Expression of GTPase-deficient Gialpha2 results in translocation of cytoplasmic RGS4 to the plasma membrane

Cardiovascular GPCR regulation by regulator of G protein signaling proteins

G protein-coupled receptors (GPCRs) play pivotal roles in regulation of cardiovascular homeostasis across all vertebrate species, including humans. In terms of normal cellular function, termination of GPCR signaling via the heterotrimeric G proteins is equally (if not more) important to its stimulation. The Regulator of G protein Signaling (RGS) protein superfamily are indispensable for GPCR signaling cessation at the cell membrane, and thus, for cellular control of GPCR signaling and function. Perturbations in both activation and termination of G protein signaling underlie many examples of cardiovascular dysfunction and heart disease pathogenesis. Despite the plethora of over 30 members comprising the mammalian RGS protein superfamily, each member interacts with a specific set of second messenger pathways and GPCR types/subtypes in a tissue/cell type-specific manner. An increasing number of studies over the past two decades have provided compelling evidence for the involvement of various RGS proteins in physiological regulation of cardiovascular GPCRs and, consequently, also in the pathophysiology of several cardiovascular ailments. This chapter summarizes the current understanding of the functional roles of RGS proteins as they pertain to cardiovascular, i.e., heart, blood vessel, and platelet GPCR function, with a particular focus on their implications for chronic heart failure pathophysiology and therapy.

R4 RGS proteins as fine tuners of immature and mature hematopoietic cell trafficking

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors. They are involved in almost every physiologic process and consequently have a pivotal role in an extensive number of pathologies, including genetic, neurologic, and immune system disorders. Indeed, the vast array of GPCRs mechanisms have led to the development of a tremendous number of drug therapies and already account for about a third of marketed drugs. These receptors mediate their downstream signals primarily via G proteins. The regulators of G-protein signaling (RGS) proteins are now in the spotlight as the critical modulatory factors of active GTP-bound Gα subunits of heterotrimeric G proteins to fine-tune the biologic responses driven by the GPCRs. Also, they possess noncanonical functions by multiple mechanisms, such as protein-protein interactions. Essential roles and impacts of these RGS proteins have been revealed in physiology, including hematopoiesis and immunity, and pathologies, including asthma, cancers, and neurologic disorders. This review focuses on the largest subfamily of R4 RGS proteins and provides a brief overview of their structures and G-proteins selectivity. With particular interest, we explore and highlight, their expression in the hematopoietic system and the regulation in the engraftment of hematopoietic stem/progenitor cells (HSPCs). Distinct expression patterns of R4 RGS proteins in the hematopoietic system and their pivotal roles in stem cell trafficking pave the way for realizing new strategies for enhancing the clinical performance of hematopoietic stem cell transplantation. Finally, we discuss the exciting future trends in drug development by targeting RGS activity and expression with small molecules inhibitors and miRNA approaches.

Regulators of G protein signaling in cardiovascular function during pregnancy

G protein-coupled receptor signaling mechanisms are implicated in many aspects of cardiovascular control, and dysfunction of such signaling mechanisms is commonly associated with disease states. Investigators have identified a large number of regulator of G protein signaling (RGS) proteins that variously contribute to the modulation of intracellular second-messenger signaling kinetics. These many RGS proteins each interact with a specific set of second-messenger cascades and receptor types and exhibit tissue-specific expression patterns. Increasing evidence supports the contribution of RGS proteins, or their loss, in the pathogenesis of cardiovascular dysfunctions. This review summarizes the current understanding of the functional contributions of RGS proteins, particularly within the B/R4 family, in cardiovascular disorders of pregnancy including gestational hypertension, uterine artery dysfunction, and preeclampsia.

Regulator of G protein signaling 4 is a novel target of GATA-6 transcription factor

GATA transcription factors regulate an array of genes important in cell proliferation and differentiation. Here we report the identification of regulator of G protein signaling 4 (RGS4) as a novel target for GATA-6 transcription factor. Although three sites (a, b, c) within the proximal region of rabbit RGS4 promoter for GATA transcription factors were predicted by bioinformatics analysis, only GATA-a site (16 bp from the core TATA box) is essential for RGS4 transcriptional regulation. RT-PCR analysis demonstrated that only GATA-6 was highly expressed in rabbit colonic smooth muscle cells but GATA-4/6 were expressed in cardiac myocytes and GATA-1/2/3 expressed in blood cells. Adenovirus-mediated expression of GATA-6 but not GATA-1 significantly increased the constitutive and IL-1β-induced mRNA expression of the endogenous RGS4 in colonic smooth muscle cells. IL-1β stimulation induced GATA-6 nuclear translocation and increased GATA-6 binding to RGS4 promoter. These data suggest that GATA factor could affect G protein signaling through regulating RGS4 expression, and GATA signaling may develop as a future therapeutic target for RGS4-related diseases.

The effect of deficient muscarinic signaling on commonly reported biochemical effects in schizophrenia and convergence with genetic susceptibility loci in explaining symptom dimensions of psychosis

With the advent of DSM 5 criticism has generally centered on a lack of biological validity of the diagnostic criteria. Part of the problem in describing a nosology of psychosis is the tacit assumption of multiple genetic causes each with an incremental loading on the clinical picture that fails to differentiate a clear underlying pathophysiology of high impact. The aim of this paper is to consolidate a primary theory of deficient muscarinic signaling underlying key clinical features of schizophrenia and its regulation by several important genetic associations including neuregulin, DISC and dysbindin. Secondary reductions in markers for GABAergic function and changes in the levels of interneuron calcium binding proteins parvalbumin and calbindin can be attributed to dysfunctional muscarinic transduction. A parallel association exists for cytokine production. The convergent pathway hypothesis is likewise used to model dopaminergic and glutamatergic theories of schizophrenia. The negative symptom dimension is correlated with dysfunction of Akt and ERK transduction, a major point of convergence. The present paradigm predicts the importance of a recent finding of a deletion in a copy number variant of PLCB1 and its potential use if replicated, as one of the first testable biological markers differentiating schizophrenia from bipolar disorder and further subtyping of schizophrenia into deficit and non-deficit. Potential limitations of PLCB1 as a prospective marker are also discussed.

A physiologically required G protein-coupled receptor (GPCR)-regulator of G protein signaling (RGS) interaction that compartmentalizes RGS activity

G protein-coupled receptors (GPCRs) can interact with regulator of G protein signaling (RGS) proteins. However, the effects of such interactions on signal transduction and their physiological relevance have been largely undetermined. Ligand-bound GPCRs initiate by promoting exchange of GDP for GTP on the Gα subunit of heterotrimeric G proteins. Signaling is terminated by hydrolysis of GTP to GDP through intrinsic GTPase activity of the Gα subunit, a reaction catalyzed by RGS proteins. Using yeast as a tool to study GPCR signaling in isolation, we define an interaction between the cognate GPCR (Mam2) and RGS (Rgs1), mapping the interaction domains. This reaction tethers Rgs1 at the plasma membrane and is essential for physiological signaling response. In vivo quantitative data inform the development of a kinetic model of the GTPase cycle, which extends previous attempts by including GPCR-RGS interactions. In vivo and in silico data confirm that GPCR-RGS interactions can impose an additional layer of regulation through mediating RGS subcellular localization to compartmentalize RGS activity within a cell, thus highlighting their importance as potential targets to modulate GPCR signaling pathways.

Deletion of PLCB1 gene in schizophrenia-affected patients

A prevalence of 1% in the general population and approximately 50% concordance rate in monozygotic twins was reported for schizophrenia, suggesting that genetic predisposition affecting neurodevelopmental processes might combine with environmental risk factors. A multitude of pathways seems to be involved in the aetiology and/or pathogenesis of schizophrenia, including dopaminergic, serotoninergic, muscarinic and glutamatergic signalling. The phosphoinositide signal transduction system and related phosphoinositide-specific phospholipase C (PI-PLC) enzymes seem to represent a point of convergence in these networking pathways during the development of selected brain regions. The existence of a susceptibility locus on the short arm of chromosome 20 moved us to analyse PLCB1, the gene codifying for PI-PLC β1 enzyme, which maps on 20p12. By using interphase fluorescent in situ hybridization methodology, we found deletions of PLCB1 in orbito-frontal cortex samples of schizophrenia-affected patients.

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

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

Regulators of G protein signaling proteins as central components of G protein-coupled receptor signaling complexes

The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein-coupled receptors (GPCRs). Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs. Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins. Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor-specific signaling tasks. Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes. This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes.

Regulator of G protein signaling 4 confers selectivity to specific G proteins to modulate mu- and delta-opioid receptor signaling

In vitro studies have shown that the Regulator of G protein Signaling 4 (RGS4) interacts with the C-termini of mu- and delta-opioid receptors (mu-OR, delta-OR) (Georgoussi et al., 2006, Cell. Signal.18, 771-782). Herein we demonstrate that RGS4 associates with these receptors in living cells and forms selective complexes with Gi/Go proteins in a receptor dependent manner. This interaction occurs within the predicted fourth intracellular loop of mu, delta-ORs as part of a signaling complex consisting of the opioid receptor, activated Galpha and RGS4. RGS4 is recruited to the plasma membrane upon opioid receptor stimulation. Expression of RGS4 in HEK293 cells attenuated agonist-mediated extracellular signal regulated kinase (ERK1,2) phosphorylation for both receptors and accelerated agonist-induced internalization of the delta-OR. RGS4 lacking its N-terminal domain failed to interact with both opioid receptors and to modulate opioid receptor signaling. Our findings demonstrate that RGS4 plays a key role in G protein coupling selectivity and signaling of the mu- and delta-OmicronRs.

PLC-beta1 knockout mice as a model of disrupted cortical development and plasticity: behavioral endophenotypes and dysregulation of RGS4 gene expression

The complexity of the genetics underlying schizophrenia is highlighted by the multitude of molecular pathways that have been reported to be disrupted in the disorder including muscarinic, serotonergic, and glutamatergic signaling systems. It is of interest, therefore, that phospholipase C-beta1 (PLC-beta1) acts as a point of convergence for these pathways during cortical development and plasticity. These signaling pathways, furthermore, are susceptible to modulation by RGS4, one of the more promising candidate genes for schizophrenia. PLC-beta1 knockout mice were behaviorally assessed on tests including fear conditioning, elevated plus maze, and the Y maze. In situ hybridization was used to assess RGS4 expression. We found that PLC-beta1 knockout mice display abnormal anxiety profiles on some, but not all measures assessed, including decreased anxiety on the elevated plus maze. We also show memory impairment and a complete absence of acquisition of hippocampal-dependent fear conditioning. Furthermore, at a molecular level, we demonstrate dramatic changes in expression of RGS4 mRNA in selective regions of the PLC-beta1 knockout mouse brain, particularly the CA1 region of the hippocampus. These results validate the utility of the PLC-beta1 knockout mouse as a model of schizophrenia, including molecular and cellular evidence for disrupted cortical maturation and associated behavioral endophenotypes.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Selective interactions between Giα1 and Giα3 and the GoLoco/GPR domain of RGS14 influence its dynamic subcellular localization

RGS14 is a multifunctional protein that contains an RGS domain, which binds active Gi/o alpha-GTP, a GoLoco/GPR domain, which binds inactive Gi alpha-GDP, and a tandem Rap1/2 binding domain (RBD). Studies were initiated to determine the roles of these domains and their interactions with Gi alpha on RGS14 subcellular localization. We report that RGS14 dynamic subcellular localization in HeLa cells depends on distinct domains and selective interactions with preferred Gi alpha isoforms. RGS14 shuttles rapidly between the nucleus and cytoplasm, and associates with centrosomes during interphase and mitosis. RGS14 localization to the nucleus depends on the RGS and RBD domains, its translocation out of the nucleus depends on the GoLoco/GPR domain, and its localization to centrosomes depends on the RBD domain. Gi alpha subunits (Gi alpha1, 2 and 3) localize predominantly at the plasma membrane. RGS14 binds directly to inactive and active forms of Gi alpha1 and Gi alpha3, but not Gi alpha2, both as a purified protein and when recovered from cells. RGS14 localizes predominantly at the plasma membrane in cells with inactive Gi alpha1 and Gi alpha3, but not Gi alpha2, whereas less RGS14 associates with active Gi alpha1/3 at the plasma membrane. RGS14 binding to inactive, but not active Gi alpha1/3 also prevents association with centrosomes or nuclear localization. Removal or functional inactivation of the GoLoco/GPR domain causes RGS14 to accumulate at centrosomes and in the nucleus, but renders it insensitive to recruitment to the plasma membrane by Gi alpha1/3. These findings highlight the importance of the GoLoco/GPR domain and its interactions with Gi alpha1/3 in determining RGS14 subcellular localization and linked functions.

Mammalian RGS proteins: Multifunctional regulators of cellular signalling

Regulators of G-protein signalling (RGS) proteins are a large and diverse family initially identified as GTPase activating proteins (GAPs) of heterotrimeric G-protein Galpha-subunits. At least some can also influence Galpha activity through either effector antagonism or by acting as guanine nucleotide dissociation inhibitors (GDIs). As our understanding of RGS protein structure and function has developed, so has the realisation that they play roles beyond G-protein regulation. Such diversity of function is enabled by the variety of RGS protein structure and their ability to interact with other cellular molecules including phospholipids, receptors, effectors and scaffolds. The activity, sub-cellular distribution and expression levels of RGS proteins are dynamically regulated, providing a layer of complexity that has yet to be fully elucidated.

Differential effects of melatonin and its downstream effector PKCalpha on subcellular localization of RGS proteins

Regulators of G protein signaling (RGS) are proteins that bind specifically to activated Galpha subunits of heterotrimeric G proteins to terminate signaling by both Galpha and Gbetagamma subunits. Signal-induced RGS redistribution may affect their activity in G protein-mediated signaling. We have previously shown that melatonin and the cell permeable cGMP analog 8-bromo cGMP, which lead to protein kinase C (PKC) activation, enhanced cytoplasmic distribution of RGS10 and RGS2 in prostate carcinoma PC3-AR cells. In the present study, we transfected PC3-AR cells with myc-tagged Galphai/Galphaq specific RGS proteins RGS2, RGS4 and RGS10 and examined the effects of melatonin, 8-bromo cGMP and PKC inhibitors on their nuclear-cytoplasmic partitioning. RGS10 and RGS2 were predominantly localized in the nucleus and perinuclear regions whereas RGS4 was mostly cytoplasmic in the PC3-AR cells. Melatonin and the cell permeable cGMP analog 8-bromo cGMP, previously found to activate PKCalpha in the PC3-AR cells, enhanced cytoplasmic localization of RGS10 and RGS2 but induced nuclear accumulation of RGS4. The isozyme specific PKC inhibitor GO6976 (PKCalpha and PKCbeta1) but not hispidin (PKCbeta) negated the effects of melatonin on RGS10, RGS2 and RGS4 localization. These findings indicate that PKCalpha, a downstream effector of the melatonin receptor, differentially affects nuclear/cytoplasmic localization of both Galphai and Galphaq specific RGS proteins. These observations provide further insight into melatonin's ability to fine-tune multiple membrane G-proteins signaling in cells.

Psychostimulants, madness, memory... and RGS proteins?

The ingestion of psychostimulant drugs by humans imparts a profound sense of alertness and well-being. However, repeated use of these drugs in some individuals will induce a physiological state of dependence, characterized by compulsive behavior directed toward the acquisition and ingestion of the drug, at the expense of customary social obligations. Drugs of abuse and many other types of experiences share the ability to alter the morphology and density of neuronal dendrites and spines. Dopaminergic modulation of corticostriatal synaptic plasticity is necessary for these morphological changes. Changes in the density of dendritic spines on striatal neurons may underlie the development of this pathological pattern of drug-seeking behavior. Identifying proteins that regulate dopaminergic signaling are of value. A family of proteins, the regulators of G protein signaling (RGS) proteins, which regulate signaling from G protein-coupled receptors, such as dopamine and glutamate, may be important in this regard. By regulating corticostriatal synaptic plasticity, RGS proteins can influence presynaptic activity, neurotransmitter release, and postsynaptic depolarization and thereby play a key role in the development of this plasticity. Pharmacological agents that modify RGS activity in humans could be efficacious in ameliorating the dependence on psychostimulant drugs.

Morphine alters the selective association between mu-opioid receptors and specific RGS proteins in mouse periaqueductal gray matter

In the CNS, several regulators of G-protein signalling (RGS) modulate the activity of mu-opioid receptors. In pull-down assays performed on membranes from mouse periaqueductal gray matter (PAG), mu-opioid receptors co-precipitated with delta-opioid receptors, Gi/o/z/q proteins, and the regulators of G-protein signalling RGS4, RGS9-2, RGS14, RGSZ1 and RGSZ2. No RGS2, RGS7, RGS10 and RGS11 proteins were associated with the mu receptors in these PAG membranes. In mice, an intracerebroventricular dose of 10 nmol morphine produced acute tolerance at mu receptors but did not disrupt the co-precipitation of mu-delta receptor complexes. However, this opioid reduced by more than 50% the co-precipitation of G alpha i/o/z subunits with mu receptors, and altered their association with some of the RGS proteins at 30 min, 3 h and 24 h after its administration. The association of RGS9-2 with mu receptors diminished by 30-40% 24 h after the administration of morphine, while that of RGSZ2 and of RGSZ1 increased. Morphine treatment recruited RGS4 to the PAG membranes, and 30 min and 3 h after the opioid challenge its association with mu receptors had increased. However, 24 h after morphine administration, the co-precipitation of RGS4 had decreased by about 30%. The opioid produced no change in the membrane levels of RGS9-2, RGS14, RGSZ1 and RGSZ2. Thus, in PAG synaptosomal membranes, a dynamic and selective link exists between, mu-opioid receptors, Gi/o/z proteins and certain RGS proteins.

Regulation of the extracellular signal-regulated kinase pathway in adult myocardium: differential roles of G(q/11), Gi and G(12/13) proteins in signalling by alpha1-adrenergic, endothelin-1 and thrombin-sensitive protease-activated receptors

Using adenoviruses encoding RGS2, RGS4 and Lsc (regulator of G protein signalling (RGS) domain of p115 RhoGEF), we investigated the contributions of G(q/11), Gi and G(12/13) proteins to G protein-coupled receptor (GPCR)-mediated activation of the extracellular signal-regulated kinase (ERK) pathway in adult rat ventricular myocytes (ARVM). Exposure to phenylephrine, endothelin-1 (ET-1) or thrombin induced significant activation of ERK1/2 and their downstream target 90 kDa ribosomal S6 kinase (p90RSK), which was abolished by overexpression of RGS4 (inhibits signalling via G(q/11) and Gi) or RGS2 (inhibits signalling via G(q/11)). Pertussis toxin (inhibits signalling via Gi) only partially attenuated the activation of ERK1/2 and p90(RSK) by phenylephrine and ET-1, but abolished such activation by thrombin. Overexpression of Lsc (inhibits signalling via G(12/13)) did not affect the responses to phenylephrine and ET-1, but suppressed the activation of ERK1/2 and p90RSK by thrombin. We conclude that full activation of the ERK pathway in ARVM by alpha1-adrenergic, ET-1 and thrombin receptors requires the activation of distinct families of heterotrimeric G proteins.

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Galpha.GDP/Gbetagamma heterotrimers to promote GDP release and GTP binding, resulting in liberation of Galpha from Gbetagamma. Galpha.GTP and Gbetagamma target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Galpha and heterotrimer reformation - a cycle accelerated by 'regulators of G-protein signaling' (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) beta is activated by Galpha(q) and Gbetagamma, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Galpha nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways.

GIPC recruits GAIP (RGS19) to attenuate dopamine D2 receptor signaling

Pleiotropic G proteins are essential for the action of hormones and neurotransmitters and are activated by stimulation of G protein-coupled receptors (GPCR), which initiates heterotrimer dissociation of the G protein, exchange of GDP for GTP on its Galpha subunit and activation of effector proteins. Regulator of G protein signaling (RGS) proteins regulate this cascade and can be recruited to the membrane upon GPCR activation. Direct functional interaction between RGS and GPCR has been hypothesized. We show that recruitment of GAIP (RGS19) by the dopamine D2 receptor (D2R), a GPCR, required the scaffold protein GIPC (GAIP-interacting protein, C terminus) and that all three were coexpressed in neurons and neuroendocrine cells. Dynamic translocation of GAIP to the plasma membrane and coassembly in a protein complex in which GIPC was a required component was dictated by D2R activation and physical interactions. In addition, two different D2R-mediated responses were regulated by the GTPase activity of GAIP at the level of the G protein coupling in a GIPC-dependent manner. Since GIPC exclusively interacted with GAIP and selectively with subsets of GPCR, this mechanism may serve to sort GPCR signaling in cells that usually express a large repertoire of GPCRs, G proteins, and RGS.

Palmitoylation and Plasma Membrane Targeting of RGS7 Are Promoted by αo

Regulator of G protein signaling (RGS) proteins modulate G protein signaling by acting as GTPase-activating proteins for G protein alpha-subunits. RGS7 belongs to a subfamily of RGS proteins that exist as dimers with the G protein beta(5)-subunit. In this report, we addressed the mechanisms of plasma membrane localization of beta(5)RGS7. When expressed in human embryonic kidney 293 cells, beta(5)RGS7 was found to be cytoplasmic and soluble. Expression of alpha(o) promoted a strong redistribution of beta(5)RGS7 to the plasma membrane. Expression of alpha(q), however, failed to affect the subcellular localization of beta(5)RGS7. The constitutively active mutant alpha(o)R179C, like wild-type alpha(o), strongly recruited beta(5)RGS7 to plasma membranes; however, inactive alpha(o)G204A, RGS-insensitive alpha(o)G184S, and lipidation-deficient alpha(o)G2A were all defective in the ability to promote plasma membrane localization of beta(5)RGS7. In addition, palmitoylation of RGS7 was demonstrated, and palmitoylation required expression of alpha(o) or alpha(o)R179C. To examine potential palmitoylation sites of RGS7, several cysteines were substituted with serines. beta(5)RGS7C133S failed to localize to plasma membranes when coexpressed with alpha(o), suggesting cysteine 133 of RGS7 as a putative palmitoylation site. Finally, deletion of amino acids 76 to 128 of RGS7, which includes part of the disheveled, EGL-10, pleckstrin (DEP) domain, prevented alpha(o)-mediated plasma membrane recruitment of beta(5)RGS7. These findings are the first to demonstrate Galpha-regulated plasma membrane localization and palmitoylation of beta(5)RGS7 and suggest that membrane targeting of beta(5)RGS7 is a complex process requiring at least RGS domain-mediated interaction with alpha(o) and RGS7 palmitoylation.

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channel activation kinetics via Galphai and Galphao-coupled receptors are determined by Galpha-specific interdomain interactions that affect GDP release rates

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channels have distinct gating properties when activated by receptors coupled specifically to Galpha(o) versus Galpha(i) subunit isoforms, with Galpha(o)-coupled currents having approximately 3-fold faster agonist-evoked activation kinetics. To identify the molecular determinants in Galpha subunits mediating these kinetic differences, chimeras were constructed using pertussis toxin (PTX)-insensitive Galpha(oA) and Galpha(i2) mutant subunits (Galpha(oA(C351G)) and Galpha(i2(C352G))) and examined in PTX-treated Xenopus oocytes expressing muscarinic m2 receptors and Kir3.1/3.2a channels. These experiments revealed that the alpha-helical N-terminal region (amino acids 1-161) and the switch regions of Galpha(i2) (amino acids 162-262) both partially contribute to slowing the GIRK activation time course when compared with the Galpha(oA(C351G))-coupled response. When present together, they fully reproduce Galpha(i2(C352G))-coupled GIRK kinetics. The Galpha(i2) C-terminal region (amino acids 263-355) had no significant effect on GIRK kinetics. Complementary responses were observed with chimeras substituting the Galpha(o) switch regions into the Galpha(i2(C352G)) subunit, which partially accelerated the GIRK activation rate. The Galpha(oA)/Galpha(i2) chimera results led us to examine an interaction between the alpha-helical domain and the Ras-like domain previously implicated in mediating a 4-fold slower in vitro basal GDP release rate in Galpha(i1) compared with Galpha(o). Mutations disrupting the interdomain contact in Galpha(i2(C352G)) at either the alphaD-alphaE loop (R145A) or the switch III loop (L233Q/A236H/E240T/M241T), significantly accelerated the GIRK activation kinetics consistent with the Galpha(i2) interdomain interface regulating receptor-catalyzed GDP release rates in vivo. We propose that differences in Galpha(i) versus Galpha(o)-coupled GIRK activation kinetics are due to intrinsic differences in receptor-catalyzed GDP release that rate-limit Gbetagamma production and is attributed to heterogeneity in Galpha(i) and Galpha(o) interdomain contacts.

RGS2 binds directly and selectively to the M1 muscarinic acetylcholine receptor third intracellular loop to modulate Gq/11alpha signaling

RGS proteins serve as GTPase-activating proteins and/or effector antagonists to modulate Galpha signaling events. In live cells, members of the B/R4 subfamily of RGS proteins selectively modulate G protein signaling depending on the associated receptor (GPCR). Here we examine whether GPCRs selectively recruit RGS proteins to modulate linked G protein signaling. We report the novel finding that RGS2 binds directly to the third intracellular (i3) loop of the G(q/11)-coupled M1 muscarinic cholinergic receptor (M1 mAChR; M1i3). This interaction is selective because closely related RGS16 does not bind M1i3, and neither RGS2 nor RGS16 binds to the G(i/o)-coupled M2i3 loop. When expressed in cells, RGS2 and M1 mAChR co-localize to the plasma membrane whereas RGS16 does not. The N-terminal region of RGS2 is both necessary and sufficient for binding to M1i3, and RGS2 forms a stable heterotrimeric complex with both activated G(q)alpha and M1i3. RGS2 potently inhibits M1 mAChR-mediated phosphoinositide hydrolysis in cell membranes by acting as an effector antagonist. Deletion of the N terminus abolishes this effector antagonist activity of RGS2 but not its GTPase-activating protein activity toward G(11)alpha in membranes. These findings predict a model where the i3 loops of GPCRs selectively recruit specific RGS protein(s) via their N termini to regulate the linked G protein. Consistent with this model, we find that the i3 loops of the mAChR subtypes (M1-M5) exhibit differential profiles for binding distinct B/R4 RGS family members, indicating that this novel mechanism for GPCR modulation of RGS signaling may generally extend to other receptors and RGS proteins.

Differentially Regulated Expression of Endogenous RGS4 and RGS7

Regulators of G protein signaling (RGS proteins) constitute a family of newly appreciated components of G protein-mediated signal transduction. With few exceptions, most information available on mammalian RGS proteins was gained by transfection/overexpression or in vitro experiments, with relatively little known about the endogenous counterparts. Transfection studies, typically of tagged RGS proteins, have been conducted to overcome the low natural abundance of endogenous RGS proteins. Because transfection studies can lead to imprecise or erroneous conclusions, we have developed antibodies of high specificity and sensitivity to focus study on endogenous proteins. Expression of both RGS4 and RGS7 was detected in rat brain tissue and cultured PC12 and AtT-20 cells. Endogenous RGS4 presented as a single 27-28-kDa protein. By contrast, cultured cells transfected with a plasmid encoding RGS4 expressed two observable forms of the protein, apparently due to utilization of distinct sites of initiation of protein synthesis. Subcellular localization of endogenous RGS4 revealed predominant association with membrane fractions, rather than in cytosolic fractions, where most heterologously expressed RGS4 has been found. Endogenous levels of RGS7 exceeded RGS4 by 30-40-fold, and studies of cultured cells revealed regulatory differences between the two proteins. We observed that RGS4 mRNA and protein were concomitantly augmented with increased cell density and decreased by exposure of PC12M cells to nerve growth factor, whereas RGS7 was unaffected. Endogenous RGS7 was relatively stable, whereas proteolysis of endogenous RGS4 was a strong determinant of its lower level expression and short half-life. Although we searched without finding evidence for regulation of RGS4 proteolysis, the possibility remains that alterations in the degradation of this protein could provide a means to promptly alter patterns of signal transduction.

The mechanism of membrane-translocation of regulator of G-protein signaling (RGS) 8 induced by Gα expression

RGS (regulators of G-protein signaling) proteins comprise a large family that modulates heterotrimeric G-protein signaling. This protein family has a common RGS domain and functions as GTPase-activating proteins for the alpha-subunits of heterotrimeric G-proteins located at the plasma membrane. RGS8 was identified as a neuron-specific RGS protein, which belongs to the B/R4 subfamily. We previously showed that RGS8 protein was translocated to the plasma membrane from the nucleus on coexpression of GTPase-deficient Galphao (GalphaoQL). Here, we first examined which subtypes of Galpha can induce the translocation of RGS8. When the Galphai family was expressed, the translocation of RGS8 did occur. To investigate the mechanism of this translocation, we generated a mutant RGS8 with reduced affinity to Galphao and an RGS-insensitive (RGS-i) mutant of GalphaoQL. Co-expression experiments with both mutants revealed that disruption of the Galpha-RGS8 interaction abolished the membrane-translocation of RGS8 despite the apparent membrane localization of RGS-i GalphaoQL. These results demonstrated that RGS8 is recruited to the plasma membrane where G-proteins are activated mainly by direct association with Galpha.

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

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

Recruitment of RGS2 and RGS4 to the plasma membrane by G proteins and receptors reflects functional interactions

N-terminally green fluorescent protein (GFP)-tagged regulator of G protein signaling (RGS) 2 and RGS4 fusion proteins expressed in human embryonic kidney 293 cells localized to the nucleus and cytosol, respectively. They were selectively recruited to the plasma membrane by G proteins and correspondingly by receptors that activate those G proteins: GFP-RGS2 when coexpressed with Galphas, beta2-adrenergic receptor, Galphaq, or AT1A angiotensin II receptor, and GFP-RGS4 when coexpressed with Galphai2 or M2 muscarinic receptor. G protein mutants with reduced RGS affinity did not produce this effect, implying that the recruitment involves direct binding to G proteins and is independent of downstream signaling events. Neither agonists nor inverse agonists altered receptor-promoted RGS association with the plasma membrane, and expressing either constitutively activated or poorly activated G protein mutants produced effects similar to those of their wild-type counterparts. Thus, intracellular interactions between these proteins seem to be relatively stable and insensitive to the activation state of the G protein, in contrast to the transient increases in RGS-G protein association known to be caused by G protein activation in solution-based assays. G protein effects on RGS localization were mirrored by RGS effects on G protein function. RGS4 was more potent than RGS2 in promoting steady-state Gi GTPase activity, whereas RGS2 inhibited Gs-dependent increases in intracellular cAMP, suggesting that G protein signaling in cells is regulated by the selective recruitment of RGS proteins to the plasma membrane.

Identification of novel ErbB3-interacting factors using the split-ubiquitin membrane yeast two-hybrid system

Analysis of membrane protein interactions is difficult because of the hydrophobic nature of these proteins, which often renders conventional biochemical and genetic assays fruitless. This is a substantial problem because proteins that are integral or associated with membranes represent approximately one-third of all proteins in a typical eukaryotic cell. We have shown previously that the modified split-ubiquitin system can be used as a genetic assay for the in vivo detection of interactions between the two characterized yeast transmembrane proteins, Ost1p and Wbp1p. This so-called split-ubiquitin membrane yeast two-hybrid (YTH) system uses the split-ubiquitin approach in which reconstitution of two ubiquitin halves is mediated by a protein-protein interaction. Here we converted the split-ubiquitin membrane YTH system into a generally applicable in vivo screening approach to identify interacting partners of a particular mammalian transmembrane protein. We have demonstrated the effectiveness of this approach by using the mammalian ErbB3 receptor as bait and have identified three previously unknown ErbB3-interacting proteins. In addition, we have confirmed one of the newly found interactions between ErbB3 and the membrane-associated RGS4 protein by coimmunoprecipitating the two proteins from human cells. We expect the split-ubiquitin membrane YTH technology to be valuable for the identification of potential interacting partners of integral membrane proteins from many model organisms.

Concerted stimulation and deactivation of pertussis toxin-sensitive G proteins by chimeric G protein-coupled receptor-regulator of G protein signaling 4 fusion proteins: analysis of the contribution of palmitoylated cysteine residues to the GAP activity of RGS4

Agonists stimulated high-affinity GTPase activity in membranes of HEK293 cells following coexpression of the alpha 2A-adrenoceptor and a pertussis toxin-resistant mutant of Go1 alpha. Enzyme kinetic analysis of Vmax and Km failed to detect regulation of the effect of agonist by a GTPase activating protein. This did occur, however, when cells were also transfected to express RGS4. Both elements of a fusion protein in which the N-terminus of RGS4 was linked to the C-terminal tail of the alpha 2A-adrenoceptor were functional, as it was able to provide concerted stimulation and deactivation of the G protein. By contrast, the alpha 2A-adrenoceptor-RGS4 fusion protein stimulated but did not enhance deactivation of a form of Go1 alpha that is resistant to the effects of regulator of G protein signaling (RGS) proteins. Employing this model system, mutation of Asn128 but not Asn88 eliminated detectable GTPase activating protein activity of RGS4 against Go1 alpha. Mutation of all three cysteine residues that are sites of post-translational acylation in RGS4 also eliminated GTPase activating protein activity but this was not achieved by less concerted mutation of these sites. These studies demonstrate that a fusion protein between a G protein-coupled receptor and an RGS protein is fully functional in providing both enhanced guanine nucleotide exchange and GTP hydrolysis of a coexpressed G protein. They also provide a direct means to assess, in mammalian cells, the effects of mutation of the RGS protein on function in circumstances in which the spatial relationship and orientation of the RGS to its target G protein is defined and maintained.

Identification of RGS2 and type V adenylyl cyclase interaction sites

The production of cAMP is controlled on many levels, notably at the level of cAMP synthesis by the enzyme adenylyl cyclase. We have recently identified a new regulator of adenylyl cyclase activity, RGS2, which decreases cAMP accumulation when overexpressed in HEK293 cells and inhibits the in vitro activity of types III, V, and VI adenylyl cyclase. In addition, RGS2 blocking antibodies lead to elevated cAMP levels in olfactory neurons. Here we examine the nature of the interaction between RGS2 and type V adenylyl cyclase. In HEK293 cells expressing type V adenylyl cyclase, RGS2 inhibited Galpha(s)-Q227L- or beta(2)-adrenergic receptor-stimulated cAMP accumulation. Deletion of the N-terminal 19 amino acids of RGS2 abolished its ability to inhibit cAMP accumulation and to bind adenylyl cyclase. Further mutational analysis indicated that neither the C terminus, RGS GAP activity, nor the RGS box domain is required for inhibition of adenylyl cyclase. Alanine scanning of the N-terminal amino acids of RGS2 identified three residues responsible for the inhibitory function of RGS2. Furthermore, we show that RGS2 interacts directly with the C(1) but not the C(2) domain of type V adenylyl cyclase and that the inhibition by RGS2 is independent of inhibition by Galpha(i). These results provide clear evidence for functional effects of RGS2 on adenylyl cyclase activity that adds a new dimension to an intricate signaling network.

GAIP participates in budding of membrane carriers at the trans-Golgi network

Galpha interacting protein (GAIP) is a regulator of G protein signaling protein that associates dynamically with vesicles and has been implicated in membrane trafficking, although its specific role is not yet known. Using an in vitro budding assay, we show that GAIP is recruited to a specific population of trans-Golgi network-derived vesicles and that these are distinct from coatomer or clathrin-coated vesicles. A truncation mutant (NT-GAIP) encoding only the N-terminal half of GAIP is recruited to trans-Golgi network membranes during the formation of vesicle carriers. Overexpression of NT-GAIP induces the formation of long, coated tubules, which are stabilized by microtubules. Results from the budding assay and from imaging in live cells show that these tubules remain attached to the Golgi stack rather than being released as carrier vesicles. NT-GAIP expression blocks membrane budding and results in the accumulation of tubular carrier intermediates. NT-GAIP-decorated tubules are competent to load vesicular stomatitis virus protein G-green fluorescent protein as post-Golgi, exocytic cargo and in cells expressing NT-GAIP there is reduced surface delivery of vesicular stomatitis virus protein G-green fluorescent protein. We conclude that GAIP functions as an essential part of the membrane budding machinery for a subset of post-Golgi exocytic carriers derived from the trans-Golgi network.

RGS13 regulates germinal center B lymphocytes responsiveness to CXC chemokine ligand (CXCL)12 and CXCL13

Normal lymphoid tissue development and function depend upon directed cell migration. Providing guideposts for cell movement and positioning within lymphoid tissues, chemokines signal through cell surface receptors that couple to heterotrimeric G proteins, which are in turn subject to regulation by regulator of G protein signaling (RGS) proteins. In this study, we report that germinal center B lymphocytes and thymic epithelial cells strongly express one of the RGS family members, RGS13. Located between Rgs1 and Rgs2, Rgs13 spans 42 kb on mouse chromosome 1. Rgs13 encodes a 157-aa protein that shares 82% amino acid identity with its 159-aa human counterpart. In situ hybridization with sense and antisense probes localized Rgs13 expression to the germinal center regions of mouse spleens and Peyer's patches and to the thymus medulla. Affinity-purified RGS13 Abs detected RGS13-expressing cells in the light zone of the germinal center. RGS13 interacted with both Gialpha and Gqalpha and strongly impaired signaling through G(i)-linked signaling pathways, including signaling through the chemokine receptors CXCR4 and CXCR5. Prolonged CD40 signaling up-regulated RGS13 expression in human tonsil B lymphocytes. These results plus previous studies of RGS1 indicate the germinal center B cells use two RGS proteins, RGS1 and RGS13, to regulate their responsiveness to chemokines.

R9AP, a membrane anchor for the photoreceptor GTPase accelerating protein, RGS9-1

The regulator of G protein signaling (RGS)-9-1.G(beta 5) complex forms the GTPase accelerating protein for G(alpha t) in vertebrate photoreceptors. Although the complex is soluble when expressed in vitro, extraction of the endogenous protein from membranes requires detergents. The detergent extracts contain a complex of RGS9-1, G(beta 5), G(alpha t), and a 25-kDa phosphoprotein, R9AP (RGS9-1-Anchor Protein). R9AP is encoded by one intronless gene in both human and mouse. Full or partial cDNA or genomic clones were obtained from mice, cattle, human, zebrafish, and Xenopus laevis. R9AP mRNA was detected only in the retina, and the protein only in photoreceptors. R9AP binds to the N-terminal domain of RGS9-1, and anchors it to the disk membrane via a C-terminal transmembrane helix.

Dependence of RGS9-1 membrane attachment on its C-terminal tail

RGS9-1 is a GTPase-accelerating protein (GAP) required for rapid recovery of the light response in vertebrate rod and cone photoreceptors. Similar to its phototransduction partners transducin (G(t)) and cGMP phosphodiesterase, it is a peripheral protein of the disc membranes, but it binds membranes much more tightly. It lacks the lipid modifications found on G(t) and cGMP phosphodiesterase, and the mechanism for membrane attachment is unknown. We have used limited proteolysis to generate a fragment of RGS9-1 that is readily removed from membranes under moderate salt conditions. Immunoblots reveal that this soluble fragment lacks a 3-kDa fragment from the C-terminal domain, the only domain within RGS9-1 that differs in sequence from the brain-specific isoform RGS9-2. Recombinant fragments of RGS9-1 with or without the partner subunit G beta(5L) were constructed with or without the C-terminal domain. Those lacking the C-terminal domain bound to photoreceptor membranes much less tightly than those containing it. Removal by urea of G beta(5L) from endogenous or recombinant RGS9-1 bound to rod outer segment membranes left RGS9-1 tightly membrane-bound, and recombinant RGS9-1 was urea-soluble in the absence of membranes. Thus the C-terminal domain of RGS9-1 is critical for membrane binding, whereas G beta(5L) does not play an important role in membrane attachment.

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

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

Second messengers regulate RGS2 expression which is targeted to the nucleus

Regulators of G-protein Signaling (RGS) proteins attenuate signaling activities of G proteins, and modulation of expression appears to be a primary mechanism for regulating RGS proteins. In human astrocytoma 1321N1 cells RGS2 expression was increased by activation of muscarinic receptors coupled to phosphoinositide signaling with carbachol, or by increased cyclic AMP production, demonstrating that both signaling systems can increase the expression of a RGS family member in a single cell type. Carbachol-stimulated increases in endogenous RGS2 protein levels appeared by immunocytochemical analysis to be largely confined to the nucleus, and this localization was confirmed by Western blot analysis which showed increased nuclear, but not cytosolic, RGS2 after carbachol treatment. Additionally, transiently expressed green fluorescent protein (GFP)-tagged, 6xHis-tagged, or unmodified RGS2 resulted in a predominant nuclear localization, as well as a distinct accumulation of RGS2 along the plasma membrane. The intranuclear localization of GFP-RGS2 was confirmed with confocal microscopy. Thus, RGS2 expression is rapidly and transiently increased by phosphoinositide signaling and by cyclic AMP, and endogenous and transfected RGS2 is largely, although not entirely, localized in the nucleus.

Up-regulation of RGS4 mRNA by opioid receptor agonists in PC12 cells expressing cloned mu- or kappa-opioid receptors

The regulators of G-protein signaling (RGS) proteins have been shown to modulate the function of some heterotrimeric G-proteins by stimulating the GTPase activity of G-protein alpha subunits. In this study, by northern blotting analysis, we investigated the regulation of RGS4 mRNA by opioid receptor agonists in PC12 cells stably expressing either cloned mu- or kappa-opioid receptors. Treatment with respective opioid receptor agonists (mu: morphine) and [D-Ala(2), MePhe(4), Gly(ol)(5)] enkephalin (DAMGO), kappa: (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro-(4,5)dec-8-y1]benzeneacetamide (U69,593)) for 0.5-24 h significantly and transiently increased the expression of RGS4 mRNA by 140-170% of the control level in a concentration-dependent manner which peaked when treated for 2 h, while treatment of non-transfected PC12 cells with opioid receptor agonists did not. The up-regulation of RGS4 mRNA was significantly blocked by co-treatment with respective opioid antagonists (mu: naloxone, kappa: norbinaltorphimine) or pretreatment with pertussis toxin. These results suggest that the activation of mu- or kappa-opioid receptors increases RGS4 mRNA level, which might contribute to opioid desentilization.

Profile of RGS expression in single rat atrial myocytes

'Regulators of G protein signaling' (RGS proteins) are members of a large family of GTPase-activating proteins that are differentially expressed in various cell types and accelerate the termination of heterotrimeric G protein signaling. To identify RGS proteins that may affect autonomic regulation of atrial excitability, we screened the expression of nineteen RGS genes (RGS subfamilies A, B, C, and D) in single spontaneously beating rat atrial myocytes maintained in primary culture. Expression profiling by single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that seven distinct RGS genes are endogenously expressed in atrial myocytes which were also identified in poly(A)(+) mRNA from rat atria (RGS2, RGS3, RGS4, RGS6, RGS10, GAIP, and RGSZ2). Other RGS transcripts were detected in atrial poly(A)(+) mRNA but not single atrial myocytes (RGS5, RGS12, RGS16, and RGS18), and therefore are likely to originate from non-myocyte sources in atrial tissue. The single-cell RT-PCR experiments also led to the identification of putative splice variants for RGS6 and GAIP. Immunocytochemistry using RGS-specific antibodies confirmed the presence of selected RGS proteins in the cultured atrial myocytes. These results demonstrate a rich diversity of RGS expression in atrial myocytes whose specific role in G-protein signaling is yet to be determined. The identification of endogenous RGS proteins in atrial myocytes will facilitate targeted suppression and/or deletion studies to determine how each RGS protein may affect atrial excitability and its short-term and long-term regulation by G-protein signaling events.

RGS14 is a bifunctional regulator of Galphai/o activity that exists in multiple populations in brain

Members of the regulators of G protein signaling (RGS) family modulate Galpha-directed signals as a result of the GTPase-activating protein (GAP) activity of their conserved RGS domain. In addition to its RGS domain, RGS14 contains a Rap binding domain (RBD) and a GoLoco motif. To define the cellular and biochemical properties of RGS14 we utilized two different affinity purified antisera that specifically recognize recombinant and native RGS14. In brain, we observed two RGS14-like immunoreactive bands of distinct size (60 kDa and 55 kDa). Both forms are present in brain cytosol and in two, biochemically distinct, membrane subpopulations: one detergent-extractable and the other detergent-insensitive. Recombinant RGS14 binds specifically to activated Galphai/o, but not Galphaq/11, Galpha12/13, or Galphas in brain membranes. In reconstitution studies, we found that RGS14 is a non-selective GAP for Galphai1 and Galphao and that full-length RGS14 is an approximately 10-fold more potent stimulator of Galpha GTPase activity than the RGS domain alone. In contrast, neither full-length RGS14 nor the isolated RBD domain is a GAP for Rap1. RGS14 is also a highly selective guanine nucleotide dissociation inhibitor (GDI) for Galphai but not Galphao, and this activity is restricted to the C-terminus containing the GoLoco domain. These findings highlight previously unknown biochemical properties of RGS14 in brain, and provide one of the first examples of an RGS protein that is a bifunctional regulator of Galpha actions.

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

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

RGS8 protein is distributed in dendrites and cell body of cerebellar Purkinje cell

RGS8 was originally identified as an RGS protein specifically expressed in neuronally differentiated P19 cells. We generated a polyclonal antibody specific to rat RGS8 using a synthetic peptide. When nonneural cells (DDT1MF2, CHO, and NIH3T3) transfected with rat RGS8 cDNA were immuno-stained with this antibody, the RGS8 protein was mainly detected in the nuclei. Since RGS8 mRNA was exclusively expressed in Purkinje cells of the cerebellum in the rat brain, we further examined the cellular distribution of the RGS8 protein in Purkinje cells using cultured cerebellar cells and tissue sections of the cerebellum. The RGS8 protein was excluded from the nuclei and distributed in the cell body and dendrites, but not in the axons of Purkinje cells. These results demonstrate the presence of a mechanism controlling the distribution of RGS8 protein in cerebellar Purkinje cells.

Mechanisms governing subcellular localization and function of human RGS2

RGS proteins negatively regulate heterotrimeric G proteins at the plasma membrane. RGS2-GFP localizes to the nucleus, plasma membrane, and cytoplasm of HEK293 cells. Expression of activated G(q) increased RGS2 association with the plasma membrane and decreased accumulation in the nucleus, suggesting that signal-induced redistribution may regulate RGS2 function. Thus, we identified and characterized a conserved N-terminal domain in RGS2 that is necessary and sufficient for plasma membrane localization. Mutational and biophysical analyses indicated that this domain is an amphipathic alpha-helix that binds vesicles containing acidic phospholipids. However, the plasma membrane targeting function of the amphipathic helical domain did not appear to be essential for RGS2 to attenuate signaling by activated G(q). Nevertheless, truncation mutants indicated that the N terminus is essential, potentially serving as a scaffold that binds receptors, signaling proteins, or nuclear components. Indeed, the RGS2 N terminus directs nuclear accumulation of GFP. Although RGS2 possesses a nuclear targeting motif, it lacks a nuclear import signal and enters the nucleus by passive diffusion. Nuclear accumulation of RGS2 does not limit its ability to attenuate G(q) signaling, because excluding RGS2 from the nucleus was without effect. RGS2 may nonetheless regulate signaling or other processes in the nucleus.

Regulator of G protein signaling 8 (RGS8) requires its NH2 terminus for subcellular localization and acute desensitization of G protein-gated K+ channels

Functional roles of the NH(2)-terminal region of RGS (regulators of G protein signaling) 8 in G protein signaling were studied. The deletion of the NH(2)-terminal region of RGS8 (DeltaNRGS8) resulted in a partial loss of the inhibitory function in pheromone response of yeasts, although Galpha binding was not affected. To examine roles in subcellular distribution, we coexpressed two fusion proteins of RGS8-RFP and DeltaNRGS8-GFP in DDT1MF2 cells. RGS8-RFP was highly concentrated in nuclei of unstimulated cells. Coexpression of constitutively active Galpha(o) resulted in translocation of RGS8 protein to the plasma membrane. In contrast, DeltaNRGS8-GFP was distributed diffusely through the cytoplasm in the presence or absence of active Galpha(o). When coexpressed with G protein-gated inwardly rectifying K(+) channels, DeltaNRGS8 accelerated both turning on and off similar to RGS8. Acute desensitization of G protein-gated inwardly rectifying K(+) current observed in the presence of RGS8, however, was not induced by DeltaNRGS8. Thus, we, for the first time, showed that the NH(2) terminus of RGS8 contributes to the subcellular localization and to the desensitization of the G protein-coupled response.

Determination of the contact energies between a regulator of G protein signaling and G protein subunits and phospholipase C beta 1

Cell signaling proteins may form functional complexes that are capable of rapid signal turnover. These contacts may be stabilized by either scaffolding proteins or multiple interactions between members of the complex. In this study, we have determined the affinities between a regulator of G protein signaling protein, RGS4, and three members of the G protein-phospholipase Cbeta (PLC-beta) signaling cascade which may allow for rapid deactivation of intracellular Ca(2+) release and activation of protein kinase C. Specifically, using fluorescence methods, we have determined the interaction energies between the RGS4, PLC-beta, G-betagamma, and both deactivated (GDP-bound) and activated (GTPgammaS-bound) Galpha(q). We find that RGS4 not only binds to activated Galpha(q), as predicted, but also to Gbetagamma and PLCbeta(1). These interactions occur through protein-protein contacts since the intrinsic membrane affinity of RGS4 was found to be very weak in the absence of the protein partner PLCbeta(1) or a lipid regulator, phosphatidylinositol-3,4,5 trisphosphate. Ternary complexes between Galpha(q), Gbetagamma and phospholipase Cbeta(1) will form, but only at relatively high protein concentrations. We propose that these interactions allow RGS4 to remain anchored to the signaling complex even in the quiescent state and allow rapid transfer to activated Galpha(q) to shut down the signal. Comparison of the relative affinities between these interacting proteins will ultimately allow us to determine whether certain complexes can form and where signals will be directed.

RGS proteins: lessons from the RGS9 subfamily

RGS proteins enhance the time resolution of G protein signaling cascades by accelerating GTP hydrolysis of G alpha subunits of heterotrimeric G proteins. RGS9-1, a photoreceptor-specific RGS protein, is the first vertebrate member of this sizeable family whose physiological function in a well-defined G protein pathway has been identified. It is essential for normal subsecond recovery kinetics of the light responses in retinal photoreceptors. Understanding this role allows RGS9-1 to serve as a useful model for understanding how specificity and regulation of RGS function are achieved. In addition to the catalytic RGS domain, shared among all members of this family, RGS9-1 contains several other domains, which are also found in a closely related subset of RGS proteins, the RGS9 subfamily. One of these domains, the G gamma-like (GGL) domain, has been identified as the attachment site for G beta 5 proteins, which act as obligate subunits for this subfamily. Results from RGS9-1 and other subfamily members suggest that specificity is achieved by cell type-specific transcription, RNA processing, and G beta 5-dependent protein stabilization. In addition, membrane localization via specific targeting domains likely plays an important role.

Regulators of G protein signaling: a bestiary of modular protein binding domains

Members of the newly discovered regulator of G protein signaling (RGS) families of proteins have a common RGS domain. This RGS domain is necessary for conferring upon RGS proteins the capacity to regulate negatively a variety of Galpha protein subunits. However, RGS proteins are more than simply negative regulators of signaling. RGS proteins can function as effector antagonists, and recent evidence suggests that RGS proteins can have positive effects on signaling as well. Many RGS proteins possess additional C- and N-terminal modular protein-binding domains and motifs. The presence of these additional modules within the RGS proteins provides for multiple novel regulatory interactions performed by these molecules. These regions are involved in conferring regulatory selectivity to specific Galpha-coupled signaling pathways, enhancing the efficacy of the RGS domain, and the translocation or targeting of RGS proteins to intracellular membranes. In other instances, these domains are involved in cross-talk between different Galpha-coupled signaling pathways and, in some cases, likely serve to integrate small GTPases with these G protein signaling pathways. This review discusses these C- and N-terminal domains and their roles in the biology of the brain-enriched RGS proteins. Methods that can be used to investigate the function of these domains are also discussed.

RGS14, a GTPase-activating protein for Gialpha, attenuates Gialpha- and G13alpha-mediated signaling pathways

Regulator of G protein signaling (RGS) proteins are a family of approximately 20 proteins that negatively regulate signaling through heterotrimeric G protein-coupled receptors. The RGS proteins act as GTPase-activating proteins (GAPs) for certain Galpha subunits and as effector antagonists for Gqalpha. Mouse RGS14 encodes a 547-amino-acid protein with an N-terminal RGS domain, which is highly expressed in lymphoid tissues. In this study, we demonstrate that RGS14 is a GAP for Gialpha subfamily members and it attenuates interleukin-8 receptor-mediated mitogen-activated protein kinase activation. However, RGS14 does not exhibit GAP activity toward Gsalpha or Gqalpha nor does it regulate Gsalpha- or Gqalpha-mediated signaling pathways. Although RGS14 does not act as a GAP for G12/13alpha, it impairs c-fos serum response element activation induced by either a constitutively active mutant of G13alpha (G13alphaQ226L) or by carbachol stimulation of muscarinic type 1 receptors. An RGS14 mutant (EN92/93AA), which does not block Gialpha-linked signaling, also inhibits serum response element activation. RGS14 localizes predominantly in the cytosol, but it can be recruited to membranes by expression of G13alphaQ226L. Although RGS14 is constitutively expressed in lymphoid cells, agents that activate B or T lymphocytes further enhance its levels. Taken together, our results suggest that signals generated after lymphocyte activation may via RGS14 directly impinge on Gialpha- or G13alpha-mediated cellular processes in lymphocytes, such as adhesion and migration.