Pheromone-induced signal transduction in Saccharomyces cerevisiae requires the sequential function of three protein kinases

Characterization and expression pattern analysis of pheromone receptor-like genes in Winter Mushroom Flammulina filiformis

Pheromone receptor-like genes (PRLGs) belong to the G protein-coupled receptors (GPCRs) family that interacts with biotic and abiotic stimulants and transmits signals to intracellular downstream pathways in eukaryotic cells. In this study, we investigated the structure and expressions patterns of PRLGs in Winter Mushroom Flammulina filiformis. Based on the alignment analysis, the structure of PRLGs was found conserved in F. filiformis strains expect few single-nucleotide polymorphism (SNP) sites. Six PRLGs were found at five different unlinked loci, scattered in the genomes of F. filiformis strains. These genes contain 2-5 introns; however, the introns were not found in the same relative positions regarding the encoded protein sequences in tested strains of F. filiformis. Three conserved motifs were identified in peptides structures of PRLGs, however, FfSte3.s6 contained only two types, suggests its difference in evolution and function. We have further analyzed the expression patterns of each PRLGs in different developmental stages of the fruiting body in F. filiformis by quantitative real-time polymerase chain reaction (qRT-PCR). The results exhibited expression variation of PRLGs at different developmental stages of the F. filiformis. Especially, FfSte3.s1 and FfSte3.s2 exhibited maximum expression level in mycelia stage. Other PRLGs exhibited high expression level in fruiting body stages. This study suggests that PRLGs could be vital genes involving in fruiting body development in F. filiformis. However, further studies could be performed to reveal their specific functional pathways in the fruiting body development.

Crosstalk Between Pheromone Signaling and NADPH Oxidase Complexes Coordinates Fungal Developmental Processes

Sexual and asexual development in filamentous ascomycetes is controlled by components of conserved signaling pathways. Here, we investigated the development of mutant strains lacking genes for kinases MAK2, MEK2, and MIK2, as well as the scaffold protein HAM5 of the pheromone response (PR) pathway. All had a defect in fruiting body development and hyphal fusion. Another phenotype was a defect in melanin-dependent ascospore germination. However, this deficiency was observed only in kinase deletion mutants, but not in strains lacking HAM5. Notably, the same developmental phenotypes were previously described for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 (NOX1) mutants, but the germination defect was only seen in NOX2 mutants. These data suggest a molecular link between the pheromone signaling pathway and both NOX complexes. Using data from yeast two-hybrid (Y2H) analysis, we found that the scaffolding protein HAM5 interacts with NOR1, the regulator of NOX1 and NOX2 complexes. This interaction was further confirmed using differently fluorescent-labeled proteins to demonstrate that NOR1 and HAM5 co-localize at cytoplasmic spots and tips of mature hyphae. This observation was supported by phenotypic characterization of single and double mutants. The oxidative stress response and the initiation of fruiting bodies were similar in Δham5Δnor1 and Δham5, but distinctly reduced in Δnor1, indicating that the double deletion leads to a partial suppression of the Δnor1 phenotype. We conclude that the PR and NOX1 complexes are connected by direct interaction between HAM5 and NOR1. In contrast, PR kinases are linked to the NOX2 complex without participation of HAM5.

A Stress-Responsive Signaling Network Regulating Pseudohyphal Growth and Ribonucleoprotein Granule Abundance in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae undergoes a stress-responsive transition to a pseudohyphal growth form in which cells elongate and remain connected in multicellular filaments. Pseudohyphal growth is regulated through conserved signaling networks that control cell growth and the response to glucose or nitrogen limitation in metazoans. These networks are incompletely understood, and our studies identify the TORC1- and PKA-regulated kinase Ksp1p as a key stress-responsive signaling effector in the yeast pseudohyphal growth response. The kinase-defective ksp1-K47D allele results in decreased pseudohyphal morphology at the cellular and colony level, indicating that Ksp1p kinase signaling is required for pseudohyphal filamentation. To determine the functional consequences of Ksp1p signaling, we implemented transcriptional profiling and quantitative phosphoproteomic analysis of ksp1-K47D on a global scale. Ksp1p kinase signaling maintains wild-type transcript levels of many pathways for amino acid synthesis and metabolism, relevant for the regulation of translation under conditions of nutrient stress. Proteins in stress-responsive ribonucleoprotein granules are regulated post-translationally by Ksp1p, and the Ksp1p-dependent phosphorylation sites S176 in eIF4G/Tif4631p and S436 in Pbp1p are required for wild-type levels of pseudohyphal growth and Protein Kinase A pathway activity. Pbp1p and Tif4631p localize in stress granules, and the ksp1 null mutant shows elevated abundance of Pbp1p puncta relative to wild-type. Collectively, the Ksp1p kinase signaling network integrates polarized pseudohyphal morphogenesis and translational regulation through the stress-responsive transcriptional control of pathways for amino acid metabolism and post-translational modification of translation factors affecting stress granule abundance.

MAPK modulation of yeast pheromone signaling output and the role of phosphorylation sites in the scaffold protein Ste5

Mitogen-activated protein kinases (MAPKs) mediate numerous eukaryotic signaling responses. They also can modulate their own signaling output via positive or negative feedback loops. In the yeast pheromone response pathway, the MAPK Fus3 triggers negative feedback that dampens its own activity. One target of this feedback is Ste5, a scaffold protein that promotes Fus3 activation. Binding of Fus3 to a docking motif (D motif) in Ste5 causes signal dampening, which was proposed to involve a central cluster of phosphorylation sites in Ste5. Here, we reanalyzed the role of these central sites. Contrary to prior claims, phosphorylation-mimicking mutations at these sites did not impair signaling. Also, the hyperactive signaling previously observed when these sites were mutated to nonphosphorylatable residues arose from their replacement with valine residues and was not observed with other substitutes. Instead, a cluster of N-terminal sites in Ste5, not the central sites, is required for the rapid dampening of initial responses. Further results suggest that the role of the Fus3 D motif is most simply explained by a tethering effect that promotes Ste5 phosphorylation, rather than an allosteric effect proposed to regulate Fus3 activity. These findings substantially revise our understanding of how MAPK feedback attenuates scaffold-mediated signaling in this model pathway.

GPCR-Gα protein precoupling: Interaction between Ste2p, a yeast GPCR, and Gpa1p, its Gα protein, is formed before ligand binding via the Ste2p C-terminal domain and the Gpa1p N-terminal domain

G protein coupled receptors bind ligands that initiate intracellular signaling cascades via heterotrimeric G proteins. In this study, involvement of the N-terminal residues of yeast G-alpha (Gpa1p) with the C-terminal residues of a full-length or C-terminally truncated Ste2p were investigated using bioluminescence resonance energy transfer (BRET), a non-radiative energy transfer phenomenon where protein-protein interactions can be quantified between a donor bioluminescent molecule and a suitable acceptor fluorophore. Constitutive and position-dependent BRET signal was observed in the absence of agonist (α-factor). Upon the activation of the receptors with α-factor, no significant change in BRET signal was observed. The location of Ste2p-Gpa1p heterodimer was investigated using confocal fluorescence microscopy and bimolecular fluorescence complementation (BiFC) assay, a technique where two non-fluorescent fragments of a fluorescent protein reassemble in vivo to restore fluorescence property thereby directly reporting a protein-protein interaction. BiFC experiments resulted in a dimerization signal intracellularly during biosynthesis on the endoplasmic reticulum (ER) and on the plasma membrane (PM). The constitutive BRET and BiFC signals observed on ER between Ste2p and Gpa1p in their quiescent and activated states are indicative of pre-coupling between these two proteins. This study is the first to show that the extreme N-terminus of yeast G protein alpha subunit is in close proximity to its receptor. The data suggests a pre-coupled heterodimer prior to receptor activation. The images presented in this study are the first direct in vivo evidence showing the localization of receptor - G protein heterodimers during biosynthesis and before reaching the plasma membrane.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.

Evolutionary history of mitogen-activated protein kinase (MAPK) genes in Lotus, Medicago, and Phaseolus

Mitogen-Activated Protein Kinase (MAPK) genes encode proteins that mediate various signaling pathways associated with biotic and abiotic stress responses in eukaryotes. The MAPK genes form a 3-tier signal transduction cascade between cellular stimuli and physiological responses. Recent identification of soybean MAPKs and availability of genome sequences from other legume species allowed us to identify their MAPK genes. The main objectives of this study were to identify MAPKs in 3 legume species, Lotus japonicus, Medicago truncatula, and Phaseolus vulgaris, and to assess their phylogenetic relationships. We used approaches in comparative genomics for MAPK gene identification and named the newly identified genes following Arabidopsis MAPK nomenclature model. We identified 19, 18, and 15 MAPKs and 7, 4, and 9 MAPKKs in the genome of Lotus japonicus, Medicago truncatula, and Phaseolus vulgaris, respectively. Within clade placement of MAPKs and MAPKKs in the 3 legume species were consistent with those in soybean and Arabidopsis. Among 5 clades of MAPKs, 4 founder clades were consistent to MAPKs of other plant species and orthologs of MAPK genes in the fifth clade-"Clade E" were consistent with those in soybean. Our results also indicated that some gene duplication events might have occurred prior to eudicot-monocot divergence. Highly diversified MAPKs in soybean relative to those in 3 other legume species are attributable to the polyploidization events in soybean. The identification of the MAPK genes in the legume species is important for the legume crop improvement; and evolutionary relationships and functional divergence of these gene members provide insights into plant genome evolution.

Combined computational and experimental analysis reveals mitogen-activated protein kinase-mediated feedback phosphorylation as a mechanism for signaling specificity

A series of mathematical models was used to quantitatively characterize pheromone-stimulated kinase activation and determine how mitogen-activated protein (MAP) kinase specificity is achieved. The findings reveal how feedback phosphorylation of a common pathway component can limit the activity of a competing MAP kinase through feedback phosphorylation of a common activator, and thereby promote signal fidelity.

Different environmental stimuli often use the same set of signaling proteins to achieve very different physiological outcomes. The mating and invasive growth pathways in yeast each employ a mitogen-activated protein (MAP) kinase cascade that includes Ste20, Ste11, and Ste7. Whereas proper mating requires Ste7 activation of the MAP kinase Fus3, invasive growth requires activation of the alternate MAP kinase Kss1. To determine how MAP kinase specificity is achieved, we used a series of mathematical models to quantitatively characterize pheromone-stimulated kinase activation. In accordance with the computational analysis, MAP kinase feedback phosphorylation of Ste7 results in diminished activation of Kss1, but not Fus3. These findings reveal how feedback phosphorylation of a common pathway component can limit the activity of a competing MAP kinase through feedback phosphorylation of a common activator, and thereby promote signal fidelity.

A framework for mapping, visualisation and automatic model creation of signal-transduction networks

An intuitive formalism for reconstructing cellular networks from empirical data is presented, and used to build a comprehensive yeast MAP kinase network. The accompanying rxncon software tool can convert networks to a range of standard graphical formats and mathematical models.

Network mapping at the granularity of empirical data that largely avoids combinatorial complexity

Automatic visualisation and model generation with the rxncon open source software tool

Visualisation in a range of formats, including all three SBGN formats, as well as contingency matrix or regulatory graph

Comprehensive and completely references map of the yeast MAP kinase network in the rxncon format

Intracellular signalling systems are highly complex. This complexity makes handling, analysis and visualisation of available knowledge a major challenge in current signalling research. Here, we present a novel framework for mapping signal-transduction networks that avoids the combinatorial explosion by breaking down the network in reaction and contingency information. It provides two new visualisation methods and automatic export to mathematical models. We use this framework to compile the presently most comprehensive map of the yeast MAP kinase network. Our method improves previous strategies by combining (I) more concise mapping adapted to empirical data, (II) individual referencing for each piece of information, (III) visualisation without simplifications or added uncertainty, (IV) automatic visualisation in multiple formats, (V) automatic export to mathematical models and (VI) compatibility with established formats. The framework is supported by an open source software tool that facilitates integration of the three levels of network analysis: definition, visualisation and mathematical modelling. The framework is species independent and we expect that it will have wider impact in signalling research on any system.

Fungal mating pheromones: choreographing the dating game

Pheromones are ubiquitous from bacteria to mammals - a testament to their importance in regulating inter-cellular communication. In fungal species, they play a critical role in choreographing interactions between mating partners during the program of sexual reproduction. Here, we describe how fungal pheromones are synthesized, their interactions with G protein-coupled receptors, and the signals propagated by this interaction, using Saccharomyces cerevisiae as a reference point. Divergence from this model system is compared amongst the ascomycetes and basidiomycetes, which reveals the wealth of information that has been gleaned from studying pheromone-driven processes across a wide spectrum of the fungal kingdom.

Membrane localization of scaffold proteins promotes graded signaling in the yeast MAP kinase cascade

BACKGROUND

Signaling through mitogen-activated protein kinase (MAPK) cascade pathways can show various input-output behaviors, including either switch-like or graded responses to increasing levels of stimulus. Prior studies suggest that switch-like behavior is promoted by positive feedback loops and nonprocessive phosphorylation reactions, but it is unclear whether graded signaling is a default behavior or whether it must be enforced by separate mechanisms. It has been hypothesized that scaffold proteins promote graded behavior.

RESULTS

Here, we experimentally probe the determinants of graded signaling in the yeast mating MAPK pathway. We find that graded behavior is robust in that it resists perturbation by loss of several negative-feedback regulators. However, the pathway becomes switch-like when activated by a crosstalk stimulus that bypasses multiple upstream components. To dissect the contributing factors, we developed a method for gradually varying the signal input at different pathway steps in vivo. Input at the beginning of the kinase cascade produced a sharp, threshold-like response. Surprisingly, the scaffold protein Ste5 increased this threshold behavior when limited to the cytosol. However, signaling remained graded whenever Ste5 was allowed to function at the plasma membrane.

CONCLUSIONS

The results suggest that the MAPK cascade module is inherently ultrasensitive but is converted to a graded system by the pathway-specific activation mechanism. Scaffold-mediated assembly of signaling complexes at the plasma membrane allows faithful propagation of weak signals, which consequently reduces pathway ultrasensitivity. These properties help shape the input-output properties of the system to fit the physiological context.

Scaffold proteins of MAP-kinase modules

Mitogen-activated protein kinases (MAPKs) regulate critical signaling pathways involved in cell proliferation, differentiation and apoptosis. Recent studies have shown that a novel class of scaffold proteins mediates the structural and functional organization of the three-tier MAPK module. By linking the MAP3K, MAP2K and MAPK into a multienzyme complex, these MAPK-specific scaffold proteins provide an insulated physical conduit through which signals from the respective MAPK can be transmitted to the appropriate spatiotemporal cellular loci. Scaffold proteins play a determinant role in modulating the signaling strength of their cognate MAPK module by regulating the signal amplitude and duration. The scaffold proteins themselves are finely regulated resulting in dynamic intra- and inter-molecular interactions that can modulate the signaling outputs of MAPK modules. This review focuses on defining the diverse mechanisms by which these scaffold proteins interact with their respective MAPK modules and the role of such interactions in the spatiotemporal organization as well as context-specific signaling of the different MAPK modules.

A systems-biology analysis of feedback inhibition in the Sho1 osmotic-stress-response pathway

BACKGROUND

A common property of signal transduction systems is that they rapidly lose their ability to respond to a given stimulus. For instance in yeast, the mitogen-activated protein (MAP) kinase Hog1 is activated and inactivated within minutes, even when the osmotic-stress stimulus is sustained.

RESULTS

Here, we used a combination of experimental and computational analyses to investigate the dynamic behavior of Hog1 activation in vivo. Computational modeling suggested that a negative-feedback loop operates early in the pathway and leads to rapid attenuation of Hog1 signaling. Experimental analysis revealed that the membrane-bound osmosensor Sho1 is phosphorylated by Hog1 and that phosphorylation occurs on Ser-166. Moreover, Sho1 exists in a homo-oligomeric complex, and phosphorylation by Hog1 promotes a transition from the oligomeric to monomeric state. A phosphorylation-site mutation (Sho1(S166E)) diminishes the formation of Sho1-oligomers, dampens activation of the Hog1 kinase, and impairs growth in high-salt or sorbitol conditions.

CONCLUSIONS

These findings reveal a novel phosphorylation-dependent feedback loop leading to diminished cellular responses to an osmotic-stress stimulus.

Inhibition of Cdc42-dependent signalling in Saccharomyces cerevisiae by phosphatase-dead SigD/SopB from Salmonella typhimurium

Heterologous expression of bacterial virulence factors in Saccharomyces cerevisiae is a feasible approach to study their molecular function. The authors have previously reported that the Salmonella typhimurium SigD protein, a phosphatidylinositol phosphatase involved in invasion of the host cell, inhibits yeast growth, presumably by depleting an essential pool of phosphatidylinositol 4,5-bisphosphate, and also that a catalytically inactive version, SigD(R468A), was able to arrest growth by a different mechanism that involved disruption of the actin cytoskeleton. This paper describes marked differences between the phenotypes elicited by expression of SigD and SigD(R468A) in yeast. First, expression of SigD(R468A) caused accumulation of large unbudded cells and loss of septin organization, while SigD expression caused none of these effects. Second, growth inhibition by SigD(R468A) was mediated by a cell cycle arrest in G2 dependent on the Swe1 morphogenetic checkpoint, but SigD-induced growth inhibition was cell cycle independent. And third, SigD caused strong activation of the yeast MAP kinase Slt2, whereas SigD(R468A) rather inactivated another MAP kinase, Kss1. In a screen for suppressors of SigD(R468A)-induced growth arrest by overexpression of a yeast cDNA library, the Cdc42 GTPase was isolated. Furthermore, SigD(R468A) was co-purified with Cdc42 from yeast lysates. It is concluded that the Salmonella SigD protein deprived of its phosphatase activity is able to disrupt yeast morphogenesis by interfering with Cdc42 function, opening the possibility that the SigD N-terminal region might directly modulate small GTPases from the host during infection.

Regulation of G protein and mitogen-activated protein kinase signaling by ubiquitination: insights from model organisms

Guanine nucleotide binding proteins (G proteins) and mitogen-activated protein kinases are highly conserved signaling molecules engaged in a wide variety of cellular processes. The strength and duration of signaling mediated by G proteins and mitogen-activated protein kinases are well known to be regulated via phosphorylation of pathway components. Over the past few years, it has become evident that many of the same signaling proteins also undergo ubiquitination, a posttranslational modification that typically leads to protein degradation. Consequently the strength and duration of signaling can also be modulated by regulating the abundance of signaling proteins. This article describes G protein- and mitogen-activated protein kinase-mediated signaling pathways that are known to be regulated by ubiquitination. The focus is on studies performed in the budding yeast Saccharomyces cerevisiae, as many principles governing this new regulatory mechanism were initially discovered in this model organism. Similar mechanisms uncovered in other model systems are also briefly discussed to illustrate the importance and universality of signaling regulation by ubiquitination.

Effect of Cryphonectria hypovirus 1 (CHV1) infection on Cpkk1, a mitogen-activated protein kinase kinase of the filamentous fungus Cryphonectria parasitica

We screened Cryphonectria parasitica genomic and cDNA libraries with a probe obtained from the amplification of a conserved region among the sequence of known mitogen activated protein kinase kinases (MAPKK) and obtained genomic and cDNA clones. Sequence comparisons of the clones obtained confirmed the identification of a C. parasitica homologue to other fungal MAPKK, which we named Cpkk1. Polyclonal antibodies raised against a purified Cpkk1 fusion protein expressed in Escherichia coli were used to detect Cpkk1 protein in extracts of CHV1-infected and uninfected C. parasitica grown in liquid culture. Differences in the dynamics of phosphorylation and dephosphorylation were noticed. Under the conditions investigated, Cpkk1 protein expression is associated with active mycelial growth, before the onset of a senescent developmental stage. We hypothesize that differences in Cpkk1 phosphorylation state between CHV1 infected and virus free strains are due to a delay of the onset of the developmental stage caused by the presence of the virus.

Scaffold proteins in MAP kinase signaling: more than simple passive activating platforms

Due to the central position of scaffold proteins in numerous signaling networks, especially in MAPK pathways, considerable efforts have been made to identify new scaffolds and to characterize their function and regulation. Most of our knowledge stems from studies of yeast MAPK scaffolds, but the identification of such scaffolds in higher eukaryotes provided a new dimension to this field and led to exciting and promising new insights into the regulation of MAPK signaling. In this review, we shortly summarize the well-established basic functions of scaffolds in yeast and highlight concepts emerging from recent studies in yeast and higher eukaryotes. In particular, we discuss how scaffolds may actively influence MAPK signaling by inducing conformational changes of bound kinases or substrates, by controlling the localization of activated MAPK and the extent and output of MAPK activation, and by modulating MAPK kinetics through the recruitment of phosphatases or ubiquitin-ligases. Finally, we summarize the current knowledge of scaffold regulation, and how these events may be functionally important for MAPK signaling.

Osmotin Is a Homolog of Mammalian Adiponectin and Controls Apoptosis in Yeast through a Homolog of Mammalian Adiponectin Receptor

The antifungal activity of the PR-5 family of plant defense proteins has been suspected to involve specific plasma membrane component(s) of the fungal target. Osmotin is a tobacco PR-5 family protein that induces apoptosis in the yeast Saccharomyces cerevisiae. We show here that the protein encoded by ORE20/PHO36 (YOL002c), a seven transmembrane domain receptor-like polypeptide that regulates lipid and phosphate metabolism, is an osmotin binding plasma membrane protein that is required for full sensitivity to osmotin. PHO36 functions upstream of RAS2 in the osmotin-induced apoptotic pathway. The mammalian homolog of PHO36 is a receptor for the hormone adiponectin and regulates cellular lipid and sugar metabolism. Osmotin and adiponectin, the corresponding "receptor" binding proteins, do not share sequence similarity. However, the beta barrel domain of both proteins can be overlapped, and osmotin, like adiponectin, activates AMP kinase in C2C12 myocytes via adiponectin receptors.

Principles of MAP Kinase Signaling Specificity inSaccharomyces cerevisiae

Cells respond to a plethora of signals using a limited set of intracellular signal transduction components. Surprisingly, pathways that transduce distinct signals can share protein components, yet avoid erroneous cross-talk. A highly tractable model system in which to study this paradox is the yeast Saccharomyces cerevisiae, which harbors three mitogen-activated protein kinase (MAPK) signal transduction cascades that share multiple signaling components. In this review we first describe potential mechanisms by which specificity could be achieved by signaling pathways that share components. Second, we summarize key features and components of the yeast MAPK pathways that control the mating pheromone response, filamentous growth, and the response to high osmolarity. Finally, we review biochemical analyses in yeast of mutations that cause cross-talk between these three MAPK pathways and their implications for the mechanistic bases for signaling specificity. Although much remains to be learned, current data indicate that scaffolding and cross pathway inhibition play key roles in the maintenance of fidelity.

Analyses of the effects of Rck2p mutants on Pbs2pDD-induced toxicity in Saccharomyces cerevisiae identify a MAP kinase docking motif, and unexpected functional inactivation due to acidic substitution of T379

Rck2p is a Ser/Thr kinase that binds to, and is activated by, Hog1p. Expression of the MAP kinase kinase Pbs2pDD from a GAL1-driven plasmid hyperactivates the HOG MAP kinase pathway, and leads to cessation of growth. This toxic effect is reduced by deletion of RCK2. We studied the structural and functional basis for the role of Rck2p in mediating the growth arrest phenotype associated with overexpression of Pbs2pDD. Rck2p kinase activity is required for the effect, because Rck2p(Delta487-610), as well as full-length Rck2p, is toxic with Pbs2pDD, but kinase-defective versions of either protein with a K201R mutation are not. Thus, the C-terminal portion of Rck2p is not required provided the protein is activated by removal of the autoinhibitory domain. Relief of inhibition in Rck2p normally requires phosphorylation by Hog1p, and Rck2p contains a putative MAP kinase docking site (TILQR589R590KKVQ) in its C-terminal segment. The Rck2p double mutant R589A/R590A expressed from a centromeric plasmid did not detectably bind Hog1p-GFP and was functionally inactive in mediating the toxic effect of Pbs2pDD, equivalent to an RCK2 deletion. However, overexpression of Rck2p R589A/R590A from a multicopy plasmid restored function. In contrast, RCK2-K201R acted as a multicopy suppressor of PBS2DD, markedly reducing its toxicity. This suppressor activity required the K201R mutation, and the effect was largely lost when the docking site was mutated, suggesting suppression by inhibition of Hog1p functions. We also studied the effect of replacing the predicted T379 and established S520 phosphorylation sites in Rck2p by glutamic acid. Surprisingly, the T379E mutant markedly reduced Pbs2pDD toxicity, and toxicity was only partially rescued by S520E. Rck2 T379E was sufficiently inactive in an rck2Delta strain to allow some cells to survive PBS2DD toxicity even when overexpressed. The significance of these findings for our understanding of Rck2p function is discussed.

Regulation of Ste7 ubiquitination by Ste11 phosphorylation and the Skp1-Cullin-F-box complex

Ste7 is a mitogen-activated protein kinase kinase that mediates pheromone signaling in Saccharomyces cerevisiae. We showed previously that Ste7 is ubiquitinated upon prolonged stimulation by pheromone and that accumulation of ubiquitinated Ste7 results in enhanced transcription and cell division arrest responses (Wang, Y., and Dohlman, H. G. (2002) J. Biol. Chem. 277, 15766-15772). We now report that ubiquitination of Ste7 requires Ste11 kinase and Skp1/Cullin/F-box (SCF) ubiquitin-conjugating activities. Ste7 is not ubiquitinated in Ste11-deficient cells or when the Ste11 phosphorylation sites have been mutated. Ste7 ubiquitination and degradation (but not phosphorylation) is specifically blocked in mutants defective for the E2 ubiquitin-conjugating enzyme Cdc34 or the cullin homologue Cdc53. Both are components of the SCF complex that ubiquitinates proteins during the G1-S transition of the cell cycle. Our findings suggest that SCF promotes the ubiquitination and degradation of Ste7, thereby favoring the resumption of cell division cycling after pheromone-induced growth arrest.

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.

Pheromone-dependent ubiquitination of the mitogen-activated protein kinase kinase Ste7

Many cell signaling pathways are regulated by phosphorylation, ubiquitination, and degradation of constituent proteins. As with phosphorylation, protein ubiquitination can be reversed, through the action of ubiquitin-specific processing proteases (UBPs). Here we have analyzed 15 UBP disruption mutants in the yeast Saccharomyces cerevisiae and identified one (ubp3 Delta) that acts specifically in the pheromone response pathway. Upon pheromone stimulation, ubp3 Delta mutants accumulate unconjugated polyubiquitin chains as well as polyubiquitinated forms of the mitogen-activated protein kinase kinase Ste7. The ubp3 Delta mutants exhibit a potentiated response to pheromone, as measured by in vivo MAP kinase activity, transcriptional induction, and cell cycle arrest. Signaling is likewise enhanced upon direct activation of Ste4 (G protein beta subunit) and Ste11 (Ste7 kinase) but not the downstream transcription factor Ste12. These findings reveal a mechanism by which pheromone-triggered ubiquitination of Ste7 can modulate the pheromone response in vivo.

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.

MAP kinase cascades: scaffolding signal specificity

Scaffold proteins organize many MAP kinase pathways by interacting with several components of these cascades. Recent studies suggest that scaffold proteins provide local activation platforms that contribute to signal specificity by insulating different MAP kinase pathways.

A filamentous growth response mediated by the yeast mating pathway

Haploid cells of the budding yeast Saccharomyces cerevisiae respond to mating pheromones by arresting their cell-division cycle in G1 and differentiating into a cell type capable of locating and fusing with mating partners. Yeast cells undergo chemotactic cell surface growth when pheromones are present above a threshold level for morphogenesis; however, the morphogenetic responses of cells to levels of pheromone below this threshold have not been systematically explored. Here we show that MATa haploid cells exposed to low levels of the alpha-factor mating pheromone undergo a novel cellular response: cells modulate their division patterns and cell shape, forming colonies composed of filamentous chains of cells. Time-lapse analysis of filament formation shows that its dynamics are distinct from that of pseudohyphal growth; during pheromone-induced filament formation, daughter cells are delayed relative to mother cells with respect to the timing of bud emergence. Filament formation requires the RSR1(BUD1), BUD8, SLK1/BCK1, and SPA2 genes and many elements of the STE11/STE7 MAP kinase pathway; this response is also independent of FAR1, a gene involved in orienting cell polarization during the mating response. We suggest that mating yeast cells undergo a complex response to low levels of pheromone that may enhance the ability of cells to search for mating partners through the modification of cell shape and alteration of cell-division patterns.

Identification of novel pheromone-response regulators through systematic overexpression of 120 protein kinases in yeast

Protein kinases are well known to transmit and regulate signaling pathways. To identify additional regulators of the pheromone signaling apparatus in yeast, we evaluated an array of 120 likely protein kinases encoded by the yeast genome. Each kinase was fused to glutathione S-transferase, overexpressed, and tested for changes in pheromone responsiveness in vivo. As expected, several known components of the pathway (YCK1, STE7, STE11, FUS3, and KSS1) impaired the growth arrest response. Seven other kinases also interfered with pheromone-induced growth arrest; in rank order they are as follows: YKL116c (renamed PRR1) = YDL214c (renamed PRR2) > YJL141c (YAK1, SRA1) > YNR047w = YCR091w (KIN82) = YIL095w (PRK1) > YCL024w (KCC4). Inhibition of pheromone signaling by PRR1, but not PRR2, required the glutathione S-transferase moiety. Both kinases inhibited gene transcription after stimulation with pheromone, a constitutively active kinase mutant STE11-4, or overexpression of the transcription factor STE12. Neither protein altered the ability of the mitogen-activated protein kinase (MAPK) Fus3 to feedback phosphorylate a known substrate, the MAPK kinase Ste7. These results reveal two new components of the pheromone-signaling cascade in yeast, each acting at a point downstream of the MAPK.

The Cryptococcus neoformans STE11alpha gene is similar to other fungal mitogen-activated protein kinase kinase kinase (MAPKKK) genes but is mating type specific

Partial sequence analysis of the Cryptococcus neoformans MATalpha mating type locus revealed the presence of a gene with substantial sequence similarity to other fungal mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK) genes. The C. neoformans gene, designated STE11alpha, showed the highest degree of similarity to the Neurospora crassa nrc-1, Schizosaccharomyces pombe byr2 and Saccharomyces cerevisiae STE11 genes. A polymerase chain reaction-mediated sib-selection technique was successfully adapted for the purpose of disrupting STE11alpha. C. neoformans ste11alphaDelta mutants were found to be sterile, consistent with the phenotypes of ste11 and byr2 mutants in S. cerevisiae and S. pombe respectively. Haploid ste11alphaDelta mutants were also found to be unable to produce hyphae, suggesting that the C. neoformans gene is functionally conserved when compared with its S. cerevisiae MAPKKK counterpart. Comparison of the wild-type STE11alpha strain with a ste11alphaDelta disruptant for virulence using the mouse model showed that the ste11alphaDelta strain was less virulent, but the difference was only minor. In spite of some of the conserved functions of STE11alpha, linkage analysis showed that STE11alpha is only found in mating type alpha strains. These results demonstrate that, although functionally conserved, the mating pathway in C. neoformans has a unique organization.

Mutational analysis suggests that activation of the yeast pheromone response mitogen-activated protein kinase pathway involves conformational changes in the Ste5 scaffold protein

Ste5 is essential for pheromone response and binds components of a mitogen-activated protein kinase (MAPK) cascade: Ste11 (MEKK), Ste7 (MEK), and Fus3 (MAPK). Pheromone stimulation releases Gbetagamma (Ste4-Ste18), which recruits Ste5 and Ste20 (p21-activated kinase) to the plasma membrane, activating the MAPK cascade. A RING-H2 domain in Ste5 (residues 177-229) negatively regulates Ste5 function and mediates its interaction with Gbetagamma. Ste5(C177A C180A), carrying a mutated RING-H2 domain, cannot complement a ste5Delta mutation, yet supports mating even in ste4Delta ste5Delta cells when artificially dimerized by fusion to glutathione S-transferase (GST). In contrast, wild-type Ste5 fused to GST permits mating of ste5Delta cells, but does not allow mating of ste4Delta ste5Delta cells. This differential behavior provided the basis of a genetic selection for STE5 gain-of-function mutations. MATa ste4Delta ste5Delta cells expressing Ste5-GST were mutagenized chemically and plasmids conferring the capacity to mate were selected. Three independent single-substitution mutations were isolated. These constitutive STE5 alleles induce cell cycle arrest, transcriptional activation, and morphological changes normally triggered by pheromone, even when Gbetagamma is absent. The first, Ste5(C226Y), alters the seventh conserved position in the RING-H2 motif, confirming that perturbation of this domain constitutively activates Ste5 function. The second, Ste5(P44L), lies upstream of a basic segment, whereas the third, Ste5(S770K), is situated within an acidic segment in a region that contacts Ste7. None of the mutations increased the affinity of Ste5 for Ste11, Ste7, or Fus3. However, the positions of these novel-activating mutations suggested that, in normal Ste5, the N terminus may interact with the C terminus. Indeed, in vitro, GST-Ste5(1-518) was able to associate specifically with radiolabeled Ste5(520-917). Furthermore, both the P44L and S770K mutations enhanced binding of full-length Ste5 to GST-Ste5(1-518), whereas they did not affect Ste5 dimerization. Thus, binding of Gbetagamma to the RING-H2 domain may induce a conformational change that promotes association of the N- and C-terminal ends of Ste5, stimulating activation of the MAPK cascade by optimizing orientation of the bound kinases and/or by increasing their accessibility to Ste20-dependent phosphorylation (or both). In accord with this model, the novel Ste5 mutants copurified with Ste7 and Fus3 in their activated state and their activation required Ste20.

Phosphorylation of the MEKK Ste11p by the PAK-like kinase Ste20p is required for MAP kinase signaling in vivo

BACKGROUND

Many signals are transduced from the cell surface to the nucleus through mitogen-activated protein (MAP) kinase cascades. Activation of MAP kinase requires phosphorylation by MEK, which in turn is controlled by Raf, Mos or a group of structurally related kinases termed MEKKs. It is not understood how MEKKs are regulated by extracellular signals. In yeast, the MEKK Ste11p functions in multiple MAP kinase cascades activated in response to pheromones, high osmolarity and nutrient starvation. Genetic evidence suggests that the p21-activated protein kinase (PAK) Ste20p functions upstream of Ste11p, and Ste20p has been shown to phosphorylate Ste11p in vitro.

RESULTS

Ste20p phosphorylated Ste11p on Ser302 and/or Ser306 and Thr307 in yeast, residues that are conserved in MEKKs of other organisms. Mutating these sites to non-phosphorylatable residues abolished Ste11p function, whereas changing them to aspartic acid to mimic the phosphorylated form constitutively activated Ste11p in vivo in a Ste20p-independent manner. The amino-terminal regulatory domain of Ste11p interacted with its catalytic domain, and overexpression of a small amino-terminal fragment of Ste11p was able to inhibit signaling in response to pheromones. Mutational analysis suggested that this interaction was regulated by phosphorylation and dependent on Thr596, which is located in the substrate cleft of the catalytic domain.

CONCLUSIONS

Our results suggest that, in response to multiple extracellular signals, phosphorylation of Ste11p by Ste20p removes an amino-terminal inhibitory domain, leading to activation of the Ste11 protein kinase. This mechanism may serve as a paradigm for the activation of mammalian MEKKs.

Fus3 controls Ty1 transpositional dormancy through the invasive growth MAPK pathway

Fus3, the mitogen-activated protein kinase (MAPK) of the mating pheromone response pathway, inhibits a post-translational step of Ty1 retrotransposition. Fus3 also inhibits haploid invasive growth by blocking cross-activation of invasive growth gene expression by the pheromone response signal cascade. Here, we show that Fus3 kinase activity and dosage co-ordinately regulate Ty1 transposition and invasive growth. A chromosomal copy of the kinase-defective fus3-K42R allele fails to inhibit either Ty1 transposition or invasive growth. When overexpressed, kinase-defective Fus3 weakly inhibits both Ty1 transposition and invasive growth, but is much less inhibitory than wild-type Fus3 expressed at the same level. Moreover, increasing the dosage of wild-type Fus3 intensifies the inhibition of both Ty1 transposition and invasive growth. To demonstrate that Fus3 regulates Ty1 transposition via its negative regulation of the invasive growth pathway, we show by epistatic analysis that the invasive growth pathway transcription factors Ste12 and Tec1 are both required for Fus3-mediated inhibition of Ty1 transposition. When haploid invasive growth is stimulated by high-copy expression of TEC1, by expression of the dominant hypermorphic allele STE11-4 or by deletion of HOG1, Ty1 transposition is concomitantly activated. In summary, these results demonstrate that the haploid invasive growth pathway activates Ty1 transposition at both transcriptional and post-transcriptional levels and that Fus3 inhibits Ty1 transposition by inhibiting the invasive growth pathway.

Feedback phosphorylation of an RGS protein by MAP kinase in yeast

Regulators of G protein signaling (RGS proteins) are well known to accelerate G protein GTPase activity in vitro and to promote G protein desensitization in vivo. Less is known about how RGS proteins are themselves regulated. To address this question we purified the RGS in yeast, Sst2, and used electrospray ionization mass spectrometry to identify post-translational modifications. This analysis revealed that Sst2 is phosphorylated at Ser-539 and that phosphorylation occurs in response to pheromone stimulation. Ser-539 lies within a consensus mitogen-activated protein (MAP) kinase phosphorylation site, Pro-X-Ser-Pro. Phosphorylation is blocked by mutations in the MAP kinase genes (FUS3, KSS1), as well as by mutations in components needed for MAP kinase activation (STE11, STE7, STE4, STE18). Phosphorylation is also blocked by replacing Ser-539 with Ala, Asp, or Glu (but not Thr). These point mutations do not alter pheromone sensitivity, as determined by growth arrest and reporter transcription assays. However, phosphorylation appears to slow the rate of Sst2 degradation. These findings indicate that the G protein-regulated MAP kinase in yeast can act as a feedback regulator of Sst2, itself a regulator of G protein signaling.

Differential transmission of G1 cell cycle arrest and mating signals bySaccharomyces cerevisiaeSte5 mutants in the pheromone pathway

Saccharomyces cerevisiae Ste5 is a scaffold protein that recruits many pheromone signaling molecules to sequester the pheromone pathway from other homologous mitogen-activated protein kinase pathways. G1 cell cycle arrest and mating are two different physiological consequences of pheromone signal transduction and Ste5 is required for both processes. However, the roles of Ste5 in G1 arrest and mating are not fully understood. To understand the roles of Ste5 better, we isolated 150 G1 cell cycle arrest defective STE5 mutants by chemical mutagenesis of the gene. Here, we found that two G1 cell cycle arrest defective STE5 mutants (ste5MD248Vand ste5delta-776) retained mating capacity. When overproduced in a wild-type strain, several ste5 mutants also showed different dominant phenotypes for G1 arrest and mating. Isolation and characterization of the mutants suggested separable roles of Ste5 in G1 arrest and mating of S. cerevisiae. In addition, the roles of Asp-248 and Tyr-421, which are important for pheromone signal transduction were further characterized by site-directed mutagenesis studies.Key words: Ste5, Saccharomyces cerevisiae, signal transduction, mating, G1 cell cycle arrest.

Inhibition of the Ca2+/calmodulin-dependent protein kinase I cascade by cAMP-dependent protein kinase

Several recent studies have shown that Ca2+/calmodulin-dependent protein kinase I (CaMKI) is phosphorylated and activated by a protein kinase (CaMKK) that is itself subject to regulation by Ca2+/calmodulin. In the present study, we demonstrate that this enzyme cascade is regulated by cAMP-mediated activation of cAMP-dependent protein kinase (PKA). In vitro, CaMKK is phosphorylated by PKA and this is associated with inhibition of enzyme activity. The major site of phosphorylation is threonine 108, although additional sites are phosphorylated with lower efficiency. In vitro, CaMKK is also phosphorylated by CaMKI at the same sites as PKA, suggesting that this regulatory phosphorylation might play a role as a negative-feedback mechanism. In intact PC12 cells, activation of PKA with forskolin resulted in a rapid inhibition of both CaMKK and CaMKI activity. In hippocampal slices CaMKK was phosphorylated under basal conditions, and activation of PKA led to an increase in phosphorylation. Two-dimensional phosphopeptide mapping indicated that activation of PKA led to increased phosphorylation of multiple sites including threonine 108. These results indicate that in vitro and in intact cells the CaMKK/CaMKI cascade is subject to inhibition by PKA-mediated phosphorylation of CaMKK. The phosphorylation and inhibition of CaMKK by PKA is likely to be involved in modulating the balance between cAMP- and Ca2+-dependent signal transduction pathways.

Relative dependence of different outputs of the Saccharomyces cerevisiae pheromone response pathway on the MAP kinase Fus3p

Fus3p and Kss1p act at the end of a conserved signaling cascade that mediates numerous cellular responses for mating. To determine the role of Fus3p in different outputs, we isolated and characterized a series of partial-function fus3 point mutants for their ability to phosphorylate a substrate (Ste7p), activate Ste12p, undergo G1 arrest, form shmoos, select partners, mate, and recover. All the mutations lie in residues that are conserved among MAP kinases and are predicted to affect either enzyme activity or binding to Ste7p or substrates. The data argue that Fus3p regulates the various outputs assayed through the phosphorylation of multiple substrates. Different levels of Fus3p function are required for individual outputs, with the most function required for shmoo formation, the terminal output. The ability of Fus3p to promote shmoo formation strongly correlates with its ability to promote G1 arrest, suggesting that the two events are coupled. Fus3p promotes recovery through a mechanism that is distinct from its ability to promote G1 arrest and may involve a mechanism that does not require kinase activity. Moreover, catalytically inactive Fus3p inhibits the ability of active Fus3p to activate Ste12p and hastens recovery without blocking G1 arrest or shmoo formation. These results raise the possibility that in the absence of sustained activation of Fus3p, catalytically inactive Fus3p blocks further differentiation by restoring mitotic growth. Finally, suppression analysis argues that Kss1p contributes to the overall pheromone response in a wild-type strain, but that Fus3p is the critical kinase for all of the outputs tested.

Cell polarity and morphogenesis in budding yeast

Eukaryotic cells respond to intracellular and extracellular cues to direct asymmetric cell growth and division. The yeast Saccharomyces cerevisiae undergoes polarized growth at several times during budding and mating and is a useful model organism for studying asymmetric growth and division. In recent years, many regulatory and cytoskeletal components important for directing and executing growth have been identified, and molecular mechanisms have been elucidated in yeast. Key signaling pathways that regulate polarization during the cell cycle and mating response have been described. Since many of the components important for polarized cell growth are conserved in other organisms, the basic mechanisms mediating polarized cell growth are likely to be universal among eukaryotes.

Hsp90 is required for pheromone signaling in yeast

The heat-shock protein 90 (Hsp90) is a cytosolic molecular chaperone that is highly abundant even at normal temperature. Specific functions for Hsp90 have been proposed based on the characterization of its interactions with certain transcription factors and kinases including Raf in vertebrates and flies. We therefore decided to address the role of Hsp90 for MAP kinase pathways in the budding yeast, an organism amenable to both genetic and biochemical analyses. We found that both basal and induced activities of the pheromone-signaling pathway depend on Hsp90. Signaling is defective in strains expressing low levels or point mutants of yeast Hsp90 (Hsp82), or human Hsp90beta instead of the wild-type protein. Ste11, a yeast equivalent of Raf, forms complexes with wild-type Hsp90 and depends on Hsp90 function for accumulation. For budding yeast, Ste11 represents the first identified endogenous "substrate" of Hsp90. Moreover, Hsp90 functions in steroid receptor and pheromone signaling can be genetically separated as the Hsp82 point mutant T525I and the human Hsp90beta are specifically defective for the former and the latter, respectively. These findings further corroborate the view that molecular chaperones must also be considered as transient or stable components of signal transduction pathways.

Potential regulation of Ste20 function by the Cln1-Cdc28 and Cln2-Cdc28 cyclin-dependent protein kinases

The activity of the Saccharomyces cerevisiae pheromone signal transduction pathway is regulated by Cln1/2-Cdc28 cyclin-dependent kinase. High level expression of CLN2 can repress activation of the pathway by mating factor or by deletion of the alpha-subunit of the heterotrimeric G-protein. We now show that CLN2 overexpression can also repress FUS1 induction if the signaling pathway is activated at the level of the beta-subunit of the G-protein (STE4) but not when activated at the level of downstream kinases (STE20 and STE11) or at the level of the transcription factor STE12. This epistatic analysis indicates that repression of pheromone signaling pathway by Cln2-Cdc28 kinase takes place at a level around STE20. In agreement with this, a marked reduction in the electrophoretic mobility of the Ste20 protein is observed at the time in the cell cycle of maximal expression of CLN2. This mobility change is constitutive in cells overexpressing CLN2 and absent in cells lacking CLN1 and CLN2. These changes in electrophoretic mobility correlate with repression of pheromone signaling and suggest Ste20 as a target for repression of signaling by G1 cyclins. Two morphogenic pathways for which Ste20 is essential, pseudohyphal differentiation and haploid-invasive growth, also require CLN1 and CLN2. Together with the previous observation that Cln1 and Cln2 are required for the function of Ste20 in cytokinesis, this suggests that Cln1 and Cln2 regulate the biological activity of Ste20 by promoting morphogenic functions, while inhibiting the mating factor signal transduction function.

Osmotin, a plant antifungal protein, subverts signal transduction to enhance fungal cell susceptibility

The plant pathogenesis-related protein osmotin is an antifungal cytotoxic agent that causes rapid cell death in the yeast S. cerevisiae. We show here that osmotin uses a signal transduction pathway to weaken defensive cell wall barriers and increase its cytotoxic efficacy. The pathway activated by osmotin includes the regulatory elements of the mating pheromone response STE4, STE18, STE20, STE5, STE11, STE7, FUS3, KSS1, and STE12. Neither the pheromone receptor nor its associated G protein alpha subunit GPA1 are required for osmotin action. However, mutation of SST2, a negative regulator of G alpha proteins, resulted in supersensitivity to osmotin. Phosphorylation of STE7 was rapidly stimulated by osmotin preceding any changes in cell vitality or morphology. These results demonstrate that osmotin subverts target cell signal transduction as part of its mechanism of action.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

Inhibitory and activating functions for MAPK Kss1 in the S. cerevisiae filamentous-growth signalling pathway

Mitogen-activated protein kinase (MAPK) cascades are conserved signalling modules that regulate responses to diverse extracellular stimuli, developmental cues and environmental stresses. A MAPK is phosphorylated and activated by a MAPK kinase (MAPKK), which is activated by an upstream protein kinase, such as Raf, Mos or a MAPKK kinase. Ste7, a MAPKK in the yeast Saccharomyces cerevisiae, is required for two developmental pathways: mating and invasive (filamentous) growth. Kss1 and Fus3, the MAPK targets of Ste7, are required for mating, but their role in invasive growth has been unclear. Because no other S. cerevisiae MAPK has been shown to function in invasive growth, it was proposed that Ste7 may have non-MAPK targets. We show instead that Kss1 is the principal target of Ste7 in the invasive-growth response in both haploids and diploids. We demonstrate further that Kss1 in its inactive form is a potent negative regulator of invasive growth. Ste7 acts to relieve this negative regulation by switching Kss1 from an inhibitor to an activator. These results indicate that this MAPK has a physiologically important function in its unactivated state. Comparison of normal and MAPK-deficient cells indicates that nitrogen starvation and activated Ras stimulate filamentous growth through both MAPK-independent and MAPK-dependent means.

A general method for identifying recessive diploid-specific mutations in Saccharomyces cerevisiae, its application to the isolation of mutants blocked at intermediate stages of meiotic prophase and characterization of a new gene SAE2

We describe a general new approach for identifying recessive mutations that affect diploid strains of yeast Saccharomyces cerevisiae and the application of this method to the identification of mutations that confer an intermediate block in meiotic prophase chromosome metabolism. The method uses a temperature-sensitive conjugation mutation ste7-1 in combination with homothallism. The mutations of interest confer a defect in spore formation that is dependent upon a gene required for initiation of meiotic recombination and development of meiosis-specific chromosome structure (SPO11). Identified in this screen were null mutations of the DMC1 gene, nonnull mutations of RAD50 (rad50S), and mutations in three new genes designed SAE1, SAE2 and SAE3 (Sporulation in the Absence of Spo Eleven). Molecular characterization of the SAE2 gene and characterization of meiotic and mitotic phenotypes of sae2 mutants are also presented. The phenotypes conferred by a sae2 null mutation are virtually indistinguishable from those conferred by the previously identified nonnull mutations of RAD50 (rad50S). Most notably, both mutations confer only weak sensitivity to the radiomimetic agent methyl methane sulfonate (MMS) but completely block resection and turnover of meiosis-specific double-strand breaks. These observations provide further evidence that this constellation of phenotypes identifies a specific molecular function.

14-3-3 proteins are essential for RAS/MAPK cascade signaling during pseudohyphal development in S. cerevisiae

14-3-3 proteins are highly conserved ubiquitous proteins whose explicit functions have remained elusive. Here, we show that the S. cerevisiae 14-3-3 homologs BMH1 and BMH2 are not essential for viability or mating MAPK cascade signaling, but they are essential for pseudohyphal-development MAPK cascade signaling and other processes. Activated alleles of RAS2 and CDC42 induce pseudohyphal development and FG(TyA)-lacZ signaling in Bmh+ strains but not in ste20 (p65PAK) or bmh1 bmh2 mutant strains. Moreover, Bmh1p and Bmh2p associate with Ste20p in vivo. Three alleles of BMH1 encode proteins defective for FG(TyA)-lacZ signaling and association with Ste20p, yet these alleles complement other 14-3-3 functions. Therefore, the 14-3-3 proteins are specifically required for RAS/MAPK cascade signaling during pseudohyphal development in S. cerevisiae.

Overexpression of the G1-cyclin gene CLN2 represses the mating pathway in Saccharomyces cerevisiae at the level of the MEKK Ste11

Basal and induced transcription of pheromone-dependent genes is regulated in a cell cycle-dependent way. FUS1, a gene strongly induced after pheromone treatment, shows high mRNA levels in mitosis and early G1 phase of the cell cycle, a decrease in G1 after START and again an increase in S phase. Overexpression of CLN2 was shown to repress the transcript number of pheromone-dependent genes (1). We asked whether the activities of components of the mating pathway fluctuate during the cell cycle. We were also interested in determining at what level Cln2 represses the signal transduction machinery. Here we show that the activity of the mitogen-activated protein kinase Fus3 indeed fluctuates during the cell cycle, reflecting the oscillations of the gene transcripts. CLN2 overexpression represses Fus3 kinase activity, independently of the phosphatase Msg5. Additionally, we show that the activity of the MEK Ste7 also fluctuates during the cell cycle. Increased Cln2 levels repress the ability of hyperactive STE11 alleles to induce the pathway. G protein-independent activation of Ste11 caused by an rga1 pbs2 mutation is resistant to high levels of Cln2 kinase. Therefore our results suggest that Cln2-dependent repression of the mating pathway occurs at the level of Ste11.

A novel MAP-kinase kinase from Candida albicans

A putative MAP-kinase kinase-encoding gene, CaSTE7, was isolated from Candida albicans by complementation of ste7 and ste11 mutants of the pheromone signal-transduction pathway of Saccharomyces cerevisiae. The nucleotide (nt) sequence revealed an ORF of 1767 nt encoding a putative protein of 589 amino acids (aa). CaSTE7 has a strong homology with MAP-kinase kinase STE7 of S. cerevisiae, the kinase domain having 45% homology with that of STE7. The deduced aa sequence contained all eleven consensus kinase subdomains found in MAP-kinase kinases. It can suppress the mating defect of ste5, ste11, ste7, and fus3 kss1 double mutants, but it cannot bypass the ste12 mutation. CaSTE7 behaves as a hyperactive allele of STE7, suppressing the mating defects of the pheromone signal-transduction pathway by constitutively stimulating STE12, and hence STE12-dependent processes.

Characterization of the basal and pheromone-stimulated phosphorylation states of Ste12p

The Saccharomyces cerevisiae transcription factor Ste12p is required for basal and activated expression of pheromone-responsive genes, and for invasive growth in haploid cells. In diploid yeast, Ste12p is implicated in pseudohyphal development. The ability of Ste12p to effect these various responses in three different cell types must require stringent regulation of its transcriptional activation function and interaction with additional transcription factors. We have examined the phosphorylation state of Ste12p in untreated and pheromone-treated haploid cells, and found eight constitutively phosphorylated peptides. Phosphorylation at the constitutive sites does not require the protein kinases of the pheromone-response pathway. Treatment of haploid yeast with mating pheromone causes the appearance of novel relatively minor phosphorylations on Ste12p. Brief [35S]methionine labeling reveals novel pheromone-dependent, electrophoretically slower migrating Ste12p species. Similarly, the sole difference we observe in tryptic phosphopeptides generated from Ste12p from pheromone-treated and untreated cells is the transient appearance of two novel minor hydrophobic phosphopeptides. The pheromone-dependent phosphorylation of Ste12p requires an intact pheromone-response pathway and localization of Ste12p to the nucleus, but does not require the Ste12p DNA-binding domain. We conclude from these experiments that the pheromone-response pathway induces the formation of specific hyperphosphorylation on Ste12p, which can only be detected as apparently minor modifications in vivo. We argue that, if Ste12p is regulated by direct pheromone-responsive phosphorylation, then that phosphorylation must be represented by the two novel phosphopeptides. However, we cannot exclude the possibility that pheromone-responsive transcription is controlled by direct phosphorylation of a target other than Ste12p.

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.

Arabidopsis thaliana cDNA isolated by functional complementation shows homology to serine/threonine protein kinases

The yeast Saccharomyces cerevisiae ste6 mutant is defective in transport of a-mating factor, resulting in an inability of ste6 a cells to mate with alpha cells. The gene encodes an ATP-binding cassette, ABC transporter. We used functional complementation of a yeast ste6 mutant with an Arabidopsis thaliana expression library in an attempt to clone an Arabidopsis homolog. Sequence analysis of the isolated Arabidopsis complementing cDNA however showed no homology to the STE6 gene. High sequence similarity was detected to members of the mitogen-activated serine/threonine protein (MAP) kinase family involved in signal transduction: STE20, STE11, BCK1, Byr2 and p65PAK. The Arabidopsis clone failed to complement a fus3/kss1 mutant of S. cerevisiae, but did complement a defect in ste11, ste20, as well as ste6. The isolated clone encodes a protein that is truncated at its amino-terminal, and might function in a similar way as a dominant STE11 truncation allele. These results suggest that the Arabidopsis cDNA encodes a putative serine/threonine kinase that can function in the mating response pathway upstream of FUS3/KSS1 in S. cerevisiae, at the level of STE11 gene. Interestingly, this clone is able to restore the ability of the ste6 yeast mutant to export a-factor.

Feedback regulation of map kinase signal pathways

Ste7 is a MEK (MAPK/ERK kinase) family member that functions in the pheromone induced mating response pathway of Saccharomyces cerevisiae. We analysed the catalytic competence and in vivo function of Ste7 variants that have alterations of stimulatory and feedback phosphorylation sites. These analyses led us to unanticipated insights into two separate feedback mechanisms that impede the output of the mating response MAPK activation pathway.

Constitutive activation of the Saccharomyces cerevisiae mating response pathway by a MAP kinase kinase from Candida albicans

The HST7 gene of Candida albicans encodes a protein with structural similarity to MAP kinase kinases. Expression of this gene in Saccharomyces cerevisiae complements disruption of the Ste7 MAP kinase kinase required for both mating in haploid cells and pseudohyphal growth in diploids. However, Hst7 expression does not complement loss of either the Pbs2 (Hog4) MAP kinase kinase required for response to high osmolarity, or loss of the Mkk1 and Mkk2 MAP kinase kinases required for proper cell wall biosynthesis. Intriguingly, HST7 acts as a hyperactive allele of STE7; expression of Hst7 activates the mating pathway even in the absence of upstream signaling components including the Ste7 regulator Ste11, elevates the basal level of the pheromone-inducible FUS1 gene, and amplifies the pseudohyphal growth response in diploid cells. Thus Hst7 appears to be at least partially independent of upstream activators or regulators, but selective in its activity on downstream target MAP kinases. Creation of Hst7/Ste7 hybrid proteins revealed that the C-terminal two-thirds of Hst7, which contains the protein kinase domain, is sufficient to confer this partial independence of upstream activators.