Degradation of a-factor by a Saccharomyces cerevisiae alpha-mating-type-specific endopeptidase: evidence for a role in recovery of cells from G1 arrest

Pheromone Guidance of Polarity Site Movement in Yeast

Cells' ability to track chemical gradients is integral to many biological phenomena, including fertilization, development, accessing nutrients, and combating infection. Mating of the yeast Saccharomyces cerevisiae provides a tractable model to understand how cells interpret the spatial information in chemical gradients. Mating yeast of the two different mating types secrete distinct peptide pheromones, called a-factor and α-factor, to communicate with potential partners. Spatial gradients of pheromones are decoded to guide mobile polarity sites so that polarity sites in mating partners align towards each other, as a prerequisite for cell-cell fusion and zygote formation. In ascomycetes including S. cerevisiae, one pheromone is prenylated (a-factor) while the other is not (α-factor). The difference in physical properties between the pheromones, combined with associated differences in mechanisms of secretion and extracellular pheromone metabolism, suggested that the pheromones might differ in the spatial information that they convey to potential mating partners. However, as mating appears to be isogamous in this species, it is not clear why any such signaling difference would be advantageous. Here we report assays that directly track movement of the polarity site in each partner as a way to understand the spatial information conveyed by each pheromone. Our findings suggest that both pheromones convey very similar information. We speculate that the different pheromones were advantageous in ancestral species with asymmetric mating systems and may represent an evolutionary vestige in yeasts that mate isogamously.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Sensory input attenuation allows predictive sexual response in yeast

Animals are known to adjust their sexual behaviour depending on mate competition. Here we report similar regulation for mating behaviour in a sexual unicellular eukaryote, the budding yeast Saccharomyces cerevisiae. We demonstrate that pheromone-based communication between the two mating types, coupled to input attenuation by recipient cells, enables yeast to robustly monitor relative mate abundance (sex ratio) within a mixed population and to adjust their commitment to sexual reproduction in proportion to their estimated chances of successful mating. The mechanism of sex-ratio sensing relies on the diffusible peptidase Bar1, which is known to degrade the pheromone signal produced by mating partners. We further show that such a response to sexual competition within a population can optimize the fitness trade-off between the costs and benefits of mating response induction. Our study thus provides an adaptive explanation for the known molecular mechanism of pheromone degradation in yeast.

Cells of the yeast Saccharomyces cerevisiae can mate with other cells of opposite mating type. Here, the authors show that the combination of a pheromone and a pheromone-degrading enzyme allows yeast cells to monitor relative mate abundance within a population and adjust their commitment to sexual reproduction.

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.

Biogenesis of the Saccharomyces cerevisiae pheromone a-factor, from yeast mating to human disease

The mating pheromone a-factor secreted by Saccharomyces cerevisiae is a farnesylated and carboxylmethylated peptide and is unusually hydrophobic compared to other extracellular signaling molecules. Mature a-factor is derived from a precursor with a C-terminal CAAX motif that directs a series of posttranslational reactions, including prenylation, endoproteolysis, and carboxylmethylation. Historically, a-factor has served as a valuable model for the discovery and functional analysis of CAAX-processing enzymes. In this review, we discuss the three modules comprising the a-factor biogenesis pathway: (i) the C-terminal CAAX-processing steps carried out by Ram1/Ram2, Ste24 or Rce1, and Ste14; (ii) two sequential N-terminal cleavage steps, mediated by Ste24 and Axl1; and (iii) export by a nonclassical mechanism, mediated by the ATP binding cassette (ABC) transporter Ste6. The small size and hydrophobicity of a-factor present both challenges and advantages for biochemical analysis, as discussed here. The enzymes involved in a-factor biogenesis are conserved from yeasts to mammals. Notably, studies of the zinc metalloprotease Ste24 in S. cerevisiae led to the discovery of its mammalian homolog ZMPSTE24, which cleaves the prenylated C-terminal tail of the nuclear scaffold protein lamin A. Mutations that alter ZMPSTE24 processing of lamin A in humans cause the premature-aging disease progeria and related progeroid disorders. Intriguingly, recent evidence suggests that the entire a-factor pathway, including all three biogenesis modules, may be used to produce a prenylated, secreted signaling molecule involved in germ cell migration in Drosophila. Thus, additional prenylated signaling molecules resembling a-factor, with as-yet-unknown roles in metazoan biology, may await discovery.

Anion activation site of insulin-degrading enzyme

Insulin-degrading enzyme (IDE) (insulysin) is a zinc metallopeptidase that metabolizes several bioactive peptides, including insulin and the amyloid β peptide. IDE is an unusual metallopeptidase in that it is allosterically activated by both small peptides and anions, such as ATP. Here, we report that the ATP-binding site is located on a portion of the substrate binding chamber wall arising largely from domain 4 of the four-domain IDE. Two variants having residues in this site mutated, IDEK898A,K899A,S901A and IDER429S, both show greatly decreased activation by the polyphosphate anions ATP and PPPi. IDEK898A,K899A,S901A is also deficient in activation by small peptides, suggesting a possible mechanistic link between the two types of allosteric activation. Sodium chloride at high concentrations can also activate IDE. There are no observable differences in average conformation between the IDE-ATP complex and unliganded IDE, but regions of the active site and C-terminal domain do show increased crystallographic thermal factors in the complex, suggesting an effect on dynamics. Activation by ATP is shown to be independent of the ATP hydrolysis activity reported for the enzyme. We also report that IDEK898A,K899A,S901A has reduced intracellular function relative to unmodified IDE, consistent with a possible role for anion activation of IDE activity in vivo. Together, the data suggest a model in which the binding of anions activates by reducing the electrostatic attraction between the two halves of the enzyme, shifting the partitioning between open and closed conformations of IDE toward the open form.

Chemical gradients and chemotropism in yeast

Chemical gradients of peptide mating pheromones are necessary for directional growth, which is critical for yeast mating. These gradients are generated by cell-type specific secretion or export and specific degradation in receiving cells. Spatial information is sensed by dedicated seven-transmembrane G-protein coupled receptors and yeast cells are able to detect extremely small differences in ligand concentration across their approximately 5-microm cell surface. Here, I will discuss our current knowledge of how cells detect and respond to such shallow chemical gradients and in particular what is known about the proteins that are involved in directional growth and the establishment of the polarity axis during yeast mating.

A common genetic system for functional studies of pitrilysin and related M16A proteases

Pitrilysin is a bacterial protease that is similar to the mammalian insulin-degrading enzyme, which is hypothesized to protect against the onset of Alzheimer's disease, and the yeast enzymes Axl1p and Ste23p, which are responsible for production of the a-factor mating pheromone in Saccharomyces cerevisiae. The lack of a phenotype associated with pitrilysin deficiency has hindered studies of this enzyme. Herein, we report that pitrilysin can be heterologously expressed in yeast such that it functionally substitutes for the shared roles of Axl1p and Ste23p in pheromone production, resulting in a readily observable phenotype. We have exploited this phenotype to conduct structure-function analyses of pitrilysin and report that residues within four sequence motifs that are highly conserved among M16A enzymes are essential for its activity. These motifs include the extended metalloprotease motif, a second motif that has been hypothesized to be important for the function of M16A enzymes, and two others not previously recognized as being important for pitrilysin function. We have also established that the two self-folding domains of pitrilysin are both required for its proteolytic activity. However, pitrilysin does not possess all the enzymatic properties of the yeast enzymes since it cannot substitute for the role of Axl1p in the repression of haploid invasive growth. These observations further support the utility of the yeast system for structure-function and comparative studies of M16A enzymes.

Effect of the pheromone-responsive G(alpha) and phosphatase proteins of Saccharomyces cerevisiae on the subcellular localization of the Fus3 mitogen-activated protein kinase

The mating-specific G(alpha) protein of Saccharomyces cerevisiae, Gpa1, stimulates adaptation to pheromone by a mechanism independent of G(beta gamma) sequestration. Genetic evidence suggests that Gpa1 targets the Fus3 mitogen-activated protein kinase, and it has recently been shown that the two proteins interact in cells responding to pheromone. To test the possibility that Gpa1 downregulates the mating signal by affecting the localization of Fus3, we created a Fus3-green fluorescent protein (GFP) fusion protein. In vegetative cells, Fus3-GFP was found in both the cytoplasm and the nucleus. Pheromone stimulated a measurable increase in the ratio of nuclear to cytoplasmic Fus3-GFP. In contrast, the relative level of nuclear Fus3-GFP decreased as cells recovered from pheromone arrest and did not increase when cells adapted to chronic stimulus were challenged again. Accumulation of Fus3-GFP in the nuclei of stimulated cells was also inhibited by overexpression of either wild-type Gpa1, the E364K hyperadaptive mutant form of Gpa1, or the Msg5 dually specific phosphatase. The effects of Gpa1 and Msg5 on Fus3 are partially interdependent. In a genetic screen for adaptive defective mutants, a nonsense allele of the nucleocytoplasmic transport receptor, Kap104, was identified. Truncation of the Kap104 cargo-binding domain blocked the effect of both Gpa1(E364K) and Msg5 on Fus3-GFP localization. Based on these results, we propose that Gpa1 and Msg5 work in concert to downregulate the mating signal and that they do so by inhibiting the pheromone-induced increase of Fus3 in the nucleus. Kap104 is required for the G(alpha)/phosphatase-mediated effect on Fus3 localization.

Receptor inhibition of pheromone signaling is mediated by the Ste4p Gbeta subunit

The pheromone response pathway of the yeast Saccharomyces cerevisiae is initiated in MATa cells by binding of alpha-factor to the alpha-factor receptor. MATa cells in which the a-factor receptor is inappropriately expressed exhibit reduced pheromone signaling, a phenomenon termed receptor inhibition. In cells undergoing receptor inhibition, activation of the signaling pathway occurs normally at early time points but decreases after prolonged exposure to pheromone. Mutations that suppress the effects of receptor inhibition were obtained in the STE4 gene, which encodes the beta-subunit of the G protein that transmits the pheromone response signal. These mutations mapped to the N terminus and second WD repeat of Ste4p in regions that are not part of its Galpha binding surface. A STE4 allele containing several of these mutations, called STE4(SD13), reversed the signaling defect seen at late times in cells undergoing receptor inhibition but had no effect on the basal activity of the pathway. Moreover, the signaling properties of STE4(SD13) were indistinguishable from those of STE4 in wild-type MATa and MATalpha cells. These results demonstrate that the effect of the STE4(SD13) allele is specific to the receptor inhibition function of STE4. STE4(SD13) suppressed the signaling defect conferred by receptor inhibition in a MATa strain containing a deletion of GPA1, the G protein alpha-subunit gene; however, STE4(SD13) had no effect in a MATalpha strain containing a GPA1 deletion. Suppression of receptor inhibition by STE4(SD13) in a MATa strain containing a GPA1 deletion was unaffected by deletion of STE2, the alpha-factor receptor gene. The results presented here are consistent with a model in which an a-specific gene product other than Ste2p detects the presence of the a-factor receptor and blocks signaling by inhibiting the function of Ste4p.

Substitutions in the pheromone-responsive Gbeta protein of Saccharomyces cerevisiae confer a defect in recovery from pheromone treatment

The pheromone-responsive Galpha protein of Saccharomyces cerevisiae, Gpa1p, stimulates an adaptive mechanism that downregulates the mating signal. In a genetic screen designed to identify signaling elements required for Gpa1p-mediated adaptation, a large collection of adaptive-defective (Adp-) mutants were recovered. Of the 49 mutants characterized thus far, approximately three-quarters exhibit a dominant defect in the negative regulation of the pheromone response. Eight of the dominant Adp- mutations showed tight linkage to the gene encoding the pheromone-responsive Gbeta, STE4. Sequence analysis of the STE4 locus in the relevant mutant strains revealed seven novel STE4 alleles, each of which was shown to disrupt proper regulation of the pheromone response. Although the STE4 mutations had only minor effects on basal mating pathway activity, the mutant forms of Gbeta dramatically affected the ability of the cell to turn off the mating response after exposure to pheromone. Moreover, the signaling activity of the aberrant Gbetagamma subunits was suppressed by G322E, a mutant form of Gpa1p that blocks the pheromone response by sequestering Gbetagamma, but not by E364K, a hyperadaptive form of Gpa1p. On the basis of these observations, we propose that Gpa1p-mediated adaptation involves the binding of an unknown negative regulator to Gbetagamma.

The mating-specific G(alpha) protein of Saccharomyces cerevisiae downregulates the mating signal by a mechanism that is dependent on pheromone and independent of G(beta)(gamma) sequestration

It has been inferred from compelling genetic evidence that the pheromone-responsive G(alpha) protein of Saccharomyces cerevisiae, Gpa1, directly inhibits the mating signal by binding to its own beta(gamma) subunit. Gpa1 has also been implicated in a distinct but as yet uncharacterized negative regulatory mechanism. We have used three mutant alleles of GPA1, each of which confers resistance to otherwise lethal doses of pheromone, to explore this possibility. Our results indicate that although the G322E allele of GPA1 completely blocks the pheromone response, the E364K allele promotes recovery from pheromone treatment rather than insensitivity to it. This observation suggests that Gpa1, like other G(alpha) proteins, interacts with an effector molecule and stimulates a positive signal--in this case, an adaptive signal. Moreover, the Gpa1-mediated adaptive signal is itself induced by pheromone, is delayed relative to the mating signal, and does not involve sequestration of G(beta)(gamma). The behavior of N388D, a mutant form of Gpa1 predicted to be activated, strongly supports these conclusions. Although N388D cannot sequester beta(gamma), as evidenced by two-hybrid analysis and its inability to complement a Gpa1 null allele under normal growth conditions, it can stimulate adaptation and rescue a gpa1(delta) strain when cells are exposed to pheromone. Considered as a whole, our data suggest that the pheromone-responsive heterotrimeric G protein of S. cerevisiae has a self-regulatory signaling function. Upon activation, the heterotrimer dissociates into its two subunits, one of which stimulates the pheromone response, while the other slowly induces a negative regulatory mechanism that ultimately shuts off the mating signal downstream of the receptor.

Schizosaccharomyces pombe sxa1+ and sxa2+ encode putative proteases involved in the mating response

The Schizosaccharomyces pombe sxa1 and sxa2 mutants showed an exaggerated response to mating pheromones, producing excessively long conjugation tubes and exhibiting mating deficiency. This phenotype was similar to phenotypes of cells bearing an activated allele of ras1, such as ras1Val-17 or ras1Leu-66, and phenotypes of cells defective in gap1. However, genetic evidence suggested that the sxa1 and sxa2 gene products are not directly involved in the Ras1 pathway. The gene products of sxa1 and sxa2, as deduced from their nucleotide sequences, were homologous to aspartyl proteases and serine carboxypeptidases, respectively. The sxa1 gene function was required for efficient mating only in h+ cells, although even disruption of sxa1 did not completely abolish the mating ability. Conversely, the sxa2 gene function was required only in h- cells. Wild-type cells produced a diffusible substance, which may be the sxa2 gene product itself, that could confer fertility to sxa2 mutant cells placed at a distance. These observations are consistent with the possibility that the sxa gene products are involved in degradation or processing of the mating pheromones and that their loss cause a persistent response to the pheromones.

Schizosaccharomyces pombe sxa1+ and sxa2+ encode putative proteases involved in the mating response

The Schizosaccharomyces pombe sxa1 and sxa2 mutants showed an exaggerated response to mating pheromones, producing excessively long conjugation tubes and exhibiting mating deficiency. This phenotype was similar to phenotypes of cells bearing an activated allele of ras1, such as ras1Val-17 or ras1Leu-66, and phenotypes of cells defective in gap1. However, genetic evidence suggested that the sxa1 and sxa2 gene products are not directly involved in the Ras1 pathway. The gene products of sxa1 and sxa2, as deduced from their nucleotide sequences, were homologous to aspartyl proteases and serine carboxypeptidases, respectively. The sxa1 gene function was required for efficient mating only in h+ cells, although even disruption of sxa1 did not completely abolish the mating ability. Conversely, the sxa2 gene function was required only in h- cells. Wild-type cells produced a diffusible substance, which may be the sxa2 gene product itself, that could confer fertility to sxa2 mutant cells placed at a distance. These observations are consistent with the possibility that the sxa gene products are involved in degradation or processing of the mating pheromones and that their loss cause a persistent response to the pheromones.

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.

Significance of C-terminal cysteine modifications to the biological activity of the Saccharomyces cerevisiae a-factor mating pheromone

We have undertaken total synthesis of the Saccharomyces cerevisiae a-factor (NH2-YIIKGVFWDPAC[S-farnesyl]-COOCH3) and several Cys-12 analogs to determine the significance of S-farnesylation and carboxy-terminal methyl esterification to the biological activity of this lipopeptide mating pheromone. Replacement of either the farnesyl group or the carboxy-terminal methyl ester by a hydrogen atom resulted in marked reduction but not total loss of bioactivity as measured by a variety of assays. Moreover, both the farnesyl and methyl ester groups could be replaced by other substituents to produce biologically active analogs. The bioactivity of a-factor decreased as the number of prenyl units on the cysteine sulfur decreased from three to one, and an a-factor analog having the S-farnesyl group replaced by an S-hexadecanyl group was more active than an S-methyl a-factor analog. Thus, with two types of modifications, a-factor activity increased as the S-alkyl group became bulkier and more hydrophobic. MATa cells having deletions of the a-factor structural genes (mfal1 mfa2 mutants) were capable of mating with either sst2 or wild-type MAT alpha cells in the presence of exogenous a-factor, indicating that it is not absolutely essential for MATa cells to actively produce a-factor in order to mate. Various a-factor analogs were found to partially restore mating to these strains as well, and their relative activities in the mating restoration assay were similar to their activities in the other assays used in this study. Mating was not restored by addition of exogenous a-factor to a cross of a wild-type MAT alpha strain and a MATaste6 mutant, indicating a role of the STE6 gene product in mating in addition to its secretion of a-factor.

Significance of C-terminal cysteine modifications to the biological activity of the Saccharomyces cerevisiae a-factor mating pheromone

We have undertaken total synthesis of the Saccharomyces cerevisiae a-factor (NH2-YIIKGVFWDPAC[S-farnesyl]-COOCH3) and several Cys-12 analogs to determine the significance of S-farnesylation and carboxy-terminal methyl esterification to the biological activity of this lipopeptide mating pheromone. Replacement of either the farnesyl group or the carboxy-terminal methyl ester by a hydrogen atom resulted in marked reduction but not total loss of bioactivity as measured by a variety of assays. Moreover, both the farnesyl and methyl ester groups could be replaced by other substituents to produce biologically active analogs. The bioactivity of a-factor decreased as the number of prenyl units on the cysteine sulfur decreased from three to one, and an a-factor analog having the S-farnesyl group replaced by an S-hexadecanyl group was more active than an S-methyl a-factor analog. Thus, with two types of modifications, a-factor activity increased as the S-alkyl group became bulkier and more hydrophobic. MATa cells having deletions of the a-factor structural genes (mfal1 mfa2 mutants) were capable of mating with either sst2 or wild-type MAT alpha cells in the presence of exogenous a-factor, indicating that it is not absolutely essential for MATa cells to actively produce a-factor in order to mate. Various a-factor analogs were found to partially restore mating to these strains as well, and their relative activities in the mating restoration assay were similar to their activities in the other assays used in this study. Mating was not restored by addition of exogenous a-factor to a cross of a wild-type MAT alpha strain and a MATaste6 mutant, indicating a role of the STE6 gene product in mating in addition to its secretion of a-factor.