Ste50p sustains mating pheromone-induced signal transduction in the yeast Saccharomyces cerevisiae

Turnover and bypass of p21-activated kinase during Cdc42-dependent MAPK signaling in yeast

Mitogen-activated protein kinase (MAPK) pathways regulate multiple cellular behaviors, including the response to stress and cell differentiation, and are highly conserved across eukaryotes. MAPK pathways can be activated by the interaction between the small GTPase Cdc42p and the p21-activated kinase (Ste20p in yeast). By studying MAPK pathway regulation in yeast, we recently found that the active conformation of Cdc42p is regulated by turnover, which impacts the activity of the pathway that regulates filamentous growth (fMAPK). Here, we show that Ste20p is regulated in a similar manner and is turned over by the 26S proteasome. This turnover did not occur when Ste20p was bound to Cdc42p, which presumably stabilized the protein to sustain MAPK pathway signaling. Although Ste20p is a major component of the fMAPK pathway, genetic approaches here identified a Ste20p-independent branch of signaling. Ste20p-independent signaling partially required the fMAPK pathway scaffold and Cdc42p-interacting protein, Bem4p, while Ste20p-dependent signaling required the 14-3-3 proteins, Bmh1p and Bmh2p. Interestingly, Ste20p-independent signaling was inhibited by one of the GTPase-activating proteins for Cdc42p, Rga1p, which unexpectedly dampened basal but not active fMAPK pathway activity. These new regulatory features of the Rho GTPase and p21-activated kinase module may extend to related pathways in other systems.

Mitogen-activated protein kinase (MAPK) cascades-A yeast perspective

Discovery of the class of protein kinase now dubbed a mitogen (or messenger)-activated protein kinase (MAPK) is an illustrative example of how disparate lines of investigation can converge and reveal an enzyme family universally conserved among eukaryotes, from single-celled microbes to humans. Moreover, elucidation of the circuitry controlling MAPK function defined a now overarching principle in enzyme regulation-the concept of an activation cascade mediated by sequential phosphorylation events. Particularly ground-breaking for this field of exploration were the contributions of genetic approaches conducted using several model organisms, but especially the budding yeast Saccharomyces cerevisiae. Notably, examination of how haploid yeast cells respond to their secreted peptide mating pheromones was crucial in pinpointing genes encoding MAPKs and their upstream activators. Fully contemporaneous biochemical analysis of the activities elicited upon stimulation of mammalian cells by insulin and other growth- and differentiation-inducing factors lead eventually to the demonstration that components homologous to those in yeast were involved. Continued studies of these pathways in yeast were integral to other foundational discoveries in MAPK signaling, including the roles of tethering, scaffolding and docking interactions.

New Features Surrounding the Cdc42-Ste20 Module that Regulates MAP Kinase Signaling in Yeast

Mitogen-activated protein kinase (MAPK) pathways regulate multiple cellular responses, including the response to stress and cell differentiation, and are highly conserved across eukaryotes from yeast to humans. In yeast, the canonical activation of several MAPK pathways includes the interaction of the small GTPase Cdc42p with the p21-activated kinase (PAK) Ste20p. We recently found that the active conformation of Cdc42p is regulated by turnover, which impacts the activity of the pathway that regulates filamentous growth (fMAPK). Here, we show that Ste20p is turned over by the 26S proteasome. Ste20p was stabilized when bound to Cdc42p, presumably to sustain MAPK pathway signaling. Ste20p is a major conduit by which signals flow through the fMAPK pathway; however, by genetic approaches we also identified a Ste20p-independent branch of the fMAPK pathway. Ste20p-dependent signaling required the 14-3-3 proteins, Bmh1p and Bmh2p, while Ste20p-independent signaling required the fMAPK pathway adaptor and Cdc42p-interacting protein, Bem4p. Ste20p-independent signaling was inhibited by one of the GTPase-activating proteins for Cdc42p in the fMAPK pathway, Rga1p, which also dampened basal but not active fMAPK pathway activity. Finally, the polarity adaptor and Cdc42p-interacting protein, Bem1p, which also regulates the fMAPK pathway, interacts with the tetra-span protein Sho1p, connecting a sensor at the plasma membrane to a protein that regulates the GTPase module. Collectively, these data reveal new regulatory features surrounding a Rho-PAK module that may extend to other pathways that control cell differentiation.

SsUbc2, a determinant of pathogenicity, functions as a key coordinator controlling global transcriptomic reprogramming during mating in sugarcane smut fungus

The basidiomycete fungus Sporisorium scitamineum is the causative agent of sugarcane smut disease. Mating between two strains of the opposite mating type is essential for filamentous growth and infection in sugarcane plants. However, the mechanisms underlying mating and pathogenicity are still not well understood. In this work we used gene disruption to investigate the role of Ssubc2, the gene encoding a kinase regulator in S. scitamineum. Deletion of Ssubc2 did not alter the haploid cell morphology or growth rate in vitro or tolerance to stress, but mutants with both alleles deleted lost mating ability and infectivity. Deletion of one Ssubc2 allele in a pair with a wild-type strain resulted in impaired mating and reduced virulence. Transcriptome profiling revealed that about a third of genes underwent reprogramming in the wild types during mating. Although gene expression reprogramming occurred in the pairing of Ssubc2-null mutants, their transcriptomic profile differed significantly from that of the wild types, in which 625 genes differed from those present in the wild types that seemed to be among the required genes for a successful mating. These genes include those known to regulate mating and pathogenicity, such as components of the MAPK pathway and hgl1. Additionally, a total of 908 genes were differentially expressed in an out-of-control manner in the mutants. We conclude that SsUbc2 functions as a key factor to coordinate the reprogramming of gene expression at the global level and is essential for the transition from monokaryotic basidial growth to dikaryotic hyphal growth through mating.

The Adaptor Protein UvSte50 Governs Fungal Pathogenicity of Ustilaginoidea virens via the MAPK Signaling Pathway

The mitogen-activated protein kinase (MAPK) signaling pathways regulate diverse cellular processes and have been partially characterized in the rice false smut fungus Ustilaginoidea virens. UvSte50 has been identified as a homolog to Saccharomyces cerevisiae Ste50, which is known to be an adaptor protein for MAPK cascades. ΔUvste50 was found to be defective in conidiation, sensitive to hyperosmotic and oxidative stresses, and non-pathogenic. The mycelial expansion of ΔUvste50 inside spikelets of rice terminated at stamen filaments, eventually resulting in a lack of formation of false smut balls on spikelets. We determined that UvSte50 directly interacts with both UvSte7 (MAPK kinase; MEK) and UvSte11 (MAPK kinase kinase; MEKK), where the Ras-association (RA) domain of UvSte50 is indispensable for its interaction with UvSte7. UvSte50 also interacts with UvHog1, a MAP kinase of the Hog1-MAPK pathway, which is known to have important roles in hyphal growth and stress responses in U. virens. In addition, affinity capture-mass spectrometry analysis and yeast two-hybrid assay were conducted, through which we identified the interactions of UvSte50 with UvRas2, UvAc1 (adenylate cyclase), and UvCap1 (cyclase-associated protein), key components of the Ras/cAMP signaling pathway in U. virens. Together, UvSte50 functions as an adaptor protein interacting with multiple components of the MAPK and Ras/cAMP signaling pathways, thus playing critical role in plant infection by U. virens.

The pheromone response module, a mitogen-activated protein kinase pathway implicated in the regulation of fungal development, secondary metabolism and pathogenicity

Mitogen-activated protein kinase (MAPK) pathways are highly conserved from yeast to human and are required for the regulation of a multitude of biological processes in eukaryotes. A pentameric MAPK pathway known as the Fus3 pheromone module was initially characterised in Saccharomyces cerevisiae and was shown to regulate cell fusion and sexual development. Individual orthologous pheromone module genes have since been found to be highly conserved in fungal genomes and have been shown to regulate a diverse array of cellular responses, such as cell growth, asexual and sexual development, secondary metabolite production and pathogenicity. However, information regarding the assembly and structure of orthologous pheromone modules, as well as the mechanisms of signalling and their biological significance is limited, specifically in filamentous fungal species. Recent studies have provided insight on the utilization of the pheromone module as a central signalling hub for the co-ordinated regulation of fungal development and secondary metabolite production. Various proteins of this pathway are also known to regulate reproduction and virulence in a range of plant pathogenic fungi. In this review, we discuss recent findings that help elucidate the structure of the pheromone module pathway in a myriad of fungal species and its implications in the control of fungal growth, development, secondary metabolism and pathogenicity.

MAP Kinase Regulation of the Candida albicans Pheromone Pathway

MAP kinases and their regulators are critical components of eukaryotic signaling pathways implicated in normal cell behavior as well as abnormal behaviors linked to diseases such as cancer. The mating pathway of the yeast Saccharomyces cerevisiae was central in establishing the MAP kinase paradigm. Here we investigate the mating pathway in a different ascomycete, the fungal pathogen C. albicans. In this dimorphic fungus MAP kinases are also implicated in the mating response, with two MAP kinases apparently playing redundant roles in the mating process. This work establishes that while some level of mating can occur in the presence of a single kinase, the Cek1 kinase is most important for mating, while the Cek2 kinase is involved in adaptation to signaling. While both kinases appear to be themselves regulated by dephosphorylation through the action of the Cpp1 phosphatase, this process appears important for mating only in the case of Cek1.

We investigated the relationships of the Cek1 and Cek2 mitogen-activated protein (MAP) kinases and the putative MAP kinase phosphatase Cpp1 in the mating process of Candida albicans. Mutants of the CPP1 gene are hyperresponsive to pheromone, generating large halos, high levels of projections, and an increase in pheromone-responsive gene expression. Mating-type-homozygous opaque cells that lack both kinases are sterile, consistent with previous observations, although several lines of evidence show that the two kinases do not simply provide redundant functions in the mating process. Loss of CEK1 reduces mating significantly, to about 0.3% of wild-type strains, and also reduces projection formation and pheromone-mediated gene expression. In contrast, loss of CEK2 has less of an effect, reducing mating to approximately one-third that of the wild-type strain and moderately reducing projection formation but having little influence on the induction of gene expression. However, loss of Cek2 function reduces adaptation to pheromone-mediated arrest. The mutation enhances pheromone response halos to a level similar to that of cpp1 mutants, although the cpp1 mutants are considerably more mating defective than the cek2 mutant. The double cek2 cpp1 mutant shows enhanced responsiveness relative to either single mutant in terms of gene expression and halo formation, suggesting the kinase and phosphatase roles in the adaptation process are independent. Analysis of protein phosphorylation shows that Cek1 undergoes pheromone-mediated phosphorylation of the activation loop, and this phosphorylation is enhanced in cells lacking either the Cpp1 phosphatase or the Cek2 kinase. In addition, Cek1-GFP shows enhanced nuclear localization in response to pheromone treatment. In contrast, Cek2 shows no evidence for pheromone-mediated phosphorylation or pheromone-mediated nuclear localization. Intriguingly, however, deletion of CPP1 enhances both the phosphorylation state and the nuclear localization of Cek2-GFP. Overall, these results identify a complex interaction among the MAP kinases and MAP kinase phosphatase that function in the C. albicans mating pathway.

IMPORTANCE MAP kinases and their regulators are critical components of eukaryotic signaling pathways implicated in normal cell behavior as well as abnormal behaviors linked to diseases such as cancer. The mating pathway of the yeast Saccharomyces cerevisiae was central in establishing the MAP kinase paradigm. Here we investigate the mating pathway in a different ascomycete, the fungal pathogen C. albicans. In this dimorphic fungus MAP kinases are also implicated in the mating response, with two MAP kinases apparently playing redundant roles in the mating process. This work establishes that while some level of mating can occur in the presence of a single kinase, the Cek1 kinase is most important for mating, while the Cek2 kinase is involved in adaptation to signaling. While both kinases appear to be themselves regulated by dephosphorylation through the action of the Cpp1 phosphatase, this process appears important for mating only in the case of Cek1.

The HamE scaffold positively regulates MpkB phosphorylation to promote development and secondary metabolism in Aspergillus nidulans

Mitogen-activated protein kinase (MAPK) pathways are conserved signalling cascades in eukaryotes which regulate a myriad of processes in fungi from sexual reproduction to stress responses. These pathways rely on recruitment of three kinases on a scaffold protein to facilitate efficient kinase phosphorylation and subsequent downstream signalling to the nucleus. The model filamentous fungus Aspergillus nidulans utilises a MAPK pathway termed the pheromone module to regulate both development and secondary metabolism. This complex consists of the MAP3K (SteC), MAP2K (MkkB), MAPK (MpkB) and adaptor protein SteD. To date, there has been no scaffold protein identified for this MAPK pathway. In this study, we characterised a protein termed HamE, which we propose as a scaffold that regulates kinase phosphorylation and signalling in the pheromone module. Mass spectrometry analysis and BIFC experiments revealed that HamE physically interacts with both MkkB and MpkB and transiently interacts with SteC. Deletion of hamE or any of the pheromone module kinases results in reduced sporulation and complete abolishment of cleistothecia production. Mutants also exhibited reductions in expression of secondary metabolite gene clusters, including the velvet complex and sterigmatocystin genes. HamE acts as a positive regulator of MpkB phosphorylation, allowing for HamE to subsequently regulate development and secondary metabolism.

The transmembrane protein FgSho1 regulates fungal development and pathogenicity via the MAPK module Ste50-Ste11-Ste7 in Fusarium graminearum

The mitogen-activated protein kinase (MAPK) signaling pathways have been characterized in Fusarium graminearum. Currently, the upstream sensors of these pathways are unknown. Biological functions of a transmembrane protein FgSho1 were investigated using a target gene deletion strategy. The relationship between FgSho1 and the MAPK cassette FgSte50-Ste11-Ste7 was analyzed in depth. The transmembrane protein FgSho1 is required for conidiation, full virulence, and deoxynivalenol (DON) biosynthesis in F. graminearum. Furthermore, FgSho1 and FgSln1 have an additive effect on virulence of F. graminearum. The yeast two-hybrid, coimmunoprecipitation, colocalization and affinity capture-mass spectrometry analyses strongly indicated that FgSho1 physically interacts with the MAPK module FgSte50-Ste11-Ste7. Similar to the FgSho1 mutant, the mutants of FgSte50, FgSte11, and FgSte7 were defective in conidiation, pathogenicity, and DON biosynthesis. In addition, FgSho1 plays a minor role in the response to osmotic stress but it is involved in the cell wall integrity pathway, which is independent of the module FgSte50-Ste11-Ste7 in F. graminearum. Collectively, results of this study strongly indicate that FgSho1 regulates fungal development and pathogenicity via the MAPK module FgSte50-Ste11-Ste7 in F. graminearum, which is different from what is known in the budding yeast Saccharomyces cerevisiae.

Cdc42p-interacting protein Bem4p regulates the filamentous-growth mitogen-activated protein kinase pathway

The ubiquitous Rho (Ras homology) GTPase Cdc42p can function in different settings to regulate cell polarity and cellular signaling. How Cdc42p and other proteins are directed to function in a particular context remains unclear. We show that the Cdc42p-interacting protein Bem4p regulates the mitogen-activated protein kinase (MAPK) pathway that controls filamentous growth in Saccharomyces cerevisiae. Bem4p controlled the filamentous-growth pathway but not other MAPK pathways (mating or high-osmolarity glycerol response [HOG]) that also require Cdc42p and other shared components. Bem4p associated with the plasma membrane (PM) protein, Sho1p, to regulate MAPK activity and cell polarization under nutrient-limiting conditions that favor filamentous growth. Bem4p also interacted with the major activator of Cdc42p, the guanine nucleotide exchange factor (GEF) Cdc24p, which we show also regulates the filamentous-growth pathway. Bem4p interacted with the pleckstrin homology (PH) domain of Cdc24p, which functions in an autoinhibitory capacity, and was required, along with other pathway regulators, to maintain Cdc24p at polarized sites during filamentous growth. Bem4p also interacted with the MAPK kinase kinase (MAPKKK) Ste11p. Thus, Bem4p is a new regulator of the filamentous-growth MAPK pathway and binds to general proteins, like Cdc42p and Ste11p, to promote a pathway-specific response.

Mate and fuse: how yeast cells do it

Many cells are able to orient themselves in a non-uniform environment by responding to localized cues. This leads to a polarized cellular response, where the cell can either grow or move towards the cue source. Fungal haploid cells secrete pheromones to signal mating, and respond by growing a mating projection towards a potential mate. Upon contact of the two partner cells, these fuse to form a diploid zygote. In this review, we present our current knowledge on the processes of mating signalling, pheromone-dependent polarized growth and cell fusion in Saccharomyces cerevisiae and Schizosaccharomyces pombe, two highly divergent ascomycete yeast models. While the global architecture of the mating response is very similar between these two species, they differ significantly both in their mating physiologies and in the molecular connections between pheromone perception and downstream responses. The use of both yeast models helps enlighten both conserved solutions and species-specific adaptations to a general biological problem.

Response to hyperosmotic stress

An appropriate response and adaptation to hyperosmolarity, i.e., an external osmolarity that is higher than the physiological range, can be a matter of life or death for all cells. It is especially important for free-living organisms such as the yeast Saccharomyces cerevisiae. When exposed to hyperosmotic stress, the yeast initiates a complex adaptive program that includes temporary arrest of cell-cycle progression, adjustment of transcription and translation patterns, and the synthesis and retention of the compatible osmolyte glycerol. These adaptive responses are mostly governed by the high osmolarity glycerol (HOG) pathway, which is composed of membrane-associated osmosensors, an intracellular signaling pathway whose core is the Hog1 MAP kinase (MAPK) cascade, and cytoplasmic and nuclear effector functions. The entire pathway is conserved in diverse fungal species, while the Hog1 MAPK cascade is conserved even in higher eukaryotes including humans. This conservation is illustrated by the fact that the mammalian stress-responsive p38 MAPK can rescue the osmosensitivity of hog1Δ mutations in response to hyperosmotic challenge. As the HOG pathway is one of the best-understood eukaryotic signal transduction pathways, it is useful not only as a model for analysis of osmostress responses, but also as a model for mathematical analysis of signal transduction pathways. In this review, we have summarized the current understanding of both the upstream signaling mechanism and the downstream adaptive responses to hyperosmotic stress in yeast.

Checkpoints in a yeast differentiation pathway coordinate signaling during hyperosmotic stress

All eukaryotes have the ability to detect and respond to environmental and hormonal signals. In many cases these signals evoke cellular changes that are incompatible and must therefore be orchestrated by the responding cell. In the yeast Saccharomyces cerevisiae, hyperosmotic stress and mating pheromones initiate signaling cascades that each terminate with a MAP kinase, Hog1 and Fus3, respectively. Despite sharing components, these pathways are initiated by distinct inputs and produce distinct cellular behaviors. To understand how these responses are coordinated, we monitored the pheromone response during hyperosmotic conditions. We show that hyperosmotic stress limits pheromone signaling in at least three ways. First, stress delays the expression of pheromone-induced genes. Second, stress promotes the phosphorylation of a protein kinase, Rck2, and thereby inhibits pheromone-induced protein translation. Third, stress promotes the phosphorylation of a shared pathway component, Ste50, and thereby dampens pheromone-induced MAPK activation. Whereas all three mechanisms are dependent on an increase in osmolarity, only the phosphorylation events require Hog1. These findings reveal how an environmental stress signal is able to postpone responsiveness to a competing differentiation signal, by acting on multiple pathway components, in a coordinated manner.

All cells can detect and respond to signals in their environment. The ability to interpret these signals with accuracy is needed for proper growth and differentiation. Moreover, cells must prioritize responses when confronted with competing signals. However the molecular mechanisms that govern signal prioritization are poorly understood. To address this question, we studied two signaling pathways in the genetic model organism budding yeast. Specifically we focused on the pheromone mating (differentiation) pathway and the high osmolarity glycerol (stress response) pathway. These pathways respond differently to each stimulus despite sharing pathway components. We find that cells must first adapt to stress before they can mate. At early times, the stress response cross-inhibits and dampens the pheromone response to suspend mating differentiation. Once cells adapt, the stress response ends and the differentiation program resumes. All signaling pathways that regulate cell fate decisions are interconnected to varying degrees. Our study highlights the importance of proper signal coordination in cell fate decisions, and it reveals new mechanisms that govern signal coordination within complex signaling networks.

Selective regulation of MAP kinase signaling by an endomembrane phosphatidylinositol 4-kinase

Multiple MAP kinase pathways share components yet initiate distinct biological processes. Signaling fidelity can be maintained by scaffold proteins and restriction of signaling complexes to discreet subcellular locations. For example, the yeast MAP kinase scaffold Ste5 binds to phospholipids produced at the plasma membrane and promotes selective MAP kinase activation. Here we show that Pik1, a phosphatidylinositol 4-kinase that localizes primarily to the Golgi, also regulates MAP kinase specificity but does so independently of Ste5. Pik1 is required for full activation of the MAP kinases Fus3 and Hog1 and represses activation of Kss1. Further, we show by genetic epistasis analysis that Pik1 likely regulates Ste11 and Ste50, components shared by all three MAP kinase pathways, through their interaction with the scaffold protein Opy2. These findings reveal a new regulator of signaling specificity functioning at endomembranes rather than at the plasma membrane.

The Gα subunit signals through the Ste50 protein during the mating pheromone response in the yeast Kluyveromyces lactis

Yeast mating signal transduction pathways require a heterotrimeric G protein composed of Gα, Gβ, and Gγ subunits connected to a mitogen-activated protein kinase (MAPK) module. While in Saccharomyces cerevisiae elimination of Gα induces constitutive activation of the mating pathway, in Kluyveromyces lactis it produces partial sterility, which indicates that K. lactis Gα (KlGα) is required to positively activate mating. We use physical interaction experiments to determine that KlGα interacts with the adaptor protein KlSte50p. The Ras association (RA) domain of KlSte50p favored interaction with the GDP-bound KlGα subunit, and when the KlGα protein is constitutively activated, the interaction drops significantly. Additionally, KlSte50p strongly associates with the MAPK kinase kinase (MAPKKK) KlSte11p through its sterile alpha motif (SAM) domain. Genetic experiments placed KlSte50p downstream of the G protein α subunit, indicating that KlGα may stimulate the mating pathway via KlSte50p. Fusion of KlSte50p to the KlGβ subunit partially eliminated the requirement of KlGα for mating, indicating that one contribution of KlGα to the mating pathway is to facilitate plasma membrane anchoring of KlSte50p.

Efficient approaches for generating GFP fusion and epitope-tagging constructs in filamentous fungi

For functional characterization of predicted genes encoding hypothetical proteins in fungal genomes, it is complementary to genetic studies to determine their expression and subcellular localization patterns in different developmental or infection stages. It is also important to identify and characterize other proteins that are physically associated with or functionally related to these genes in vivo by co-immunoprecipitation or affinity purification analyses. In this chapter, we described a set of yeast shuttle vectors and protocols to generate fusion constructs by the yeast gap repair approach. Because of the simplicity and efficiency of yeast gap repair, these vectors and the general methods described in this chapter are suitable for functional genomics studies in filamentous fungi.

Control of MAPK specificity by feedback phosphorylation of shared adaptor protein Ste50

Many different signaling pathways share common components but nevertheless invoke distinct physiological responses. In yeast, the adaptor protein Ste50 functions in multiple mitogen-activated protein (MAP) kinase pathways, each with unique dynamical and developmental properties. Although Kss1 activity is sustained and promotes invasive growth, Hog1 activity is transient and promotes cell adaptation to osmotic stress. Here we show that osmotic stress activates Kss1 as well as Hog1. We show further that Hog1 phosphorylates Ste50 and that phosphorylation of Ste50 limits the duration of Kss1 activation and prevents invasive growth under high osmolarity growth conditions. Thus feedback regulation of a shared component can restrict the activity of a competing MAP kinase to ensure signal fidelity.

Protein kinases involved in mating and osmotic stress in the yeast Kluyveromyces lactis

Systematic disruption of genes encoding kinases and mitogen-activated protein kinases (MAPKs) was performed in Kluyveromyces lactis haploid cells. The mutated strains were assayed by their capacity to mate and to respond to hyperosmotic stress. The K. lactis Ste11p (KlSte11p) MAPK kinase kinase (MAPKKK) was found to act in both mating and osmoresponse pathways while the scaffold KlSte5p and the MAPK KlFus3p appeared to be specific for mating. The p21-activated kinase KlSte20p and the kinase KlSte50p participated in both pathways. Protein association experiments showed interaction of KlSte50p and KlSte20p with Galpha and Gbeta, respectively, the G protein subunits involved in the mating pathway. Both KlSte50p and KlSte20p also showed interaction with KlSte11p. Disruption mutants of the K. lactis PBS2 (KlPBS2) and KlHOG1 genes of the canonical osmotic response pathway resulted in mutations sensitive to high salt and high sorbitol but dispensable for mating. Mutations that eliminate the MAPKK KlSte7p activity had a strong effect on mating and also showed sensitivity to osmotic stress. Finally, we found evidence of physical interaction between KlSte7p and KlHog1p, in addition to diminished Hog1p phosphorylation after a hyperosmotic shock in cells lacking KlSte7p. This study reveals novel roles for components of transduction systems in yeast.

Ste50 adaptor protein influences Ras/cAMP-driven stress-response and cell survival in Saccharomyces cerevisiae

The Ste50 adaptor protein is involved in a variety of cellular pathways that yeast cells use to adapt rapidly to environmental changes. A highly activated Ras-cyclic AMP (cAMP) pathway by deletion of the high-affinity cAMP-dependent phosphodiesterase 2 (PDE2) leads to repression of a stress mediated response and cell survival. Here we show that inactivation of STE50 confers a synthetic genetic interaction with pde2Delta. A hyperosmotic stress growth defect of ste50Delta pde2Delta cells is exacerbated by extracellular cAMP or by galactose as the sole carbon source in the medium. The inactivation of the serine/threonine protein-kinase Akt homologue Sch9 increase stress resistance and extend chronological life span. By pde2Delta-dependent increase of the Ras-cAMP pathway activity, inactivation of STE50 results in an extreme shortening of life span and oxidative stress sensitivity of sch9Delta mutants. Furthermore, sch9Delta can promote transcription of the small heat shock protein HSP26 in a PDE2-dependent manner; however, sch9Delta can promote transcription of the mitochondrial superoxide dismutase SOD2 in a PDE2- and STE50-dependent manner. These data indicate that inactivation of STE50 influences stress tolerance in mutants of the Ras-cAMP pathway, which is a major determinant of intrinsic stress tolerance and cell survival of the Saccharomyces cerevisiae.

Multiple upstream signals converge on the adaptor protein Mst50 in Magnaporthe grisea

Rice blast fungus (Magnaporthe grisea) forms a highly specialized infection structure for plant penetration, the appressorium, the formation and growth of which are regulated by the Mst11-Mst7-Pmk1 mitogen-activated protein kinase cascade. We characterized the MST50 gene that directly interacts with both MST11 and MST7. Similar to the mst11 mutant, the mst50 mutant was defective in appressorium formation, sensitive to osmotic stresses, and nonpathogenic. Expressing a dominant active MST7 allele in mst50 complemented its defects in appressorium but not lesion formation. The sterile alpha-motif (SAM) domain of Mst50 was essential for its interaction with Mst11 and for appressorium formation. Although the SAM and Ras-association domain (RAD) of Mst50 were dispensable for its interaction with Mst7, deletion of RAD reduced appressorium formation and virulence on rice (Oryza sativa) seedlings. The interaction between Mst50 and Mst7 or Mst11 was detected by coimmunoprecipitation assays in developing appressoria. Mst50 also interacts with Ras1, Ras2, Cdc42, and Mgb1 in yeast two-hybrid assays. Expressing a dominant active RAS2 allele in the wild-type strain but not in mst50 stimulated abnormal appressorium formation. These results indicate that MST50 functions as an adaptor protein interacting with multiple upstream components and plays critical roles in activating the Pmk1 cascade for appressorium formation and plant infection in M. grisea.

The RA domain of Ste50 adaptor protein is required for delivery of Ste11 to the plasma membrane in the filamentous growth signaling pathway of the yeast Saccharomyces cerevisiae

In Saccharomyces cerevisiae, pheromone response requires Ste5 scaffold protein, which ensures efficient G-protein-dependent recruitment of mitogen-activated protein kinase (MAPK) cascade components Ste11 (MAPK kinase kinase), Ste7 (MAPK kinase), and Fus3 (MAPK) to the plasma membrane for activation by Ste20 protein kinase. Ste20, which phosphorylates Ste11 to initiate signaling, is activated by binding to Cdc42 GTPase (membrane anchored via its C-terminal geranylgeranylation). Less clear is how activated and membrane-localized Ste20 contacts Ste11 to trigger invasive growth signaling, which also requires Ste7 and the MAPK Kss1, but not Ste5. Ste50 protein associates constitutively via an N-terminal sterile-alpha motif domain with Ste11, and this interaction is required for optimal invasive growth and hyperosmotic stress (high-osmolarity glycerol [HOG]) signaling but has a lesser role in pheromone response. We show that a conserved C-terminal, so-called "Ras association" (RA) domain in Ste50 is also essential for invasive growth and HOG signaling in vivo. In vitro the Ste50 RA domain is not able to associate with Ras2, but it does associate with Cdc42 and binds to a different face than does Ste20. RA domain function can be replaced by the nine C-terminal, plasma membrane-targeting residues (KKSKKCAIL) of Cdc42, and membrane-targeted Ste50 also suppresses the signaling deficiency of cdc42 alleles specifically defective in invasive growth. Thus, Ste50 serves as an adaptor to tether Ste11 to the plasma membrane and can do so via association with Cdc42, thereby permitting the encounter of Ste11 with activated Ste20.

Saccharomyces cerevisiae Ste50 Binds the MAPKKK Ste11 Through a Head-to-tail SAM Domain Interaction

In Saccharomyces cerevisiae, signal transduction through pathways governing mating, osmoregulation, and nitrogen starvation depends upon a direct interaction between the sterile alpha motif (SAM) domains of the Ste11 mitogen-activated protein kinase kinase kinase (MAPKKK) and its regulator Ste50. Previously, we solved the NMR structure of the SAM domain from Ste11 and identified two mutants that diminished binding to the Ste50 SAM domain. Building upon the Ste11 study, we present the NMR structure of the monomeric Ste50 SAM domain and a series of mutants bearing substitutions at surface-exposed hydrophobic amino acid residues. The mid-loop (ML) region of Ste11-SAM, defined by helices H3 and H4 and the end-helix (EH) region of Ste50-SAM, defined by helix H5, were sensitive to substitution, indicating that these two surfaces contribute to the high-affinity interaction. The combination of two mutants, Ste11-SAM-L72R and Ste50-SAM-L69R, formed a high-affinity heterodimer unencumbered by competing homotypic interactions that had prevented earlier NMR studies of the wild-type complex. Yeast bearing mutations that prevented the heterotypic Ste11-Ste50 association in vitro presented signaling defects in the mating and high-osmolarity growth pathways.

Mdy2, a ubiquitin-like (UBL)-domain protein, is required for efficient mating in Saccharomyces cerevisiae

MDY2, a gene required for efficient mating of the yeast Saccharomyces cerevisiae, was characterized in this study. The gene encodes a protein of 212 amino acids, which contains a ubiquitin-like (UBL) domain (residues 74-149). Deletion of MDY2 is associated with a five- to sevenfold reduction in mating efficiency, mainly due to defects in nuclear migration and karyogamy at the prezygotic stage. However, prior to mating pair fusion, shmoo formation is reduced by 30%, with a concomitant failure to form mating pairs. Strikingly, migration of the nucleus into the shmoo tip is also delayed or fails to occur. In addition, we show that in mdy2 mutants, microtubule bundles, as well as the microtubule end-binding protein Kar9, fail to localize properly to the shmoo tip, suggesting that the nuclear migration defect could be due to aberrant localization of Kar9. Pheromone signal transduction (as measured by FUS1 induction by α-factor) is not affected in mdy2Δ mutants and mitosis is also normal in these cells. MDY2 is not induced by mating pheromone. In vegetatively growing cells, GFP-Mdy2 is localized in the nucleus, and remains nuclear after exposure of cells to α-factor. His-tagged Mdy2 shows no evidence of the C-terminal processing typical of ubiquitin, and also localizes to the nucleus. Thus MDY2 is a novel gene, whose product plays a role in shmoo formation and in nuclear migration in the pre-zygote, possibly by interacting with other UBL-type proteins that possess ubiquitin association (UBA) domains.

Candida glabrata Ste11 is involved in adaptation to hypertonic stress, maintenance of wild-type levels of filamentation and plays a role in virulence

The conserved family of fungal Ste11 mitogen activated protein kinase/kinases play important roles in several signalling cascades. We have cloned the STE11 homologue from the fungal pathogen Candida glabrata. The C. glabrata gene is present in a single copy in the genome, contains a well-conserved catalytic domain typical of the serine-threonine protein kinases and a sterile alpha motif widespread in signalling and nuclear proteins. Hypothetical translation of C. glabrata STE11 suggests that the protein has 64% identity and 77% similarity at the amino acid level to Saccharomyces cerevisiae Ste11. We have shown that C. glabrata STE11 can complement the mating defect and partially rescue the reduced nitrogen starvation induced filamentation of S. cerevisiae ste11 mutants. Functional analysis of a C. glabrata ste11 null mutant demonstrates that Ste11 is required for adaptation to hypertonic stress but is largely dispensable for maintenance of cell wall integrity. It also plays a role in C. glabrata nitrogen starvation induced filamentation. Survival analysis revealed that C. glabrata ste11 mutants, while still able to cause disease, are attenuated for virulence compared to reconstituted, STE11 cells. These data suggest that C. glabrata Ste11, in a similar fashion to the S. cerevisiae protein, functions in a number of different signalling modules.

The many faces of SAM

Protein-protein interactions are essential for the assembly, regulation, and localization of functional protein complexes in the cell. SAM domains are among the most abundant protein-protein interaction motifs in organisms from yeast to humans. Although SAM domains adopt similar folds, they are remarkably versatile in their binding properties. Some identical SAM domains can interact with each other to form homodimers or polymers. In other cases, SAM domains can bind to other related SAM domains, to non-SAM domain-containing proteins, and even to RNA. Such versatility earns them functional roles in myriad biological processes, from signal transduction to transcriptional and translational regulation. In this review, we describe the structural basis of SAM domain interactions and highlight their roles in the scaffolding of protein complexes in normal and pathological processes.

Solution structure of the dimeric SAM domain of MAPKKK Ste11 and its interactions with the adaptor protein Ste50 from the budding yeast: implications for Ste11 activation and signal transmission through the Ste50-Ste11 complex

Ste11, a homologue of mammalian MAPKKKs, together with its binding partner Ste50 works in a number of MAPK signaling pathways of Saccharomyces cerevisiae. Ste11/Ste50 binding is mediated by their sterile alpha motifs or SAM domains, of which homologues are also found in many other intracellular signaling and regulatory proteins. Here, we present the solution structure of the SAM domain or residues D37-R104 of Ste11 and its interactions with the cognate SAM domain-containing region of Ste50, residues M27-Q131. NMR pulse-field-gradient (PFG) and rotational correlation time measurements (tauc) establish that the Ste11 SAM domain exists predominantly as a symmetric dimer in solution. The solution structure of the dimeric Ste11 SAM domain consists of five well-defined helices per monomer packed into a compact globular structure. The dimeric structure of the SAM domain is maintained by a novel dimer interface involving interactions between a number of hydrophobic residues situated on helix 4 and at the beginning of the C-terminal long helix (helix 5). The dimer structure may also be stabilized by potential salt bridge interactions across the interface. NMR H/2H exchange experiments showed that binding of the Ste50 SAM to the Ste11 SAM very likely involves the positively charged extreme C-terminal region as well as exposed hydrophobic patches of the dimeric Ste11 SAM domain. The dimeric structure of the Ste11 SAM and its interactions with the Ste50 SAM may have important roles in the regulation and activation of the Ste11 kinase and signal transmission and amplifications through the Ste50-Ste11 complex.

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.

The solution structure of the S.cerevisiae Ste11 MAPKKK SAM domain and its partnership with Ste50

Ste11 is a MAPKKK from Saccharomyces cerevisiae that helps mediate the response to mating pheromone and the ability to thrive in high-salt environments. These diverse functions are facilitated by a direct interaction between the SAM domain of Ste11 with the SAM domain of its regulatory partner, Ste50. We have solved the NMR structure of the Ste11 SAM domain (PDB 1OW5), which reveals a compact, five alpha-helix bundle and a high degree of structural similarity to the Polyhomeotic SAM domain. The combined study of Ste11 SAM rotational correlation times and crosslinking to Ste50-SAM has suggested a mode through which Ste11-SAM oligomerizes and selectively associates with Ste50-SAM. To probe homotypic and heterotypic interations, Ste11-SAM variants each containing a substitution of a surface-exposed hydrophobic residue were constructed. An I59R variant of Ste11-SAM, disrupted binding to Ste50-SAM in vitro. Yeast expressing full-length Ste11-I59R could neither respond to mating pheromone nor thrive in high salt media-demonstrating that the interaction between Ste11 and Ste50 SAM domains is a prerequisite for key signal transduction events.

Phosphorylation of the MAPKKK regulator Ste50p in Saccharomyces cerevisiae: a casein kinase I phosphorylation site is required for proper mating function

The Ste50 protein of Saccharomyces cerevisiae is a regulator of the Ste11p protein kinase. Ste11p is a member of the MAP3K (or MEKK) family, which is conserved from yeast to mammals. Ste50p is involved in all the signaling pathways that require Ste11p function, yet little is known about the regulation of Ste50p itself. Here, we show that Ste50p is phosphorylated on multiple serine/threonine residues in vivo. Threonine 42 (T42) is phosphorylated both in vivo and in vitro, and the protein kinase responsible has been identified as casein kinase I. Replacement of T42 with alanine (T42A) compromises Ste50p function. This mutation abolishes the ability of overexpressed Ste50p to suppress either the mating defect of a ste20 ste50 deletion mutant or the mating defect of a strain with a Ste11p deleted from its sterile-alpha motif domain. Replacement of T42 with a phosphorylation-mimetic aspartic acid residue (T42D) permits wild-type function in all assays of Ste50p function. These results suggest that phosphorylation of T42 of Ste50p is required for proper signaling in the mating response. However, this phosphorylation does not seem to have a detectable role in modulating the high-osmolarity glycerol synthesis pathway.

Structure of the Sterile α Motif (SAM) Domain of the Saccharomyces cerevisiae Mitogen-activated Protein Kinase Pathway-modulating Protein STE50 and Analysis of Its Interaction with the STE11 SAM

The sterile alpha motif (SAM) is a 65-70-amino acid domain found in over 300 proteins that are involved in either signal transduction or transcriptional activation and repression. SAM domains have been shown to mediate both homodimerization and heterodimerization and in some cases oligomerization. Here, we present the solution structure of the SAM domain of the Saccharomyces cerevisiae protein, Ste50p. Ste50p functions as a modulator of the mitogen-activated protein kinase (MAPK) cascades in S. cerevisiae, which control mating, pseudohyphal growth, and osmo-tolerance. This is the first example of the structure of a SAM domain from a MAPK module protein. We have studied the associative behavior of Ste50p SAM in solution and shown that it is monomeric. We have examined the SAM domain from Ste11p, the MAPK kinase kinase that associates with Ste50p in vivo, and shown that it forms dimers with a self-association K(d) of approximately 0.5 mm. We have also analyzed the interaction of Ste50p SAM with Ste11p SAM and the effects of mutations at Val-37, Asp-38, Pro-71, Leu-73, Leu-75, and Met-99 of STE50 on the heterodimerization properties of Ste50p SAM. We have found that L73A and L75A abrogate the Ste50p interaction with Ste11p, and we compare these data with the known interaction sites defined for other SAM domain interactions.

The role of adaptor protein Ste50-dependent regulation of the MAPKKK Ste11 in multiple signalling pathways of yeast

In Saccharomyces cerevisiae, Ste50 functions in cell signalling between the activated G protein and the mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) Ste11. ScSte50 is an essential component of three MAPK-mediated signalling pathways, which control the mating response, invasive/filamentous growth and osmotolerance (HOG pathway), respectively. ScSte50 signalling may also contribute to cell wall integrity in vegetative cells. The protein contains a sterile alpha motif (SAM) and a putative Ras-associated domain (RAD), which are essential for signal transduction. Ste50 and Ste11 interact constitutively via their SAM regions. Ste50 interacts weakly and probably transiently with the pheromone receptor-bound heterotrimeric G protein G(alpha beta gamma), and with the small G proteins Cdc42, Ras1 and Ras2. It is specifically the RAD region of Ste50 that mediates the interactions with Cdc42 and Ras. Homologues of ScSTE50 are also found in other fungi, like S. kluyveri, Hansenula polymorpha, Candida albicans and Neurospora crassa. In this review, the role of Ste50 as an adaptor that links the G protein-associated Cdc42-Ste20 kinase complex to the effector kinase Ste11 and thus modulates signal transduction, especially in the pheromone-response pathway of S. cerevisiae, is discussed.

The sensing of nutritional status and the relationship to filamentous growth in Saccharomyces cerevisiae

Heterotrophic organisms rely on the ingestion of organic molecules or nutrients from the environment to sustain energy and biomass production. Non-motile, unicellular organisms have a limited ability to store nutrients or to take evasive action, and are therefore most directly dependent on the availability of nutrients in their immediate surrounding. Such organisms have evolved numerous developmental options in order to adapt to and to survive the permanently changing nutritional status of the environment. The phenotypical, physiological and molecular nature of nutrient-induced cellular adaptations has been most extensively studied in the yeast Saccharomyces cerevisiae. These studies have revealed a network of sensing mechanisms and of signalling pathways that generate and transmit the information on the nutritional status of the environment to the cellular machinery that implements specific developmental programmes. This review integrates our current knowledge on nutrient sensing and signalling in S. cerevisiae, and suggests how an integrated signalling network may lead to the establishment of a specific developmental programme, namely pseudohyphal differentiation and invasive growth.

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.

The Ste5p scaffold

An emerging theme of mitogen-activated protein kinase (MAPK) cascades is that they form molecular assemblies within cells; the spatial organization of which is provided by scaffold proteins. Yeast Ste5p was the first MAPK cascade scaffold to be described. Early work demonstrated that Ste5p selectively tethers the MAPKKK, MAPKK and MAPK of the yeast mating pathway and is essential for efficient activation of the MAPK by the pheromone stimulus. Recent work indicates that Ste5p is not a passive scaffold but plays a direct role in the activation of the MAPKKK by a heterotrimeric G protein and PAK-type kinase. This activation event requires the formation of an active Ste5p oligomer and proper recruitment of Ste5p to a Gβγ dimer at the submembrane of the cell cortex, which suggests that Ste5p forms a stable Ste5p signalosome linked to a G protein. Additional studies underscore the importance of regulated localization of Ste5p to the plasma membrane and have revealed nuclear shuttling as a regulatory device that controls the access of Ste5p to the plasma membrane. A model that links Ste5p oligomerization with stable membrane recruitment is presented. In this model, pathway activation is coordinated with the conversion of a less active closed form of Ste5 containing a protected RING-H2 domain into an active Ste5p dimer that can bind to Gβγ and form a multimeric scaffold lattice upon which the MAPK cascade can assemble.

A protein interaction map for cell polarity development

Many genes required for cell polarity development in budding yeast have been identified and arranged into a functional hierarchy. Core elements of the hierarchy are widely conserved, underlying cell polarity development in diverse eukaryotes. To enumerate more fully the protein-protein interactions that mediate cell polarity development, and to uncover novel mechanisms that coordinate the numerous events involved, we carried out a large-scale two-hybrid experiment. 68 Gal4 DNA binding domain fusions of yeast proteins associated with the actin cytoskeleton, septins, the secretory apparatus, and Rho-type GTPases were used to screen an array of yeast transformants that express approximately 90% of the predicted Saccharomyces cerevisiae open reading frames as Gal4 activation domain fusions. 191 protein-protein interactions were detected, of which 128 had not been described previously. 44 interactions implicated 20 previously uncharacterized proteins in cell polarity development. Further insights into possible roles of 13 of these proteins were revealed by their multiple two-hybrid interactions and by subcellular localization. Included in the interaction network were associations of Cdc42 and Rho1 pathways with proteins involved in exocytosis, septin organization, actin assembly, microtubule organization, autophagy, cytokinesis, and cell wall synthesis. Other interactions suggested direct connections between Rho1- and Cdc42-regulated pathways; the secretory apparatus and regulators of polarity establishment; actin assembly and the morphogenesis checkpoint; and the exocytic and endocytic machinery. In total, a network of interactions that provide an integrated response of signaling proteins, the cytoskeleton, and organelles to the spatial cues that direct polarity development was revealed.

"Mutagenesis" by peptide aptamers identifies genetic network members and pathway connections

We selected peptide aptamers from combinatorial libraries that disrupted cell-cycle arrest caused by mating pheromone in yeast. We used these aptamers as baits in two-hybrid hunts to identify genes involved in cell-cycle arrest. These experiments identified genes known to function in the pathway, as well as a protein kinase, the CBK1 product, whose function was not known. We used a modified two-hybrid system to identify specific interactions disrupted by these aptamers. These experiments demonstrate a means to perform "genetics" on the protein complement of a cell without altering its genetic material. Peptide aptamers can be identified that disrupt a process. These aptamers can then be used as affinity reagents to identify individual proteins and protein interactions needed for the process. Forward genetic analysis with peptide aptamer "mutagens" should be particularly useful in elucidating genetic networks in organisms and processes for which classical genetics is not feasible.

Functional characterization of the interaction of Ste50p with Ste11p MAPKKK in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Ste11p protein kinase is a homologue of mammalian MAPK/extracellular signal-regulated protein kinase kinase kinases (MAPKKKs or MEKKs) as well as the Schizosaccharomyces pombe Byr2p kinase. Ste11p functions in several signaling pathways, including those for mating pheromone response and osmotic stress response. The Ste11p kinase has an N-terminal domain that interacts with other signaling molecules to regulate Ste11p function and direct its activity in these pathways. One of the Ste11p regulators is Ste50p, and Ste11p and Ste50p associate through their respective N-terminal domains. This interaction relieves a negative activity of the Ste11p N terminus, and removal of this negative function is required for Ste11p function in the high-osmolarity glycerol (HOG) pathway. The Ste50p/Ste11p interaction is also important (but not essential) for Ste11p function in the mating pathway; in this pathway binding of the Ste11p N terminus with both Ste50p and Ste5p is required, with the Ste5p association playing the major role in Ste11p function. In vitro, Ste50p disrupts an association between the catalytic C terminus and the regulatory N terminus of Ste11p. In addition, Ste50p appears to modulate Ste11p autophosphorylation and is itself a substrate of the Ste11p kinase. Therefore, both in vivo and in vitro data support a role for Ste50p in the regulation of Ste11p activity.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Requirement of STE50 for osmostress-induced activation of the STE11 mitogen-activated protein kinase kinase kinase in the high-osmolarity glycerol response pathway

Exposure of yeast cells to increases in extracellular osmolarity activates the HOG1 mitogen-activated protein (MAP) kinase cascade, which is composed of three tiers of protein kinases: (i) the SSK2, SSK22, and STE11 MAP kinase kinase kinases (MAPKKKs), (ii) the PBS2 MAPKK, and (iii) the HOG1 MAP kinase. Activation of the MAP kinase cascade is mediated by two upstream mechanisms. The SLN1-YPD1-SSK1 two-component osmosensor activates the SSK2 and SSK22 MAPKKKs by direct interaction of the SSK1 response regulator with these MAPKKKs. The second mechanism of HOG1 MAP kinase activation is independent of the two-component osmosensor and involves the SHO1 transmembrane protein and the STE11 MAPKKK. Only PBS2 and HOG1 are common to the two mechanisms. We conducted an exhaustive mutant screening to identify additional elements required for activation of STE11 by osmotic stress. We found that strains with mutations in the STE50 gene, in combination with ssk2Delta ssk22Delta mutations, were unable to induce HOG1 phosphorylation after osmotic stress. Both two-hybrid analyses and coprecipitation assays demonstrated that the N-terminal domain of STE50 binds strongly to the N-terminal domain of STE11. The binding of STE50 to STE11 is constitutive and is not affected by osmotic stress. Furthermore, the two proteins relocalize similarly after osmotic shock. It was concluded that STE50 fulfills an essential role in the activation of the high-osmolarity glycerol response pathway by acting as an integral subunit of the STE11 MAPKKK.

The Hog1 MAPK prevents cross talk between the HOG and pheromone response MAPK pathways in Saccharomyces cerevisiae

The MAPKKK Ste11p functions in three Saccharomyces cerevisiae MAPK cascades [the high osmolarity glycerol (HOG), pheromone response, and pseudohyphal/invasive growth pathways], but its activation in response to high osmolarity stimulates only the HOG pathway. To determine what restricts cross-activation of MAPK cascades (cross talk), we have studied mutants in which the pheromone response pathway is activated by high osmolarity (1 M sorbitol). We found that mutations in the HOG1 gene, encoding the p38-type MAPK of the HOG pathway, and in the PBS2 gene, encoding the activating kinase for Hog1p, allowed osmolarity-induced activation of the pheromone response pathway. This cross talk required the osmosensor Sho1p, as well as Ste20p, Ste50p, the pheromone response MAPK cascade (Ste11p, Ste7p, and Fus3p or Kss1p), and Ste12p but not Ste4p or the MAPK scaffold protein, Ste5p. The cross talk in hog1 mutants induced multiple responses of the pheromone response pathway: induction of a FUS1::lacZ reporter, morphological changes, and mating in ste4 and ste5 mutants. We suggest that Hog1p may prevent osmolarity-induced cross talk by inhibiting Sho1p, perhaps as part of a feedback control on the HOG pathway. We have also shown that Ste20p and Ste50p function in the Sho1p branch of the HOG pathway and that a second osmosensor in addition to Sho1p may activate Ste11p. Finally, we have found that pseudohyphal growth exhibited by wild-type (HOG1) strains depends on SHO1, suggesting that Sho1p may be a receptor that feeds into the pseudohyphal growth pathway.

Transdominant genetic analysis of a growth control pathway

Genetic selections that use proteinaceous transdominant inhibitors encoded by DNA libraries to cause mutant phenocopies may facilitate genetic analysis in traditionally nongenetic organisms. We performed a selection for random short peptides and larger protein fragments (collectively termed "perturbagens") that inhibit the yeast pheromone response pathway. Peptide and protein fragment perturbagens that permit cell division in the presence of pheromone were recovered. Two perturbagens were derived from proteins required for pheromone response, and an additional two were derived from proteins that may negatively influence the pheromone response pathway. Furthermore, three known components of the pathway were identified as probable perturbagen targets based on physical interaction assays. Thus, by selection for transdominant inhibitors of pheromone response, multiple pathway components were identified either directly as gene fragments or indirectly as the likely targets of specific perturbagens. These results, combined with the results of previous work [Holzmayer, T. A., Pestov, D. G. & Roninson, I. B. (1992) Nucl. Acids. Res. 20, 711-717; Whiteway, M., Dignard, D. & Thomas, D. Y. (1992) Proc. Natl. Acad. Sci. USA 89, 9410-9414; and Gudkov, A. V., Kazarov, A. R., Thimmapaya, R., Axenovich, S. A., Mazo, I. A. & Roninson, I. B. (1994) Proc. Natl. Acad. Sci. USA 91, 3744-3748], suggest that transdominant genetic analysis of the type described here will be broadly applicable.

The Byr2 kinase translocates to the plasma membrane in a Ras1-dependent manner

The activation of mitogen-activated protein kinase cascades by the Ras GTPase is an evolutionarily conserved signal transduction mechanism. To better understand the interaction between Ras and its target kinase, we study the yeast Schizosaccharomyces pombe where the Ras1 GTPase activates the Byr2 kinase. Cell fractionation and immunofluorescence showed that Ras1 was localized to the plasma membrane and that Byr2 was in the cytoplasm. When Ras1 was overexpressed, Byr2 was translocated to the plasma membrane. Byr2 translocation was dependent on binding to Ras1 since Ras1-V12, an activated mutant of Ras1, caused more Byr2 translocation than Ras1, since Ras1-D38E, an effector domain mutant, did not cause Byr2 translocation, and since the Ras1-binding domain of Byr2 was necessary and sufficient to cause Byr2 translocation. The Byr2 protein was usually not uniform around the plasma membrane, but was frequently enriched at the cell ends and at the region of septal deposition. This uneven membrane localization depended upon regions of the Byr2 regulatory domain, in addition to those required for Ras1 binding, suggesting that these Byr2 domains participate in protein-protein interactions.

Functional binding between Gbeta and the LIM domain of Ste5 is required to activate the MEKK Ste11

BACKGROUND

In the budding yeast Saccharomyces cerevisiae, the pheromones that induce haploid cells of opposite cell types to mate activate the Gbeta and Ggamma subunits of a heterotrimeric G protein. These subunits signal through the PAK kinase Ste20 to activate a mitogen-activated protein (MAP) kinase cascade comprising the MEKK Ste11, the MEK Ste7 and two MAP kinases, Fus3 and Kss1. The pathway requires Ste5, a scaffold protein that tethers the MAP kinase cascade enzymes into a high molecular weight complex. Ste5 is thought to associate with Gbeta in a pheromone-independent manner, but it is not known if this interaction affects signaling.

RESULTS

A ste5C180A mutant - which expresses Ste5 disrupted in the LIM domain, a putative metal-binding motif that has been proposed to be essential for Ste5 oligomerization - could not transmit the pheromone signal from Gbeta through Ste20 to Ste11. The Ste5C180A protein was impaired in binding Gbeta, although it could oligomerize, bind Ste11, Ste7 and Fus3, facilitate the basal activation of Ste11, and relay the Ste11 signal to MAP kinases. Ste5 bound to Gbeta in a pheromone-dependent manner and preferentially associated with a phosphorylated form of Gbeta in wild-type and ste20Delta, but not in ste5C180A, strains.

CONCLUSIONS

Pheromone induces binding of Gbeta to Ste5 through its LIM domain. This binding is essential for activation of Ste11 and is distinct from the ability of Ste5 to oligomerize or to serve as a scaffold and relay the signal from Ste11 to the MAP kinases. Pheromone also induces Ste5-dependent phosphorylation of Gbeta.

Pheromone signalling and polarized morphogenesis in yeast

Yeast cells respond to mating pheromones by activating a signal transduction pathway involving a seven transmembrane receptor/G protein complex linked to a mitogen-activated protein kinase module. Regulation of the G protein signal is controlled by the receptor and Sst2p; Sst2p may function as a GTPase-activating protein for the G protein alpha subunit. The Ste20 kinase acts in the linkage between the G protein and the MAP kinase module. Experiments suggest that binding of the Rho-like GTPase Cdc42p to Ste20p is not required for the mating response, yet is needed for the pseudohyphal growth response which involves many of the same kinases.