Regional and Temporal Population Structure of Pseudoperonospora cubensis in Michigan and Ontario.

Cucurbit downy mildew (CDM), caused by the oomycete pathogen Pseudoperonospora cubensis, is a devastating disease that affects cucurbit species worldwide. This obligate, wind-dispersed pathogen does not overwinter in Michigan or other northern regions and new isolates can enter the state throughout the growing season. To evaluate the regional and temporal population structure of P. cubensis, sporangia from CDM lesions were collected from cucurbit foliage grown in Michigan and Ontario field locations in 2011. Population structure and genetic diversity were assessed in 257 isolates using nine simple sequence repeat markers. Genetic diversity was high for isolates from Michigan and Canada (0.6627 and 0.6131, respectively). Five genetic clusters were detected and changes in population structure varied by site and sampling date within a growing season. The Michigan and Canada populations were significantly differentiated, and a unique genetic cluster was detected in Michigan.

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.

Alpha-factor structural gene mutations in Saccharomyces cerevisiae: effects on alpha-factor production and mating.

The role of alpha-factor structural genes MF alpha 1 and MF alpha 2 in alpha-factor production and mating has been investigated by the construction of mf alpha 1 and mf alpha 2 mutations that totally eliminate gene function. An mf alpha 1 mutant in which the entire coding region is deleted shows a considerable decrease in alpha-factor production and a 75% decrease in mating. Mutations in mf alpha 2 have little or no effect on alpha-factor production or mating. The mf alpha 1 mf alpha 2 double mutants are completely defective in mating and alpha-factor production. These results indicate that at least one alpha-factor structural gene product is required for mating in MAT alpha cells, that MF alpha 1 is responsible for the majority of alpha-factor production, and that MF alpha 1 and MF alpha 2 are the only active alpha-factor genes.

Analysis of the receptor binding domain of Gpa1p, the G(alpha) subunit involved in the yeast pheromone response pathway.

The Saccharomyces cerevisiae G protein alpha subunit Gpa1p is involved in the response of both MATa and MAT alpha cells to pheromone. We mutagenized the GPA1 C terminus to characterize the receptor-interacting domain and to investigate the specificity of the interactions with the a- and alpha-factor receptors. The results are discussed with respect to a structural model of the Gpa1p C terminus that was based on the crystal structure of bovine transducin. Some mutants showed phenotypes different than the pheromone response and mating defects expected for mutations that affect receptor interactions, and therefore the mutations may affect other aspects of Gpa1p function. Most of the mutations that resulted in pheromone response and mating defects had similar effects in MATa and MAT alpha cells, suggesting that they affect the interactions with both receptors. Overexpression of the pheromone receptors increased the mating of some of the mutants tested but not the wild-type strain, consistent with defects in mutant Gpa1p-receptor interactions. The regions identified by the mating-defective mutants correlated well with the regions of mammalian G(alpha) subunits implicated in receptor interactions. The strongest mating type-specific effects were seen for mutations to proline and a mutation of a glycine residue predicted to form a C-terminal beta turn. The analogous beta turn in mammalian G(alpha) subunits undergoes a conformational change upon receptor interaction. We propose that the conformation of this region of Gpa1p differs during the interactions with the a- and alpha-factor receptors and that these mating type-specific mutations preclude the orientation necessary for interaction with one of the two receptors.

Saccharomyces cerevisiae Mpt5p interacts with Sst2p and plays roles in pheromone sensitivity and recovery from pheromone arrest.

SST2 plays an important role in the sensitivity of yeast cells to pheromone and in recovery from pheromone-induced G1 arrest. Recently, a family of Sst2p homologs that act as GTPase-activating proteins (GAPs) for G alpha subunits has been identified. We have identified an interaction between Sst2p and the previously identified Mpt5p by using the two-hybrid system. Loss of Mpt5p function resulted in a temperature-sensitive growth phenotype, an increase in pheromone sensitivity, and a defect in recovery from pheromone-induced G1 arrest, although the effects on pheromone response and recovery were mild in comparison to those of sst2 mutants. Overexpression of either Sst2p or Mpt5p promoted recovery from G1 arrest. Promotion of recovery by overexpression of Mpt5p required Sst2p, but the effect of overexpression of Sst2p was only partially dependent on Mpt5p. Mpt5p was also found to interact with the mitogen-activated protein kinase homologs Fus3p and Kss1p, and an mpt5 mutation was able to suppress the pheromone arrest and mating defects of a fus3 mutant. Because either mpt5 or cln3 mutations suppressed the fus3 phenotypes, interactions of Mpt5p with the G1 cyclins and Cdc28p were tested. An interaction between Mpt5p and Cdc28p was detected. We discuss these results with respect to a model in which Sst2p plays a role in pheromone sensitivity and recovery that acts through Mpt5p in addition to a role as a G alpha GAP suggested by the analysis of the Sst2p homologs.

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.

Mutations at the Saccharomyces cerevisiae SUP4 tRNA(Tyr) locus: isolation, genetic fine-structure mapping, and correlation with physical structure.

The SUP4 tRNA(Tyr) locus in Saccharomyces cerevisiae has been studied by the isolation and characterization of mutations at the SUP4 gene which result in the loss of suppressor function. Most of the mutations act as single-site mutations, whereas about a third of the mutations are deletions of the entire gene. Two meiotic fine-structure maps of the gene were made. The first mapping technique placed 10 mutations plus the sup4+ anticodon on a map by a measurement of levels of recombination between pairs of mutations. The second map utilized a more qualitative estimate of recombination frequency, allowing 69 mutations and the sup4+ anticodon to be mapped. The maps were compared with the physical structure of the gene for the 34 mutations whose nucleotide alteration has been determined by DNA sequencing (Koski et al., Cell 22:415-425, 1980; Kurjan et al., Cell 20:701-709, 1980). Both maps show a good correlation with the physical structure of the gene, even though certain properties of genetic fine-structure maps, such as marker effects and "map expansion," were seen.

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.

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.

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.

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.

Effects of expression of mammalian G alpha and hybrid mammalian-yeast G alpha proteins on the yeast pheromone response signal transduction pathway.

Scg1, the product of the Saccharomyces cerevisiae SCG1 (also called GPA1) gene, is homologous to the alpha subunits of G proteins involved in signal transduction in mammalian cells. Scg1 negatively controls the pheromone response pathway in haploid cells. Either pheromonal activation or an scg1 null mutation relieves the negative control and leads to an arrest of cell growth in the G1 phase of the cell cycle. Expression of rat G alpha s was previously shown to complement the growth defect of scg1 null mutants while not allowing mating. We have extended this analysis to examine the effects of the short form of G alpha s (which lacks 15 amino acids present in the long form), G alpha i2, G alpha o, and Scg1-mammalian G alpha hybrids. In addition, we have found that constructs able to complement scg1 are also able to inhibit the response to pheromone and mating when expressed in a wild-type SCG1 strain. Overexpression of Scg1 has a similar inhibitory effect. These results are consistent with a model proposed for the action of Scg1 as the alpha component of a heterotrimeric G protein in which the beta gamma component (Ste4/Ste18) activates the pheromone response after dissociation from Scg1. They suggest that the G alpha constructs able to complement scg1 can interact with beta gamma to prevent activation of the pathway but are unable to interact with pheromone receptors to activate the pathway.

Evidence that mating by the Saccharomyces cerevisiae gpa1Val50 mutant occurs through the default mating pathway and a suggestion of a role for ubiquitin-mediated proteolysis.

The yeast G alpha subunit, Gpa1p, plays a negative role in the pheromone response pathway. The gpa1Val50 mutant was previously shown to have a growth defect, consistent with the GTPase defect predicted for this mutation, and greatly reduced mating. Various explanations for the mating defect have been proposed. One approach to analyze the gpa1Val50 mating defect involved epistasis analysis. The low mating of the gpa1Val50 mutant was independent of the pheromone receptor; therefore, it results from intracellular activation of the pathway, consistent with a GTPase defect. This result suggests that gpa1Val50 mating occurs through the default rather than the chemotropic pathway involved in pheromone response. We therefore tested the effect of a spa2 mutation on gpa1Val50 mating, because Spa2p has been implicated in the default pathway. The spa2 mutation greatly reduced the mating of the gpa1Val50 mutant, suggesting that gpa1Val50 mating occurs predominantly through the default pathway. In a second approach to investigate the gpa1Val50 phenotypes, suppressors of the gpa1Val50 mating defect were isolated. Two suppressor genes corresponded to SON1/UFD5 and SEN3, which are implicated in ubiquitin-mediated proteolysis. On the basis of these results, we suggest that a positive component of the default mating pathway is subject to ubiquitin-mediated degradation.

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.

Multiple sex pheromones and receptors of a mushroom-producing fungus elicit mating in yeast.

The mushroom-producing fungus Schizophyllum commune has thousands of mating types defined, in part, by numerous lipopeptide pheromones and their G protein-linked receptors. Compatible combinations of pheromones and receptors encoded by different mating types regulate a pathway of sexual development leading to mushroom formation and meiosis. A complex set of pheromone-receptor interactions maximizes the likelihood of outbreeding; for example, a single pheromone can activate more than one receptor and a single receptor can be activated by more than one pheromone. The current study demonstrates that the sex pheromones and receptors of Schizophyllum, when expressed in Saccharomyces cerevisiae, can substitute for endogenous pheromone and receptor and induce the yeast pheromone response pathway through the yeast G protein. Secretion of active Schizophyllum pheromone requires some, but not all, of the biosynthetic machinery used by the yeast lipopeptide pheromone a-factor. The specificity of interaction among pheromone-receptor pairs in Schizophyllum was reproduced in yeast, thus providing a powerful system for exploring molecular aspects of pheromone-receptor interactions for a class of seven-transmembrane-domain receptors common to a wide range of organisms.

Role of alpha-factor and the MF alpha 1 alpha-factor precursor in mating in yeast.

The peptide pheromones secreted by a and alpha cells (called a-factor and alpha-factor, respectively) are each encoded by two structural genes. For strains of either mating type, addition of exogenous pheromone does not alleviate the mating defect of mutants with disruptions of both structural genes. In addition, a particular insertion mutation in the major alpha-factor structural gene (MF alpha 1) that should result in an altered product inhibits alpha mating. These results suggested that the pheromone precursors (the MF alpha 1 pro region in particular) might play a second role in mating separate from the role of pheromone production. To analyze the role of alpha-factor and the MF alpha 1 precursor in alpha mating, we have constructed two classes of mutants. The mating defects of mutants that should produce the MF alpha 1 pro region peptide but no alpha-factor could not be alleviated by addition of exogenous alpha-factor in crosses to a wild-type a strain, indicating that the previous results were not due to an inability of the disruption mutants to produce the pro region peptide. Mutants able to produce alpha-factor, but with a variety of alterations in MF alpha 1 precursor structure, mated at levels proportional to the levels of alpha-factor produced, suggesting that the only role of the alpha-factor precursor in mating is to produce alpha-factor. Both of these results argue against a role for the MF alpha 1 pro region separate from its role in alpha-factor production.(ABSTRACT TRUNCATED AT 250 WORDS)

The N terminus of Saccharomyces cerevisiae Sst2p plays an RGS-domain-independent, Mpt5p-dependent role in recovery from pheromone arrest.

The Saccharomyces cerevisiae RGS protein Sst2p is involved in desensitization to pheromone and acts as a GTPase-activating protein for the Galpha subunit Gpa1p. Other results indicate that Sst2p acts through Mpt5p and that this action occurs downstream of Fus3p and through Cln3p/Cdc28p. Our results indicate that the interaction of Sst2p with Mpt5p requires the N-terminal MPI (Mpt5p-interacting) domain of Sst2p and is independent of the C-terminal RGS domain. Overexpression of the MPI domain results in an Mpt5p-dependent increase in recovery from pheromone arrest. Overexpression of either intact Sst2p or the MPI domain leads to partial suppression of a gpa1 growth defect, and this suppression is dependent on Mpt5p, indicating that MPI function occurs downstream of Gpa1p and through Mpt5p. Combination of an mpt5 mutation with the GPA1(G302S) mutation, which uncouples Gpa1p from Sst2p, results in pheromone supersensitivity similar to the sst2 mutant, and promotion of recovery by overexpression of Sst2p is dependent on both Mpt5p and the Gpa1p interaction. These results indicate that Sst2p is a bifunctional protein and that the MPI domain acts through Mpt5p independently of the RGS domain. RGS family members from other fungi contain N-terminal domains with sequence similarity to the Sst2p MPI domain, suggesting that MPI function may be conserved.

Genetic relationships between the G protein beta gamma complex, Ste5p, Ste20p and Cdc42p: investigation of effector roles in the yeast pheromone response pathway.

The Saccharomyces cerevisiae G protein beta gamma dimer, Ste4p/Ste18p, acts downstream of the alpha subunit, Gpa1p, to activate the pheromone response pathway and therefore must interact with a downstream effector. Synthetic sterile mutants that exacerbate the phenotype of ste4-ts mutations were isolated to identify proteins that functionally interact with Ste4p. The identification of a ste18 mutant indicated that this screen could identify proteins that interact directly with Ste4p. The other mutations were in STE5 and the STE20 kinase gene, which act near Ste4p in the pathway, and a new gene called STE21. ste20 null mutants showed residual mating, suggesting that another kinase may provide some function. Overexpression of Ste5p under galactose control activated the pheromone response pathway. This activation was dependent on Ste4p and Ste18p and partially dependent on Ste20p. These results cannot be explained by the linear pathway of Ste4p-->Ste20p-->Ste5p. Overexpression of Cdc42p resulted in a slight increase in pheromone induction of a reporter gene, and overexpression of activated forms of Cdc42p resulted in a further twofold increase. Mutations in pheromone response pathway components did not suppress the lethality associated with the activated CDC42 mutations, suggesting that this effect is independent of the pheromone response pathway.

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 SCG1 gene: a G alpha-like protein implicated in the a- and alpha-factor response pathway.

We have identified the SCG1 gene by its ability to suppress the pheromone-supersensitive sst2-1 mutation. The nucleotide sequence of SCG1 suggests that it encodes a 54 kd protein homologous to the alpha subunit of the vertebrate G proteins transducin, Gs, Gi, and Go. SCG1 expression and function are haploid-specific; haploid scg1 cells grow into very small colonies consisting of large, abnormally shaped cells, whereas a/alpha scg1/scg1 diploids show wild-type morphology, growth, and sporulation. We postulate that the SCG1 product is involved in the pheromone response pathway, and propose two models for the function of the SCG1 product. Expression of the rat alpha s gene in yeast partially complements both the sst2 and scg1 defects, indicating a high level of conservation of sequence and function between SCG1 and mammalian G alpha subunits.