but the fatty acid and inositol components of the anchor are lost before cell wall association (Lu, C.-F., J. Kurjan, and P. N. Lipke, 1994. A pathway for cell wall anchorage of Saccharomyces cerevisiae alpha-agglutinin. Mol. Cell. Biol. 14:4825-4833). Cell wall association of alpha-agglutinin was accompanied by an increase in size and a gain in reactivity to antibodies directed against beta 1,6-glucan. Several kre mutants, which have defects in synthesis of cell wall beta 1,6-glucan, had reduced molecular size of cell wall alpha-agglutinin. These findings demonstrate that the cell wall form of alpha-agglutinin is covalently associated with beta 1,6-glucan. The alpha-agglutinin biosynthetic precursors did not react with antibody to beta 1,6-glucan, and the sizes of these forms were unaffected in kre mutants. A COOH-terminal truncated form of alpha-agglutinin, which is not GPI anchored and is secreted into the medium, did not react with the anti-beta 1,6-glucan. We propose that extracellular cross-linkage to beta 1,6-glucan mediates covalent association of alpha-agglutinin with the cell wall in a manner that is dependent on prior addition of a GPI anchor to alpha-agglutinin.

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.

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

alpha-Agglutinin is a cell adhesion glycoprotein expressed on the cell wall of Saccharomyces cerevisiae alpha cells. Binding of alpha-agglutinin to its ligand a-agglutinin, expressed by a cells, mediates cell-cell contact during mating. Analysis of truncations of the 650-amino-acid alpha-agglutinin structural gene AG alpha 1 delineated functional domains of alpha-agglutinin. Removal of the C-terminal hydrophobic sequence allowed efficient secretion of the protein and loss of cell surface attachment. This cell surface anchorage domain was necessary for linkage to a glycosyl phosphatidylinositol anchor. A construct expressing the N-terminal 350 amino acid residues retained full a-agglutinin-binding activity, localizing the binding domain to the N-terminal portion of alpha-agglutinin. A 278-residue N-terminal peptide was inactive; therefore, the binding domain includes residues between 278 and 350. The segment of alpha-agglutinin between amino acid residues 217 and 308 showed significant structural and sequence similarity to a consensus sequence for immunoglobulin superfamily variable-type domains. The similarity of the alpha-agglutinin-binding domain to mammalian cell adhesion proteins suggests that this structure is a highly conserved feature of adhesion proteins in diverse eukaryotes.

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

alpha-Agglutinin is a cell adhesion glycoprotein expressed on the cell wall of Saccharomyces cerevisiae alpha cells. Binding of alpha-agglutinin to its ligand a-agglutinin, expressed by a cells, mediates cell-cell contact during mating. Analysis of truncations of the 650-amino-acid alpha-agglutinin structural gene AG alpha 1 delineated functional domains of alpha-agglutinin. Removal of the C-terminal hydrophobic sequence allowed efficient secretion of the protein and loss of cell surface attachment. This cell surface anchorage domain was necessary for linkage to a glycosyl phosphatidylinositol anchor. A construct expressing the N-terminal 350 amino acid residues retained full a-agglutinin-binding activity, localizing the binding domain to the N-terminal portion of alpha-agglutinin. A 278-residue N-terminal peptide was inactive; therefore, the binding domain includes residues between 278 and 350. The segment of alpha-agglutinin between amino acid residues 217 and 308 showed significant structural and sequence similarity to a consensus sequence for immunoglobulin superfamily variable-type domains. The similarity of the alpha-agglutinin-binding domain to mammalian cell adhesion proteins suggests that this structure is a highly conserved feature of adhesion proteins in diverse eukaryotes.

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 AGA1 product is involved in cell surface attachment of the Saccharomyces cerevisiae cell adhesion glycoprotein a-agglutinin

Saccharomyces cerevisiae a and alpha cells express the complementary cell surface glycoproteins a-agglutinin and alpha-agglutinin, respectively, which interact with one another to promote cellular aggregation during mating. Treatment of S. cerevisiae a cells with reducing agents releases the binding subunit of a-agglutinin, which has been purified and characterized; little biochemical information on the overall structure of a-agglutinin is available. To characterise a-agglutinin structure and function, we have used a genetic approach to clone an a-agglutinin structural gene (AGAI). Mutants with a-specific agglutination defects were isolated, the majority of which fell into a single complementation group, called aga1. The aga1 mutants showed wild-type pheromone production and response, efficient mating on solid medium, and a mating defect in liquid medium; these phenotypes are characteristic of agglutinin mutants. The AGA1 gene was cloned by complementation; the gene sequence indicated that it could encode a protein of 725 amino acids with high serine and threonine content, a putative N-terminal signal sequence, and a C-terminal hydrophobic sequence similar to signals for the attachment to glycosyl phosphatidylinositol anchors. Active a-agglutinin binding subunit is secreted by aga1 mutants, indicating that AGA1 is involved in cells surface attachment of a-agglutinin. This result suggests that AGA1 encodes a protein with functional similarity to the core subunits of a-agglutinin analogs from other budding yeasts. Unexpectedly, the AGA1 transcript was expressed and induced by pheromone in both a and alpha cells, suggesting that the a-specific expression of active a-agglutinin results only from a-specific regulation of the a-agglutinin binding subunit.

The AGA1 product is involved in cell surface attachment of the Saccharomyces cerevisiae cell adhesion glycoprotein a-agglutinin

Saccharomyces cerevisiae a and alpha cells express the complementary cell surface glycoproteins a-agglutinin and alpha-agglutinin, respectively, which interact with one another to promote cellular aggregation during mating. Treatment of S. cerevisiae a cells with reducing agents releases the binding subunit of a-agglutinin, which has been purified and characterized; little biochemical information on the overall structure of a-agglutinin is available. To characterise a-agglutinin structure and function, we have used a genetic approach to clone an a-agglutinin structural gene (AGAI). Mutants with a-specific agglutination defects were isolated, the majority of which fell into a single complementation group, called aga1. The aga1 mutants showed wild-type pheromone production and response, efficient mating on solid medium, and a mating defect in liquid medium; these phenotypes are characteristic of agglutinin mutants. The AGA1 gene was cloned by complementation; the gene sequence indicated that it could encode a protein of 725 amino acids with high serine and threonine content, a putative N-terminal signal sequence, and a C-terminal hydrophobic sequence similar to signals for the attachment to glycosyl phosphatidylinositol anchors. Active a-agglutinin binding subunit is secreted by aga1 mutants, indicating that AGA1 is involved in cells surface attachment of a-agglutinin. This result suggests that AGA1 encodes a protein with functional similarity to the core subunits of a-agglutinin analogs from other budding yeasts. Unexpectedly, the AGA1 transcript was expressed and induced by pheromone in both a and alpha cells, suggesting that the a-specific expression of active a-agglutinin results only from a-specific regulation of the a-agglutinin binding subunit.

Expression of the HTLV-I tax transactivator in yeast: correlation between phenotypic alterations and tax function in higher eukaryotes

The transcriptional activator protein (Tax) from human T-cell leukemia virus type 1 was expressed in yeast using several different promoters in several strains: In all instances, expression of Tax resulted in very strong aggregation of the yeast cells. This phenotype appears to be identical by all criteria tested to the flocculation phenotype of the dominant mutation flo 1. Of most significance, mutations in Tax that affect transactivation of the IL-2R alpha regulatory sequences, but retain their ability to activate the viral long terminal repeat also fail to yield the aggregation phenotype. Based on these findings, expression of Tax in yeast may prove to be a simple primordial system for examining the regulatory mechanisms and cellular functions involved in regulation of gene expression.

The yeast cell fusion protein FUS1 is O-glycosylated and spans the plasma membrane

Previous work has shown that efficient cell fusion during conjugation in Saccharomyces cerevisiae requires a pheromone-induced surface protein encoded by FUS1. We show that the FUS1 protein migrates on SDS/polyacrylamide gels with an apparent molecular mass of 80 kDa, although the mass is predicted to be 58 kDa from the gene coding capacity. This discrepancy results from the presence of O-linked mannose oligosaccharides attached to the clustered serines and threonines at the amino terminus of the protein. The addition of mannose is completely abolished in the early secretory mutant sec53, attenuated in the late-endoplasmic reticulum-blocked sec18, and unaffected in sec7, which is blocked late in the Golgi phase of secretion. Membrane fractionation and protease protection experiments indicate that FUS1 spans the plasma membrane, with its glycosylated amino terminus projecting into the periplasmic space.

AG alpha 1 is the structural gene for the Saccharomyces cerevisiae alpha-agglutinin, a cell surface glycoprotein involved in cell-cell interactions during mating

We have cloned the alpha-agglutinin structural gene, AG alpha 1, by the isolation of alpha-specific agglutination-defective mutants, followed by isolation of a complementing plasmid. Independently isolated alpha-specific agglutination-defective mutations were in a single complementation group, consistent with biochemical results indicating that the alpha-agglutinin is composed of a single polypeptide. Mapping results suggested that the complementation group identified by these mutants is allelic to the ag alpha 1 mutation identified previously. Expression of AG alpha 1 RNA was alpha specific and inducible by a-factor. Sequences similar to the consensus sequences for positive control by MAT alpha 1 and pheromone induction were found upstream of the AG alpha 1 initiation codon. The AG alpha 1 gene could encode a 650-amino-acid protein with a putative signal sequence, 12 possible N-glycosylation sites, and a high proportion of serine and threonine residues, all of which are features expected for the alpha-agglutinin sequence. Disruption of the AG alpha 1 gene resulted in failure to express alpha-agglutinin and loss of cellular agglutinability in alpha cells. An Escherichia coli fusion protein containing 229 amino acids of the AG alpha 1 sequence was recognized by an anti-alpha-agglutinin antibody. In addition, the ability of this antibody to inhibit agglutination was prevented by this fusion protein. These results indicate that AG alpha 1 encodes alpha-agglutinin. Features of the AG alpha 1 gene product suggest that the amino-terminal half of the protein contains the a-agglutinin binding domain and that the carboxy-terminal half contains a cell surface localization domain, possibly including a glycosyl phosphatidylinositol anchor.

Purification of the inducible alpha-agglutinin of S. cerevisiae and molecular cloning of the gene

The alpha-agglutinin responsible for mating type-specific agglutination of S. cerevisiae alpha-cells has been purified to homogeneity. The glycoprotein released from the cell surface under mild conditions has a relative molecular mass of 200 to 300 kDa as determined by SDS-gel electrophoresis. The protein moiety corresponds to 68.2 kDa. With an oligonucleotide corresponding to the N-terminal amino acid sequence, the alpha-agglutinin gene has been cloned and sequenced. From the DNA sequence, a protein of 631 amino acids with 12 potential N-glycosylation sites is predicted. The carboxy terminal one-third of the protein is not required for agglutination activity.

AG alpha 1 is the structural gene for the Saccharomyces cerevisiae alpha-agglutinin, a cell surface glycoprotein involved in cell-cell interactions during mating

We have cloned the alpha-agglutinin structural gene, AG alpha 1, by the isolation of alpha-specific agglutination-defective mutants, followed by isolation of a complementing plasmid. Independently isolated alpha-specific agglutination-defective mutations were in a single complementation group, consistent with biochemical results indicating that the alpha-agglutinin is composed of a single polypeptide. Mapping results suggested that the complementation group identified by these mutants is allelic to the ag alpha 1 mutation identified previously. Expression of AG alpha 1 RNA was alpha specific and inducible by a-factor. Sequences similar to the consensus sequences for positive control by MAT alpha 1 and pheromone induction were found upstream of the AG alpha 1 initiation codon. The AG alpha 1 gene could encode a 650-amino-acid protein with a putative signal sequence, 12 possible N-glycosylation sites, and a high proportion of serine and threonine residues, all of which are features expected for the alpha-agglutinin sequence. Disruption of the AG alpha 1 gene resulted in failure to express alpha-agglutinin and loss of cellular agglutinability in alpha cells. An Escherichia coli fusion protein containing 229 amino acids of the AG alpha 1 sequence was recognized by an anti-alpha-agglutinin antibody. In addition, the ability of this antibody to inhibit agglutination was prevented by this fusion protein. These results indicate that AG alpha 1 encodes alpha-agglutinin. Features of the AG alpha 1 gene product suggest that the amino-terminal half of the protein contains the a-agglutinin binding domain and that the carboxy-terminal half contains a cell surface localization domain, possibly including a glycosyl phosphatidylinositol anchor.

Alpha-agglutinin expression in Saccharomyces cerevisiae

A polyclonal antiserum raised against purified alpha-agglutinin was made specific for alpha-agglutinin after adsorption with a cells. The adsorbed antiserum identified alpha-agglutinin peptides on Western blots and bound to cell surface alpha-agglutinin, inhibiting the binding of alpha cells to a cells. Using the antibody, we have determined that 1) the surface distribution of alpha-agglutinin on alpha cells is polar, 2) about 5 x 10(4) molecules/cell are constitutively expressed on strain X2180-1B (alpha) cells, and 3) treatment of alpha cells with the sex pheromone a-factor causes an increase in cell surface alpha-agglutinin, consistent with the a-factor induced increase in cell agglutinability.

Mutations in a gene encoding the alpha subunit of a Saccharomyces cerevisiae G protein indicate a role in mating pheromone signaling

Mutations which allowed conjugation by Saccharomyces cerevisiae cells lacking a mating pheromone receptor gene were selected. One of the genes defined by such mutations was isolated from a yeast genomic library by complementation of a temperature-sensitive mutation and is identical to the gene GPA1 (also known as SCG1), recently shown to be highly homologous to genes encoding the alpha subunits of mammalian G proteins. Physiological analysis of temperature-sensitive gpa1 mutations suggests that the encoded G protein is involved in signaling in response to mating pheromones. Mutational disruption of G-protein activity causes cell-cycle arrest in G1, deposition of mating-specific cell surface agglutinins, and induction of pheromone-specific mRNAs, all of which are responses to pheromone in wild-type cells. In addition, mutants can conjugate without the benefit of mating pheromone or pheromone receptor. A model is presented where the activated G protein has a negative impact on a constitutive signal which normally keeps the pheromone response repressed.

Mutations in a gene encoding the alpha subunit of a Saccharomyces cerevisiae G protein indicate a role in mating pheromone signaling

Mutations which allowed conjugation by Saccharomyces cerevisiae cells lacking a mating pheromone receptor gene were selected. One of the genes defined by such mutations was isolated from a yeast genomic library by complementation of a temperature-sensitive mutation and is identical to the gene GPA1 (also known as SCG1), recently shown to be highly homologous to genes encoding the alpha subunits of mammalian G proteins. Physiological analysis of temperature-sensitive gpa1 mutations suggests that the encoded G protein is involved in signaling in response to mating pheromones. Mutational disruption of G-protein activity causes cell-cycle arrest in G1, deposition of mating-specific cell surface agglutinins, and induction of pheromone-specific mRNAs, all of which are responses to pheromone in wild-type cells. In addition, mutants can conjugate without the benefit of mating pheromone or pheromone receptor. A model is presented where the activated G protein has a negative impact on a constitutive signal which normally keeps the pheromone response repressed.

Pheromonal regulation and sequence of the Saccharomyces cerevisiae SST2 gene: a model for desensitization to pheromone

Strains of both haploid mating types containing sst2 mutations are altered in response to pheromone; MATa sst2 cells are supersensitive to alpha-factor, and MAT alpha sst2 cells are supersensitive to a-factor. This phenotype suggests that SST2 encodes a component of the pheromone response pathway that is common to both mating types. We have cloned the SST2 gene by isolation of multicopy plasmids that complement the sst2-1 mutation. One such plasmid contained a 4.5-kilobase HindIII fragment that was able to complement the sst2-1 mutation in high or low copy number, integrated at the SST2 locus, and resulted in an sst2 phenotype when disrupted, indicating that this fragment contained the SST2 gene. We identified the functional region of the complementing DNA fragment by transposon mutagenesis. Sequencing of this fragment identified an open reading frame encoding 698 amino acids at a position that correlated well with the functional region. Expression of an Sst2-beta-galactosidase fusion was haploid specific and induced by exposure to pheromone. We discuss a model in which induction of the SST2 product results in inhibition of a component of the pheromone response pathway, resulting in desensitization to pheromone.

Pheromonal regulation and sequence of the Saccharomyces cerevisiae SST2 gene: a model for desensitization to pheromone

Strains of both haploid mating types containing sst2 mutations are altered in response to pheromone; MATa sst2 cells are supersensitive to alpha-factor, and MAT alpha sst2 cells are supersensitive to a-factor. This phenotype suggests that SST2 encodes a component of the pheromone response pathway that is common to both mating types. We have cloned the SST2 gene by isolation of multicopy plasmids that complement the sst2-1 mutation. One such plasmid contained a 4.5-kilobase HindIII fragment that was able to complement the sst2-1 mutation in high or low copy number, integrated at the SST2 locus, and resulted in an sst2 phenotype when disrupted, indicating that this fragment contained the SST2 gene. We identified the functional region of the complementing DNA fragment by transposon mutagenesis. Sequencing of this fragment identified an open reading frame encoding 698 amino acids at a position that correlated well with the functional region. Expression of an Sst2-beta-galactosidase fusion was haploid specific and induced by exposure to pheromone. We discuss a model in which induction of the SST2 product results in inhibition of a component of the pheromone response pathway, resulting in desensitization to pheromone.

Pheromone induction of agglutination in Saccharomyces cerevisiae a cells

a-Agglutinin, the cell surface sexual agglutinin of yeast a cells, was assayed by its ability to bind its complementary agglutinin, alpha-agglutinin. The specific binding of 125I-alpha-agglutinin to a cells treated with the sex pheromone alpha-factor was 2 to 2.5 times that of binding to a cells not treated with alpha-factor. Competition with unlabeled alpha-agglutinin revealed that the increased binding was due to increased cell surface expression of a-agglutinin, with no apparent change in the binding constant. The increase in site number was similar to the increase in cellular agglutinability. Increased expression of a-agglutinin followed the same kinetics as the increase in cellular agglutinability, with a 10-min lag followed by a 15- to 20-min response time. Induction kinetics were similar in cells in phases G1 and G2 of the cell cycle. Maximal expression levels were similar in cells treated with excess pheromone and in cells exposed to pheromone after destruction of constitutively expressed a-agglutinin.

Two genes required for cell fusion during yeast conjugation: evidence for a pheromone-induced surface protein

We characterized two genes, FUS1 and FUS2, which are required for fusion of Saccharomyces cerevisiae cells during conjugation. Mutations in these genes lead to an interruption of the mating process at a point just before cytoplasmic fusion; the partition dividing the mating pair remains undissolved several hours after the cells have initially formed a stable "prezygote." Fusion is only moderately impaired when the two parents together harbor one or two mutant fus genes, and it is severely compromised only when three or all four fus genes are inactivated. Cloning of FUS1 and FUS2 revealed that they share some functional homology; FUS1 on a high-copy number plasmid can partially suppress a fus2 mutant, and vice versa. FUS1 remains essentially unexpressed in vegetative cells, but is strongly induced by incubation of haploid cells with the appropriate mating pheromone. Immunofluorescence microscopy of alpha factor-induced a cells harboring a fus1-LACZ fusion showed the fusion protein to be localized at the cell surface, concentrated at one end of the cell (the shmoo tip). FUS1 maps near HIS4, and the intervening region (including BIK1, a gene required for nuclear fusion) was sequenced along with FUS1. The sequence of FUS1 revealed the presence of three copies of a hexamer (TGAAAC) conserved in the 5' noncoding regions of other pheromone-inducible genes. The deduced FUS1 protein sequence exhibits a striking concentration of serines and threonines at the amino terminus (46%; 33 of 71), followed by a 25-amino acid hydrophobic stretch and a predominantly hydrophilic carboxy terminus, which contains several potential N-glycosylation sites (Asn-X-Ser/Thr). This sequence suggests that FUS1 encodes a membrane-anchored glycoprotein with both N- and O-linked sugars.

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

Sex-specific agglutinins from the cell surface of haploid cells of Saccharomyces cerevisiae (X2180, mta and mt alpha) were purified and analysed. The constitutive agglutinin from mta cells was extractable with 3 mM dithiothreitol. It was shown to be a glycoprotein (3% mannose) with an apparent Mr of 43,000 based on gel filtration, but in SDS-PAGE it behaved as a much smaller molecule (Mr between 18,000 and 26,000). About one in three amino acids was a hydroxyamino acid. Its biological activity was resistant to boiling for 1 h, but sensitive to pronase. Intact mt alpha cells retained their agglutinability in the presence of dithiothreitol but limited trypsinizing released a biologically active agglutinin fragment. It had an apparent Mr of 320,000 (gel filtration). When analysed by SDS-PAGE, a single diffuse band with an apparent Mr of 225,000 was observed. The protein was 94% (w/w) mannose with a trace of N-acetyl glucosamine. Its biological activity was almost completely lost after boiling for 1 h. Both agglutinins behaved as monovalent molecules and specifically inhibited the biological activity of both noninduced and pheromone-induced cells. Pheromone treatment of mta cells resulted in an apparent 32-fold increase in agglutinin activity at the cell surface, whereas pheromone treatment of mt alpha cells only doubled the apparent agglutinin activity.

Two genes required for cell fusion during yeast conjugation: evidence for a pheromone-induced surface protein

We characterized two genes, FUS1 and FUS2, which are required for fusion of Saccharomyces cerevisiae cells during conjugation. Mutations in these genes lead to an interruption of the mating process at a point just before cytoplasmic fusion; the partition dividing the mating pair remains undissolved several hours after the cells have initially formed a stable "prezygote." Fusion is only moderately impaired when the two parents together harbor one or two mutant fus genes, and it is severely compromised only when three or all four fus genes are inactivated. Cloning of FUS1 and FUS2 revealed that they share some functional homology; FUS1 on a high-copy number plasmid can partially suppress a fus2 mutant, and vice versa. FUS1 remains essentially unexpressed in vegetative cells, but is strongly induced by incubation of haploid cells with the appropriate mating pheromone. Immunofluorescence microscopy of alpha factor-induced a cells harboring a fus1-LACZ fusion showed the fusion protein to be localized at the cell surface, concentrated at one end of the cell (the shmoo tip). FUS1 maps near HIS4, and the intervening region (including BIK1, a gene required for nuclear fusion) was sequenced along with FUS1. The sequence of FUS1 revealed the presence of three copies of a hexamer (TGAAAC) conserved in the 5' noncoding regions of other pheromone-inducible genes. The deduced FUS1 protein sequence exhibits a striking concentration of serines and threonines at the amino terminus (46%; 33 of 71), followed by a 25-amino acid hydrophobic stretch and a predominantly hydrophilic carboxy terminus, which contains several potential N-glycosylation sites (Asn-X-Ser/Thr). This sequence suggests that FUS1 encodes a membrane-anchored glycoprotein with both N- and O-linked sugars.

Interaction of alpha-agglutinin with Saccharomyces cerevisiae a cells

Binding of Saccharomyces cerevisiae alpha-agglutinin to target a cells was assayed by agglutination inhibition and 125I-alpha-agglutinin binding. The assays showed characteristics of equilibrium binding, namely saturability, competability, and the establishment of a kinetic endpoint in the presence of free alpha-agglutinin and free receptor. The binding was heterogeneous, displaying strong binding (10(9) liters/mol) and a weaker interaction. There were about 2 X 10(4) strong binding sites per a cell. Denaturing gels displayed identical labeled species binding to the a cells in the weak and strong interactions. Furthermore, weakly bound material could subsequently bind tightly to fresh a cells, implying that the same species of alpha-agglutinin was bound in the two states.

Identification of glycoprotein components of alpha-agglutinin, a cell adhesion protein from Saccharomyces cerevisiae

Several glycoproteins which inhibit the agglutinability of Saccharomyces cerevisiae mating type a cells were partially purified from extracts of mating type alpha cells. These proteins, called alpha-agglutinin, were labeled with 125I-Bolton-Hunter reagent. The labeled alpha-agglutinin showed specific binding to a cells. Such specific binding approached saturation with respect to agglutinin or cells and was inhibited in the presence of excess unlabeled alpha-agglutinin. Nonspecific binding was similar in a and alpha cells, was neither saturable nor competable, and was three- to fourfold less than the specific binding to a cells at maximum tested agglutinin concentrations. The major a-specific binding species had a low electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gels and had an apparent molecular weight of 155,000 by rate zonal centrifugation. Endo-N-acetylglucosaminidase H digestion of the purified glycoprotein complex converted the low-mobility material to four major and several minor bands which were resolved by polyacrylamide gel electrophoresis. All but two minor peptides bound specifically to a cells. Analyses of agglutinin from mnn mutants confirmed the deglycosylation results in suggesting that the N-linked carbohydrate portion of alpha-agglutinin was not necessary for activity.

Biological activity of the Asn-5,Arg-7 tridecapeptide encoded by MF alpha 2 of Saccharomyces cerevisiae

The precursor predicted by the nucleotide sequence of the MF alpha 2 gene of Saccharomyces cerevisiae contains one copy of the tridecapeptide alpha-factor previously characterized (H2N-Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr-COOH) and one copy of a peptide that contains two conservative amino acid substitutions (H2N-Trp-His-Trp-Leu-Asn-Leu-Arg-Pro-Gly-Gln-Pro-Met-Tyr-COOH). To determine whether the novel molecule possesses biological activity, the Asn-5,Arg-7 tridecapeptide was prepared chemically by solid-phase peptide synthesis. Growth arrest and morphogenesis assays gave identical activity profiles for the Asn-5,Arg-7 peptide and the other gene product, the Gln-5,Lys-7 peptide. The activities of the two peptides were additive and indistinguishable for S. cerevisiae X2180-1A. When present in fourfold molar excess, the biologically inactive desTrp-1,Ala-3 dodecapeptide reversed activity of the Asn-5,Arg-7 and Gln-5,Lys-7 tridecapeptides. Furthermore, neither peptide caused growth arrest of a MATa ste2(Ts) mutant when assayed at the restrictive temperature. These studies suggest that both pheromones interact with the alpha-factor receptor in a similar manner.

Agglutination and mating activity of the MF alpha 2-encoded alpha-factor analog in Saccharomyces cerevisiae

The MF alpha 2-encoded Asn-5,Arg-7 alpha-factor-like peptide has been shown shown to have similar activity to Gln-5,Lys-7 alpha-factor in morphogenesis and growth arrest studies (S. Raths, P. Shenbagamurthi, F. Naider, and J. M. Becker, J. Bacteriol. 168:1468-1471, 1986). We tested the Asn-5,Arg-7 peptide in agglutination and mating assays and found that its activity was similar to or slightly less than that of the Gln-5,Lys-7 alpha-factor. The Asn-5,Arg-7 alpha-factor-like peptide is thus the most active analog of the Gln-5,Lys-7 alpha-factor known.

Structure-activity relationships of the yeast alpha-factor

The yeast Saccharomyces cerevisiae produces a peptide pheromone, termed the alpha-factor, as a prelude to sexual conjugation. Haploid MAT alpha-cells, but not haploid MAT a-cells or MAT a/alpha-diploids, produce this tridecapeptide of the structure: Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr. Structural analogues of the alpha-factor have been prepared with alterations in many of the residues, derivatized peptides have been synthesized, and truncated and elongated peptides have been studied. These peptides have been analyzed for their biological activities by various assays. Mutants of S. cerevisiae have been isolated that do not respond to alpha-factor or are supersensitive to the pheromone and its analogues. The mating system of S. cerevisiae provides a powerful model in which genetics, biochemistry, and molecular biology can be used to unravel the mysteries of peptide hormone structure and function.

Flocculation of Saccharomyces cerevisiae tup1 mutants

Strains of Saccharomyces cerevisiae carrying a mutation in the TUP1 locus exhibited calcium-dependent flocculation. The flocculation had none of the characteristics of sexual agglutination. The flocculation differed from that exhibited by a FLO1 strain in the effect of pH on cation dependence and sensitivity to chemical inactivation.

Different structure-function relationships for alpha-factor-induced morphogenesis and agglutination in Saccharomyces cerevisiae

Eight synthetic analogs of the mating pheromone alpha-factor-induced morphogenesis and increased agglutinability in a cells. Most analogs induced increased agglutinability at lower concentrations than those at which they induced morphogenesis, but the ratio of the potencies for the two effects varied 140-fold among different analogs. Morphological response to pheromone required exposure for at least 90 min, but increased agglutinability followed exposures of 20 s. Two synthetic analogs induced neither response. In competition experiments, both of these analogs prevented induction of increased agglutinability and morphogenesis by active alpha factor. The inactive peptides blocked increased agglutinability at lower concentrations than those at which they blocked morphogenesis. alpha factors exhibited different structure-function relationships for morphogenesis as compared with agglutinability. Thus, response of Saccharomyces cerevisiae to alpha factor is complex and may be mediated by more than one receptor.

Activation of yeast mannan synthetase by alpha factor pheromone

The Saccharomyces cerevisiae mating pheromone alpha factor induces the activation of yeast mannan synthetase both in vivo and in vitro. In vivo the activating effect of the pheromone on the synthetase is specific for cells of the alpha mating type, while in vitro alpha factor is able to exert its action on the synthetase of either cell type (MAT a, MAT alpha and MAT a/MAT alpha).

Induction of yeast mating pheromone a-factor by alpha cells

Saccharomyces cerevisiae cells of a and alpha mating types constitutively secrete cell-specific peptide mating pheromones. a-Factor is secreted by a cells and acts on alpha cells, while alpha-factor is secreted by alpha cells and acts on a cells. Confirming preliminary studies, we demonstrate here that cultures of a cells contain higher than constitutive levels of a-factor activity when grown with alpha cells or alpha-factor. This induction of a-factor may result from increased synthesis or increased secretion of a-factor, as opposed to modification or stabilization of preexisting a-factor, as part of the a cell response to alpha-factor, as an a ste2 mutant (which cannot respond to alpha-factor) is not induced by alpha-factor. In mixed cultures inoculated with equal numbers of a cells and alpha cells, a cells predominate by stationary phase. Thus, a series of sequential interactions between a and alpha cells may be involved in establishing optimal hormone concentrations and cell ratios for conjugation.