Asg7p-Ste3p inhibition of pheromone signaling: regulation of the zygotic transition to vegetative growth

Limits to transcriptional silencing in Saccharomyces cerevisiae

Mating-type switching in the budding yeast Saccharomyces cerevisiae relies on the Sir protein complex to silence HML and HMR, the two loci containing copies of the alleles of the mating type locus, MAT. Sir-based transcriptional silencing has been considered locus-specific, but the recent discovery of rare and transient escapes from silencing at HMLα2 with a sensitive assay called to question if these events extend to the whole locus. Adapting the same assay, we measured that transient silencing failures at HML were more frequent for the α2 gene than α1, similarly to their expression level in unsilenced cells. By coupling a mating assay, at HML we found that one of the two genes at that locus can be transiently expressed while the other gene is maintained silent. Thus, transient silencing loss can be a property of the gene rather than the locus. Cells lacking the SIR1 gene experience epigenetic bistability at HML and HMR. Our previous result led us to ask if HML could allow for two independent epigenetic states within the locus in a sir1Δ mutant. A simple construct using a double fluorescent reporter at HMLα1 and HMLα2 ruled out this possibility. Each HML locus displayed a single epigenetic state. We revisited the question of the correlation between the states of two HML loci in diploid cells, and showed they were independent. Finally, we determined the relative strength of gene repression achieved by Sir-based silencing with that achieved by the a1-α2 repressor.

Post-Transcriptional Control of Mating-Type Gene Expression during Gametogenesis in Saccharomyces cerevisiae

Gametogenesis in diploid cells of the budding yeast Saccharomyces cerevisiae produces four haploid meiotic products called spores. Spores are dormant until nutrients trigger germination, when they bud asexually or mate to return to the diploid state. Each sporulating diploid produces a mix of spores of two haploid mating types, a and α. In asexually dividing haploids, the mating types result from distinct, mutually exclusive gene expression programs responsible for production of mating pheromones and the receptors to sense them, all of which are silent in diploids. It was assumed that spores only transcribe haploid- and mating-type-specific genes upon germination. We find that dormant spores of each mating type harbor transcripts representing all these genes, with the exception of Mata1, which we found to be enriched in a spores. Mata1 transcripts, from a rare yeast gene with two introns, were mostly unspliced. If the retained introns reflect tethering to the MATa locus, this could provide a mechanism for biased inheritance. Translation of pheromones and receptors were repressed at least until germination. We find antisense transcripts to many mating genes that may be responsible. These findings add to the growing number of examples of post-transcriptional regulation of gene expression during gametogenesis.

Mechanisms that ensure monogamous mating in Saccharomyces cerevisiae

Haploid cells of the budding yeast Saccharomyces cerevisiae communicate using secreted pheromones and mate to form diploid zygotes. Mating is monogamous, resulting in the fusion of precisely one cell of each mating type. Monogamous mating in crowded conditions, where cells have access to more than one potential partner, raises the question of how multiple-mating outcomes are prevented. Here we identify mutants capable of mating with multiple partners, revealing the mechanisms that ensure monogamous mating. Before fusion, cells develop polarity foci oriented toward potential partners. Competition between these polarity foci within each cell leads to disassembly of all but one focus, thus favoring a single fusion event. Fusion promotes the formation of heterodimeric complexes between subunits that are uniquely expressed in each mating type. One complex shuts off haploid-specific gene expression, and the other shuts off the ability to respond to pheromone. Zygotes able to form either complex remain monogamous, but zygotes lacking both can re-mate.

Genetically engineered transvestites reveal novel mating genes in budding yeast

Haploid budding yeast has two mating types, defined by the alleles of the MAT locus, MATa and MATα. Two haploid cells of opposite mating types mate by signaling to each other using reciprocal pheromones and receptors, polarizing and growing toward each other, and eventually fusing to form a single diploid cell. The pheromones and receptors are necessary and sufficient to define a mating type, but other mating-type-specific proteins make mating more efficient. We examined the role of these proteins by genetically engineering "transvestite" cells that swap the pheromone, pheromone receptor, and pheromone processing factors of one mating type for another. These cells mate with each other, but their mating is inefficient. By characterizing their mating defects and examining their transcriptomes, we found Afb1 (a-factor barrier), a novel MATα-specific protein that interferes with a-factor, the pheromone secreted by MATa cells. Strong pheromone secretion is essential for efficient mating, and the weak mating of transvestites can be improved by boosting their pheromone production. Synthetic biology can characterize the factors that control efficiency in biological processes. In yeast, selection for increased mating efficiency is likely to have continually boosted pheromone levels and the ability to discriminate between partners who make more and less pheromone. This discrimination comes at a cost: weak mating in situations where all potential partners make less pheromone.

Asymmetry in sexual pheromones is not required for ascomycete mating

BACKGROUND

We investigated the determinants of sexual identity in the budding yeast Saccharomyces cerevisiae. The higher fungi are divided into the ascomycetes and the basidiomycetes. Most ascomycetes have two mating types: one (called α in yeasts and MAT1-1 in filamentous fungi) produces a small, unmodified, peptide pheromone, and the other (a in yeasts and MAT1-2 in filamentous fungi) produces a peptide pheromone conjugated to a C-terminal farnesyl group that makes it very hydrophobic. In the basidiomycetes, all pheromones are lipid-modified, and this difference is a distinguishing feature between the phyla. We asked whether the asymmetry in pheromone modification is required for successful mating in ascomycetes.

RESULTS

We cloned receptor and pheromone genes from a filamentous ascomycete and a basidiomycete and expressed these in the budding yeast, Saccharomyces cerevisiae, to generate novel, alternative mating pairs. We find that two yeast cells can mate even when both cells secrete a-like or α-like peptides. Importantly, this is true regardless of whether the cells express the a- or α-mating-type loci, which control the expression of other, sex-specific genes, in addition to the pheromones and pheromone receptors.

CONCLUSIONS

We demonstrate that the asymmetric pheromone modification is not required for successful mating of ascomycete fungi and confirm that, in budding yeast, the primary determinants of mating are the specificity of the receptors and their corresponding pheromones.

Interspecies variation reveals a conserved repressor of alpha-specific genes in Saccharomyces yeasts

The mating-type determination circuit in Saccharomyces yeast serves as a classic paradigm for the genetic control of cell type in all eukaryotes. Using comparative genetics, we discovered a central and conserved, yet previously undetected, component of this genetic circuit: active repression of alpha-specific genes in a cells. Upon inactivation of the SUM1 gene in Saccharomyces bayanus, a close relative of Saccharomyces cerevisiae, a cells acquired mating characteristics of alpha cells and displayed autocrine activation of their mating response pathway. Sum1 protein bound to the promoters of alpha-specific genes, repressing their transcription. In contrast to the standard model, alpha1 was important but not required for alpha-specific gene activation and mating of alpha cells in the absence of Sum1. Neither Sum1 protein expression, nor its association with target promoters was mating-type-regulated. Thus, the alpha1/Mcm1 coactivators did not overcome repression by occluding Sum1 binding to DNA. Surprisingly, the mating-type regulatory function of Sum1 was conserved in S. cerevisiae. We suggest that a comprehensive understanding of some genetic pathways may be best attained through the expanded phenotypic space provided by study of those pathways in multiple related organisms.

A walk-through of the yeast mating pheromone response pathway

The intracellular signal transduction pathway by which the yeast Saccharomyces cerevisiae responds to the presence of peptide mating pheromone in its surroundings is one of the best understood signaling pathways in eukaryotes, yet continues to generate new surprises and insights. In this review, we take a brief walk down the pathway, focusing on how the signal is transmitted from the cell-surface receptor-coupled G protein, via a MAP kinase cascade, to the nucleus.

A walk-through of the yeast mating pheromone response pathway

The intracellular signal transduction pathway by which the yeast Saccharomyces cerevisiae responds to the presence of peptide mating pheromone in its surroundings is one of the best understood signaling pathways in eukaryotes, yet continues to generate new surprises and insights. In this review, we take a brief walk down the pathway, focusing on how the signal is transmitted from the cell-surface receptor-coupled G protein, via a MAP kinase cascade, to the nucleus.

Autocrine activation of the pheromone response pathway in matalpha2- cells is attenuated by SST2- and ASG7-dependent mechanisms

Yeast mat alpha2 mutants express both mating pheromones and both mating pheromone receptors. They show modest signaling in the pheromone response pathway, as revealed by increased levels of FUS1 transcript, yet are resistant to pheromone treatment. Together, these phenotypes suggest that alpha2- cells undergo autocrine activation of the pheromone response pathway, which is subsequently attenuated. We constructed a regulatable version of the alpha2 gene (GALalpha2) and showed that, upon loss of alpha2 activity, cells exhibit an initial robust response to pheromone that is attenuated within 3 h. We reasoned that the viability of alpha2- cells might be due to attenuation, and therefore performed a genome-wide synthetic lethal screen to identify potential adaptation components. We identified two genes, SST2 and ASG7. Loss of either of these attenuation components results in activation of the pheromone pathway in alpha2- cells. Loss of both proteins causes a more severe phenotype. Sst2 functions as a GTPase activating protein (GAP) for the Galpha subunit of the trimeric G protein. Asg7 is an a -cell specific protein that acts in concert with the alpha-cell specific a -factor receptor, Ste3, to inhibit signaling by Gbetagamma. Hence, our results suggest that mat alpha2 mutants mimic the intracellular signaling events that occur in newly fused zygotes.

Ubiquitin-dependent degradation of the yeast Mat(alpha)2 repressor enables a switch in developmental state

Developmental transitions in eukaryotic cell lineages revolve around two general processes: the dismantling of the regulatory program specifying an initial differentiated state and its replacement by a new system of regulators. However, relatively little is known about the mechanisms by which a previous regulatory state is inactivated. Protein degradation is implicated in a few examples, but the molecular reasons that a formerly used regulator must be removed are not understood. Many yeast strains undergo a developmental transition in which cells of one mating type differentiate into a distinct cell type by a programmed genetic rearrangement at the MAT locus. We find that Mat(alpha)2, a MAT-encoded transcriptional repressor that is key to creating several cell types, must be rapidly degraded for cells to switch their mating phenotype properly. Strikingly, ubiquitin-dependent proteolysis of alpha2 is required for two mechanistically distinct purposes: It allows the timely inactivation of one transcriptional repressor complex, and it prevents the de novo assembly of a different, inappropriate regulatory complex. Analogous epigenetic mechanisms for reprogramming transcription are likely to operate in many developmental pathways.

Cell-type-dependent repression of yeast a-specific genes requires Itc1p, a subunit of the Isw2p-Itc1p chromatin remodelling complex

In Saccharomyces cerevisiae MATa haploid cells, the a-specific genes are expressed, whereas in the MATalpha haploid and MATa/alpha diploid cell types their transcription is repressed. It is shown in this report that Itc1p, a component of the ATP-dependent Isw2p-Itc1p chromatin remodelling complex, is required for the repression of a-specific genes. It has previously been reported that disruption of the ITC1 gene leads, in MATalpha cells, to an aberrant cell morphology resembling the polarized mating projection of cells responding to pheromone. The activation of the pheromone signalling pathway in itc1 mutants of both mating types was examined and found to be constitutively active in MATalpha itc1 but not in MATa itc1 cells. Furthermore, unlike the wild-type, MATalpha itc1 and MATa/alpha itc1/itc1 cells secrete a-factor and express significant levels of other a-specific genes. The results indicate that the inappropriate a-factor production in a MATalpha context, due to the derepression of the a-specific genes, produces an autocrine signalling loop that leads to the aberrant morphology displayed by MATalpha itc1 cells. It is suggested that the Isw2p-Itc1p complex contributes to maintain the repressive chromatin structure described for the asg operator present in the promoters of a-specific genes.

Rst1 and Rst2 are required for the a/alpha diploid cell type in yeast

In the budding yeast Saccharomyces cerevisiae, the preservation of the mating competent haploid (a or alpha) and the mating incompetent diploid (a/alpha) is necessary to prevent aneuploidy. Once haploid cells respond to pheromone, the mating-specific signal transduction pathway is activated, and the MAP kinase Fus3 phosphorylates two specific repressor proteins Rst1 and Rst2 (also known as Dig1 and Dig2) to promote Ste12-dependent transcription of mating-specific genes. In contrast, diploid cells cannot mate because genes that encode components of the mating pathway are repressed through the combined action of the Mata1-Matalpha2 and Matalpha2-Mcm1 repressors. Surprisingly, repression of Ste12 by Rst1 and Rst2 is essential for diploid sterility. Homozygous deletion of both RST1 and RST2 (rst-) causes a/alpha diploid cells constitutively to express a-specific genes and mate preferentially as a-cells. This phenotype is sensitive to Ste12 dosage, as removal of one copy of STE12 completely reduces the ectopic activation of a-specific genes. The Matalpha2-Mcm1 complex, which normally represses a-specific genes, is defective in rst- diploids because Matalpha2 is destabilized in rst- diploids, possibly as a consequence of its relocalization from the nucleus to the cytoplasm. This study finds that Rst1 and Rst2 are necessary for the a/alpha diploid cell type. Rst1 and Rst2 are required in order to prevent the amplification of a robust Ste12 transcriptional programme that appears to over-ride Matalpha2-dependent repression of haploid and a-specific genes.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

G proteins and pheromone signaling

All cells have the capacity to respond to chemical and sensory stimuli. Central to many such signaling pathways is the heterotrimeric G protein, which transmits a signal from cell surface receptors to intracellular effectors. Recent studies using the yeast Saccharomyces cerevisiae have produced important advances in our understanding of G protein activation and inactivation. This review focuses on the mechanisms by which G proteins transmit a signal from peptide pheromone receptors to the mating response in yeast and how mechanisms elucidated in yeast can provide insights to signaling events in more complex organisms.

Regulation of G protein-initiated signal transduction in yeast: paradigms and principles

All cells have the capacity to evoke appropriate and measured responses to signal molecules (such as peptide hormones), environmental changes, and other external stimuli. Tremendous progress has been made in identifying the proteins that mediate cellular response to such signals and in elucidating how events at the cell surface are linked to subsequent biochemical changes in the cytoplasm and nucleus. An emerging area of investigation concerns how signaling components are assembled and regulated (both spatially and temporally), so as to control properly the specificity and intensity of a given signaling pathway. A related question under intensive study is how the action of an individual signaling pathway is integrated with (or insulated from) other pathways to constitute larger networks that control overall cell behavior appropriately. This review describes the signal transduction pathway used by budding yeast (Saccharomyces cerevisiae) to respond to its peptide mating pheromones. This pathway is comprised by receptors, a heterotrimeric G protein, and a protein kinase cascade all remarkably similar to counterparts in multicellular organisms. The primary focus of this review, however, is recent advances that have been made, using primarily genetic methods, in identifying molecules responsible for regulation of the action of the components of this signaling pathway. Just as many of the constituent proteins of this pathway and their interrelationships were first identified in yeast, the functions of some of these regulators have clearly been conserved in metazoans, and others will likely serve as additional models for molecules that carry out analogous roles in higher organisms.

Localization and signaling of G(beta) subunit Ste4p are controlled by a-factor receptor and the a-specific protein Asg7p

Haploid yeast cells initiate pheromone signaling upon the binding of pheromone to its receptor and activation of the coupled G protein. A regulatory process termed receptor inhibition blocks pheromone signaling when the a-factor receptor is inappropriately expressed in MATa cells. Receptor inhibition blocks signaling by inhibiting the activity of the G protein beta subunit, Ste4p. To investigate how Ste4p activity is inhibited, its subcellular location was examined. In wild-type cells, alpha-factor treatment resulted in localization of Ste4p to the plasma membrane of mating projections. In cells expressing the a-factor receptor, alpha-factor treatment resulted in localization of Ste4p away from the plasma membrane to an internal compartment. An altered version of Ste4p that is largely insensitive to receptor inhibition retained its association with the membrane in cells expressing the a-factor receptor. The inhibitory function of the a-factor receptor required ASG7, an a-specific gene of previously unknown function. ASG7 RNA was induced by pheromone, consistent with increased inhibition as the pheromone response progresses. The a-factor receptor inhibited signaling in its liganded state, demonstrating that the receptor can block the signal that it initiates. ASG7 was required for the altered localization of Ste4p that occurs during receptor inhibition, and the subcellular location of Asg7p was consistent with its having a direct effect on Ste4p localization. These results demonstrate that Asg7p mediates a regulatory process that blocks signaling from a G protein beta subunit and causes its relocalization within the cell.