Feedback phosphorylation of an RGS protein by MAP kinase in yeast

Global quantitative understanding of non-equilibrium cell fate decision-making in response to pheromone

Cell-cycle arrest and polarized growth are commonly used to characterize the response of yeast to pheromone. However, the quantitative decision-making processes underlying time-dependent changes in cell fate remain unclear. In this study, we conducted single-cell level experiments to observe multidimensional responses, uncovering diverse fates of yeast cells. Multiple states are revealed, along with the kinetic switching rates and pathways among them, giving rise to a quantitative landscape of mating response. To quantify the experimentally observed cell fates, we developed a theoretical framework based on non-equilibrium landscape and flux theory. Additionally, we performed stochastic simulations of biochemical reactions to elucidate signal transduction and cell growth. Notably, our experimental findings have provided the first global quantitative evidence of the real-time synchronization between intracellular signaling, physiological growth, and morphological functions. These results validate the proposed underlying mechanism governing the emergence of multiple cell fate states. This study introduces an emerging mechanistic approach to understand non-equilibrium cell fate decision-making in response to pheromone.

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.

Phosphorylation of RGS regulates MAP kinase localization and promotes completion of cytokinesis

Phosphorylation of the RGS Sst2 alters its subcellular distribution, MAPK localization, and interaction with Kel1, which promotes coordination of polarized growth with completion of cytokinesis.

Yeast use the G-protein–coupled receptor signaling pathway to detect and track the mating pheromone. The G-protein–coupled receptor pathway is inhibited by the regulator of G-protein signaling (RGS) Sst2 which induces Gα GTPase activity and inactivation of downstream signaling. G-protein signaling activates the MAPK Fus3, which phosphorylates the RGS; however, the role of this modification is unknown. We found that pheromone-induced RGS phosphorylation peaks early; the phospho-state of RGS controls its localization and influences MAPK spatial distribution. Surprisingly, phosphorylation of the RGS promotes completion of cytokinesis before pheromone-induced growth. Completion of cytokinesis in the presence of pheromone is promoted by the kelch-repeat protein, Kel1 and antagonized by the formin Bni1. We found that RGS complexes with Kel1 and prefers the unphosphorylatable RGS mutant. We also found overexpression of unphosphorylatable RGS exacerbates cytokinetic defects, whereas they are rescued by overexpression of Kel1. These data lead us to a model where Kel1 promotes completion of cytokinesis before pheromone-induced polarity but is inhibited by unphosphorylated RGS binding.

The rice blast fungus MoRgs1 functioning in cAMP signaling and pathogenicity is regulated by casein kinase MoCk2 phosphorylation and modulated by membrane protein MoEmc2

GTP-binding protein (G-protein) and regulator of G-protein signaling (RGS) mediated signal transduction are critical in the growth and virulence of the rice blast pathogen Magnaporthe oryzae. We have previously reported that there are eight RGS and RGS-like proteins named MoRgs1 to MoRgs8 playing distinct and shared regulatory functions in M. oryzae and that MoRgs1 has a more prominent role compared to others in the fungus. To further explore the unique regulatory mechanism of MoRgs1, we screened a M. oryzae cDNA library for genes encoding MoRgs1-interacting proteins and identified MoCkb2, one of the two regulatory subunits of the casein kinase (CK) 2 MoCk2. We found that MoCkb2 and the sole catalytic subunit MoCka1 are required for the phosphorylation of MoRgs1 at the plasma membrane (PM) and late endosome (LE). We further found that an endoplasmic reticulum (ER) membrane protein complex (EMC) subunit, MoEmc2, modulates the phosphorylation of MoRgs1 by MoCk2. Interestingly, this phosphorylation is also essential for the GTPase-activating protein (GAP) function of MoRgs1. The balance among MoRgs1, MoCk2, and MoEmc2 ensures normal operation of the G-protein MoMagA-cAMP signaling required for appressorium formation and pathogenicity of the fungus. This has been the first report that an EMC subunit is directly linked to G-protein signaling through modulation of an RGS-casein kinase interaction.

G-proteins play a significant role in signal perception and transduction during pathogen and host interactions. In the rice blast fungus M. oryzae, previous studies demonstrated that G-protein/cAMP signaling are important for appressorium formation and pathogenicity. One of the eight regulator of G-protein signaling (RGS) and RGS-like proteins, MoRgs1, targets G-protein MoMagA to regulate cAMP levels and growth and virulence of the fungus; however, how MoRgs1 exhibits this function and its own regulation indifferent from other RGS and RGS-like proteins are not clear. We here demonstrated that MoRgs1 is subject to regulation by the casein kinase 2 MoCk2 through protein phosphorylation, and this regulation is also essential for the GTPase-activating protein (GAP) function of MoRgs1. We also showed that the endoplasmic reticulum (ER) membrane complex (EMC) subunit MoEmc2 modulates MoCk2-mediated MoRgs1 phosphorylation. Balanced interactions among MoRgs1, MoEmc2, and MoCk2 ensure normal appressorium formation and pathogenicity of M. oryzae.

Negative feedback-loop mechanisms regulating HOG- and pheromone-MAPK signaling in yeast

The pheromone response and the high osmolarity glycerol (HOG) pathways are considered the prototypical MAPK signaling systems. They are the best-understood pathways in eukaryotic cells, yet they continue to provide insights in how cells relate with the environment. These systems are subjected to tight regulatory circuits to prevent hyperactivation in length and intensity. Failure to do this may be a matter of life or death specially for unicellular organisms such as Saccharomyces cerevisiae. The signaling pathways are fine-tuned by positive and negative feedback loops exerted by pivotal control elements that allow precise responses to specific stimuli, despite the fact that some elements of the systems are common to different signaling pathways. Here we describe the experimentally proven negative feedback loops that modulate the pheromone response and the HOG pathways. As described in this review, MAP kinases are central mechanistic components of these feedback loops. They have the capacity to modulate basal signaling activity, a fast extranuclear response, and a longer-lasting transcriptional process.

Spatial and temporal signal processing and decision making by MAPK pathways

Recent studies show that MAPK pathways perform exquisite spatial and temporal signal processing. This review discusses the mechanisms that process dynamic inputs into graded output responses, the role of positive and negative feedbacks, and feedforward regulation.

Mitogen-activated protein kinase (MAPK) pathways are conserved from yeast to man and regulate a variety of cellular processes, including proliferation and differentiation. Recent developments show how MAPK pathways perform exquisite spatial and temporal signal processing and underscores the importance of studying the dynamics of signaling pathways to understand their physiological response. The importance of dynamic mechanisms that process input signals into graded downstream responses has been demonstrated in the pheromone-induced and osmotic stress–induced MAPK pathways in yeast and in the mammalian extracellular signal-regulated kinase MAPK pathway. Particularly, recent studies in the yeast pheromone response have shown how positive feedback generates switches, negative feedback enables gradient detection, and coherent feedforward regulation underlies cellular memory. More generally, a new wave of quantitative single-cell studies has begun to elucidate how signaling dynamics determine cell physiology and represents a paradigm shift from descriptive to predictive biology.

Engineering dynamical control of cell fate switching using synthetic phospho-regulons

Many cells can sense and respond to time-varying stimuli, selectively triggering changes in cell fate only in response to inputs of a particular duration or frequency. A common motif in dynamically controlled cells is a dual-timescale regulatory network: although long-term fate decisions are ultimately controlled by a slow-timescale switch (e.g., gene expression), input signals are first processed by a fast-timescale signaling layer, which is hypothesized to filter what dynamic information is efficiently relayed downstream. Directly testing the design principles of how dual-timescale circuits control dynamic sensing, however, has been challenging, because most synthetic biology methods have focused solely on rewiring transcriptional circuits, which operate at a single slow timescale. Here, we report the development of a modular approach for flexibly engineering phosphorylation circuits using designed phospho-regulon motifs. By then linking rapid phospho-feedback with slower downstream transcription-based bistable switches, we can construct synthetic dual-timescale circuits in yeast in which the triggering dynamics and the end-state properties of the ON state can be selectively tuned. These phospho-regulon tools thus open up the possibility to engineer cells with customized dynamical control.

Signal inhibition by a dynamically regulated pool of monophosphorylated MAPK

MAPKs are activated by dual phosphorylation. Much of the MAPK Fus3 is monophosphorylated and acts to inhibit signaling in vivo. Computational models reveal how a kinase scaffold and phosphatase act together to dynamically regulate dual-phosphorylated and monophosphorylated MAPKs and the downstream signal.

Protein kinases regulate a broad array of cellular processes and do so through the phosphorylation of one or more sites within a given substrate. Many protein kinases are themselves regulated through multisite phosphorylation, and the addition or removal of phosphates can occur in a sequential (processive) or a stepwise (distributive) manner. Here we measured the relative abundance of the monophosphorylated and dual-phosphorylated forms of Fus3, a member of the mitogen-activated protein kinase (MAPK) family in yeast. We found that upon activation with pheromone, a substantial proportion of Fus3 accumulates in the monophosphorylated state. Introduction of an additional copy of Fus3 lacking either phosphorylation site leads to dampened signaling. Conversely, cells lacking the dual-specificity phosphatase (msg5Δ) or that are deficient in docking to the MAPK-scaffold (Ste5^ND) accumulate a greater proportion of dual-phosphorylated Fus3. The double mutant exhibits a synergistic, or “synthetic,” supersensitivity to pheromone. Finally, we present a predictive computational model that combines MAPK scaffold and phosphatase activities and is sufficient to account for the observed MAPK profiles. These results indicate that the monophosphorylated and dual-phosphorylated forms of the MAPK act in opposition to one another. Moreover, they reveal a new mechanism by which the MAPK scaffold acts dynamically to regulate signaling.

Dynamic ubiquitination of the mitogen-activated protein kinase kinase (MAPKK) Ste7 determines mitogen-activated protein kinase (MAPK) specificity

Ubiquitination is a post-translational modification that tags proteins for proteasomal degradation. In addition, there is a growing appreciation that ubiquitination can influence protein activity and localization. Ste7 is a prototype MAPKK in yeast that participates in both the pheromone signaling and nutrient deprivation/invasive growth pathways. We have shown previously that Ste7 is ubiquitinated upon pheromone stimulation. Here, we show that the Skp1/Cullin/F-box ubiquitin ligase SCF(Cdc4) and the ubiquitin protease Ubp3 regulate Ste7 ubiquitination and signal specificity. Using purified components, we demonstrate that SCF(Cdc4) ubiquitinates Ste7 directly. Using gene deletion mutants, we show that SCF(Cdc4) and Ubp3 have opposing effects on Ste7 ubiquitination. Although SCF(Cdc4) is necessary for proper activation of the pheromone MAPK Fus3, Ubp3 is needed to limit activation of the invasive growth MAPK Kss1. Finally, we show that Fus3 phosphorylates Ubp3 directly and that phosphorylation of Ubp3 is necessary to limit Kss1 activation. These results reveal a feedback loop wherein one MAPK limits the ubiquitination of an upstream MAPKK and thereby prevents spurious activation of a second competing MAPK.

A finer tuning of G-protein signaling through regulated control of RGS proteins

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAP) for various Gα subunits of heterotrimeric G proteins. Through this mechanism, RGS proteins regulate the magnitude and duration of G-protein-coupled receptor signaling and are often referred to as fine tuners of G-protein signaling. Increasing evidence suggests that RGS proteins themselves are regulated through multiple mechanisms, which may provide an even finer tuning of G-protein signaling and crosstalk between G-protein-coupled receptors and other signaling pathways. This review summarizes the current data on the control of RGS function through regulated expression, intracellular localization, and covalent modification of RGS proteins, as related to cell function and the pathogenesis of diseases.

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.

How can yeast cells decide between three activated MAP kinase pathways? A model approach

In yeast (Saccharomyces cerevisiae), the regulation of three MAP kinase pathways responding to pheromones (Fus3 pathway), carbon/nitrogen starvation (Kss1 pathway), and high osmolarity/osmotic stress (Hog1 pathway) is the subject of intensive research. We were interested in the question how yeast cells would respond when more than one of the MAP kinase pathways are activated simultaneously. Here, we give a brief overview over the regulatory mechanisms of the yeast MAP kinase pathways and investigate a kinetic model based on presently known molecular interactions and feedbacks within and between the three mitogen-activated protein kinases (MAPK) pathways. When two pathways are activated simultaneously with the osmotic stress response as one of them, the model predicts that the osmotic stress response (Hog1 pathway) is turned on first. The same is true when all three pathways are activated at the same time. When testing simultaneous stimulations by low nitrogen and pheromones through the Kss1 and Fus3 pathways, respectively, the low nitrogen response dominates over the pheromone response. Due to its autocatalytic activation mechanism, the pheromone response (Fus3 pathway) shows typical sigmoid response kinetics and excitability. In the presence of a small but sufficient amount of activated Fus3, a stimulation by pheromones will lead to a rapid self-amplification of the pheromone response. This 'excitability' appears to be a feature of the pheromone pathway that has specific biological significance.

The effects of replacing Sst2 with the heterologous RGS4 on polarization and mating in yeast

RGS proteins stimulate the deactivation of heterotrimeric G-proteins. The yeast RGS protein Sst2 is regulated at both the transcriptional and posttranscriptional levels. We replaced the SST2 gene with the distantly related human RGS4 gene, which consists of the catalytic domain and an N-terminal membrane attachment peptide, and replaced the native promoter (P(SST2)) with the heterologous tetracycline-repressible promoter (P(TET)). We then measured the effect of the substitutions on pheromone sensitivity, mating, and polarization. Although the pheromone sensitivity was essentially normal, there were differences in mating and polarization. In particular, the RGS4-substituted strains did not form multiple mating projections at high levels of alpha-factor, but instead formed a single malformed projection, which frequently gave rise to a bud. We provide evidence that this phenotype arose because unlike Sst2, RGS4 did not localize to the projection. We use mathematical modeling to argue that localization of Sst2 to the projection prevents excess G-protein activation during the pheromone response. In addition, modeling and experiments demonstrate that the dose of Sst2 influences the frequency of mating projection formation.

Regulation of RGS2 and Second Messenger Signaling in Vascular Smooth Muscle Cells by cGMP-dependent Protein Kinase

RGS2, a GTPase-activating protein (GAP) for G(q)alpha, regulates vascular relaxation and blood pressure. RGS2 can be phosphorylated by type Ialpha cGMP-dependent protein kinase (cGKIalpha), increasing its GAP activity. To understand how RGS2 and cGKIalpha regulate vascular smooth muscle signaling and function, we identified signaling pathways that are controlled by cGMP in an RGS2-dependent manner and discovered new mechanisms whereby cGK activity regulates RGS2. We show that RGS2 regulates vasoconstrictor-stimulated Ca(2+) store release, capacitative Ca(2+) entry, and noncapacitative Ca(2+) entry and that RGS2 is required for cGMP-mediated inhibition of vasoconstrictor-elicited phospholipase Cbeta activation, Ca(2+) store release, and capacitative Ca(2+) entry. RGS2 is degraded in vascular smooth muscle cells via the proteasome. Inhibition of cGK activity blunts RGS2 degradation. However, inactivation of the cGKIalpha phosphorylation sites in RGS2 does not stabilize the protein, suggesting that cGK activity regulates RGS2 degradation by other mechanisms. cGK activation promotes association of RGS2 with the plasma membrane by a mechanism requiring its cGKIalpha phosphorylation sites. By regulating GAP activity, plasma membrane association, and degradation, cGKIalpha therefore may control a cycle of RGS2 activation and inactivation. By diminishing cGK activity, endothelial dysfunction may impair RGS2 activation, thereby blunting vascular relaxation and contributing to hypertension.

A role of RGS proteins in drug addiction

The diverse family of Regulators of G protein signaling (RGS) proteins are widely distributed proteins with multiple functions, including GAP activity for heterotrimeric G protein alpha subunits. Three members of the RGS family, RGS9-2, RGS4 and RGSz, have been shown to play an essential modulatory role in psychostimulant and opiate drug actions. Interestingly, these proteins show distinct structure, distribution pattern and cellular localization. In addition, each of these proteins is differentially regulated by drugs of abuse in particular brain networks and appears to modulate distinct signal transduction events. The striatal enriched RGS9 plays a prominent role in opiate and psychostimulant drug reward; RGS4 appears to modulate opiate dependence via actions in the locus coeruleus, whereas RGSz modulates analgesia via activation of the PKC pathway.

Systems biology analysis of G protein and MAP kinase signaling in yeast

Approximately a third of all drugs act by binding directly to cell surface receptors coupled to G proteins. Other drugs act indirectly on these same pathways, for example, by inhibiting neurotransmitter reuptake or by blocking the inactivation of intracellular second messengers. These drugs have revolutionized the treatment of human disease. However, the complexity of G protein signaling mechanisms has significantly hampered our ability to identify additional new drug targets. Moreover, today's molecular pharmacologists are accustomed to working on narrowly focused problems centered on a single protein or enzymatic process. Here we describe emerging efforts in yeast aimed at identifying proteins and processes that modulate the function of receptors, G proteins and MAP kinase effectors. The scope of these efforts is far more systematic, comprehensive and quantitative than anything attempted previously, and includes integrated approaches in genetics, proteomics and computational biology.

Pheromone-induced degradation of Ste12 contributes to signal attenuation and the specificity of developmental fate

The Ste12 transcription factor of Saccharomyces cerevisiae regulates transcription programs controlling two different developmental fates. One is differentiation into a mating-competent form that occurs in response to mating pheromone. The other is the transition to a filamentous-growth form that occurs in response to nutrient deprivation. These two distinct roles for Ste12 make it a focus for studies into regulatory mechanisms that impart biological specificity. The transient signal characteristic of mating differentiation led us to test the hypothesis that regulation of Ste12 turnover might contribute to attenuation of the mating-specific transcription program and restrict activation of the filamentation program. We show that prolonged pheromone induction leads to ubiquitin-mediated destabilization and decreased amounts of Ste12. This depletion in pheromone-stimulated cultures is dependent on the mating-pathway-dedicated mitogen-activated protein kinase Fus3 and its target Cdc28 inhibitor, Far1. Attenuation of pheromone-induced mating-specific gene transcription (FUS1) temporally correlates with Ste12 depletion. This attenuation is abrogated in the deletion backgrounds (fus3Delta or far1Delta) where Ste12 is found to persist. Additionally, pheromone induces haploid invasion and filamentous-like growth instead of mating differentiation when Ste12 levels remain high. These observations indicate that loss of Ste12 reinforces the adaptive response to pheromone and contributes to the curtailing of a filamentation response.

RGSZ1 interacts with protein kinase C interacting protein PKCI-1 and modulates mu opioid receptor signaling

Protein kinase C interacting protein (PKCI-1) was identified among the potential interactors from a yeast two hybrid screen of human brain library using N terminal of RGSZ1 as a bait. The cysteine string region, unique to the RZ subfamily, contributes to the observed interaction because PKCI-1 interacted with N-terminus of RGS17 and GAIP, but not with that of RGS2 or RGS7 where cysteine string motif is absent. The interaction between RGSZ1 and PKCI-1 was confirmed by coimmunoprecipitation and immunofluorescence. PKCI-1 and RGSZ1 could be detected by coimmunoprecipitation using 14-3-3 antibody in cells transfected with PKCI-1 or RGSZ1 respectively, but when transfected with PKCI-1 and RGSZ1 together, only RGSZ1 could be detected. Phosphorylation of Galphaz by protein kinase C (PKC) reduces the ability of the RGS to effectively function as GTPase accelerating protein for Galphaz, and interferes with ability of Galphaz to interact with betagamma complex. We investigated the roles of 14-3-3 and PKCI-1 in phosphorylation of Galphaz. Phosphorylation of Galphaz by PKC was inhibited by 14-3-3 and the presence of PKCI-1 did not provide any further inhibition. PKCI-1 interacts with mu opioid receptor and suppresses receptor desensitization and PKC related mu opioid receptor phosphorylation [W. Guang, H. Wang, T. Su, I.B. Weinstein, J.B. Wang, Mol. Pharmacol. 66 (2004) 1285.]. Previous studies have also shown that mu opioid receptor co-precipitates with RGSZ1 and influence mu receptor signaling by acting as effector antagonists [J. Garzon, M. Rodriguez-Munoz, P. Sanchez-Blazquez, Neuropharmacology 48 (2005) 853., J. Garzon, M. Rodriguez-Munoz, A. Lopez-Fando, P. Sanchez-Blazquez Neuropsychopharmacology 30 (2005) 1632.]. Inhibition of cAMP by mu opioid receptor was significantly reduced by RGSZ1 and this effect was enhanced in combination with PKCI-1. Our studies thus provide a link between the previous observations mentioned above and indicate that the major function of PKCI-1 is to modulate mu opioid receptor signaling pathway along with RGSZ1, rather than directly mediating the Galphaz RGSZ1 interaction.

Dynamic studies of scaffold-dependent mating pathway in yeast

The mating pathway in Saccharomyces cerevisiae is one of the best understood signal transduction pathways in eukaryotes. It transmits the mating signal from plasma membrane into the nucleus through the G-protein coupled receptor and the mitogen-activated protein kinase (MAPK) cascade. According to current understanding of the mating pathway, we construct a system of ordinary differential equations to describe the process. Our model is consistent with a wide range of experiments, indicating that it captures some main characteristics of the signal transduction along the pathway. Investigation with the model reveals that the shuttling of the scaffold protein and the dephosphorylation of kinases involved in the MAPK cascade cooperate to regulate the response upon pheromone induction and to help preserve the fidelity of the mating signaling. We explored factors affecting the dose-response curves of this pathway and found that both negative feedback and concentrations of the proteins involved in the MAPK cascade play crucial roles. Contrary to some other MAPK systems where signaling sensitivity is being amplified successively along the cascade, here the mating signal is transmitted through the cascade in an almost linear fashion.

Investigating RGS proteins in yeast

Regulator of G protein signalling (RGS) proteins are vital in the adaptation of cells to stimulation via G protein-coupled receptors. Yeast have been integral in elucidating the important role that RGS proteins play within cellular processes. In addition to extensive characterisation of the endogenous RGS proteins, these organisms have enabled the identification and analysis of numerous mammalian homologues. The simplicity and plasticity of the yeast pheromone-response pathway has facilitated studies which would have been impossible in mammalian systems and it is certain that yeast will continue to have a great impact on this field of research in the future.

Genome-scale analysis reveals Sst2 as the principal regulator of mating pheromone signaling in the yeast Saccharomyces cerevisiae

A common property of G protein-coupled receptors is that they become less responsive with prolonged stimulation. Regulators of G protein signaling (RGS proteins) are well known to accelerate G protein GTPase activity and do so by stabilizing the transition state conformation of the G protein alpha subunit. In the yeast Saccharomyces cerevisiae there are four RGS-homologous proteins (Sst2, Rgs2, Rax1, and Mdm1) and two Galpha proteins (Gpa1 and Gpa2). We show that Sst2 is the only RGS protein that binds selectively to the transition state conformation of Gpa1. The other RGS proteins also bind Gpa1 and modulate pheromone signaling, but to a lesser extent and in a manner clearly distinct from Sst2. To identify other candidate pathway regulators, we compared pheromone responses in 4,349 gene deletion mutants representing nearly all nonessential genes in yeast. A number of mutants produced an increase (sst2, bar1, asc1, and ygl024w) or decrease (cla4) in pheromone sensitivity or resulted in pheromone-independent signaling (sst2, pbs2, gas1, and ygl024w). These findings suggest that Sst2 is the principal regulator of Gpa1-mediated signaling in vivo but that other proteins also contribute in distinct ways to pathway regulation.

Phosphorylation of the RGS protein Sst2 by the MAP kinase Fus3 and use of Sst2 as a model to analyze determinants of substrate sequence specificity

Previously, we used mass spectrometry to demonstrate pheromone-stimulated phosphorylation of Ser-539 in Sst2, a regulator of G protein signaling in yeast Saccharomyces cerevisiae [Garrison, T. R., et al. (1999) J. Biol. Chem. 274, 36387-36391]. Here, we show that Sst2 phosphorylation is mediated by the mitogen-activated protein (MAP) kinase Fus3. Phosphorylation occurs within a canonical MAP kinase phosphorylation site (Pro-X-Ser/Thr-Pro, where "X" at the -1 position can be any amino acid), a consensus sequence deduced earlier from analysis of synthetic peptide substrates. In a direct test of the model, we compared Sst2 phosphorylation following systematic substitution of the -1 residue His-538. Each of the substitution mutants was suitable as a MAP kinase substrate, as shown by phosphorylation-dependent mobility shifts in vivo and/or by direct phosphorylation in vitro followed by peptide mapping and mass spectrometry sequencing. This analysis documents phosphorylation of Sst2 by Fus3 and demonstrates that the prevailing model for MAP kinase recognition is valid for a native substrate protein in vivo as well as for small synthetic peptides tested in vitro.

Quantitative Phosphoproteomics Applied to the Yeast Pheromone Signaling Pathway

Cellular processes such as proliferation, differentiation, and adaptation to environmental changes are regulated by protein phosphorylation. Development of sensitive and comprehensive analytical methods for determination of protein phosphorylation is therefore a necessity in the pursuit of a detailed molecular view of complex biological processes. We present a quantitative modification-specific proteomic approach that combines stable isotope labeling by amino acids in cell culture (SILAC) for quantitation with IMAC for phosphopeptide enrichment and three stages of mass spectrometry (MS/MS/MS) for identification. This integrated phosphoproteomic technology identified and quantified phosphorylation in key regulator and effector proteins of a prototypical G-protein-coupled receptor signaling pathway, the yeast pheromone response. SILAC encoding of yeast proteomes was achieved by incorporation of [(13)C(6)]arginine and [(13)C(6)]lysine in a double auxotroph yeast strain. Pheromone-treated yeast cells were mixed with SILAC-encoded cells as the control and lysed, and extracted proteins were digested with trypsin. Phosphopeptides were enriched by a combination of strong cation exchange chromatography and IMAC. Phosphopeptide fractions were analyzed by LC-MS using a linear ion trap-Fourier transform ion cyclotron resonance mass spectrometer. MS/MS and neutral loss-directed MS/MS/MS analysis allowed detection and sequencing of phosphopeptides with exceptional accuracy and specificity. Of more than 700 identified phosphopeptides, 139 were differentially regulated at least 2-fold in response to mating pheromone. Among these regulated proteins were components belonging to the mitogen-activated protein kinase signaling pathway and to downstream processes including transcriptional regulation, the establishment of polarized growth, and the regulation of the cell cycle.

A walk-through of the yeast mating pheromone response pathway

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

Differential regulation of G protein alpha subunit trafficking by mono- and polyubiquitination

Previously we used mass spectrometry to show that the yeast G protein alpha subunit Gpa1 is ubiquitinated at Lys-165, located within a subdomain not present in other G alpha proteins (Marotti, L. A., Jr., Newitt, R., Wang, Y., Aebersold, R., and Dohlman, H. G. (2002) Biochemistry 41, 5067-5074). Here we describe the functional role of Gpa1 ubiquitination. We find that Gpa1 expression is elevated in mutants deficient in either proteasomal or vacuolar protease function. Vacuolar protease pep4 mutants accumulate monoubiquitinated Gpa1, and much of the protein is localized within the vacuolar compartment. In contrast, proteasome-defective rpt6/cim3 mutants accumulate polyubiquitinated Gpa1, and in this case the protein exhibits cytoplasmic localization. Cells that lack Ubp12 ubiquitin-processing protease activity accumulate both mono- and polyubiquitinated forms of Gpa1. In this case, Gpa1 accumulates in both the cytoplasm and vacuole. Finally, a Gpa1 mutant that lacks the ubiquitinated subdomain remains unmodified and is predominantly localized at the plasma membrane. These data reveal a strong relationship between the extent of ubiquitination and trafficking of the G protein alpha subunit to its site of degradation.

A walk-through of the yeast mating pheromone response pathway

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

RGS21 is a novel regulator of G protein signalling selectively expressed in subpopulations of taste bud cells

Abstract G-protein-mediated signalling processes are involved in sweet and bitter taste transduction. In particular, the G protein alpha-subunit gustducin has been implicated in these processes. One of the limiting factors for the time-course of cellular responses induced by tastants is therefore the intrinsic GTPase activity of alpha-gustducin, which determines the lifetime of the active G protein complex. In several signalling systems specific 'regulator of G protein signalling' (RGS) proteins accelerate the GTPase activity of G protein alpha-subunits. Using differential screening approaches, we have identified a novel RGS protein termed RGS21, which represents the smallest known member of this protein family. Reverse transcription polymerase chain reaction and in situ hybridization experiments demonstrated that RGS21 is expressed selectively in taste tissue where it is found in a subpopulation of sensory cells. Furthermore, it is coexpressed in individual taste cells with bitter and sweet transduction components including alpha-gustducin, phospholipase Cbeta2, T1R2/T1R3 sweet taste receptors and T2R bitter taste receptors. In vitro binding assays demonstrate that RGS21 binds alpha-gustducin in a conformation-dependent manner and has the potential to interact with the same Galpha subtypes as T1R receptors. These results suggest that RGS21 could play a regulatory role in bitter as well as sweet taste transduction processes.

Differential Contribution of GTPase Activation and Effector Antagonism to the Inhibitory Effect of RGS Proteins on Gq-mediated Signaling in Vivo

RGS proteins act as negative regulators of G protein signaling by serving as GTPase-activating proteins (GAP) for alpha subunits of heterotrimeric G proteins (Galpha), thereby accelerating G protein inactivation. RGS proteins can also block Galpha-mediated signal production by competing with downstream effectors for Galpha binding. Little is known about the relative contribution of GAP and effector antagonism to the inhibitory effect of RGS proteins on G protein-mediated signaling. By comparing the inhibitory effect of RGS2, RGS3, RGS5, and RGS16 on Galpha(q)-mediated phospholipase Cbeta (PLCbeta) activation under conditions where GTPase activation is possible versus nonexistent, we demonstrate that members of the R4 RGS subfamily differ significantly in their dependence on GTPase acceleration. COS-7 cells were transiently transfected with either muscarinic M3 receptors, which couple to endogenous Gq protein and mediate a stimulatory effect of carbachol on PLCbeta, or constitutively active Galphaq*, which is inert to GTP hydrolysis and activates PLCbeta independent of receptor activation. In M3-expressing cells, all of the RGS proteins significantly blunted the efficacy and potency of carbachol. In contrast, Galphaq* -induced PLCbeta activation was inhibited by RGS2 and RGS3 but not RGS5 and RGS16. The observed differential effects were not due to changes in M3, Galphaq/Galphaq*, PLCbeta, or RGS expression, as shown by receptor binding assays and Western blots. We conclude that closely related R4 RGS family members differ in their mechanism of action. RGS5 and RGS16 appear to depend on G protein inactivation, whereas GAP-independent mechanisms (such as effector antagonism) are sufficient to mediate the inhibitory effect of RGS2 and RGS3.

Phosphoinositide 3-kinase cascade facilitates mu-opioid desensitization in sensory neurons by altering G-protein-effector interactions

Signaling via G-protein-coupled receptors undergoes desensitization after prolonged agonist exposure. Here we investigated the role of phosphoinositide 3-kinase (PI3K) and its downstream pathways in desensitization of micro-opioid inhibition of neuronal Ca2+ channels. In cultured mouse dorsal root ganglion neurons, two mechanistically different forms of desensitization were observed after acute or chronic treatment with the micro agonist [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO). Chronic DAMGO desensitization was heterologous in nature and significantly attenuated by blocking the activity of PI3K or mitogen-activated protein kinase (MAPK). A combined application of PI3K and MAPK inhibitors showed no additive effect, suggesting that these two kinases act in a common pathway to facilitate chronic desensitization. Acute DAMGO desensitization, however, was not affected by the inhibitors. Furthermore, upregulation of the PI3K-Akt pathway in mutant mice lacking phosphatase and tensin homolog, a lipid phosphatase counteracting PI3K, selectively enhanced chronic desensitization in a PI3K- and MAPK-dependent manner. Using the prepulse facilitation (PPF) test, we further examined changes in the voltage-dependent component of DAMGO action that requires direct interactions between betagamma subunits of G-proteins and Ca2+ channels. DAMGO-induced PPF was diminished after chronic treatment, suggesting disruption of G-protein-channel interactions. Such disruption could occur at the postreceptor level, because chronic DAMGO also reduced GTPgammaS-induced PPF that was independent of receptor activation. Again, inhibition of PI3K or MAPK reduced desensitization of PPF. Our data suggest that the PI3Kcascade involving MAPK and Akt enhances micro-opioid desensitization via postreceptor modifications that interfere with G-protein-effector interactions.

In through the out door: nuclear localization of the regulators of G protein signaling

The regulators of G protein signaling (RGS) are an extraordinary class of diverse multifunctional signaling proteins best known for their potent capacity to down-regulate the activity of Galpha subunits at the plasma membrane. In certain circumstances, some RGS proteins undergo translocation to the nucleus or plasma membrane from the cytoplasm. Translocation demonstrates a potentially dynamic alternative mechanism for Galpha subunit or effector regulation. The nuclear localization of the regulators of G protein signaling proteins further suggests these proteins possess even greater functional heterogeneity than that envisioned previously, as regulators of transcription and cell cycle control.

Palmitoylation regulates regulator of G-protein signaling (RGS) 16 function. II. Palmitoylation of a cysteine residue in the RGS box is critical for RGS16 GTPase accelerating activity and regulation of Gi-coupled signalling

Palmitoylation is a reversible post-translational modification used by cells to regulate protein activity. The regulator of G-protein signaling (RGS) proteins RGS4 and RGS16 share conserved cysteine (Cys) residues that undergo palmitoylation. In the accompanying article (Hiol, A., Davey, P. C., Osterhout, J. L., Waheed, A. A., Fischer, E. R., Chen, C. K., Milligan, G., Druey, K. M., and Jones, T. L. Z. (2003) J. Biol. Chem. 278, 19301-19308), we determined that mutation of NH2-terminal cysteine residues in RGS16 (Cys-2 and Cys-12) reduced GTPase accelerating (GAP) activity toward a 5-hydroxytryptamine (5-HT1A)/G alpha o1 receptor fusion protein in cell membranes. NH2-terminal acylation also permitted palmitoylation of a cysteine residue in the RGS box of RGS16 (Cys-98). Here we investigated the role of internal palmitoylation in RGS16 localization and GAP activity. Mutation of RGS16 Cys-98 or RGS4 Cys-95 to alanine reduced GAP activity on the 5-HT1A/G alpha o1 fusion protein and regulation of adenylyl cyclase inhibition. The C98A mutation had no effect on RGS16 localization or GAP activity toward purified G-protein alpha subunits. Enzymatic palmitoylation of RGS16 resulted in internal palmitoylation on residue Cys-98. Palmitoylated RGS16 or RGS4 WT but not C98A or C95A preincubated with membranes expressing 5-HT1a/G alpha o1 displayed increased GAP activity over time. These results suggest that palmitoylation of a Cys residue in the RGS box is critical for RGS16 and RGS4 GAP activity and their ability to regulate Gi-coupled signaling in mammalian cells.

Src-mediated RGS16 tyrosine phosphorylation promotes RGS16 stability

The amplitude of signaling evoked by stimulation of G protein-coupled receptors may be controlled in part by the GTPase accelerating activity of the regulator of G protein signaling (RGS) proteins. In turn, subcellular targeting, protein-protein interactions, or post-translational modifications such as phosphorylation may shape RGS activity and specificity. We found previously that RGS16 undergoes tyrosine phosphorylation on conserved tyrosine residues in the RGS box. Phosphorylation on Tyr(168) was mediated by the epidermal growth factor receptor (EGFR). We show here that endogenous RGS16 is phosphorylated after epidermal growth factor stimulation of MCF-7 cells. In addition, p60-Src or Lyn kinase phosphorylated recombinant RGS16 in vitro, and RGS16 underwent phosphorylation in the presence of constitutively active Src (Y529F) in EGFR(-) CHO-K1 cells. Blockade of endogenous Src activity by selective inhibitors attenuated RGS16 phosphorylation induced by pervanadate or receptor stimulation. Furthermore, the rate of RGS16 degradation was reduced in cells expressing active Src or treated with pervanadate or a G protein-coupled receptor ligand (CXCL12). Induction of RGS16 tyrosine phosphorylation was associated with increased RGS16 protein levels and enhanced GAP activity in cell membranes. These results suggest that Src mediates RGS16 tyrosine phosphorylation, which may promote RGS16 stability.

Phosphorylation of RGS14 by protein kinase A potentiates its activity toward G alpha i

Regulators of G protein signaling (RGS proteins) modulate Galpha-directed signals because of the GTPase activating protein (GAP) activity of their conserved RGS domain. RGS14 and RGS12 are unique among RGS proteins in that they also regulate Galpha(i) signals because of the guanine nucleotide dissociation inhibitor (GDI) activity of a GoLoco motif near their carboxy-termini. Little is known about cellular regulation of RGS proteins, although several are phosphorylated in response to G-protein directed signals. Here we show for the first time the phosphorylation of native and recombinant RGS14 in host cells. Direct stimulation of adenylyl cyclase or introduction of dibutyryl-cAMP induces phosphorylation of RGS14 in cells. This phosphorylation occurs through activation of cAMP-dependent protein kinase (PKA) since phosphate incorporation is completely blocked by a selective inhibitor of PKA but only partially or not at all blocked by inhibitors of other G-protein regulated kinases. We show that purified PKA phosphorylates two specific sites on recombinant RGS14, one of which, threonine 494 (Thr494), is immediately adjacent to the GoLoco motif. Because of this proximity, we focused on the possible effects of PKA phosphorylation on the GDI activity of RGS14. We found that mimicking phosphorylation on Thr494 enhanced the GDI activity of RGS14 toward Galpha(i) nearly 3-fold, with no associated effect on the GAP activity toward either Galpha(i) or Galpha(o). These findings implicate cAMP-induced phosphorylation as an important modulator of RGS14 function since phosphorylation could enhance RGS14 binding to Galpha(i)-GDP, thereby limiting Galpha(i) interactions with downstream effector(s) and/or enhancing Gbetagamma-dependent signals.

Transcription profiling of Candida albicans cells undergoing the yeast-to-hyphal transition

The ability of the pathogenic fungus Candida albicans to switch from a yeast to a hyphal morphology in response to external signals is implicated in its pathogenicity. We used glass DNA microarrays to investigate the transcription profiles of 6333 predicted ORFs in cells undergoing this transition and their responses to changes in temperature and culture medium. We have identified several genes whose transcriptional profiles are similar to those of known virulence factors that are modulated by the switch to hyphal growth caused by addition of serum and a 37 degrees C growth temperature. Time course analysis of this transition identified transcripts that are induced before germ tube initiation and shut off later in the developmental process. A strain deleted for the Efg1p and Cph1p transcription factors is defective in hyphae formation, and its response to serum and increased temperature is almost identical to the response of a wild-type strain grown at 37 degrees C in the absence of serum. Thus Efg1p and Cph1p are needed for the activation of the transcriptional program that is induced by the presence of serum.

Interaction of 14-3-3 protein with regulator of G protein signaling 7 is dynamically regulated by tumor necrosis factor-alpha

Regulators of G protein signaling (RGS) constitute a family of proteins with a conserved RGS domain of approximately 120 amino acids that accelerate the intrinsic GTP hydrolysis of activated Galpha(i) and Galpha(q) subunits. The phosphorylation-dependent interaction of 14-3-3 proteins with a subset of RGS proteins inhibits their GTPase-accelerating activity in vitro. The inhibitory interaction between 14-3-3 and RGS7 requires phosphorylation of serine 434 of RGS7. We now show that phosphorylation of serine 434 is dynamically regulated by TNF-alpha. Cellular stimulation by TNF-alpha transiently decreased the phosphorylation of serine 434 of RGS7, abrogating the inhibitory interaction with 14-3-3. We examined the effect of 14-3-3 on RGS-mediated deactivation kinetics of G protein-coupled inwardly rectifying K(+) channels (GIRKs) in Xenopus oocytes. 14-3-3 inhibited the function of wild-type RGS7, but not that of either RSG7(P436R) or RGS4, two proteins that do not bind 14-3-3. Our findings are the first evidence that extracellular signals can modulate the activity of RGS proteins by regulating their interaction with 14-3-3.

RGS3 interacts with 14-3-3 via the N-terminal region distinct from the RGS (regulator of G-protein signalling) domain

RGS3 belongs to a family of the regulators of G-protein signalling (RGS), which bind and inhibit the G alpha subunits of heterotrimeric G-proteins via a homologous RGS domain. Increasing evidence suggests that RGS proteins can also interact with targets other than G-proteins. Employing yeast two-hybrid screening of a cDNA library, we identified an interaction between RGS3 and the phosphoserine-binding protein 14-3-3. This interaction was confirmed by in vitro binding and co-immunoprecipitation experiments. RGS3-deletion analysis revealed the presence of a single 14-3-3-binding site located outside of the RGS domain. Ser(264) was then identified as the 14-3-3-binding site of RGS3. The S(264)A mutation resulted in the loss of RGS3 binding to 14-3-3, without affecting its ability to bind G alpha(q). Signalling studies showed that the S(264)A mutant was more potent than the wild-type RGS3 in inhibition of G-protein-mediated signalling. Binding experiments revealed that RGS3 exists in two separate pools, either 14-3-3-bound or G-protein-bound, and that the 14-3-3-bound RGS3 is unable to interact with G-proteins. These data are consistent with the model wherein 14-3-3 serves as a scavenger of RGS3, regulating the amounts of RGS3 available for binding G-proteins. This study describes a new level in the regulation of G-protein signalling, in which the inhibitors of G-proteins, RGS proteins, can themselves be regulated by phosphorylation and binding 14-3-3.

Regulation of stress response signaling by the N-terminal dishevelled/EGL-10/pleckstrin domain of Sst2, a regulator of G protein signaling in Saccharomyces cerevisiae

All members of the regulator of G protein signaling (RGS) family contain a conserved core domain that can accelerate G protein GTPase activity. The RGS in yeast, Sst2, can inhibit a G protein signal leading to mating. In addition, some RGS proteins contain an N-terminal domain of unknown function. Here we use complementary whole genome analysis methods to investigate the function of the N-terminal Sst2 domain. To identify a signaling pathway regulated by N-Sst2, we performed genome-wide transcription profiling of cells expressing this fragment alone and found differences in 53 transcripts. Of these, 40 are induced by N-Sst2, and nearly all contain a stress response element (STRE) in the promoter region. To identify components of a signaling pathway leading from N-Sst2 to STREs, we performed a genome-wide two-hybrid analysis using N-Sst2 as bait and found 17 interacting proteins. To identify the functionally relevant interacting proteins, we analyzed all of the available gene deletion mutants and found three (vps36 Delta, pep12 Delta, and tlg2 Delta) that induce STRE and also repress pheromone-dependent transcription. We selected VPS36 for further characterization. A vps36 Delta mutation diminishes signaling by pheromone as well as by downstream components including the G protein, effector kinase (Ste11), and transcription factor (Ste12). Conversely, overexpression of Vps36 enhances the pheromone response in sst2 Delta cells but not in wild type. These findings indicate that Vps36 and Sst2 have opposite and opposing effects on the pheromone and stress response pathways, with Vps36 acting downstream of the G protein and independently of Sst2 RGS activity.

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.

Identification and characterization of alternatively spliced murine Rgs11 isoforms: genomic structure and gene analysis

The RGS proteins comprise a large family of proteins which were recently identified as negative Regulators of G-protein Signaling. They have been shown to act as GTPase Activating Proteins (GAPs) towards the G(alpha) subunits of heterotrimeric G-proteins. In addition to this GAP activity, which has been shown to occur through the RGS domain, RGS proteins are likely to possess other functions due to the existence of other domains in these molecules (De Vries and Farquhar, 1999; Hepler, 1999). Here, we report the molecular characterization of the murine Rgs11 gene. The gene encodes a protein with high homology to human RGS11 (79.9%), containing conserved DEP (Dishevelled/EGL-10/Pleckstrin) and GGL (G protein gamma-like) domains. The gene is comprised of at least 13 exons, spanning 8-9 kb. Spliced transcript variants were identified which are co-expressed with 5A3, a transcript that contains the largest ORF. Expression of mouse Rgs11 was found to be restricted to specific tissues with a unique pattern of expression observed in brain.

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.

Inhibition of somatostatin receptor 5-signaling by mammalian regulators of G-protein signaling (RGS) in yeast

Regulators of G-protein signaling (RGSs) are negative regulators of G-protein coupled receptor (GPCR)-mediated signaling that function to limit the lifetime of receptor-activated G(alpha)-proteins. Here we show that four mammalian RGSs differentially inhibit the activation of a FUS1--LacZ reporter gene by the STE2 encoded GPCR in yeast. In order to examine the role of the GPCR in modulating RGS function, we functionally expressed the human somatostatin receptor 5 (SST(5)) in yeast. In the absence of RGSs, FUS1--LacZ activation in response to somatostatin increased in a dose-dependent manner in cells expressing SST(5). In contrast to the results obtained with Ste2p, all RGSs completely inhibited SST(5)-mediated signaling even at concentrations of agonist as high as 10(minus sign5) M. The ability of RGSs to inhibit SST(5) signaling was further assessed in cells expressing modified Gpa1 proteins. Even though SST(5)-mediated FUS1--LacZ activation was 5-fold more efficient with a Gpa1p/G(i3alpha) chimera, response to somatostatin was completely abolished by all four RGSs. Furthermore, we demonstrate that RGS1, RGS2 and RGS5 have reduced ability to inhibit SST(5)-mediated activation of the RGS-resistant Gpa1p(Gly302Ser) mutant suggesting that the ability to interact with the G(alpha)-protein is required for the inhibition of signaling. Taken together, our results indicate that RGSs serve as better GAPs for Gpa1p when activated by SST(5) than when this G-protein is activated by Ste2p.

RGS16 function is regulated by epidermal growth factor receptor-mediated tyrosine phosphorylation

Galpha(i)-coupled receptor stimulation results in epidermal growth factor receptor (EGFR) phosphorylation and MAPK activation. Regulators of G protein signaling (RGS proteins) inhibit G protein-dependent signal transduction by accelerating Galpha(i) GTP hydrolysis, shortening the duration of G protein effector stimulation. RGS16 contains two conserved tyrosine residues in the RGS box, Tyr(168) and Tyr(177), which are predicted sites of phosphorylation. RGS16 underwent phosphorylation in response to m2 muscarinic receptor or EGFR stimulation in HEK 293T or COS-7 cells, which required EGFR kinase activity. Mutational analysis suggested that RGS16 was phosphorylated on both tyrosine residues (Tyr(168) Tyr(177)) after EGF stimulation. RGS16 co-immunoprecipitated with EGFR, and the interaction did not require EGFR activation. Purified EGFR phosphorylated only recombinant RGS16 wild-type or Y177F in vitro, implying that EGFR-mediated phosphorylation depended on residue Tyr(168). Phosphorylated RGS16 demonstrated enhanced GTPase accelerating (GAP) activity on Galpha(i). Mutation of Tyr(168) to phenylalanine resulted in a 30% diminution in RGS16 GAP activity but completely eliminated its ability to regulate G(i)-mediated MAPK activation or adenylyl cyclase inhibition in HEK 293T cells. In contrast, mutation of Tyr(177) to phenylalanine had no effect on RGS16 GAP activity but also abolished its regulation of G(i)-mediated signal transduction in these cells. These data suggest that tyrosine phosphorylation regulates RGS16 function and that EGFR may potentially inhibit Galpha(i)-dependent MAPK activation in a feedback loop by enhancing RGS16 activity through tyrosine phosphorylation.

RGSZ1 and Ret RGS: two of several splice variants from the gene RGS20

RGSZ1 and Ret RGS, members of the regulator of G-protein signaling (RGS) family, are GTPase-activating proteins (GAPs) with high selectivity for G alpha(z). We show here that RGSZ1 and Ret RGSZ1 are products of two of several splice variants of one gene, RGS20. RGS20 spans approximately 107 kb and contains at least seven exons. Five exons account for RGSZ1, including a single exon distinct to RGSZ1 that encodes a newly identified amino-terminal region. The previously described open reading frame (ORF) and 3' untranslated region are encoded by four downstream exons that also encode about half of Ret RGS. The 5' end of the RGSZ1 ORF contains several in-frame ATG codons (3-5 depending on the species), and multiple translational start sites may help explain the molecular weight heterogeneity of purified bovine brain RGSZ. Ret RGS replaces the 24 N-terminal amino acid residues of RGSZ1 with a large, N-terminal region that initially distinguished the bovine Ret RGS from human and mouse RGSZ1. This N-terminal domain is encoded by two distinct 5' exons that are variably combined with the four downstream exons shared with RGSZ1 to produce at least six mRNAs. They encode proteins with N termini that vary in size, hydrophobicity, and the presence of a cysteine string. At least two mRNAs that include the exon that encodes the N-terminal region unique to RGSZ1 were found in brain and a few other tissues, but not retina. RGS20 thus can account for multiple G(z)-selective GAPs in different tissues.

MAP kinase dynamics in response to pheromones in budding yeast

Although scaffolding is a major regulator of mitogen-activated protein kinase (MAPK) pathways, scaffolding proteins are poorly understood. During yeast mating, MAPK Fus3p is phosphorylated by MAPKK Ste7p, which is activated by MAPKKK Ste11p. This MAPK module interacts with the scaffold molecule Ste5p. Here we show that Ste11p and Ste7p were predominantly cytoplasmic proteins, while Ste5p and Fus3p were found in the nucleus and the cytoplasm. Ste5p, Ste7p and Fus3p also localized to tips of mating projections in pheromone-treated cells. Using fluorescence recovery after photobleaching (FRAP), we demonstrate that Fus3p rapidly shuttles between the nucleus and the cytoplasm independently of pheromones, Fus3p phosphorylation and Ste5p. Membrane-bound Ste5p can specifically recruit Fus3p and Ste7p to the cell cortex. Ste5p remains stably bound at the plasma membrane, unlike activated Fus3p, which dissociates from Ste5p and translocates to the nucleus.

Phosphorylation of the regulator of G protein signaling RGS9-1 by protein kinase A is a potential mechanism of light- and Ca2+-mediated regulation of G protein function in photoreceptors

In vertebrate photoreceptors, photoexcited rhodopsin interacts with the G protein transducin, causing it to bind GTP and stimulate the enzyme cGMP phosphodiesterase. The rapid termination of the active state of this pathway is dependent upon a photoreceptor-specific regulator of G protein signaling RGS9-1 that serves as a GTPase activating protein (GAP) for transducin. Here, we show that, in preparations of photoreceptor outer segments (OS), RGS9-1 is readily phosphorylated by an endogenous Ser/Thr protein kinase. Protein kinase C and MAP kinase inhibitors reduced labeling by about 30%, while CDK5 and CaMK II inhibitors had no effect. cAMP-dependent protein kinase (PKA) inhibitor H89 reduced RGS9-1 labeling by more than 90%, while dibutyryl-cAMP stimulated it 3-fold, implicating PKA as the major kinase responsible for RGS9-1 phosphorylation in OS. RGS9-1 belongs to an RGS subfamily also including RGS6, RGS7, and RGS11, which exist as heterodimers with the G protein beta subunit Gbeta5. Phosphorylated RGS9-1 remains associated with Gbeta5L, a photoreceptor-specific splice form, which itself was not phosphorylated. RGS9-1 immunoprecipitated from OS was in vitro phosphorylated by exogenous PKA. The PKA catalytic subunit could also phosphorylate recombinant RGS9-1, and mutational analysis localized phosphorylation sites to Ser(427) and Ser(428). Substitution of these residues for Glu, to mimic phosphorylation, resulted in a reduction of the GAP activity of RGS9-1. In OS, RGS9-1 phosphorylation required the presence of free Ca(2+) ions and was inhibited by light, suggesting that RGS9-1 phosphorylation could be one of the mechanisms mediating a stronger photoresponse in dark-adapted cells.

Phosphorylation and nuclear translocation of a regulator of G protein signaling (RGS10)

Heterotrimeric G proteins are involved in the transduction of hormonal and sensory signals across plasma membranes of eukaryotic cells. Hence, they are a critical point of control for a variety of agents that modulate cellular function. Activation of these proteins is dependent on GTP binding to their alpha (Galpha) subunits. Regulators of G protein signaling (RGS) bind specifically to activated Galpha proteins, potentiating the intrinsic GTPase activity of the Galpha proteins and thus expediting the termination of Galpha signaling. Although there are several points in most G protein controlled signaling pathways that are affected by reversible covalent modification, little evidence has been shown addressing whether or not the functions of RGS proteins are themselves regulated by such modifications. We report in this study the acute functional regulation of RGS10 thru the specific and inducible phosphorylation of RGS10 protein at serine 168 by cAMP-dependent kinase A. This phosphorylation nullifies the RGS10 activity at the plasma membrane, which controls the G protein-dependent activation of the inwardly rectifying potassium channel. Surprisingly, the phosphorylation-mediated attenuation of RGS10 activity was not manifested in an alteration of its ability to accelerate GTPase activity of Galpha. Rather, the phosphorylation event correlates with translocation of RGS10 from the plasma membrane and cytosol into the nucleus.