Mechanism of RGS4, a GTPase-activating protein for G protein alpha subunits

TOR signaling regulates GPCR levels on the plasma membrane and suppresses the Saccharomyces cerevisiae mating pathway

Saccharomyces cerevisiae respond to mating pheromone through the GPCRs Ste2 and Ste3, which promote growth of a mating projection in response to ligand binding. This commitment to mating is nutritionally and energetically taxing, and so we hypothesized that the cell may suppress mating signaling during starvation. We set out to investigate negative regulators of the mating pathway in nutritionally depleted environments. Here, we report that nutrient deprivation led to loss of Ste2 from the plasma membrane. Recapitulating this effect with nitrogen starvation led us to hypothesize that it was due to TORC1 signaling. Rapamycin inhibition of TORC1 impacted membrane levels of all yeast GPCRs. Inhibition of TORC1 also dampened mating pathway output. Deletion analysis revealed that TORC1 repression leads to α-arrestin-directed CME through TORC2-Ypk1 signaling. We then set out to determine whether major downstream effectors of the TOR complexes also downregulate pathway output during mating. We found that autophagy contributes to pathway downregulation through analysis of strains lacking ATG8 . We also show that Ypk1 significantly reduced pathway output. Thus, both autophagy machinery and TORC2-Ypk1 signaling serve as attenuators of pheromone signaling during mating. Altogether, we demonstrate that the stress-responsive TOR complexes coordinate GPCR endocytosis and reduce the magnitude of pheromone signaling, in ligand-independent and ligand-dependent contexts.

One Sentence Summary

TOR signaling regulates the localization of all Saccharomyces cerevisiae GPCRs during starvation and suppress the mating pathway in the presence and absence of ligand.

Heterotrimeric G protein signaling without GPCRs: The Gα-binding-and-activating (GBA) motif

Heterotrimeric G proteins (Gαβγ) are molecular switches that relay signals from 7-transmembrane receptors located at the cell surface to the cytoplasm. The function of these receptors is so intimately linked to heterotrimeric G proteins that they are named G protein-coupled receptors (GPCRs), showcasing the interdependent nature of this archetypical receptor-transducer axis of transmembrane signaling in eukaryotes. It is generally assumed that activation of heterotrimeric G protein signaling occurs exclusively by the action of GPCRs, but this idea has been challenged by the discovery of alternative mechanisms by which G proteins can propagate signals in the cell. This review will focus on a general principle of G protein signaling that operates without the direct involvement of GPCRs. The mechanism of G protein signaling reviewed here is mediated by a class of G protein regulators defined by containing an evolutionarily conserved sequence named the Gα-binding-and-activating (GBA) motif. Using the best characterized proteins with a GBA motif as examples, Gα-interacting vesicle-associated protein (GIV)/Girdin and dishevelled-associating protein with a high frequency of leucine residues (DAPLE), this review will cover (i) the mechanisms by which extracellular cues not relayed by GPCRs promote the coupling of GBA motif-containing regulators with G proteins, (ii) the structural and molecular basis for how GBA motifs interact with Gα subunits to facilitate signaling, (iii) the relevance of this mechanism in different cellular and pathological processes, including cancer and birth defects, and (iv) strategies to manipulate GBA-G protein coupling for experimental therapeutics purposes, including the development of rationally engineered proteins and chemical probes.

Class VI G protein-coupled receptors in Aspergillus oryzae regulate sclerotia formation through GTPase-activating activity

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in eukaryotes that sense and transduce extracellular signals into cells. In Aspergillus oryzae, 16 canonical GPCR genes are identified and classified into nine classes based on the sequence similarity and proposed functions. Class VI GPCRs (AoGprK-1, AoGprK-2, and AoGprR in A. oryzae), unlike other GPCRs, feature a unique hybrid structure containing both the seven transmembrane (7-TM) and regulator of G-protein signaling (RGS) domains, which is not found in animal GPCRs. We report here that the mutants with double or triple deletion of class VI GPCR genes produced significantly increased number of sclerotia compared to the control strain when grown on agar plates. Interestingly, complementation analysis demonstrated that the expression of the RGS domain without the 7-TM domain is sufficient to restore the phenotype. In line with this, among the three Gα subunits in A. oryzae, AoGpaA, AoGpaB, and AoGanA, forced expression of GTPase-deficient mutants of either AoGpaA or AoGpaB caused an increase in the number of sclerotia formed, suggesting that RGS domains of class VI GPCRs are the negative regulators of these two GTPases. Finally, we measured the expression of velvet complex genes and sclerotia formation-related genes and found that the expression of velB was significantly increased in the multiple gene deletion mutants. Taken together, these results demonstrate that class VI GPCRs negatively regulate sclerotia formation through their GTPase-activating activity in the RGS domains. KEY POINTS: • Class VI GPCRs in A. oryzae regulate sclerotia formation in A. oryzae • RGS function of class VI GPCRs is responsible for regulation of sclerotia formation • Loss of class VI GPCRs resulted in increased expression of sclerotia-related genes.

Articulating target-mining techniques to disinter Alzheimer's specific targets for drug repurposing

BACKGROUND AND OBJECTIVES

Alzheimer's Disease (AD), an extremely progressive neurodegenerative disorder is an amalgamation of numerous intricate pathological networks. This century old disease is still an unmet medical condition owing to the modest efficacy of existing therapeutic agents in antagonizing the multi-targeted pathological pathways underlying AD. Given the paucity in AD specific drugs, fabricating comprehensive research strategies to envision disease specific targets to channelize and expedite drug discovery are mandated. However, the dwindling approval rates and stringent regulatory constraints concerning the approval of a new chemical entity is daunting the pharmaceutical industries from effectuating de novo research. To bridge the existing gaps in AD drug research, a promising contemporary way out could be drug repurposing. This drug repurposing investigation is intended to envisage AD specific targets and create drug libraries pertinent to the shortlisted targets via a series of avant-garde bioinformatics and computational strategies.

METHODS

Transcriptomic analysis of three AD specific datasets viz., GSE122063, GSE15222 and GSE5281 revealed significant Differentially Expressed Genes (DEGs) and subsequent Protein-Protein Interactions (PPI) network analysis captured crucial AD targets. Later, homology model was constructed through I-TASSER for a shortlisted target protein which lacked X-ray crystallographic structure and the built protein model was validated by molecular dynamic simulations. Further, drug library was created for the shortlisted target based on structural and side effect similarity with respective standard drugs. Finally, molecular docking, binding energy calculations and molecular dynamics studies were carried out to unravel the interactions exhibited by drugs from the created library with amino acids in active binding pocket of RGS4.

RESULTS

SST and RGS4 were shortlisted as potentially significant AD specific targets, however, the less explored target RGS4 was considered for further sequential analysis. Homology model constructed for RGS4 displayed best quality when validated through Ramachandran plot and ERRAT plot. Subsequent docking and molecular dynamics studies showcased substantial affinity demonstrated by three drugs viz., Ziprasidone, Melfoquine and Metaxalone from the created drug libraries, towards RGS4.

CONCLUSION

This virtual analysis forecasted the repurposable potential of Ziprasidone, Melfoquine and Metaxalone against AD based on their affinity towards RGS4, a key AD-specific target.

Mixed-solvent molecular dynamics simulation-based discovery of a putative allosteric site on regulator of G protein signaling 4

Regulator of G protein signaling 4 (RGS4) is an intracellular protein that binds to the G_α subunit ofheterotrimeric G proteins and aids in terminating G protein coupled receptor signaling. RGS4 has been implicated in pain, schizophrenia, and the control of cardiac contractility. Inhibitors of RGS4 have been developed but bind covalently to cysteine residues on the protein. Therefore, we sought to identify alternative druggable sites on RGS4 using mixed-solvent molecular dynamics simulations, which employ low concentrations of organic probes to identify druggable hotspots on the protein. Pseudo-ligands were placed in consensus hotspots, and perturbation with normal mode analysis led to the identification and characterization of a putative allosteric site, which would be invaluable for structure-based drug design of non-covalent, small molecule inhibitors. Future studies on the mechanism of this allostery will aid in the development of novel therapeutics targeting RGS4.

The G protein signaling regulator RGS3 enhances the GTPase activity of KRAS

Recently reported to be effective in patients with lung cancer, KRASG12C inhibitors bind to the inactive, or guanosine diphosphate (GDP)–bound, state of the oncoprotein and require guanosine triphosphate (GTP) hydrolysis for inhibition. However, KRAS mutations prevent the catalytic arginine of GTPase-activating proteins (GAPs) from enhancing an otherwise slow hydrolysis rate. If KRAS mutants are indeed insensitive to GAPs, it is unclear how KRASG12C hydrolyzes sufficient GTP to allow inactive state–selective inhibition. Here, we show that RGS3, a GAP previously known for regulating G protein–coupled receptors, can also enhance the GTPase activity of mutant and wild-type KRAS proteins. Our study reveals an unexpected mechanism that inactivates KRAS and explains the vulnerability to emerging clinically effective therapeutics.

Residue-level determinants of RGS R4 subfamily GAP activity and specificity towards the Gi subfamily

The structural basis for the GTPase-accelerating activity of regulators of G protein signaling (RGS) proteins, as well as the mechanistic basis for their specificity in interacting with the heterotrimeric (αβγ) G proteins they inactivate, is not sufficiently understood at the family level. Here, we used biochemical assays to compare RGS domains across the RGS family and map those individual residues that favorably contribute to GTPase-accelerating activity, and those residues responsible for attenuating RGS domain interactions with Gα subunits. We show that conserved interactions of RGS residues with both the Gα switch I and II regions are crucial for RGS activity, while the reciprocal effects of "modulatory" and "disruptor" residues selectively modulate RGS activity. Our results quantify how specific interactions between RGS domains and Gα subunits are set by a balance between favorable RGS residue interactions with particular Gα switch regions, and unfavorable interactions with the Gα helical domain.

Strategies towards Targeting Gαi/s Proteins: Scanning of Protein-Protein Interaction Sites To Overcome Inaccessibility

Heterotrimeric G proteins are classified into four subfamilies and play a key role in signal transduction. They transmit extracellular signals to intracellular effectors subsequent to the activation of G protein-coupled receptors (GPCRs), which are targeted by over 30 % of FDA-approved drugs. However, addressing G proteins as drug targets represents a compelling alternative, for example, when G proteins act independently of the corresponding GPCRs, or in cases of complex multifunctional diseases, when a large number of different GPCRs are involved. In contrast to Gαq, efforts to target Gαi/s by suitable chemical compounds has not been successful so far. Here, a comprehensive analysis was conducted examining the most important interface regions of Gαi/s with its upstream and downstream interaction partners. By assigning the existing compounds and the performed approaches to the respective interfaces, the druggability of the individual interfaces was ranked to provide perspectives for selective targeting of Gαi/s in the future.

Allosteric Pathways Originating at Cysteine Residues in Regulators of G-Protein Signaling Proteins

Regulators of G-protein signaling (RGS) proteins play a central role in modulating signaling via G-protein coupled receptors (GPCRs). Specifically, RGS proteins bind to activated Gα subunits in G-proteins, accelerate the GTP hydrolysis, and thereby rapidly dampen GPCR signaling. Therefore, covalent molecules targeting conserved cysteine residues among RGS proteins have emerged as potential candidates to inhibit the RGS/Gα protein-protein interaction and enhance GPCR signaling. Although these inhibitors bind to conserved cysteine residues among RGS proteins, we have previously suggested [J. Am. Chem. Soc. 2018;140:3454–3460] that their potencies and specificities are related to differential protein dynamics among RGS proteins. Using data from all-atom molecular dynamics simulations, we reveal these differences in dynamics of RGS proteins by partitioning the protein structural space into a network of communities that allow allosteric signals to propagate along unique pathways originating at inhibitor binding sites and terminating at the RGS/Gα protein-protein interface.

A Global Map of G Protein Signaling Regulation by RGS Proteins

The control over the extent and timing of G protein signaling is provided by the regulator of G protein signaling (RGS) proteins that deactivate G protein α subunits (Gα). Mammalian genomes encode 20 canonical RGS and 16 Gα genes with key roles in physiology and disease. To understand the principles governing the selectivity of Gα regulation by RGS, we examine the catalytic activity of all canonical human RGS proteins and their selectivity for a complete set of Gα substrates using real-time kinetic measurements in living cells. The data reveal rules governing RGS-Gα recognition, the structural basis of its selectivity, and provide principles for engineering RGS proteins with defined selectivity. The study also explores the evolution of RGS-Gα selectivity through ancestral reconstruction and demonstrates how naturally occurring non-synonymous variants in RGS alter signaling. These results provide a blueprint for decoding signaling selectivity and advance our understanding of molecular recognition principles.

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Systematic analysis reveals G protein selectivity of all canonical RGS proteins

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RGS proteins rely on selectivity bar codes for selective G protein recognition

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Transplantation of bar codes across RGS proteins switches their G protein preferences

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Natural variants, mutations, and evolution shape RGS selectivity

MOS1 Negatively Regulates Sugar Responses and Anthocyanin Biosynthesis in Arabidopsis

Sugars, which are important signaling molecules, regulate diverse biological processes in plants. However, the convergent regulatory mechanisms governing these physiological activities have not been fully elucidated. MODIFIER OF snc1-1 (MOS1), a modulator of plant immunity, also regulates floral transition, cell cycle control, and other biological processes. However, there was no evidence of whether this protein was involved in sugar responses. In this study, we found that the loss-of-function mutant mos1-6 (mos1) was hypersensitive to sugar and was characterized by defective germination and shortened roots when grown on high-sugar medium. The expression of MOS1 was enhanced by sucrose. Hexokinase 1, an important gene involved in sugar signaling, was upregulated in the mos1 mutant compared to wild-type Col-0 in response to sugar. Furthermore, the mos1 mutant accumulated more anthocyanin than did wild-type Col-0 when grown on high-sugar concentration medium or under high light. MOS1 was found to regulate the expression of flavonoid and anthocyanin biosynthetic genes in response to exogenous sucrose and high-light stress but with different underlying mechanisms, showing multiple functions in addition to immunity regulation in plant development. Our results suggest that the immune regulator MOS1 serves as a coordinator in the regulatory network, governing immunity and other physiological processes.

Probing the mutational landscape of regulators of G protein signaling proteins in cancer

The advent of deep-sequencing techniques has revealed that mutations in G protein-coupled receptor (GPCR) signaling pathways in cancer are more prominent than was previously appreciated. An emergent theme is that cancer-associated mutations tend to cause enhanced GPCR pathway activation to favor oncogenicity. Regulators of G protein signaling (RGS) proteins are critical modulators of GPCR signaling that dampen the activity of heterotrimeric G proteins through their GTPase-accelerating protein (GAP) activity, which is conferred by a conserved domain dubbed the "RGS-box." Here, we developed an experimental pipeline to systematically assess the mutational landscape of RGS GAPs in cancer. A pan-cancer bioinformatics analysis of the 20 RGS domains with GAP activity revealed hundreds of low-frequency mutations spread throughout the conserved RGS domain structure with a slight enrichment at positions that interface with G proteins. We empirically tested multiple mutations representing all RGS GAP subfamilies and sampling both G protein interface and noninterface positions with a scalable, yeast-based assay. Last, a subset of mutants was validated using G protein activity biosensors in mammalian cells. Our findings reveal that a sizable fraction of RGS protein mutations leads to a loss of function through various mechanisms, including disruption of the G protein-binding interface, loss of protein stability, or allosteric effects on G protein coupling. Moreover, our results also validate a scalable pipeline for the rapid characterization of cancer-associated mutations in RGS proteins.

BCL9 provides multi-cellular communication properties in colorectal cancer by interacting with paraspeckle proteins

Colorectal cancer (CRC) is the third most commonly diagnosed cancer, which despite recent advances in treatment, remains incurable due to molecular heterogeneity of tumor cells. The B-cell lymphoma 9 (BCL9) oncogene functions as a transcriptional co-activator of the Wnt/β-catenin pathway, which plays critical roles in CRC pathogenesis. Here we have identified a β-catenin-independent function of BCL9 in a poor-prognosis subtype of CRC tumors characterized by expression of stromal and neural associated genes. In response to spontaneous calcium transients or cellular stress, BCL9 is recruited adjacent to the interchromosomal regions, where it stabilizes the mRNA of calcium signaling and neural associated genes by interacting with paraspeckle proteins. BCL9 subsequently promotes tumor progression and remodeling of the tumor microenvironment (TME) by sustaining the calcium transients and neurotransmitter-dependent communication among CRC cells. These data provide additional insights into the role of BCL9 in tumor pathogenesis and point towards additional avenues for therapeutic intervention.

Gαi2 Signaling Regulates Inflammasome Priming and Cytokine Production by Biasing Macrophage Phenotype Determination

Macrophages exist as innate immune subsets that exhibit phenotypic heterogeneity and functional plasticity. Their phenotypes are dictated by inputs from the tissue microenvironment. G-protein-coupled receptors are essential in transducing signals from the microenvironment, and heterotrimeric Gα signaling links these receptors to downstream effectors. Several Gα_i-coupled G-protein-coupled receptors have been implicated in macrophage polarization. In this study, we use genetically modified mice to investigate the role of Gα_i2 on inflammasome activity and macrophage polarization. We report that Gα_i2 in murine bone marrow-derived macrophages (BMDMs) regulates IL-1β release after activation of the NLRP3, AIM2, and NLRC4 inflammasomes. We show this regulation stems from the biased polarity of Gα_i2 deficient (Gnai2 ^-/-) and RGS-insensitive Gα_i2 (Gnai2 ^G184S/G184S) BMDMs. We determined that although Gnai2 ^G184S/G184S BMDMs (excess Gα_i2 signaling) have a tendency toward classically activated proinflammatory (M1) phenotype, Gnai2^-/- BMDMs (Gα_i2 deficient) are biased toward alternatively activated anti-inflammatory (M2) phenotype. Finally, we find that Gα_i2-deficient macrophages have increased Akt activation and IFN-β production but defects in ERK1/2 and STAT3 activation after LPS stimulation. Gα_i2-deficient macrophages also exhibit increased STAT6 activation after IL-4 stimulation. In summary, our data indicates that excess Gα_i2 signaling promotes an M1 macrophage phenotype, whereas Gα_i2 signaling deficiency promotes an M2 phenotype. Understanding Gα_i2-mediated effects on macrophage polarization may bring to light insights regarding disease pathogenesis and the reprogramming of macrophages for the development of novel therapeutics.

Gα and regulator of G-protein signaling (RGS) protein pairs maintain functional compatibility and conserved interaction interfaces throughout evolution despite frequent loss of RGS proteins in plants

Signaling pathways regulated by heterotrimeric G-proteins exist in all eukaryotes. The regulator of G-protein signaling (RGS) proteins are key interactors and critical modulators of the Gα protein of the heterotrimer. However, while G-proteins are widespread in plants, RGS proteins have been reported to be missing from the entire monocot lineage, with two exceptions. A single amino acid substitution-based adaptive coevolution of the Gα:RGS proteins was proposed to enable the loss of RGS in monocots. We used a combination of evolutionary and biochemical analyses and homology modeling of the Gα and RGS proteins to address their expansion and its potential effects on the G-protein cycle in plants. Our results show that RGS proteins are widely distributed in the monocot lineage, despite their frequent loss. There is no support for the adaptive coevolution of the Gα:RGS protein pair based on single amino acid substitutions. RGS proteins interact with, and affect the activity of, Gα proteins from species with or without endogenous RGS. This cross-functional compatibility expands between the metazoan and plant kingdoms, illustrating striking conservation of their interaction interface. We propose that additional proteins or alternative mechanisms may exist which compensate for the loss of RGS in certain plant species.

Expression of the ZNT1 Zinc Transporter from the Metal Hyperaccumulator Noccaea caerulescens Confers Enhanced Zinc and Cadmium Tolerance and Accumulation to Arabidopsis thaliana

Prompt regulation of transition metal transporters is crucial for plant zinc homeostasis. NcZNT1 is one of such transporters, found in the metal hyperaccumulator Brassicaceae species Noccaea caerulescens. It is orthologous to AtZIP4 from Arabidopsis thaliana, an important actor in Zn homeostasis. We examined if the NcZNT1 function contributes to the metal hyperaccumulation of N. caerulescens. NcZNT1 was found to be a plasma-membrane located metal transporter. Constitutive overexpression of NcZNT1 in A. thaliana conferred enhanced tolerance to exposure to excess Zn and Cd supply, as well as increased accumulation of Zn and Cd and induction of the Fe deficiency response, when compared to non-transformed wild-type plants. Promoters of both genes were induced by Zn deficiency in roots and shoots of A. thaliana. In A. thaliana, the AtZIP4 and NcZNT1 promoters were mainly active in cortex, endodermis and pericycle cells under Zn deficient conditions. In N. caerulescens, the promoters were active in the same tissues, though the activity of the NcZNT1 promoter was higher and not limited to Zn deficient conditions. Common cis elements were identified in both promoters by 5' deletion analysis. These correspond to the previously determined Zinc Deficiency Responsive Elements found in A. thaliana to interact with two redundantly acting transcription factors, bZIP19 and bZIP23, controlling the Zn deficiency response. In conclusion, these results suggest that NcZNT1 is an important factor in contributing to Zn and Cd hyperaccumulation in N. caerulescens. Differences in cis- and trans-regulators are likely to account for the differences in expression between A. thaliana and N. caerulescens. The high, constitutive NcZNT1 expression in the stele of N. caerulescens roots implicates its involvement in long distance root-to-shoot metal transport by maintaining a Zn/Cd influx into cells responsible for xylem loading.

Heterotrimeric G protein signaling via GIV/Girdin: Breaking the rules of engagement, space, and time

Canonical signal transduction via heterotrimeric G proteins is spatially and temporally restricted, that is, triggered exclusively at the plasma membrane (PM), only by agonist activation of G protein-coupled receptors (GPCRs) via a process that completes within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a non-canonical pathway for activation of heterotrimeric G proteins by the non-receptor guanidine-nucleotide exchange factor (GEF), GIV/Girdin. This pathway has distinctive temporal and spatial features and an unusual profile of receptor engagement: diverse classes of receptors, not just GPCRs can engage with GIV to trigger such activation. Such activation is spatially and temporally unrestricted, that is, can occur both at the PM and on internal membranes discontinuous with the PM, and can continue for prolonged periods of time. Here, we provide the most complete up-to-date review of the molecular mechanisms that govern the unique spatiotemporal aspects of non-canonical G protein activation by GIV and the relevance of this new paradigm in health and disease.

Potent inhibition of angiotensin AT1 receptor signaling by RGS8: importance of the C-terminal third exon part of its RGS domain

R4/B subfamily RGS (regulator of G protein signaling) proteins play roles in regulation of many GPCR-mediated responses. Multiple RGS proteins are usually expressed in a cell, and it is difficult to point out which RGS protein species are functionally important in the cell. To evaluate intrinsic potency of these RGS proteins, we compared inhibitory effects of RGS1, RGS2, RGS3, RGS4, RGS5, RGS8 and RGS16 on AT1 receptor signaling. Intracellular Ca(2+) responses to angiotensin II were markedly attenuated by transiently expressed RGS2, RGS3 and RGS8, compared to weak inhibition by RGS1, RGS4, RGS5 and RGS16. N-terminally deleted RGS2 (RGS2 domain) lost this potent inhibitory effect, whereas RGS domains of RGS3 and RGS8 showed strong inhibition similar to those of the full-length proteins. To investigate key determinants that specify the differences in potency, we constructed chimeric domains by replacing one or two of three exon parts of RGS8 domain with the corresponding part of RGS5. The chimeric RGS8 domains containing the first or the second exon part of RGS5 showed strong inhibitory effects similar to that of wild type RGS8, but the chimeric domain with the third exon part of RGS5 lost its activity. On the contrary, replacement of the third exon part of RGS5 with the corresponding residues of RGS8 increased the inhibitory effect. The role of the third exon part of RGS8 domain was further confirmed with the chimeric RGS8/RGS4 domains. These results indicate the potent inhibitory activity of RGS8 among R4/B subfamily proteins and importance of the third exon.

Gαi3-Dependent Inhibition of JNK Activity on Intracellular Membranes

Heterotrimeric G-protein signaling has been shown to modulate a wide variety of intracellular signaling pathways, including the mitogen-activated protein kinase (MAPK) family. The activity of one MAPK family class, c-Jun N-terminal kinases (JNKs), has been traditionally linked to the activation of G-protein coupled receptors (GPCRs) at the plasma membrane. Using a unique set of G-protein signaling tools developed in our laboratory, we show that subcellular domain-specific JNK activity is inhibited by the activation of Gαi3, the Gαi isoform found predominantly within intracellular membranes, such as the endoplasmic reticulum (ER)–Golgi interface, and their associated vesicle pools. Regulators of intracellular Gαi3, including activator of G-protein signaling 3 (AGS3) and the regulator of G-protein signaling protein 4 (RGS4), have a marked impact on the regulation of JNK activity. Together, these data support the existence of unique intracellular signaling complexes that control JNK activity deep within the cell. This work highlights some of the cellular pathways that are regulated by these intracellular complexes and identifies potential strategies for their regulation in mammalian cells.

GIV/Girdin transmits signals from multiple receptors by triggering trimeric G protein activation

Activation of trimeric G proteins has been traditionally viewed as the exclusive job of G protein-coupled receptors (GPCRs). This view has been challenged by the discovery of non-receptor activators of trimeric G proteins. Among them, GIV (a.k.a. Girdin) is the first for which a guanine nucleotide exchange factor (GEF) activity has been unequivocally associated with a well defined motif. Here we discuss how GIV assembles alternative signaling pathways by sensing cues from various classes of surface receptors and relaying them via G protein activation. We also describe the dysregulation of this mechanism in disease and how its targeting holds promise for novel therapeutics.

Regulator of G protein signaling transcript expression in human neural progenitor differentiation: R7 subfamily regulation by DNA methylation

G protein-coupled receptors (GPCRs) and their ligands are critical regulators of neural progenitor differentiation, and GPCR signaling pathways are regulated by regulator of G protein signaling (RGS) proteins. RGS protein expression is dynamically regulated, and we have recently described the epigenetic regulation of RGS transcript expression. Given the potential of RGS proteins to regulate GPCR signaling and the established role of epigenetic regulation in progenitor differentiation, we explored the impact of epigenetic regulation of RGS transcripts during in vitro differentiation of human neural progenitors. Here, we demonstrate robust upregulation of the RGS transcripts RGS4, RGS5, RGS6, RGS7, and RGS11 during neuronal differentiation, while DNA methyltransferase (DNMT) and histone deacetylase enzyme expression is suppressed during differentiation. Transcripts encoding R7 subfamily RGS proteins and the R7-binding partners R7BP and R9AP showed the greatest upregulation. Further, we showed that direct pharmacological inhibition of DNMT activity enhances expression of RGS2, RGS4, RGS5, RGS6, RGS7, RGS8, RGS9L, RGS10, and RGS14 as well as R7BP and R9AP transcripts in progenitors, consistent with regulation by DNMTs. Our results reveal marked upregulation of RGS expression during neuronal differentiation and suggest that decreased expression of DNMT enzymes during differentiation contributes to upregulation.

RGS4 regulates partial agonism of the M2 muscarinic receptor-activated K+ currents

Partial agonists are used clinically to avoid overstimulation of receptor-mediated signalling, as they produce a submaximal response even at 100% receptor occupancy. The submaximal efficacy of partial agonists is due to conformational change of the agonist-receptor complex, which reduces effector activation. In addition to signalling activators, several regulators help control intracellular signal transductions. However, it remains unclear whether these signalling regulators contribute to partial agonism. Here we show that regulator of G-protein signalling (RGS) 4 is a determinant for partial agonism of the M2 muscarinic receptor (M2R). In rat atrial myocytes, pilocarpine evoked smaller G-protein-gated K(+) inwardly rectifying (KG) currents than those evoked by ACh. In a Xenopus oocyte expression system, pilocarpine acted as a partial agonist in the presence of RGS4 as it did in atrial myocytes, while it acted like a full agonist in the absence of RGS4. Functional couplings within the agonist-receptor complex/G-protein/RGS4 system controlled the efficacy of pilocarpine relative to ACh. The pilocarpine-M2R complex suppressed G-protein-mediated activation of KG currents via RGS4. Our results demonstrate that partial agonism of M2R is regulated by the RGS4-mediated inhibition of G-protein signalling. This finding helps us to understand the molecular components and mechanism underlying the partial agonism of M2R-mediated physiological responses.

Transcriptional suppression, DNA methylation, and histone deacetylation of the regulator of G-protein signaling 10 (RGS10) gene in ovarian cancer cells

RGS10 regulates ovarian cancer cell growth and survival, and RGS10 expression is suppressed in cell models of ovarian cancer chemoresistance. However, the mechanisms governing RGS10 expression in ovarian cancer are poorly understood. Here we report RGS10 suppression in primary ovarian cancer and CAOV-3 ovarian cancer cells compared to immortalized ovarian surface epithelial (IOSE) cells, and in A2780-AD chemoresistant cells compared to parental A2780 cells. RGS10-1 and RGS10-2 transcripts are expressed in ovarian cancer cells, but only RGS10-1 is suppressed in A2780-AD and CAOV-3 cells, and the RGS10-1 promoter is uniquely enriched in CpG dinucleotides. Pharmacological inhibition of DNA methyl-transferases (DNMTs) increased RGS10 expression, suggesting potential regulation by DNA methylation. Bisulfite sequencing analysis identified a region of the RGS10-1 promoter with significantly enhanced DNA methylation in chemoresistant A2780-AD cells relative to parental A2780 cells. DNA methylation in CAOV-3 and IOSE cells was similar to A2780 cells. More marked differences were observed in histone acetylation of the RGS10-1 promoter. Acetylated histone H3 associated with the RGS10-1 promoter was significantly lower in A2780-AD cells compared to parental cells, with a corresponding increase in histone deacetylase (HDAC) enzyme association. Similarly, acetylated histone levels at the RGS10-1 promoter were markedly lower in CAOV-3 cells compared to IOSE cells, and HDAC1 binding was doubled in CAOV-3 cells. Finally, we show that pharmacological inhibition of DNMT or HDAC enzymes in chemoresistant A2780-AD cells increases RGS10 expression and enhances cisplatin toxicity. These data suggest that histone de-acetylation and DNA methylation correlate with RGS10 suppression and chemoresistance in ovarian cancer. Markers for loss of RGS10 expression may identify cancer cells with unique response to therapeutics.

Structural and functional analysis of the regulator of G protein signaling 2-gαq complex

The heterotrimeric G protein Gαq is a key regulator of blood pressure, and excess Gαq signaling leads to hypertension. A specific inhibitor of Gαq is the GTPase activating protein (GAP) known as regulator of G protein signaling 2 (RGS2). The molecular basis for how Gαq/11 subunits serve as substrates for RGS proteins and how RGS2 mandates its selectivity for Gαq is poorly understood. In crystal structures of the RGS2-Gαq complex, RGS2 docks to Gαq in a different orientation from that observed in RGS-Gαi/o complexes. Despite its unique pose, RGS2 maintains canonical interactions with the switch regions of Gαq in part because its α6 helix adopts a distinct conformation. We show that RGS2 forms extensive interactions with the α-helical domain of Gαq that contribute to binding affinity and GAP potency. RGS subfamilies that do not serve as GAPs for Gαq are unlikely to form analogous stabilizing interactions.

Differential role of Axin RGS domain function in Wnt signaling during anteroposterior patterning and maternal axis formation

Axin is a critical component of the β-catenin destruction complex and is also necessary for Wnt signaling initiation at the level of co-receptor activation. Axin contains an RGS domain, which is similar to that of proteins that accelerate the GTPase activity of heterotrimeric Gα/Gna proteins and thereby limit the duration of active G-protein signaling. Although G-proteins are increasingly recognized as essential components of Wnt signaling, it has been unclear whether this domain of Axin might function in G-protein regulation. This study was performed to test the hypothesis that Axin RGS-Gna interactions would be required to attenuate Wnt signaling. We tested these ideas using an axin1 genetic mutant (masterblind) and antisense oligo knockdowns in developing zebrafish and Xenopus embryos. We generated a point mutation that is predicted to reduce Axin-Gna interaction and tested for the ability of the mutant forms to rescue Axin loss-of-function function. This Axin point mutation was deficient in binding to Gna proteins in vitro, and was unable to relocalize to the plasma membrane upon Gna overexpression. We found that the Axin point mutant construct failed to rescue normal anteroposterior neural patterning in masterblind mutant zebrafish, suggesting a requirement for G-protein interactions in this context. We also found that the same mutant was able to rescue deficiencies in maternal axin1 loss-of-function in Xenopus. These data suggest that maternal and zygotic Wnt signaling may differ in the extent of Axin regulation of G-protein signaling. We further report that expression of a membrane-localized Axin construct is sufficient to inhibit Wnt/β-catenin signaling and to promote Axin protein turnover.

Fluorescence-based assay probing regulator of G protein signaling partner proteins

The regulator of G protein signaling (RGS) proteins are one of the essential modulators for the G protein system. Besides regulating G protein signaling by accelerating the GTPase activity of Gα subunits, RGS proteins are implicated in exerting other functions; they are also known to be involved in several diseases. Moreover, the existence of a single RGS protein in plants and its seven-transmembrane domain found in 2003 triggered efforts to unveil detailed structural and functional information of RGS proteins. We present a method for real-time examination of the protein-protein interactions between RGS and Gα subunits. AtRGS1 from plants and RGS4 from mammals were site-directedly labeled with the fluorescent probe Lucifer yellow on engineered cysteine residues and used to interact with different Gα subunits. The physical interactions can be revealed by monitoring the real-time fluorescence changes (8.6% fluorescence increase in mammals and 27.6% in plants); their correlations to functional exertion were shown with a GTPase accelerating activity assay and further confirmed by measurement of K(d). We validate the effectiveness of this method and suggest its application to the exploration of more RGS signaling partner proteins in physiological and pathological studies.

RGS proteins maintain robustness of GPCR-GIRK coupling by selective stimulation of the G protein subunit Gαo

Termination of heterotrimeric guanine nucleotide-binding protein (G protein) signaling downstream of activated G protein-coupled receptors (GPCRs) is accelerated by regulator of G protein signaling (RGS) proteins, which act as guanosine triphosphatase (GTPase)-activating proteins (GAPs). Using a Xenopus oocyte expression system, we found that although RGS proteins had a negative effect of accelerating the kinetics of GPCR-coupled potassium ion (K+) channel (GIRK) deactivation, they also had positive effects of increasing the amplitudes and activation kinetics of neurotransmitter-evoked GIRK currents. The RGS box domain alone was sufficient to stimulate neurotransmitter-dependent activation of GIRK currents. Moreover, RGS4 mutants with compromised GAP activity augmented GPCR-GIRK coupling (as assessed by measurement of the GIRK current elicited by neurotransmitter). By accelerating G protein activation kinetics, RGS4 specifically stimulated Gα₀, which stimulated GPCR-GIRK coupling despite its GAP activity. Opposing actions of RGS proteins thus both stimulate and inhibit G proteins to modulate the amplitude and kinetics of neurotransmitter-induced GIRK currents, thereby distinguishing the responses to activation of different G protein isoforms.

D(2)-Dopamine receptors target regulator of G protein signaling 9-2 to detergent-resistant membrane fractions

Detergent-resistant membranes (DRM) are thought to contain structures such as lipid rafts that are involved in compartmentalizing cell membranes. We report that the majority of D(2)-dopamine receptors (D(2)R) expressed endogenously in mouse striatum or expressed in immortalized cell-lines is found in DRM. In addition, exogenous co-expression of D(2)R in a cell line shifted the expression of regulator of G protein signaling 9-2 (RGS9-2) into DRM. RGS9-2 is a protein that is highly enriched in the striatum and specifically regulates striatal D(2)R. In the striatum, RGS9-2 is mostly associated with DRMs but when expressed in cell lines, RGS9-2 is present in the soluble cytoplasmic fraction. In contrast, the majority of mu opioid receptors and delta opioid receptors are found in detergent-soluble membrane and there was no shift of RGS9-2 into DRM after co-expression of mu opioid receptor. These data suggest that the targeting of RGS9-2 to DRM in the striatum is mediated by D(2)R and that DRM is involved in the formation of a D(2)R signaling complex. D(2)R-mediated targeting of RGS9-2 to DRM was blocked by the deletion of the RGS9-2 DEP domain or by a point mutation that abolishes the GTPase accelerating protein function of RGS9-2.

Structural analysis of the Ras-like G protein MglA and its cognate GAP MglB and implications for bacterial polarity

The small G protein MglA and its cognate GAP MglB exemplify a new type of GTPase activation mechanism. In contrast to other Ras-like proteins, the key 'arginine finger' is provided not by the GAP, but by MglA itself.

The bacterium Myxococcus xanthus uses a G protein cycle to dynamically regulate the leading/lagging pole polarity axis. The G protein MglA is regulated by its GTPase-activating protein (GAP) MglB, thus resembling Ras family proteins. Here, we show structurally and biochemically that MglA undergoes a dramatic, GDP–GTP-dependent conformational change involving a screw-type forward movement of the central β2-strand, never observed in any other G protein. This movement and complex formation with MglB repositions the conserved residues Arg53 and Gln82 into the active site. Residues required for catalysis are thus not provided by the GAP MglB, but by MglA itself. MglB is a Roadblock/LC7 protein and functions as a dimer to stimulate GTP hydrolysis in a 2:1 complex with MglA. In vivo analyses demonstrate that hydrolysis mutants abrogate Myxococcus' ability to regulate its polarity axis changing the reversal behaviour from stochastic to oscillatory and that both MglA GTPase activity and MglB GAP catalysis are essential for maintaining a proper polarity axis.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

Integrating energy calculations with functional assays to decipher the specificity of G protein-RGS protein interactions

The diverse Regulator of G protein Signaling (RGS) family sets the timing of G protein signaling. To understand how the structure of RGS proteins determines their common ability to inactivate G proteins and their selective G protein recognition, we combined structure-based energy calculations with biochemical measurements of RGS activity. We found a previously unidentified group of variable 'Modulatory' residues that reside at the periphery of the RGS domain-G protein interface and fine-tune G protein recognition. Mutations of Modulatory residues in high-activity RGS proteins impaired RGS function, whereas redesign of low-activity RGS proteins in critical Modulatory positions yielded complete gain of function. Therefore, RGS proteins combine a conserved core interface with peripheral Modulatory residues to selectively optimize G protein recognition and inactivation. Finally, we show that our approach can be extended to analyze interaction specificity across other large protein families.

Regulation of stem cell pluripotency and differentiation by G protein coupled receptors

Stem cell-based therapeutics have the potential to effectively treat many terminal and debilitating human diseases, but the mechanisms by which their growth and differentiation are regulated are incompletely defined. Recent data from multiple systems suggest major roles for G protein coupled receptor (GPCR) pathways in regulating stem cell function in vivo and in vitro. The goal of this review is to illustrate common ground between the growing field of stem cell therapeutics and the long-established field of G protein coupled receptor signaling. Herein, we briefly introduce basic stem cell biology and discuss how several conserved pathways regulate pluripotency and differentiation in mouse and human stem cells. We further discuss general mechanisms by which GPCR signaling may impact these pluripotency and differentiation pathways, and summarize specific examples of receptors from each of the major GPCR subfamilies that have been shown to regulate stem cell function. Finally, we discuss possible therapeutic implications of GPCR regulation of stem cell function.

Regulation of the AGS3·G{alpha}i signaling complex by a seven-transmembrane span receptor

G-protein signaling modulators (GPSM) play diverse functional roles through their interaction with G-protein subunits. AGS3 (GPSM1) contains four G-protein regulatory motifs (GPR) that directly bind Gα(i) free of Gβγ providing an unusual scaffold for the "G-switch" and signaling complexes, but the mechanism by which signals track into this scaffold are not well understood. We report the regulation of the AGS3·Gα(i) signaling module by a cell surface, seven-transmembrane receptor. AGS3 and Gα(i1) tagged with Renilla luciferase or yellow fluorescent protein expressed in mammalian cells exhibited saturable, specific bioluminescence resonance energy transfer indicating complex formation in the cell. Activation of α(2)-adrenergic receptors or μ-opioid receptors reduced AGS3-RLuc·Gα(i1)-YFP energy transfer by over 30%. The agonist-mediated effects were inhibited by pertussis toxin and co-expression of RGS4, but were not altered by Gβγ sequestration with the carboxyl terminus of GRK2. Gα(i)-dependent and agonist-sensitive bioluminescence resonance energy transfer was also observed between AGS3 and cell-surface receptors typically coupled to Gα(i) and/or Gα(o) indicating that AGS3 is part of a larger signaling complex. Upon receptor activation, AGS3 reversibly dissociates from this complex at the cell cortex. Receptor coupling to both Gαβγ and GPR-Gα(i) offer additional flexibility for systems to respond and adapt to challenges and orchestrate complex behaviors.

Regulator of g protein signaling 3 modulates wnt5b calcium dynamics and somite patterning

Vertebrate development requires communication among cells of the embryo in order to define the body axis, and the Wnt-signaling network plays a key role in axis formation as well as in a vast array of other cellular processes. One arm of the Wnt-signaling network, the non-canonical Wnt pathway, mediates intracellular calcium release via activation of heterotrimeric G proteins. Regulator of G protein Signaling (RGS) proteins can accelerate inactivation of G proteins by acting as G protein GTPase-activating proteins (GAPs), however, the possible role of RGS proteins in non-canonical Wnt signaling and development is not known. Here, we identify rgs3 as having an overlapping expression pattern with wnt5b in zebrafish and reveal that individual knockdown of either rgs3 or wnt5b gene function produces similar somite patterning defects. Additionally, we describe endogenous calcium release dynamics in developing zebrafish somites and determine that both rgs3 and wnt5b function are required for appropriate frequency and amplitude of calcium release activity. Using rescue of gene knockdown and in vivo calcium imaging assays, we demonstrate that the activity of Rgs3 requires its ability to interact with Gα subunits and function as a G protein GAP. Thus, Rgs3 function is necessary for appropriate frequency and amplitude of calcium release during somitogenesis and is downstream of Wnt5 activity. These results provide the first evidence for an essential developmental role of RGS proteins in modulating the duration of non-canonical Wnt signaling.

Vertebrate development requires communication among cells in order to define the body axis (front/back, head/tail, or left/right). Secreted factors such as Wnts play key roles in a vast array of cellular processes, including patterning of the body axis. One arm of the Wnt-signaling network, the non-canonical pathway, mediates intracellular calcium release via activation of heterotrimeric G proteins. Regulator of G protein Signaling (RGS) proteins can accelerate inactivation of G proteins by acting as G protein GAPs and are uniquely situated to control the amplitude of a Wnt signal. Here, we combine cellular, molecular, and genetic analyses with high resolution calcium imaging to identify a role for RGS modulation of Wnt-mediated calcium release dynamics and developmental patterning events. We find that loss of rgs3 gene function produced body patterning defects like those observed with loss of wnt5b gene function. Analysis of endogenous calcium release dynamics in developing zebrafish revealed that both rgs3 and wnt5b are required for appropriate frequency and amplitude of calcium release. Our results provide new evidence that a member of the RGS protein family is essential for modulating the non-canonical Wnt network to assure normal tissue patterning during development.

Allosteric inhibition of the regulator of G protein signaling-Galpha protein-protein interaction by CCG-4986

Regulator of G protein signaling (RGS) proteins act to temporally modulate the activity of G protein subunits after G protein-coupled receptor activation. RGS proteins exert their effect by directly binding to the activated Galpha subunit of the G protein, catalyzing the accelerated hydrolysis of GTP and returning the G protein to its inactive, heterotrimeric form. In previous studies, we have sought to inhibit this GTPase-accelerating protein activity of the RGS protein by using small molecules. In this study, we investigated the mechanism of CCG-4986 [methyl-N-[(4-chlorophenyl)sulfonyl]-4-nitro-benzenesulfinimidoate], a previously reported small-molecule RGS inhibitor. Here, we find that CCG-4986 inhibits RGS4 function through the covalent modification of two spatially distinct cysteine residues on RGS4. We confirm that modification of Cys132, located near the RGS/Galpha interaction surface, modestly inhibits Galpha binding and GTPase acceleration. In addition, we report that modification of Cys148, a residue located on the opposite face of RGS4, can disrupt RGS/Galpha interaction through an allosteric mechanism that almost completely inhibits the Galpha-RGS protein-protein interaction. These findings demonstrate three important points: 1) the modification of the Cys148 allosteric site results in significant changes to the RGS interaction surface with Galpha; 2) this identifies a "hot spot" on RGS4 for binding of small molecules and triggering an allosteric change that may be significantly more effective than targeting the actual protein-protein interaction surface; and 3) because of the modification of a positional equivalent of Cys148 in RGS8 by CCG-4986, lack of inhibition indicates that RGS proteins exhibit fundamental differences in their responses to small-molecule ligands.

RGS9-2 mediates specific inhibition of agonist-induced internalization of D2-dopamine receptors

Regulator of G protein signaling 9-2 (RGS9-2), a member of the RGS family of GTPase accelerating proteins, is expressed specifically in the striatum, a brain region involved in controlling movement, motivation, mood and addiction. RGS9-2 can be found co-localized with D(2)-class dopamine receptors in medium spiny striatal neurons and altered functioning of both RGS9-2 and D(2)-like dopamine receptors have been implicated in schizophrenia, movement disorders and reward responses. Previously we showed that RGS9-2 can specifically co-localize with D(2)-dopamine receptors (D2R). Here we provide further evidence of the specificity of RGS9-2 for regulating D2R cellular functions: the expression of RGS9-2 inhibits dopamine-mediated cellular internalization of D2R, while the expression of another RGS protein, RGS4, had no effect. In addition, the agonist-mediated internalization of the G protein coupled delta opioid receptor was unaffected by RGS9-2 expression. We utilized mutant constructs of RGS9-2 to show that the RGS9-2 DEP (for Disheveled, EGL-10, Pleckstrin homology) domain and the GTPase accelerating activity of RGS9-2 were necessary for mediating specific inhibition of D2R internalization.

Galpha q allosterically activates and relieves autoinhibition of p63RhoGEF

Galpha(q) directly activates p63RhoGEF and closely related catalytic domains found in Trio and Kalirin, thereby linking G(q)-coupled receptors to the activation of RhoA. Although the crystal structure of G alpha(q) in complex with the catalytic domains of p63RhoGEF is available, the molecular mechanism of activation has not yet been defined. In this study, we show that membrane translocation does not appear to play a role in G alpha(q)-mediated activation of p63RhoGEF, as it does in some other RhoGEFs. G alpha(q) instead must act allosterically. We next identify specific structural elements in the PH domain that inhibit basal nucleotide exchange activity, and provide evidence that G alpha(q) overcomes this inhibition by altering the conformation of the alpha 6-alpha N linker that joins the DH and PH domains, a region that forms direct contacts with RhoA. We also identify residues in G alpha(q) that are important for the activation of p63RhoGEF and that contribute to G alpha subfamily selectivity, including a critical residue in the G alpha(q) C-terminal helix, and demonstrate the importance of these residues for RhoA activation in living cells.

Structure and function of regulator of G protein signaling homology domains

All regulator of G protein signaling (RGS) proteins contain a conserved domain of approximately 130 amino acids that binds to activated heterotrimeric G protein α subunits (Gα) and accelerates their rate of GTP hydrolysis. Homologous domains are found in at least six other protein families, including a family of Rho guanine nucleotide exchange factors (RhoGEFs) and the G protein-coupled receptor kinases (GRKs). Although some of the RhoGEF and GRK RGS-like domains can also bind to activated Gα subunits, they do so in distinct ways and with much lower levels of GTPase activation. In other protein families, the domains have as of yet no obvious relationship to heterotrimeric G protein signaling. These RGS homology (RH) domains are now recognized as mediators of extraordinarily diverse protein-protein interactions. Through these interactions, they play roles that range from enzyme to molecular scaffold to signal transducing module. In this review, the atomic structures of RH domains from RGS proteins, Axins, RhoGEFs, and GRKs are compared in light of what is currently known about their functional roles.

Structural determinants of G-protein alpha subunit selectivity by regulator of G-protein signaling 2 (RGS2)

"Regulator of G-protein signaling" (RGS) proteins facilitate the termination of G protein-coupled receptor (GPCR) signaling via their ability to increase the intrinsic GTP hydrolysis rate of Galpha subunits (known as GTPase-accelerating protein or "GAP" activity). RGS2 is unique in its in vitro potency and selectivity as a GAP for Galpha(q) subunits. As many vasoconstrictive hormones signal via G(q) heterotrimer-coupled receptors, it is perhaps not surprising that RGS2-deficient mice exhibit constitutive hypertension. However, to date the particular structural features within RGS2 determining its selectivity for Galpha(q) over Galpha(i/o) substrates have not been completely characterized. Here, we examine a trio of point mutations to RGS2 that elicits Galpha(i)-directed binding and GAP activities without perturbing its association with Galpha(q). Using x-ray crystallography, we determined a model of the triple mutant RGS2 in complex with a transition state mimetic form of Galpha(i) at 2.8-A resolution. Structural comparison with unliganded, wild type RGS2 and of other RGS domain/Galpha complexes highlighted the roles of these residues in wild type RGS2 that weaken Galpha(i) subunit association. Moreover, these three amino acids are seen to be evolutionarily conserved among organisms with modern cardiovascular systems, suggesting that RGS2 arose from the R4-subfamily of RGS proteins to have specialized activity as a potent and selective Galpha(q) GAP that modulates cardiovascular function.

Sugar sensing and signaling

Plants, restricted by their environment, need to integrate a wide variety of stimuli with their metabolic activity, growth and development. Sugars, generated by photosynthetic carbon fixation, are central in coordinating metabolic fluxes in response to the changing environment and in providing cells and tissues with the necessary energy for continued growth and survival. A complex network of metabolic and hormone signaling pathways are intimately linked to diverse sugar responses. A combination of genetic, cellular and systems analyses have uncovered nuclear HXK1 (hexokinase1) as a pivotal and conserved glucose sensor, directly mediating transcription regulation, while the KIN10/11 energy sensor protein kinases function as master regulators of transcription networks under sugar and energy deprivation conditions. The involvement of disaccharide signals in the regulation of specific cellular processes and the potential role of cell surface receptors in mediating sugar signals add to the complexity. This chapter gives an overview of our current insight in the sugar sensing and signaling network and describes some of the molecular mechanisms involved.

Isolation and characterization of a novel human RGS mutant displaying gain-of-function activity

Regulator of G protein signaling (RGS) proteins play a crucial role in the adaptation of cells to stimulation by G protein-coupled receptors via heterotrimeric G proteins. Alterations in RGS function have been implicated in a wide range of disease states, leading to many researchers focusing on controlling the action of these regulatory proteins. Previous studies have centered on reducing or inhibiting the action of RGS proteins, utilizing inactive mutants or small molecular RGS inhibitors. Here we describe the isolation and characterization of a novel human RGS4 mutant which displays enhanced or gain-of-function (GOF) activity. RGS4(S30C) demonstrates GOF activity both in an in vivo yeast-based signalling pathway and in vitro against the Galpha(o1) subunit contained in an alpha(2A)-adrenoreceptor-Galpha(o1)(C351I) fusion protein. Mutational analysis of serine 30 identified a number of alternative substitutions that result in GOF activity. GOF activity was retained upon transposition of the serine 30-cysteine mutation to the equivalent serine residue in human RGS16. As with previously identified GOF mutants, RGS4(S30C/S30F/S30K) demonstrate increased steady state protein levels, however these mutants also demonstrate enhanced GAP activity through an additional mechanism distinct from the increased protein content. The identification of human RGS mutants with GOF activity may provide novel therapeutic agents for the treatment of signaling-based diseases and the ability to transpose these mutations to other human RGS proteins extends their application to multiple pathways.

Receptor-mediated activation of heterotrimeric G-proteins: current structural insights

G-protein-coupled receptors (GPCRs) serve as catalytic activators of heterotrimeric G-proteins (Galphabetagamma) by exchanging GTP for the bound GDP on the Galpha subunit. This guanine nucleotide exchange factor activity of GPCRs is the initial step in the G-protein cycle and determines the onset of various intracellular signaling pathways that govern critical physiological responses to extracellular cues. Although the structural basis for many steps in the G-protein nucleotide cycle have been made clear over the past decade, the precise mechanism for receptor-mediated G-protein activation remains incompletely defined. Given that these receptors have historically represented a set of rich drug targets, a more complete understanding of their mechanism of action should provide further avenues for drug discovery. Several models have been proposed to explain the communication between activated GPCRs and Galphabetagamma leading to the structural changes required for guanine nucleotide exchange. This review is focused on the structural biology of G-protein signal transduction with an emphasis on the current hypotheses regarding Galphabetagamma activation. We highlight several recent results shedding new light on the structural changes in Galpha that may underlie GDP release.

The G protein-coupled receptor kinase (GRK) interactome: role of GRKs in GPCR regulation and signaling

G protein-coupled receptor kinases (GRKs) and arrestins are key participants in the canonical pathways leading to phosphorylation-dependent GPCR desensitization, endocytosis, intracellular trafficking and resensitization as well as in the modulation of important intracellular signaling cascades by GPCR. Novel studies have revealed a phosphorylation-independent desensitization mechanism operating through their RGS-homology (RH) domain and the recent determination of the crystal structures of GRK2 and GRK6 has uncovered interesting details on the structure-function relationships of these kinases. Emerging evidence indicates that the activity of GRKs is tightly modulated by mechanisms including phosphorylation by different kinases and interaction with several cellular proteins such as calmodulin, caveolin or RKIP. In addition, GRKs are involved in multiple interactions with non-receptor proteins (PI3K, Akt, GIT or MEK) that point to novel GRK cellular roles. In this article, our purpose is to describe the ever increasing map of functional interactions for GRK proteins as a basis to better understand its contribution to cellular processes.

Palmitoylation and its effect on the GTPase-activating activity and conformation of RGS2

Regulator of G protein signaling (RGS) proteins act as negative regulators of G protein coupled signaling by accelerating the GTPase activity of the G proteins alpha subunits. Reversible palmitoylation, a common post-translational modification for various components of the G protein-coupled signaling pathway, plays an important role in the modulation of protein activity. RGS2 appears to act selectively to increase the GTPase activity of Gqalpha when single turnover assays are preformed in solution. However, less attention has been paid to the effects of palmitoylation of RGS2 on its conformation and GTPase-activating activity. Studies of palmitoylation on a series of RGS2 mutants in which alanine was substituted for cysteine revealed cysteine 106, 116 and 199 to be multiple putative palmitoylation sites in RGS2, the efficiency of palmitate incorporation being about 60% at each individual palmitoylation site. Palmitoylation of RGS2 inhibited the GTPase-activating activity toward a GTPase-deficient R183C mutant of Gqalpha in vitro, but mutation of cysteine 116 eliminated the inhibition of palmitoylation on GTPase-activating activity of RGS2. The effect of palmitoylation on conformation of RGS2 was examined by monitoring spectra of the intrinsic fluorescence and Circular Dichroism. The results suggested that GTPase-activating activity change of RGS2 might be related to conformational change of RGS2 upon palmitoylation. Taken together, these results provided clear and strong experimental evidence for palmitoylation sites in RGS2 as well as for effect of palmitoylation on the GTPase-activating activity and conformation of RGS2.

The selective interactions and functions of regulators of G-protein signalling

There is accumulating evidence that regulators of G-protein signalling (RGS) can have roles in signal transduction that are not related to GAP activity. Furthermore, RGSs have much more selective effects in vivo than might be anticipated from their behaviour in in vitro assays. I discuss the molecular mechanisms by which these phenomena might be explained including specific interactions between the RGS and G-protein coupled receptor, G-protein and effector.

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.