Thinking outside of the "RGS box": new approaches to therapeutic targeting of regulators of G protein signaling

Regulators of G protein signaling (RGS) proteins are emerging as potentially important drug targets. The mammalian RGS protein family has more than 20 members and they share a common ∼120-residue RGS homology domain or "RGS box." RGS proteins regulate signaling via G protein-coupled receptors by accelerating GTPase activity at active α subunits of G proteins of the G(q) and G(i/o) families. Most studies searching for modulators of RGS protein function have been focused on inhibiting the GTPase accelerating protein activity. However, many RGS proteins contain additional domains that serve other functions, such as interactions with proteins or subcellular targeting. Here, we discuss a rationale for therapeutic targeting of RGS proteins by regulation of expression or allosteric modulation to permit either increases or decreases in RGS function. Several RGS proteins have reduced expression or function in pathophysiological states, so strategies to increase RGS function would be useful. Because several RGS proteins are rapidly degraded by the N-end rule pathway, finding ways to stabilize them may prove to be an effective way to enhance RGS protein function.

Analyzing binding data

Measuring the rate and extent of radioligand binding provides information on the number of binding sites, and their affinity and accessibility of these binding sites for various drugs. This unit explains how to design and analyze such experiments.

Reversible, allosteric small-molecule inhibitors of regulator of G protein signaling proteins

Regulators of G protein signaling (RGS) proteins are potent negative modulators of G protein signaling and have been proposed as potential targets for small-molecule inhibitor development. We report a high-throughput time-resolved fluorescence resonance energy transfer screen to identify inhibitors of RGS4 and describe the first reversible small-molecule inhibitors of an RGS protein. Two closely related compounds, typified by CCG-63802 [((2E)-2-(1,3-benzothiazol-2-yl)-3-[9-methyl-2-(3-methylphenoxy)-4-oxo-4H-pyrido[1,2-a]pyrimidin-3-yl]prop-2-enenitrile)], inhibit the interaction between RGS4 and Galpha(o) with an IC(50) value in the low micromolar range. They show selectivity among RGS proteins with a potency order of RGS 4 > 19 = 16 > 8 >> 7. The compounds inhibit the GTPase accelerating protein activity of RGS4, and thermal stability studies demonstrate binding to the RGS but not to Galpha(o). On RGS4, they depend on an interaction with one or more cysteines in a pocket that has previously been identified as an allosteric site for RGS regulation by acidic phospholipids. Unlike previous small-molecule RGS inhibitors identified to date, these compounds retain substantial activity under reducing conditions and are fully reversible on the 10-min time scale. CCG-63802 and related analogs represent a useful step toward the development of chemical tools for the study of RGS physiology.

RGS inhibition at G(alpha)i2 selectively potentiates 5-HT1A-mediated antidepressant effects

Elevating serotonin (5-HT) levels with selective serotonin reuptake inhibitors (SSRIs) is the most widely used treatment for depression. However, current therapies are ineffective, have delayed benefit, or cause side effects in many patients. Here, we define a mechanism downstream of 5-HT1A receptors that mediates antidepressant-like behavior and is profoundly and selectively enhanced by genetic disruption of regulators of G protein signaling (RGS) activity at G(alpha)i2. Animals rendered insensitive to RGS protein regulation through a mutation in G(alpha)i2 (G184S) exhibited spontaneous antidepressant- and anxiolytic-like behaviors. Mice expressing RGS-insensitive G(alpha)i2 also exhibited increased cortical and hippocampal phosphorylation of glycogen synthase kinase-3beta, a constitutively active proapoptotic kinase that is inhibited through phosphorylation in response to serotonin, SSRIs, and 5-HT1 receptor agonists. Both behavioral and biochemical phenotypes were blocked by treatment with WAY 100635, a 5-HT1A-selective antagonist. RGS-insensitive mice were also 5-10 times more responsive to the antidepressant-like effects of the SSRI fluvoxamine and 5-HT1A-selective agonist 8-hydroxy-2-dipropylaminotetralin. In contrast, the antidepressant potency of agents acting through nonserotonergic mechanisms was unchanged as was 5-HT1A action on body temperature. The findings point to a critical role for endogenous RGS proteins to suppress the antidepressant-like effects of 5-HT1A receptor activation. By selectively enhancing the beneficial effects of serotonin, inhibition of RGS proteins represents a therapeutic approach for the treatment of mood disorders.

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.

GPCR-OKB: the G Protein Coupled Receptor Oligomer Knowledge Base

SUMMARY

Rapid expansion of available data about G Protein Coupled Receptor (GPCR) dimers/oligomers over the past few years requires an effective system to organize this information electronically. Based on an ontology derived from a community dialog involving colleagues using experimental and computational methodologies, we developed the GPCR-Oligomerization Knowledge Base (GPCR-OKB). GPCR-OKB is a system that supports browsing and searching for GPCR oligomer data. Such data were manually derived from the literature. While focused on GPCR oligomers, GPCR-OKB is seamlessly connected to GPCRDB, facilitating the correlation of information about GPCR protomers and oligomers.

AVAILABILITY AND IMPLEMENTATION

The GPCR-OKB web application is freely available at http://www.gpcr-okb.org

Use of flow cytometric methods to quantify protein-protein interactions

A method is described for the quantitative analysis of protein-protein interactions using the flow cytometry protein interaction assay (FCPIA). This method is based upon immobilizing protein on a polystyrene bead, incubating these beads with a fluorescently labeled binding partner, and assessing the sample for bead-associated fluorescence in a flow cytometer. This method can be used to calculate protein-protein interaction affinities or to perform competition experiments with unlabeled binding partners or small molecules. Examples described in this protocol highlight the use of this assay in the quantification of the affinity of binding partners of the regulator of G-protein signaling protein, RGS19, in either a saturation or a competition format. An adaptation of this method that is compatible for high-throughput screening is also provided.

Design, synthesis and prostate cancer cell-based studies of analogs of the Rho/MKL1 transcriptional pathway inhibitor, CCG-1423

We recently identified bis(amide) CCG-1423 (1) as a novel inhibitor of RhoA/C-mediated gene transcription that is capable of inhibiting invasion of PC-3 prostate cancer cells in a Matrigel model of metastasis. An initial structure-activity relationship study focusing on bioisosteric replacement of the amides and conformational restriction identified two compounds, 4g and 8, with improved selectivity for inhibition of RhoA/C-mediated gene transcription and attenuated cytotoxicity relative to 1. Both compounds were also capable of inhibiting cell invasion with equal efficacy to 1 but with less attendant cytotoxicity.

Differential modulation of mu-opioid receptor signaling to adenylyl cyclase by regulators of G protein signaling proteins 4 or 8 and 7 in permeabilised C6 cells is Galpha subtype dependent

Regulators of G protein signaling (RGS) proteins act as GTPase-accelerating protein to negatively modulate G protein signaling and are defined by a conserved RGS domain with considerable amino acid diversity. To determine the effects of specific, purified RGS proteins on mu-opioid signaling, C6 cells stably expressing a mu-opioid receptor were rendered permeable to proteins by treatment with digitonin. Mu-opioid inhibition of forskolin-stimulated adenylyl cyclase by [D-Ala(2),N-Me-Phe(4),Gly-ol]-enkephalin (DAMGO), a mu-specific opioid peptide, remained fully intact in permeabilized cells. Purified RGS domain of RGS4 added to permeabilized cells resulted in a twofold loss in DAMGO potency but had no effect in cells expressing RGS-insensitive G proteins. The inhibitory effect of DAMGO was reduced to the same extent by purified RGS4 and RGS8. In contrast, the RGS domain of RGS7 had no effect and inhibited the action of RGS8 as a result of weak physical association with Galphai2 and minimal GTPase-accelerating protein activity in C6 cell membranes. These data suggest that differences in conserved RGS domains of specific RGS proteins contribute to differential regulation of opioid signaling to adenylyl cyclase and that a permeabilized cell model is useful for studying the effects of specific RGS proteins on aspects of G protein-coupled receptor signaling.

Isoflurane-induced changes in righting response and breathing are modulated by RGS proteins

BACKGROUND

Recent evidence suggests that G protein-coupled receptors, especially those linked to G(alpha)(i), contribute to the mechanisms of anesthetic action. Regulator of G protein signaling (RGS) proteins bind to activated G(alpha)(i) and inhibit signal transduction. Genomic knock-in mice with an RGS-insensitive G(alpha)(i2) G184S (G(alpha)(i2) GS) allele exhibit enhanced G(alpha)(i2) signaling and provide a novel approach for investigating the role of G(alpha)(i2) signaling and RGS proteins in general anesthesia.

METHODS

We anesthetized homozygous G(alpha)(i2) GS/GS and wild-type (WT) mice with isoflurane and quantified time (in seconds) to loss and resumption of righting response. During recovery from isoflurane anesthesia, breathing was quantified in a plethysmography chamber for both lines of mice.

RESULTS

G(alpha)(i2) GS/GS mice required significantly less time for loss of righting and significantly more time for resumption of righting than WT mice. During recovery from isoflurane anesthesia, G(alpha)(i2) GS/GS mice exhibited significantly greater respiratory depression. Poincaré analyses show that GS/GS mice have diminished respiratory variability compared with WT mice.

CONCLUSION

Modulation of G(alpha)(i2) signaling by RGS proteins alters loss and resumption of wakefulness and state-dependent changes in breathing.

Polyplexed flow cytometry protein interaction assay: a novel high-throughput screening paradigm for RGS protein inhibitors

Intracellular signaling cascades are a series of regulated protein-protein interactions that may provide a number of targets for potential drug discovery. Here, the authors examine the interaction of regulators of G-protein signaling (RGS) proteins with the G-protein Galphao, using a flow cytometry protein interaction assay (FCPIA). FCPIA accurately measures nanomolar binding constants of this protein-protein interaction and has been used in high-throughput screening. This report focuses on 5 RGS proteins (4, 6, 7, 8, and 16). To increase the content of screens, the authors assessed high-throughput screening of these RGS proteins in multiplex, by establishing binding constants of each RGS with Galphao in isolation, and then in a multiplex format with 5 RGS proteins present. To use this methodology as a higher-content multiplex protein-protein interaction screen, they established Z-factor values for RGS proteins in multiplex of 0.73 to 0.92, indicating this method is suitable for screening using FCPIA. To increase throughput, they also compressed a set of 8000 compounds by combining 4 compounds in a single assay well. Subsequent deconvolution of the compounds mixtures verified the identification of active compounds at specific RGS targets in their mixtures using the polyplexed FCPIA method.

A covalent peptide inhibitor of RGS4 identified in a focused one-bead, one compound library screen

Background

Regulators of G protein signaling (RGSs) accelerate GTP hydrolysis by Gα subunits and profoundly inhibit signaling by G protein-coupled receptors (GPCRs). The distinct expression patterns and pathophysiologic regulation of RGS proteins suggest that inhibitors may have therapeutic potential. We recently described a focused one-bead, one-compound (OBOC) library screen to identify peptide inhibitors of RGS4. Here we extend our observations to include another peptide with a different mechanism of action.

Results

Peptide 5nd (Tyr-Trp-c [Cys-Lys-Gly-Leu-Cys]-Lys-NH₂, S-S) blocks the RGS4-Gα_o interaction with an IC₅₀ of 28 μM. It forms a covalent, dithiothreitol (DTT) sensitive adduct with a mass consistent with the incorporation of one peptide per RGS. Peptide 5nd activity is abolished by either changing its disulfide bridge to a methylene dithioether bridge, which cannot form disulfide bridges to the RGS, or by removing all cysteines from the RGS protein. However, no single cysteine in RGS4 is completely necessary or sufficient for 5nd activity.

Conclusion

Though it has some RGS selectivity, 5nd appears to be a partially random cysteine modifier. These data suggest that it inhibits RGS4 by forming disulfide bridges with the protein.

Identification of thieno[3,2-b]pyrrole derivatives as novel small molecule inhibitors of neurotropic alphaviruses

Neurotropic alphaviruses such as western, eastern, and Venezuelan equine encephalitis viruses cause serious and potentially fatal central nervous system infections in humans and are high-priority potential bioterrorism agents. There are currently no widely available vaccines or licensed therapies for these virulent pathogens. To identify potential novel antiviral drugs, we developed a cell-based assay with a western equine encephalitis virus replicon that expresses a luciferase reporter gene and screened a small molecule diversity library of 51,028 compounds. We identified and validated a thieno[3,2-b]pyrrole compound with a half maximal inhibitory concentration of <10 micromol/L, a selectivity index>20, and potent activity against live virus in cultured neuronal cells. Furthermore, a structure-activity relationship analysis with 20 related compounds identified several with enhanced activity profiles, including 6 with submicromolar half maximal inhibitory concentrations. In conclusion, we have identified a novel class of promising inhibitors with potent activity against virulent neurotropic alphaviruses.

International Union of Pharmacology. LXXII. Recommendations for trace amine receptor nomenclature

Trace amines such as p-tyramine and beta-phenylethylamine are found endogenously as well as in the diet. Concomitant ingestion of these foodstuffs with monoamine oxidase inhibitors may result in the hypertensive crisis known as the "beer, wine, and cheese effect" attributed to their sympathomimetic action. Trace amines have been shown to act on one of a novel group of mammalian seven transmembrane spanning G protein-coupled receptors belonging to the rhodopsin superfamily, cloned in 2001. This receptor encoded by the human TAAR1 gene is also present in rat and mouse genomes (Taar1) and has been shown to be activated by endogenous trace amine ligands, including p-tyramine and beta-phenylethylamine. A number of drugs, most notably amphetamine and its derivatives, act as agonists at this receptor. This review proposes an official nomenclature designating TAAR1 as the trace amine 1 receptor following the convention of naming receptors after the endogenous agonist, abbreviated to TA(1) where necessary. It goes on to discuss briefly the significance of the receptor, agents acting upon it, its distribution, and currently hypothesized physiological and pathophysiological roles. In humans, a further five genes are thought to encode functional receptors (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9). TAAR3 seems to be a pseudogene in some individuals but not others. TAAR4 is a pseudogene in humans, but occurs with TAAR3 as a functional gene in rodents. Nine further genes are present in rats and mice. The endogenous ligands are not firmly established but some may respond to odorants consistent with their expression in olfactory epithelium.

GNAI2 and regulators of G protein signaling as a potential Noonan syndrome mechanism

Noonan syndrome (NS OMIM 163950) is a relatively common autosomal dominant developmental disorder characterized by short stature, specific facial features, and congenital cardiac anomalies. Approximately 50-66% of cases have defined mutations in the K-ras/Raf/MEK/ERK pathway that lead to constitutive signaling, but a significant number remain unexplained. We hypothesize that enhanced signaling through Galpha(i2) (from the GNAI2 gene) may also produce a NS-like phenotype. This is based on a recently described mouse model in which RGS-mediated inhibition of Galpha(i2) is prevented by a knock-in mutation (G184S) that blocks RGS binding [Huang et al., Mol. Cell. Biol. 2006;26:6870-9]. The mice have short body length, cardiac hypertrophy, a triangular face with wide-set eyes and ears, and hematologic alterations. There is a slight increase in ERK activation and a pronounced enhancement of PI3K/Akt phosphorylation in MEFs from these mice suggesting that abnormal increases in Galpha(i2) signaling could represent a novel upstream mechanism for NS. This suggests a novel set of candidate genes for NS (GNAI2 and RGS proteins) and if validated could have important implications for therapy as well.

Regulator of G protein signaling protein suppression of Galphao protein-mediated alpha2A adrenergic receptor inhibition of mouse hippocampal CA3 epileptiform activity

Activation of G protein-coupled alpha(2) adrenergic receptors (ARs) inhibits epileptiform activity in the hippocampal CA3 region. The specific mechanism underlying this action is unclear. This study investigated which subtype(s) of alpha(2)ARs and G proteins (Galpha(o) or Galpha(i)) are involved in this response using recordings of mouse hippocampal CA3 epileptiform bursts. Application of epinephrine (EPI) or norepinephrine (NE) reduced the frequency of bursts in a concentration-dependent manner: (-)EPI > (-)NE >>> (+)NE. To identify the alpha(2)AR subtype involved, equilibrium dissociation constants (pK(b)) were determined for the selective alphaAR antagonists atipamezole (8.79), rauwolscine (7.75), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101; 6.87), and prazosin (5.71). Calculated pK(b) values correlated best with affinities determined previously for the mouse alpha(2A)AR subtype (r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from alpha(2A)AR-but not alpha(2C)AR-knockout mice. Pretreatment with pertussis toxin also reduced the EPI-mediated inhibition of epileptiform bursts. Finally, using knock-in mice with point mutations that disrupt regulator of G protein signaling (RGS) binding to Galpha subunits to enhance signaling by that G protein, the EPI-mediated inhibition of bursts was significantly more potent in slices from RGS-insensitive Galpha(o)(G184S) heterozygous (Galpha(o)+/GS) mice compared with either Galpha(i2)(G184S) heterozygous (Galpha(i2)+/GS) or control mice (EC(50) = 2.5 versus 19 and 23 nM, respectively). Together, these findings indicate that the inhibitory effect of EPI on hippocampal CA3 epileptiform activity uses an alpha(2A)AR/Galpha(o) protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective alpha(2A)AR agonists as a novel antiepileptic drug therapy.

High-throughput screening for small-molecule inhibitors of LARG-stimulated RhoA nucleotide binding via a novel fluorescence polarization assay

Guanine nucleotide exchange factors (GEFs) stimulate guanine nucleotide exchange and the subsequent activation of Rho-family proteins in response to extracellular stimuli acting upon cytokine, tyrosine kinase, adhesion, integrin, and G-protein-coupled receptors (GPCRs). Upon Rho activation, several downstream events occur, such as morphological and cytoskeletal changes, motility, growth, survival, and gene transcription. The leukemia-associated RhoGEF (LARG) is a member of the regulators of G-protein signaling homology domain (RH) family of GEFs originally identified as a result of chromosomal translocation in acute myeloid leukemia. Using a novel fluorescence polarization guanine nucleotide-binding assay using BODIPY-Texas Red-GTPgammaS (BODIPY-TR-GTPgammaS), the authors performed a 10,000-compound high-throughput screen for inhibitors of LARG-stimulated RhoA nucleotide binding. Five compounds identified from the high-throughput screen were confirmed in a nonfluorescent radioactive guanine nucleotide-binding assay measuring LARG-stimulated [( 35)S] GTPgammaS binding to RhoA, thus ruling out nonspecific fluorescent effects. All 5 compounds selectively inhibited LARG-stimulated RhoA [( 35)S] GTPgammaS binding but had little to no effect on RhoA or Galpha( o) [(35)S] GTPgammaS binding. Therefore, these 5 compounds should serve as promising starting points for the development of small-molecule inhibitors of LARG-mediated nucleotide exchange as both pharmacological tools and therapeutics. In addition, the fluorescence polarization guanine nucleotide-binding assay described here should serve as a useful approach for both high-throughput screening and general biological applications.

A juxtamembrane mutation in the N terminus of the dopamine transporter induces preference for an inward-facing conformation

The human dopamine transporter (hDAT) regulates synaptic dopamine (DA) levels and is the site of action of abused and therapeutic drugs. Here we study the effect of a threonine residue (Thr62 in hDAT) that is highly conserved within a canonical phosphorylation site (RETW) in the juxtamembrane N-terminal region of monoamine transporters. In stably transfected human embryonic kidney 293T cells, expression of T62D-hDAT was reduced compared with hDAT or T62A-hDAT. T62D-hDAT displayed dramatically reduced [(3)H]dopamine up-take but exhibited a higher basal dopamine efflux compared with hDAT or T62A-hDAT, as determined by measurements of [(3)H]dopamine efflux and amperometry. The high constitutive efflux in T62D-hDAT precluded the measurement of amphetamine-stimulated [(3)H]dopamine efflux, but when dopamine was added internally into voltage-clamped T62D-hDAT cells, amphetamine-induced efflux comparable with hDAT was detected by amperometry. In accordance with findings that Zn(2+) can rescue reduced DA uptake in mutant transporters that are predominantly inward-facing, micromolar concentrations of Zn(2+) markedly potentiated [(3)H]dopamine uptake in T62D-hDAT and permitted the measurement of amphetamine-stimulated dopamine efflux. These results suggest that T62D-hDAT prefers an inward-facing conformation in the transition between inward- and outward-facing conformations. For T62A-hDAT, however, the measured 50% reduction in both [(3)H]dopamine uptake and [(3)H]dopamine efflux was consistent with a slowed transition between inward- and outward-facing conformations. The mechanism underlying the important functional role of Thr62 in hDAT activity suggested by these findings is examined in a structural context using dynamic simulations of a three-dimensional molecular model of DAT.

IUPHAR-DB: the IUPHAR database of G protein-coupled receptors and ion channels

The IUPHAR database (IUPHAR-DB) integrates peer-reviewed pharmacological, chemical, genetic, functional and anatomical information on the 354 nonsensory G protein-coupled receptors (GPCRs), 71 ligand-gated ion channel subunits and 141 voltage-gated-like ion channel subunits encoded by the human, rat and mouse genomes. These genes represent the targets of approximately one-third of currently approved drugs and are a major focus of drug discovery and development programs in the pharmaceutical industry. IUPHAR-DB provides a comprehensive description of the genes and their functions, with information on protein structure and interactions, ligands, expression patterns, signaling mechanisms, functional assays and biologically important receptor variants (e.g. single nucleotide polymorphisms and splice variants). In addition, the phenotypes resulting from altered gene expression (e.g. in genetically altered animals or in human genetic disorders) are described. The content of the database is peer reviewed by members of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR); the data are provided through manual curation of the primary literature by a network of over 60 subcommittees of NC-IUPHAR. Links to other bioinformatics resources, such as NCBI, Uniprot, HGNC and the rat and mouse genome databases are provided. IUPHAR-DB is freely available at http://www.iuphar-db.org.

Assembly of high order G alpha q-effector complexes with RGS proteins

Transmembrane signaling through G alpha(q)-coupled receptors is linked to physiological processes such as cardiovascular development and smooth muscle function. Recent crystallographic studies have shown how G alpha(q) interacts with two activation-dependent targets, p63RhoGEF and G protein-coupled receptor kinase 2 (GRK2). These proteins bind to the effector-binding site of G alpha(q) in a manner that does not appear to physically overlap with the site on G alpha(q) bound by regulator of G-protein signaling (RGS) proteins, which function as GTPase-activating proteins (GAPs). Herein we confirm the formation of RGS-G alpha(q)-GRK2/p63RhoGEF ternary complexes using flow cytometry protein interaction and GAP assays. RGS2 and, to a lesser extent, RGS4 are negative allosteric modulators of Galpha(q) binding to either p63RhoGEF or GRK2. Conversely, GRK2 enhances the GAP activity of RGS4 but has little effect on that of RGS2. Similar but smaller magnitude responses are induced by p63RhoGEF. The fact that GRK2 and p63RhoGEF respond similarly to these RGS proteins supports the hypothesis that GRK2 is a bona fide G alpha(q) effector. The results also suggest that signal transduction pathways initiated by GRK2, such as the phosphorylation of G protein-coupled receptors, and by p63RhoGEF, such as the activation of gene transcription, can be regulated by RGS proteins via both allosteric and GAP mechanisms.

Novel peptide ligands of RGS4 from a focused one-bead, one-compound library

Regulators of G protein signaling accelerate GTP hydrolysis by G alpha subunits and profoundly inhibit signaling by G protein-coupled receptors. The distinct expression patterns and pathophysiologic regulation of regulators of G protein signaling proteins suggest that inhibitors may have therapeutic potential. We previously reported the design, mechanistic evaluation, and structure-activity relationships of a disulfide-containing cyclic peptide inhibitor of RGS4, YJ34 (Ac-Val-Lys-c[Cys-Thr-Gly-Ile-Cys]-Glu-NH(2), S-S) (Roof et al., Chem Biol Drug Des, 67, 2006, 266). Using a focused one-bead, one-compound peptide library that contains features known to be necessary for the activity of YJ34, we now identify peptides that bind to RGS4. Six peptides showed confirmed binding to RGS4 by flow cytometry. Two analogs of peptide 2 (Gly-Thr-c[Cys-Phe-Gly-Thr-Cys]-Trp-NH(2), S-S with a free or acetylated N-terminus) inhibited RGS4-stimulated G alpha(o) GTPase activity at 25-50 microM. They selectively inhibit RGS4 but not RGS7, RGS16, and RGS19. Their inhibition of RGS4 does not depend on cysteine-modification of RGS4, as they do not lose activity when all cysteines are removed from RGS4. Peptide 2 has been modeled to fit in the same binding pocket predicted for YJ34 but in the reverse orientation.