The core domain of a new retina specific RGS protein stimulates the GTPase activity of transducin in vitro

Functions of regulators of G protein signaling 16 in immunity, inflammation, and other diseases

Regulators of G protein signaling (RGS) act as guanosine triphosphatase activating proteins to accelerate guanosine triphosphate hydrolysis of the G protein α subunit, leading to the termination of the G protein-coupled receptor (GPCR) downstream signaling pathway. RGS16, which is expressed in a number of cells and tissues, belongs to one of the small B/R4 subfamilies of RGS proteins and consists of a conserved RGS structural domain with short, disordered amino- and carboxy-terminal extensions and an α-helix that classically binds and de-activates heterotrimeric G proteins. However, with the deepening of research, it has been revealed that RGS16 protein not only regulates the classical GPCR pathway, but also affects immune, inflammatory, tumor and metabolic processes through other signaling pathways including the mitogen-activated protein kinase, phosphoinositide 3-kinase/protein kinase B, Ras homolog family member A and stromal cell-derived factor 1/C-X-C motif chemokine receptor 4 pathways. Additionally, the RGS16 protein may be involved in the Hepatitis B Virus -induced inflammatory response. Therefore, given the continuous expansion of knowledge regarding its role and mechanism, the structure, characteristics, regulatory mechanisms and known functions of the small RGS proteinRGS16 are reviewed in this paper to prepare for diagnosis, treatment, and prognostic evaluation of different diseases such as inflammation, tumor, and metabolic disorders and to better study its function in other diseases.

High-resolution structure of RGS17 suggests a role for Ca2+ in promoting the GTPase-activating protein activity by RZ subfamily members

Regulator of G protein signaling (RGS) proteins are negative regulators of G protein-coupled receptor (GPCR) signaling through their ability to act as GTPase-activating proteins (GAPs) for activated Gα subunits. Members of the RZ subfamily of RGS proteins bind to activated Gα_o, Gα_z, and Gα_i1-3 proteins in the nervous system and thereby inhibit downstream pathways, including those involved in Ca²⁺-dependent signaling. In contrast to other RGS proteins, little is known about RZ subfamily structure and regulation. Herein, we present the 1.5-Å crystal structure of RGS17, the most complete and highest-resolution structure of an RZ subfamily member to date. RGS17 cocrystallized with Ca²⁺ bound to conserved positions on the predicted Gα-binding surface of the protein. Using NMR chemical shift perturbations, we confirmed that Ca²⁺ binds in solution to the same site. Furthermore, RGS17 had greater than 55-fold higher affinity for Ca²⁺ than for Mg²⁺ Finally, we found that Ca²⁺ promotes interactions between RGS17 and activated Gα and decreases the K_m for GTP hydrolysis, potentially by altering the binding mechanism between these proteins. Taken together, these findings suggest that Ca²⁺ positively regulates RGS17, which may represent a general mechanism by which increased Ca²⁺ concentration promotes the GAP activity of the RZ subfamily, leading to RZ-mediated inhibition of Ca²⁺ signaling.

Regulator of G Protein Signaling 17 as a Negative Modulator of GPCR Signaling in Multiple Human Cancers

Regulators of G protein signaling (RGS) proteins modulate G protein-coupled receptor (GPCR) signaling networks by terminating signals produced by active Gα subunits. RGS17, a member of the RZ subfamily of RGS proteins, is typically only expressed in appreciable amounts in the human central nervous system, but previous works have shown that RGS17 expression is selectively upregulated in a number of malignancies, including lung, breast, prostate, and hepatocellular carcinoma. In addition, this upregulation of RGS17 is associated with a more aggressive cancer phenotype, as increased proliferation, migration, and invasion are observed. Conversely, decreased RGS17 expression diminishes the response of ovarian cancer cells to agents commonly used during chemotherapy. These somewhat contradictory roles of RGS17 in cancer highlight the need for selective, high-affinity inhibitors of RGS17 to use as chemical probes to further the understanding of RGS17 biology. Based on current evidence, these compounds could potentially have clinical utility as novel chemotherapeutics in the treatment of lung, prostate, breast, and liver cancers. Recent advances in screening technologies to identify potential inhibitors coupled with increasing knowledge of the structural requirements of RGS-Gα protein-protein interaction inhibitors make the future of drug discovery efforts targeting RGS17 promising. This review highlights recent findings related to RGS17 as both a canonical and atypical RGS protein, its role in various human disease states, and offers insights on small molecule inhibition of RGS17.

RGS Protein Regulation of Phototransduction

First identified in yeast and worm and later in other species, the physiological importance of Regulators of G-protein Signaling (RGS) in mammals was first demonstrated at the turn of the century in mouse retinal photoreceptors, in which RGS9 is needed for timely recovery of rod phototransduction. The role of RGS in vision has also been established a synapse away in retinal depolarizing bipolar cells (DBCs), where RGS7 and RGS11 work redundantly and in a complex with Gβ5-S as GAPs for Goα in the metabotropic glutamate receptor 6 pathway situated at DBC dendritic tips. Much less is known on how RGS protein subserves vision in the rest of the visual system. The research into the roles of RGS proteins in vision holds great potential for many exciting new discoveries.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Timing is everything: GTPase regulation in phototransduction

As the molecular mechanisms of vertebrate phototransduction became increasingly clear in the 1980s, a persistent problem was the discrepancy between the slow GTP hydrolysis catalyzed by the phototransduction G protein, transducin, and the much more rapid physiological recovery of photoreceptor cells from light stimuli. Beginning with a report published in 1989, a series of studies revealed that transducin GTPase activity could approach the rate needed to explain physiological recovery kinetics in the presence of one or more factors present in rod outer segment membranes. One by one, these factors were identified, beginning with PDEγ, the inhibitory subunit of the cGMP phosphodiesterase activated by transducin. There followed the discovery of the crucial role played by the regulator of G protein signaling, RGS9, a member of a ubiquitous family of GTPase-accelerating proteins, or GAPs, for heterotrimeric G proteins. Soon after, the G protein β isoform Gβ5 was identified as an obligate partner subunit, followed by the discovery or R9AP, a transmembrane protein that anchors the RGS9 GAP complex to the disk membrane, and is essential for the localization, stability, and activity of this complex in vivo. The physiological importance of all of the members of this complex was made clear first by knockout mouse models, and then by the discovery of a human visual defect, bradyopsia, caused by an inherited deficiency in one of the GAP components. Further insights have been gained by high-resolution crystal structures of subcomplexes, and by extensive mechanistic studies both in vitro and in animal models.

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.

Gbeta5-RGS complexes co-localize with mGluR6 in retinal ON-bipolar cells

The time course of G-protein-coupled responses is largely determined by the kinetics of GTP hydrolysis by the G protein alpha subunit, which is accelerated by interaction with regulator of G-protein signaling (RGS) proteins. Light responses of ON-bipolar cells of the vertebrate retina require rapid inactivation of the G protein Galphao, which is activated in the dark by metabotropic glutamate receptor, mGluR6, in their dendritic tips. It is not yet known, however, which RGS protein(s) might be responsible for rapid inactivation kinetics. By immunofluorescence and co-immunoprecipitation, we have identified complexes of the Galphao-selective RGS proteins RGS7 and RGS11, with their obligate binding partner, Gbeta5, that are localized to the dendritic tips of murine rod and cone ON-bipolar cells, along with mGluR6. Experiments using pre- and post-synaptic markers, and a dissociated bipolar cell preparation, clearly identified the location of these complexes as the ON-bipolar cell dendritic tips and not the adjacent photoreceptor terminals or horizontal cell dendrites. In mice lacking mGluR6, the distribution of RGS11, RGS7 and Gbeta5 shifts away from the dendritic tips, implying a functional relationship with mGluR6. The precise co-localization of Gbeta5-RGS7 and Gbeta5-RGS11 with mGluR6, and the dependence of localization on the presence of mGluR6, suggests that Gbeta5-RGS7 and Gbeta5-RGS11 function specifically in the mGluR6 signal transduction pathway, where they may stimulate the GTPase activity of Galphao, thus accelerating the ON-bipolar cell light response, in a manner analogous to the acceleration of photoreceptor light responses by the Gbeta5-RGS9-1 complex.

RGS17/RGSZ2 and the RZ/A family of regulators of G-protein signaling

Regulators of G-protein signaling (RGS proteins) comprise over 20 different proteins that have been classified into subfamilies on the basis of structural homology. The RZ/A family includes RGSZ2/RGS17 (the most recently discovered member of this family), GAIP/RGS19, RGSZ1/RGS20, and the RGSZ1 variant Ret-RGS. The RGS proteins are GTPase activating proteins (GAPs) that turn off G-proteins and thus negatively regulate the signaling of G-protein coupled receptors (GPCRs). In addition, some RZ/A family RGS proteins are able to modify signaling through interactions with adapter proteins (such as GIPC and GIPN). The RZ/A proteins have a simple structure that includes a conserved amino-terminal cysteine string motif, RGS box and short carboxyl-terminal, which confer GAP activity (RGS box) and the ability to undergo covalent modification and interact with other proteins (amino-terminal). This review focuses on RGS17 and its RZ/A sibling proteins and discusses the similarities and differences among these proteins in terms of their palmitoylation, phosphorylation, intracellular localization and interactions with GPCRs and adapter proteins. The specificity of these RGS protein for different Galpha proteins and receptors, and the consequences for signaling are discussed. The tissue and brain distribution, and the evolving understanding of the roles of this family of RGS proteins in receptor signaling and brain function are highlighted.

A retinal-specific regulator of G-protein signaling interacts with Galpha(o) and accelerates an expressed metabotropic glutamate receptor 6 cascade

G(o) is the most abundant G-protein in the brain, but its regulators are essentially unknown. In retina, Galpha(o1) is obligatory in mediating the metabotropic glutamate receptor 6 (mGluR6)-initiated ON response. To identify the interactors of G(o), we conducted a yeast two-hybrid screen with constituitively active Galpha(o) as a bait. The screen frequently identified a regulator of G-protein signaling (RGS), Ret-RGS1, the interaction of which we confirmed by coimmunoprecipitation with Galpha(o) in transfected cells and in retina. Ret-RGS1 localized to the dendritic tips of ON bipolar neurons, along with mGluR6 and Galpha(o1). When Ret-RGS1 was coexpressed in Xenopus oocytes with mGluR6, Galpha(o1), and a GIRK (G-protein-gated inwardly rectifying K+) channel, it accelerated the deactivation of the channel response to glutamate in a concentration-dependent manner. Because light onset suppresses glutamate release from photoreceptors onto the ON bipolar dendrites, Ret-RGS1 should accelerate the rising phase of the light response of the ON bipolar cell. This would tend to match its kinetics to that of the OFF bipolar that arises directly from ligand-gated channels.

Albumin: a Galpha(s)-specific guanine nucleotide dissociation inhibitor and GTPase activating protein

Heterotrimeric GTP binding protein (G protein)-mediated signal transduction events are regulated by their effectors and regulators of G protein signaling (RGS) protein family. The latter proteins function as GTPase activating proteins (GAPs) for G protein alpha subunits and terminate signaling events. In a search for proteins that modulate the activity of the stimulatory G protein of adenylyl cyclase (Galpha(s)), we found that bovine serum albumin (BSA) inhibits the steady-state GTPase activity of Galpha(s), but not the inhibitory G protein (Galpha(i1)). This effect of BSA is mediated by decreasing the rate of GDP dissociation from Galpha(s) and decreasing the rate of GTP binding. Thus, BSA functions as a guanine nucleotide dissociation inhibitor for Galpha(s). Moreover, BSA also increased the intrinsic GTPase activity of Galpha(s), but not Galpha(i1), demonstrating that BSA functions as a Galpha(s)-specific GAP. Using mutants of Galpha(s) (Q227L, Q227N, R201C, and R201K), we demonstrate that BSA mediates its GAP function by modulating the ability of R201 to increase GTPase activity. Moreover, using wild-type and Q227N forms of Galpha(s), our studies demonstrate that the GDI function of BSA decreases the ability of Galpha(s) to stimulate adenylyl cyclase. These findings assign a novel function to BSA as a regulator of G protein signaling.

Promotion of G alpha i3 subunit down-regulation by GIPN, a putative E3 ubiquitin ligase that interacts with RGS-GAIP

We have isolated an RGS-GAIP interacting protein that links RGS proteins to protein degradation. GIPN (GAIP interacting protein N terminus) is a 38-kDa protein with an N-terminal leucine-rich region, a central RING finger-like domain, and a putative C-terminal transmembrane domain. GIPN binds exclusively to RGS proteins of subfamily A, RGS-GAIP, RGSZ1, and RGSZ2. The N-terminal leucine-rich region of GIPN interacts with the cysteine-rich motif of RGS-GAIP. GIPN mRNA is ubiquitously expressed, and GIPN is found on the plasma membrane of transfected HEK293 cells. Endogenous GIPN is concentrated along the basolateral plasma membrane of proximal and distal tubules in rat kidney, where many G protein-coupled receptors and some G proteins are also located. Two immunoreactive species are found in rat kidney, a 38-kDa cytosolic form and an approximately 94-kDa membrane form. GIPN shows Zn2+- and E1/E2-dependent autoubiquitination in vitro, suggesting that it has E3 ubiquitin ligase activity. Overexpression of GIPN stimulates proteasome-dependent reduction of endogenous G alpha i3 in HEK293 cells and reduces the half-life of overexpressed G alpha i3-YFP. Thus, our findings suggest that GIPN is involved in the degradation of G alpha i3 subunits via the proteasome pathway. RGS-GAIP functions as a bifunctional adaptor that binds to G alpha subunits through its RGS domain and to GIPN through its cysteine string motif.

G beta 5.RGS7 inhibits G alpha q-mediated signaling via a direct protein-protein interaction

A subfamily of regulators of G protein signaling (RGS) proteins consisting of RGS6, -7, -9, and -11 is characterized by the presence of a unique Ggamma-like domain through which they form obligatory dimers with the G protein subunit Gbeta5 in vivo. In Caenorhabditis elegans, orthologs of Gbeta5.RGS dimers are implicated in regulating both Galphai and Galphaq signaling, and in cell-based assays these dimers regulate Galphai/o- and Galphaq/11-mediated pathways. However, initial studies with purified Gbeta5.RGS6 or Gbeta5.RGS7 showed that they only serve as GTPase activating proteins for Galphao. Pull-down assays and co-immunoprecipitation with these dimers failed to detect their binding to either Galphao or Galphaq, indicating that the interaction might require additional factors present in vivo. Here, we asked if the RGS7.Gbeta5 complex binds to Galphaq using fluorescence resonance energy transfer (FRET) in transiently transfected mammalian cells. RGS7, Gbeta5, and Galpha subunits were tagged with yellow variants of green fluorescent protein. First we confirmed the functional activity of the fusion proteins by co-immunoprecipitation and also their effect on signaling. Second, we again demonstrate the interaction between RGS7 and Gbeta5 using FRET. Finally, using both FRET spectroscopy on cell suspensions and microscopy of individual cells, we showed FRET between the yellow fluorescence protein-tagged RGS7.Gbeta5 complex and cyan fluorescence protein-tagged Galphaq, indicating a direct interaction between these molecules.

Regulators of G-protein signalling: multifunctional proteins with impact on signalling in the cardiovascular system

Regulator of G-protein signalling (RGS) proteins form a superfamily of at least 25 proteins, which are highly diverse in structure, expression patterns, and function. They share a 120 amino acid homology domain (RGS domain), which exhibits GTPase accelerating activity for alpha-subunits of heterotrimeric G-proteins, and thus, are negative regulators of G-protein-mediated signalling. Based on the organisation of the Rgs genes, structural similarities, and differences in functions, they can be divided into at least six subfamilies of RGS proteins and three more families of RGS-like proteins. Many of these proteins regulate signalling processes within cells, not only via interaction with G-protein alpha-subunits, but are G-protein-regulated effectors, Gbetagamma scavenger, or scaffolding proteins in signal transduction complexes as well. The expression of at least 16 different RGS proteins in the mammalian or human myocardium have been described. A subgroup of at least eight was detected in a single atrial myocyte. The exact functions of these proteins remain mostly elusive, but RGS proteins such as RGS4 are involved in the regulation of G(i)-protein betagamma-subunit-gated K(+) channels. An up-regulation of RGS4 expression has been consistently found in human heart failure and some animal models. Evidence is increasing that the enhanced RGS4 expression counter-regulates the G(q/11)-induced signalling caused by hypertrophic stimuli. In the vascular system, RGS5 seems to be an important signalling regulator. It is expressed in vascular endothelial cells, but not in cultured smooth muscle cells. Its down-regulation, both in a model of capillary morphogenesis and in an animal model of stroke, render it a candidate gene, which may be involved in the regulation of capillary growth, angiogenesis, and in the pathophysiology of stroke.

Palmitoyl protein thioesterase 1 is targeted to the axons in neurons

Palmitoyl protein thioesterase 1 (PPT1) is a depalmitoylating enzyme whose deficiency leads to infantile neuronal ceroid lipofuscinosis. The disease is characterized by early loss of vision and massive neuronal death. Although PPT1 is expressed in many tissues, a deficiency of PPT1 damages neurons only in the cerebral and cerebellar cortexes and retina; other cell types remain relatively unaffected. We previously demonstrated that PPT1 is present in the synaptosomes and synaptic vesicles of neurons. To understand the crucial role of PPT1 for neuronal cells, we further investigated the expression and targeting of PPT1 in retinal, hippocampal, and cortical neurons during their maturation in culture. We found that PPT1 activity increases by neuronal maturation and is highest in retinal neuron cultures. In retinal neurons the expression of PPT1 precedes that of the synaptic vesicle protein 2 and synaptophysin, indicating a significant role for PPT1 in the early development of neuronal cells. We also found by quantitative confocal immunofluorescence microscopy that PPT1 is targeted preferably to axons in mature neurons, as indicated by its colocalization with the axonal marker microtubule-associated protein 1. In axons PPT1 is targeted specifically to axonal varicosities and presynaptic terminals, as indicated by its significant colocalization with growth-associated protein 43 and synaptophysin. Axonal localization of PPT1 was confirmed by double labeling with synaptophysin and postembedding immunoelectron microscopy. The polarized axonal targeting of PPT1 may well indicate a role for PPT1 in the exocytotic pathway of neurons.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

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

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

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

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

Inducible RGS2 is a cross-talk regulator for parathyroid hormone signaling in rat osteoblast-like UMR106 cells

Parathyroid hormone (PTH) activates dual signal transduction systems via Galphas and Galphaq proteins. We now report a novel mechanism by which "cross-talk" may occur between the Galphas and Galphaq signaling pathways. RGS2 (Regulator of G protein Signaling 2) mRNA was rapidly and transiently increased only by PTH analogs (PTH1-84, 1-34, 1-31, and PTHrP) that activated the Galphas-mediated cAMP/PKA signaling pathway, whereas activation of the Galphaq-mediated Ca(2+)/PKC signaling pathway by PTH3-34 had no effect on RGS2 expression. Treatment of UMR106 cells with nonPTH activators of the cAMP/PKA signaling pathway such as cholera toxin, forskolin, 8-Br-cAMP, and dibutyryl-cAMP also significantly elevated RGS2 mRNA levels, while activator of the Galphaq pathway PMA did not. Pretreatment using the Galphas signaling pathway inhibitors SQ22536 and H89 significantly blocked PTH-induced RGS2 expression, but the Galphaq signaling pathway inhibitor bisindolylmaleimide I had no effect. Therefore, RGS2 expression is governed solely by the Galphas signaling pathway. Additionally, we demonstrate for the first time that RGS2 binds to both Galphas and Galphaq subunits in their transition state (GDP/AlF(-4)-bound) forms, suggesting that RGS2 has the potential to act as a bridge between the cAMP/PKA and Ca(2+)/PKC pathways, and that it may act as a cross-talk regulator for these two PTH signaling pathways.

Binding of regulator of G protein signaling (RGS) proteins to phospholipid bilayers. Contribution of location and/or orientation to Gtpase-activating protein activity

Regulator of G protein signaling (RGS) proteins must bind membranes in an orientation that permits the protein-protein interactions necessary for regulatory activity. RGS4 binds to phospholipid surfaces in a slow, multistep process that leads to maximal GTPase-activating protein (GAP) activity. When RGS4 is added to phospholipid vesicles that contain m2 or m1 muscarinic receptor and G(i), G(z), or G(q), GAP activity increases approximately 3-fold over 4 h at 30 degrees C and more slowly at 20 degrees C. This increase in GAP activity is preceded by several other events that suggest that, after binding, optimal interaction with G protein and receptor requires reorientation of RGS4 on the membrane surface, a conformational change, or both. Binding of RGS4 is initially reversible but becomes irreversible within 5 min. Onset of irreversibility parallels initial quenching of tryptophan fluorescence (t(12) approximately 30 s). Further quenching occurs after binding has become irreversible (t(12) approximately 6 min) but is complete well before maximal GAP activity is attained. These processes all appear to be energetically driven by the amphipathic N-terminal domain of RGS4 and are accelerated by palmitoylation of cysteine residues in this region. The RGS4 N-terminal domain confers similar membrane binding behavior on the RGS domains of either RGS10 or RGSZ1.

A novel regulator of G-protein signaling bearing GAP activity for Galphai and Galphaq in megakaryocytes

The regulator of G-protein signaling (RGS) negatively regulates the alpha subunit of G proteins by accelerating their intrinsic guanosine triphosphatase (GTPase) activity. Here are reported the isolation and characterization of a novel mouse RGS, termed RGS18, which is a new member of RGS subfamily B. Northern blot analysis showed that RGS18 messenger RNA was detected predominantly in spleen and hematopoietic cells, and immunohistochemical studies demonstrated that RGS18 was expressed in megakaryocytes, platelets, granulocytes/monocytes, and, weakly, in hematopoietic stem cells, but not in lymphocytes or erythrocytes. Although various subcellular localizations of RGS have been reported, RGS18 was found to be localized in cytoplasm in megakaryocytes. In vitro binding assays of RGS18 with megakaryocyte cell lysates with or without AlF(4)(-) treatment demonstrated that RGS18 specifically binds to 2 alpha subunits of the G protein, Galphai and Galphaq. Furthermore, RGS18 clearly exhibited GTPase-activating protein (GAP) activity for Galphai and Galphaq but not for Galphas or Galpha12. In addition, chemokine stromal-derived factor 1 (SDF-1), which has been reported to stimulate megakaryocyte colony formation in the presence of thrombopoietin, affected the binding of RGS18 to Galphai but not to Galphaq. Therefore, the newly isolated RGS18 turned out to be a new member of the RGS family bearing GAP activity for Galphai, which might be stimulated by SDF-1 in megakaryocytes, as well as for Galphaq. Thus, RGS18 may play an important role in proliferation, differentiation, and/or migration of megakaryocytes.

G(z) signaling: emerging divergence from G(i) signaling

A large variety of neurotransmitters, hormones, and chemokines regulate cellular functions via cell surface receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins) belonging to the G(i) subfamily. All members of the G(i) subfamily, with the sole exception of G(z), are substrates for the pertussis toxin ADP-ribosyl transferase. G(z) also exhibits unique biochemical and regulatory properties. Initial portrayals of the cellular functions of G(z) bear high resemblance to those of other G(i) proteins both in terms of the receptors and effectors linked to G(z). However, recent discoveries have begun to insinuate a distinct role for G(z) in cellular communication. Functional interactions of the alpha subunit of G(z) (Galpha(z)) with the NKR-P1 receptor, Galpha(z)-specific regulator of G protein signaling, p21-activated kinase, G protein-regulated inducers of neurite outgrowth, and the Eya2 transcription cofactor have been demonstrated. These findings provide possible links for G(z) to participate in cellular development, survival, proliferation, differentiation and even apoptosis. In this review, we have drawn a sketch of a signaling network with G(z) as the centerpiece. The emerging picture is one that distinguishes G(z) from other members of the G(i) subfamily.

Expression levels of RGS7 and RGS4 proteins determine the mode of regulation of the G protein-activated K(+) channel and control regulation of RGS7 by G beta 5

Regulators of G protein signaling RGS4 and RGS7 accelerate the kinetics of K(+) channels (GIRKs) in the Xenopus oocyte system. Here, via quantitative analysis of RGS expression, we reveal biphasic effects of RGSs on GIRK regulation. At low concentrations, RGS4 inhibited basal GIRK activity, but stimulated it at high concentrations. RGS7, which is associated with the G protein subunit G beta 5, is regulated by G beta 5 by two distinct mechanisms. First, G beta 5 augments RGS7 activity, and second, it increases its expression. These dual effects resolve previous controversies regarding RGS4 and RGS7 function and indicate that they modulate signaling by mechanisms supplementary to their GTPase-activating protein activity.

GTPase-activating proteins for heterotrimeric G proteins: regulators of G protein signaling (RGS) and RGS-like proteins

GTPase-activating proteins (GAPs) regulate heterotrimeric G proteins by increasing the rates at which their subunits hydrolyze bound GTP and thus return to the inactive state. G protein GAPs act allosterically on G subunits, in contrast to GAPs for the Ras-like monomeric GTP-binding proteins. Although they do not contribute directly to the chemistry of GTP hydrolysis, G protein GAPs can accelerate hydrolysis >2000-fold. G protein GAPs include both effector proteins (phospholipase C-¿, p115RhoGEF) and a growing family of regulators of G protein signaling (RGS proteins) that are found throughout the animal and fungal kingdoms. GAP activity can sharpen the termination of a signal upon removal of stimulus, attenuate a signal either as a feedback inhibitor or in response to a second input, promote regulatory association of other proteins, or redirect signaling within a G protein signaling network. GAPs are regulated by various controls of their cellular concentrations, by complex interactions with G¿ or with G¿5 through an endogenous G-like domain, and by interaction with multiple other proteins.

Molecular cloning and characterization of a novel regulator of G-protein signaling from mouse hematopoietic stem cells

A novel regulator of G-protein signaling (RGS) has been isolated from a highly purified population of mouse long-term hematopoietic stem cells, and designated RGS18. It has 234 amino acids consisting of a central RGS box and short divergent NH(2) and COOH termini. The calculated molecular weight of RGS18 is 27,610 and the isoelectric point is 8.63. Mouse RGS18 is expressed from a single gene and shows tissue specific distribution. It is most highly expressed in bone marrow followed by fetal liver, spleen, and then lung. In bone marrow, RGS18 level is highest in long-term and short-term hematopoietic stem cells, and is decreased as they differentiate into more committed multiple progenitors. The human RGS18 ortholog has a tissue-specific expression pattern similar to that of mouse RGS18. Purified RGS18 interacts with the alpha subunit of both G(i) and G(q) subfamilies. The results of in vitro GTPase single-turnover assays using Galpha(i) indicated that RGS18 accelerates the intrinsic GTPase activity of Galpha(i). Transient overexpression of RGS18 attenuated inositol phosphates production via angiotensin receptor and transcriptional activation through cAMP-responsive element via M1 muscarinic receptor. This suggests RGS18 can act on G(q)-mediated signaling pathways in vivo.

Endoproteolytic processing of Sst2, a multidomain regulator of G protein signaling in yeast

Regulators of G protein signaling (RGS proteins) constitute a large family of G protein-binding proteins. All RGS proteins contain a conserved core domain that can accelerate G protein GTPase activity. In addition, many family members contain a unique N-terminal domain of unknown function. Here, we demonstrate that the RGS protein in yeast, Sst2, is proteolytically processed in vivo to yield separate but functional N-terminal and RGS core domain fragments. In whole cell lysates, the full-length SST2 product (82 kDa) as well as a prominent 36-kDa species are specifically recognized by antibodies against the C terminus of the Sst2 protein. Purification and chemical sequencing of the 36-kDa species revealed cleavage sites after Ser-414 and Ser-416, just preceding the region of RGS homology. Expression of a mutationally truncated form of the protein (C-Sst2) could not restore function to an sst2Delta mutant strain. In contrast, co-expression of C-Sst2 with the N-terminal domain (N-Sst2) partially restored the ability to regulate the growth arrest response but not the transcription induction response. Whereas the full-length protein was localized to the microsomal and plasma membrane fractions, the N-Sst2 species was predominantly in the microsomal fraction, and C-Sst2 was in the soluble fraction. Mutations that block proteasome or vacuolar protease function, or mutations in the cleavage site Ser residues of Sst2, did not alter processing. However, Sst2 processing did require expression of other components of the pheromone response pathway, including the receptor and the G protein. These results indicate that Sst2 is proteolytically processed, that this event is regulated by the signaling pathway, and that processing can profoundly alter the function and subcellular localization of the protein.

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.

Functional roles of the two domains of phosducin and phosducin-like protein

Phosducin and phosducin-like protein regulate G protein signaling pathways by binding the betagamma subunit complex (Gbetagamma) and blocking Gbetagamma association with Galpha subunits, effector enzymes, or membranes. Both proteins are composed of two structurally independent domains, each constituting approximately half of the molecule. We investigated the functional roles of the two domains of phosducin and phosducin-like protein in binding retinal G(t)betagamma. Kinetic measurements using surface plasmon resonance showed that: 1) phosducin bound G(t)betagamma with a 2. 5-fold greater affinity than phosducin-like protein; 2) phosphorylation of phosducin decreased its affinity by 3-fold, principally as a result of a decrease in k(1); and 3) most of the free energy of binding comes from the N-terminal domain with a lesser contribution from the C-terminal domain. In assays measuring the association of G(t)betagamma with G(t)alpha and light-activated rhodopsin, both N-terminal domains inhibited binding while neither of the C-terminal domains had any effect. In assays measuring membrane binding of G(t)betagamma, both the N- and C-terminal domains inhibited membrane association, but much less effectively than the full-length proteins. This inhibition could only be described by models that included a change in G(t)betagamma to a conformation that did not bind the membrane. These models yielded a free energy change of +1.5 +/- 0.25 kcal/mol for the transition from the G(t)alpha-binding to the Pd-binding conformation of G(t)betagamma.

The regulator of G protein signaling family

Regulator of G protein signaling (RGS) proteins are responsible for the rapid turnoff of G protein-coupled receptor signaling pathways. The major mechanism whereby RGS proteins negatively regulate G proteins is via the GTPase activating protein activity of their RGS domain. Structural and mutational analyses have characterized the RGS/G alpha interaction in detail, explaining the molecular mechanisms of the GTPase activating protein activity of RGS proteins. More than 20 RGS proteins have been isolated, and there are indications that specific RGS proteins regulate specific G protein-coupled receptor pathways. This specificity is probably created by a combination of cell type-specific expression, tissue distribution, intracellular localization, posttranslational modifications, and domains other than the RGS domain that link them to other signaling pathways. In this review we discuss what has been learned so far about the role of RGS proteins in regulating G protein-coupled receptor signaling and point out areas that may be fruitful for future research.

Complexes of the G protein subunit gbeta 5 with the regulators of G protein signaling RGS7 and RGS9. Characterization in native tissues and in transfected cells

A novel protein class, termed regulators of G protein signaling (RGS), negatively regulates G protein pathways through a direct interaction with Galpha subunits and stimulation of GTP hydrolysis. An RGS subfamily including RGS6, -7, -9, and -11, which contain a characteristic Ggamma -like domain, also has the unique ability to interact with the G protein beta subunit Gbeta(5). Here, we examined the behavior of Gbeta(5), RGS7, RGS9, and Galpha in tissue extracts using immunoprecipitation and conventional chromatography. Native Gbeta(5) and RGS7 from brain, as well as photoreceptor-specific Gbeta(5)L and RGS9, always co-purified as tightly associated dimers, and neither RGS-free Gbeta(5) nor Gbeta(5)-free RGS could be detected. Co-expression in COS-7 cells of Gbeta(5) dramatically increased the protein level of RGS7 and vice versa, indicating that cells maintain Gbeta(5):RGS stoichiometry in a manner similar to Gbetagamma complexes. This mechanism is non-transcriptional and is based on increased protein stability upon dimerization. Thus, analysis of native Gbeta(5)-RGS and their coupled expression argue that in vivo, Gbeta(5) and Ggamma-like domain-containing RGSs only exist as heterodimers. Native Gbeta(5)-RGS7 did not co-precipitate or co-purify with Galpha(o) or Galpha(q); nor did Gbeta(5)L-RGS9 with Galpha(t). However, in transfected cells, RGS7 and Gbeta(5)-RGS7 inhibited Galpha(q)-mediated Ca(2+) response to muscarinic M3 receptor activation. Thus, Gbeta(5)-RGS dimers differ from other RGS proteins in that they do not bind to Galpha with high affinity, but they can still inhibit G protein signaling.

Ca2+/Calmodulin reverses phosphatidylinositol 3,4, 5-trisphosphate-dependent inhibition of regulators of G protein-signaling GTPase-activating protein activity

Regulators of G protein signaling (RGS proteins) are GTPase-activating proteins (GAPs) for G(i) and/or G(q) class G protein alpha subunits. RGS GAP activity is inhibited by phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) but not by other lipid phosphoinositides or diacylglycerol. Both the negatively charged head group and long chain fatty acids (C16) are required for binding and inhibition of GAP activity. Amino acid substitutions in helix 5 within the RGS domain of RGS4 reduce binding affinity and inhibition by PIP(3) but do not affect inhibition of GAP activity by palmitoylation. Conversely, the GAP activity of a palmitoylation-resistant mutant RGS4 is inhibited by PIP(3). Calmodulin binds all RGS proteins we tested in a Ca(2+)-dependent manner but does not directly affect GAP activity. Indeed, Ca(2+)/calmodulin binds a complex of RGS4 and a transition state analog of Galpha(i1)-GDP-AlF(4)(-). Ca(2+)/calmodulin reverses PIP(3)-mediated but not palmitoylation-mediated inhibition of GAP activity. Ca(2+)/calmodulin competition with PIP(3) may provide an intracellular mechanism for feedback regulation of Ca(2+) signaling evoked by G protein-coupled agonists.

RGS4 binds to membranes through an amphipathic alpha -helix

RGS4, a mammalian GTPase-activating protein for G protein alpha subunits, requires its N-terminal 33 amino acids for plasma membrane localization and biological activity (Srinivasa, S. P., Bernstein, L. S., Blumer, K. J., and Linder, M. E. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 5584-5589). In this study, we tested the hypothesis that the N-terminal domain mediates membrane binding by forming an amphipathic alpha-helix. RGS4 bound to liposomes containing anionic phospholipids in a manner dependent on the first 33 amino acids. Circular dichroism spectroscopy of a peptide corresponding to amino acids 1-31 of RGS4 revealed that the peptide adopted an alpha-helical conformation in the presence of anionic phospholipids. Point mutations that either neutralized positive charges on the hydrophilic face or substituted polar residues on the hydrophobic face of the model helix disrupted plasma membrane targeting and biological activity of RGS4 expressed in yeast. Recombinant mutant proteins were active as GTPase-activating proteins in solution but exhibited diminished binding to anionic liposomes. Peptides corresponding to mutants with the most pronounced phenotypes were also defective in forming an alpha-helix as measured by circular dichroism spectroscopy. These results support a model for direct interaction of RGS4 with membranes through hydrophobic and electrostatic interactions of an N-terminal alpha-helix.

Molecular cloning and characterization of Xenopus RGS5

We identified six genes that encode putative RGS proteins (XRGSI-VI) in developing Xenopus embryos using PCR amplification with degenerate primers corresponding to the conserved region (RGS domain) of known RGS proteins. RT-PCR analysis revealed that mRNAs of these XRGSs are differentially expressed during embryogenesis. At stage 1, only XRGSII mRNA was detected. On the other hand, expression of XRGSVI mRNA increased apparently at stage 14 and expression of three of other XRGS (III, IV, V) elevated between stage 25 and 40. To further characterize XRGS proteins expressed in Xenopus embryos, we isolated a cDNA clone for XRGSIII. Based on determined nucleotide sequence, XRGSIII was considered as a Xenopus homologue of mammalian RGS5 (XRGS5). Genetic analysis using the pheromone response halo assay showed that expression of XRGS5 inhibits yeast response to alpha-factor, suggesting that XRGS5 negatively regulates the G-protein-mediated signaling pathway in developing Xenopus embryos.

Slowed recovery of rod photoresponse in mice lacking the GTPase accelerating protein RGS9-1

Timely deactivation of the alpha-subunit of the rod G-protein transducin (Galphat) is essential for the temporal resolution of rod vision. Regulators of G-protein signalling (RGS) proteins accelerate hydrolysis of GTP by the alpha-subunits of heterotrimeric G proteins in vitro. Several retinal RGS proteins can act in vitro as GTPase accelerating proteins (GAP) for Galphat. Recent reconstitution experiments indicate that one of these, RGS9-1, may account for much of the Galphat GAP activity in rod outer segments (ROS). Here we report that ROS membranes from mice lacking RGS9-1 hydrolyse GTP more slowly than ROS membranes from control mice. The Gbeta5-L protein that forms a complex with RGS9-1 was absent from RGS9-/- retinas, although Gbeta5-L messenger RNA was still present. The flash responses of RGS9-/- rods rose normally, but recovered much more slowly than normal. We conclude that RGS9-1, probably in a complex with Gbeta5-L, is essential for acceleration of hydrolysis of GTP by Galphat and for normal recovery of the photoresponse.

Emerging roles for RGS proteins in cell signalling

Regulators of G-protein signalling (RGS proteins) are a family of highly diverse, multifunctional signalling proteins that share a conserved 120 amino acid domain (RGS domain). RGS domains bind directly to activated Galpha subunits and act as GTPase-activating proteins (GAPs) to attenuate and/or modulate hormone and neurotransmitter receptor-initiated signalling by both Galpha-GTP and Gbetagamma. Apart from this structural domain, which is shared by all known RGS proteins, these proteins differ widely in their overall size and amino acid identity and possess a remarkable variety of structural domains and motifs. These biochemical features impart signalling functions and/or enable RGS proteins to interact with a growing list of unexpected protein-binding partners with diverse cellular roles. New appreciation for the broader cellular functions of RGS proteins challenges established models of G-protein signalling and serves to identify these proteins as central participants in receptor signalling and cell physiology.

Solution structure of human GAIP (Galpha interacting protein): a regulator of G protein signaling

The solution structure of the human protein GAIP (Galpha interacting protein), a regulator of G protein signaling, has been determined by NMR techniques. Dipolar couplings of the oriented protein in two different liquid crystal media have been used in the structure calculation. The solution structure of GAIP is compared to the crystal structure of an homologous protein from rat (RGS4) complexed to the alpha-subunit of a G protein. Some of RGS4 residues involved in the Galpha-RGS binding interface have similar orientations in GAIP (free form), indicating that upon binding these residues do not suffer conformational rearrangements, and therefore, their role does not seem to be restricted to Galpha interaction but also to RGS folding and stability. We suggest that other structural differences between the two proteins may be related to the process of binding as well as to a distinct efficiency in their respective GTPase activating function.

An RGS protein regulates the pheromone response in the fission yeast Schizosaccharomyces pombe

The rate and extent of a cell's response to an extracellular stimulus is influenced by regulators that act on the intracellular signalling machinery. Although not directly involved in propagating the intracellular signal, regulators control the activity of the proteins that transmit the signals. To understand this aspect of cell signalling, we have studied the pheromone response pathway in the fission yeast Schizosaccharomyces pombe, a relatively simple signalling system in a genetically tractable organism. We demonstrate this approach by investigating the role of Rgs1, a member of the Regulator of G protein Signalling (RGS) family of proteins. The rgs1 gene was identified through the Sz. pombe genome sequencing project (accession number Q09777) and recognized as having similarity to RGS proteins [Tesmer et al. (1997) Cell 89: 251-261], but this is the first report concerning the activity of the protein. Strains lacking rgs1 (Deltargs1) are hypersensitive to pheromone stimulation and unable to conjugate with a mating partner. Inhibition of mating occurs at a relative late stage in the process as Deltargs1 strains exhibit pheromone-dependent transcription and form shmoos. Expression of SST2 (an RGS protein that regulates pheromone signalling in the budding yeast Saccharomyces cerevisiae) overcomes the hypersensitivity of the Deltargs1 strains but fails to rescue their mating defect.

Antagonism between G(o)alpha and G(q)alpha in Caenorhabditis elegans: the RGS protein EAT-16 is necessary for G(o)alpha signaling and regulates G(q)alpha activity

To elucidate the cellular role of the heterotrimeric G protein G(o), we have taken a molecular genetic approach in Caenorhabditis elegans. We screened for suppressors of activated GOA-1 (G(o)alpha) that do not simply decrease its expression and found mutations in only two genes, sag-1 and eat-16. Animals defective in either gene display a hyperactive phenotype similar to that of goa-1 loss-of-function mutants. Double-mutant analysis indicates that both sag-1 and eat-16 act downstream of, or parallel to, G(o)alpha and negatively regulate EGL-30 (G(q)alpha) signaling. eat-16 encodes a regulator of G protein signaling (RGS) most similar to the mammalian RGS7 and RGS9 proteins and can inhibit endogenous mammalian G(q)/G(11) in COS-7 cells. Animals defective in both sag-1 and eat-16 are inviable, but reducing function in egl-30 restores viability, indicating that the lethality of the eat-16; sag-1 double mutant is due to excessive G(q)alpha activity. Analysis of these mutations indicates that the G(o) and G(q) pathways function antagonistically in C. elegans, and that G(o)alpha negatively regulates the G(q) pathway, possibly via EAT-16 or SAG-1. We propose that a major cellular role of G(o) is to antagonize signaling by G(q).

The membrane association domain of RGS16 contains unique amphipathic features that are conserved in RGS4 and RGS5

Regulators of G protein signaling (RGS proteins) modulate G protein-mediated signaling pathways by acting as GTPase-activating proteins for Gi, Gq, and G12 alpha-subunits of heterotrimeric G proteins. Although it is known that membrane association is critical for the biological activities of many RGS proteins, the mechanism underlying this requirement remains unclear. We reported recently that the NH2 terminus of RGS16 is required for its function in vivo. In this study, we show that RGS16 lacking the NH2 terminus is no longer localized to the plasma membrane as is the wild type protein, suggesting that membrane association is important for biological function. The region of amino acids 7-32 is sufficient to confer the membrane-targeting activity, of which amino acids 12-30 are predicted to adopt an amphipathic alpha-helix. Site-directed mutagenesis experiments showed that the hydrophobic residues of the nonpolar face of the helix and the strips of positively charged side chains positioned along the polar/nonpolar interface of the helix are crucial for membrane association. Subcellular fractionation by differential centrifugation followed by conditions that distinguish peripheral membrane proteins from integral ones indicate that RGS16 is a peripheral membrane protein. We show further that RGS16 membrane association does not require palmitoylation. Our results, together with other recent findings, have defined a unique membrane association domain with amphipathic features. We believe that these structural features and the mechanism of membrane association of RGS16 are likely to apply to the homologous domains in RGS4 and RGS5.

RET-RGS, a retina-specific regulator of G-protein signaling, is located in synaptic regions of the rat retina

RGS (regulators of G protein signaling) proteins negatively regulate the alpha subunit of G proteins by accelerating their intrinsic GTPase activity. In a previous work, we reported the cloning of a cDNA encoding for a new RGS protein, RET-RGS. We showed that it is specifically expressed in the retina, notably by photoreceptor cells and that it has an in vitro GAP activity on transducin. To understand the role of RET-RGS, and in particular to determine whether it regulates the phototransduction cascade in photoreceptor cells, RET-RGS was immunolocalized on rat retina sections. Whereas no labeling was detected in outer nor inner segments of photoreceptors cells, dense immunoreactive products were localized in the outer and inner plexiform layers which correspond to the regions of synaptic interplay between the different neurons of the retina including the photoreceptor cells. These results rule out a role of RET-RGS on the phototransduction cascade and suggest that it may participate in retina specific synaptic transductions.

Amino-terminal cysteine residues of RGS16 are required for palmitoylation and modulation of Gi- and Gq-mediated signaling

RGS proteins (Regulators of G protein Signaling) are a recently discovered family of proteins that accelerate the GTPase activity of heterotrimeric G protein alpha subunits of the i, q, and 12 classes. The proteins share a homologous core domain but have divergent amino-terminal sequences that are the site of palmitoylation for RGS-GAIP and RGS4. We investigated the function of palmitoylation for RGS16, which shares conserved amino-terminal cysteines with RGS4 and RGS5. Mutation of cysteine residues at residues 2 and 12 blocked the incorporation of [3H]palmitate into RGS16 in metabolic labeling studies of transfected cells or into purified RGS proteins in a cell-free palmitoylation assay. The purified RGS16 proteins with the cysteine mutations were still able to act as GTPase-activating protein for Gialpha. Inhibition or a decrease in palmitoylation did not significantly change the amount of protein that was membrane-associated. However, palmitoylation-defective RGS16 mutants demonstrated impaired ability to inhibit both Gi- and Gq-linked signaling pathways when expressed in HEK293T cells. These findings suggest that the amino-terminal region of RGS16 may affect the affinity of these proteins for Galpha subunits in vivo or that palmitoylation localizes the RGS protein in close proximity to Galpha subunits on cellular membranes.

Structural basis for the selectivity of the RGS protein, GAIP, for Galphai family members. Identification of a single amino acid determinant for selective interaction of Galphai subunits with GAIP

GAIP is a regulator of G protein signaling (RGS) that accelerates the rate of GTP hydrolysis by some G protein alpha subunits. In the present studies, we have examined the structural basis for the ability of GAIP to discriminate among members of the Galphai family. Galphai1, Galphai3, and Galphao interacted strongly with GAIP, whereas Galphai2 interacted weakly and Galphas did not interact at all. A chimeric G protein composed of a Galphai2 N terminus and a Galphai1 C terminus interacted as strongly with GAIP as native Galphai1, whereas a chimeric N-terminal Galphai1 with a Galphai2 C terminus did not interact. These results suggest that the determinants responsible for GAIP selectivity between these two Galphais reside within the C-terminal GTPase domain of the G protein. To further localize residues contributing to G protein-GAIP selectivity, a panel of 15 site-directed Galphai1 and Galphai2 mutants were assayed. Of the Galphai1 mutants tested, only that containing a mutation at aspartate 229 located at the N terminus of Switch 3 did not interact with GAIP. Furthermore, the only Galphai2 variant that interacted strongly with GAIP contained a replacement of the corresponding Galphai2 Switch 3 residue (Ala230) with aspartate. To determine whether GAIP showed functional preferences for Galpha subunits that correlate with the binding data, the ability of GAIP to enhance the GTPase activity of purified alpha subunits was tested. GAIP catalyzed a 3-5-fold increase in the rate of GTP hydrolysis by Galphai1 and Galphai2(A230D) but no increase in the rate of Galphai2 and less than a 2-fold increase in the rate of Galphai1(D229A) under the same conditions. Thus, GAIP was able to discriminate between Galphai1 and Galphai2 in both binding and functional assays, and in both cases residue 229/230 played a critical role in selective recognition.

Clathrin-coated vesicles bearing GAIP possess GTPase-activating protein activity in vitro

Galpha-interacting protein (GAIP) is a member of the RGS (regulators of G protein signaling) family, which serve as GAPs (GTPase-activating proteins) for Galpha subunits. Previously, we demonstrated that GAIP is localized on clathrin-coated vesicles (CCVs). Here, we tested whether GAIP-enriched vesicles could accelerate the GTPase activity of Galphai proteins. A rat liver fraction containing vesicular carriers (CV2) was enriched (4.5x) for GAIP by quantitative immunoblotting, and GAIP was detected on some of the vesicles in the CV2 fraction by immunoelectron microscopy. When liver fractions were added to recombinant Galphai3 and tested for GAP activity, only the CV2 fraction contained GAP activity. Increasing amounts of CV2 increased the activity, whereas immunodepletion of the CV2 fraction with an antibody against the C terminus of GAIP decreased GAP activity. CCV fractions were prepared from rat liver by using a protocol that maintains the clathrin coats. GAIP was enriched in these fractions and was detected on CCVs by immunogold labeling. Addition of increasing amounts of CCV to recombinant Galphai3 protein increased the GTPase activity. We conclude that CCVs possess GAP activity for Galphai3 and that membrane-associated GAIP is capable of interacting with Galphai3. The reconstitution of the interaction between a heterotrimeric G protein and GAIP on CCVs provides biochemical evidence for a model whereby the G protein and its GAP are compartmentalized on different membranes and come into contact at the time of vesicle fusion. Alternatively, they may be located on the same membrane and segregate at the time of vesicle budding.

RGS7 and RGS8 differentially accelerate G protein-mediated modulation of K+ currents

The recently discovered family of RGS (regulators of G protein signaling) proteins acts as GTPase activating proteins which bind to alpha subunits of heterotrimeric G proteins. We previously showed that a brain-specific RGS, RGS8 speeds up the activation and deactivation kinetics of the G protein-coupled inward rectifier K+ channel (GIRK) upon receptor stimulation (Saitoh, O., Kubo, Y., Miyatani, Y., Asano, T., and Nakata, H. (1997) Nature 390, 525-529). Here we report the isolation of a full-length rat cDNA of another brain-specific RGS, RGS7. In situ hybridization study revealed that RGS7 mRNA is predominantly expressed in Golgi cells within granule cell layer of cerebellar cortex. We observed that RGS7 recombinant protein binds preferentially to Galphao, Galphai3, and Galphaz. When co-expressed with GIRK1/2 in Xenopus oocytes, RGS7 and RGS8 differentially accelerate G protein-mediated modulation of GIRK. RGS7 clearly accelerated activation of GIRK current similarly with RGS8 but the acceleration effect of deactivation was significantly weaker than that of RGS8. These acceleration properties of RGS proteins may play important roles in the rapid regulation of neuronal excitability and the cellular responses to short-lived stimulations.

Modulation of renal tubular cell function by RGS3

The recently discovered family of regulators of G protein signaling (RGS) accelerates the intrinsic GTPase activity of certain Galpha subunits, thereby terminating G protein signaling. Particularly high mRNA levels of one family member, RGS3, are found in the adult kidney. To establish the temporal and spatial renal expression pattern of RGS3, a polyclonal antiserum was raised against the COOH terminus of RGS3. Staining of mouse renal tissue at different gestational stages revealed high levels of RGS3 within the developing and mature tubular epithelial cells. We tested whether RGS3 can modulate tubular migration, an important aspect of tubular development, in response to G protein-mediated signaling. Several mouse intermedullary collecting duct (mIMCD-3) cell lines were generated that expressed RGS3 under the control of an inducible promoter. Lysophosphatidic acid (LPA) is a potent chemoattractant that mediates its effects through heterotrimeric G proteins. We found that induction of RGS3 significantly reduced LPA-mediated cell migration in RGS3-expressing mIMCD-3 clones, whereas chemotaxis induced by hepatocyte growth factor remained unaffected by RGS3. Our findings suggest that RGS3 modulates tubular functions during renal development and in the adult kidney.

The alpha-helical domain of Galphat determines specific interaction with regulator of G protein signaling 9

RGS proteins (regulators of G protein signaling) are potent accelerators of the intrinsic GTPase activity of G protein alpha subunits (GAPs), thus controlling the response kinetics of a variety of cell signaling processes. Most RGS domains that have been studied have relatively little GTPase activating specificity especially for G proteins within the Gi subfamily. Retinal RGS9 is unique in its ability to act synergistically with a downstream effector cGMP phosphodiesterase to stimulate the GTPase activity of the alpha subunit of transducin, Galphat. Here we report another unique property of RGS9: high specificity for Galphat. The core (RGS) domain of RGS9 (RGS9) stimulates Galphat GTPase activity by 10-fold and Galphai1 GTPase activity by only 2-fold at a concentration of 10 microM. Using chimeric Galphat/Galphai1 subunits we demonstrated that the alpha-helical domain of Galphat imparts this specificity. The functional effects of RGS9 were well correlated with its affinity for activated Galpha subunits as measured by a change in fluorescence of a mutant Galphat (Chi6b) selectively labeled at Cys-210. Kd values for RGS9 complexes with Galphat and Galphai1 calculated from the direct binding and competition experiments were 185 nM and 2 microM, respectively. The gamma subunit of phosphodiesterase increases the GAP activity of RGS9. We demonstrate that this is because of the ability of Pgamma to increase the affinity of RGS9 for Galphat. A distinct, nonoverlapping pattern of RGS and Pgamma interaction with Galphat suggests a unique mechanism of effector-mediated GAP function of the RGS9.

Gbeta5 prevents the RGS7-Galphao interaction through binding to a distinct Ggamma-like domain found in RGS7 and other RGS proteins

The G protein beta subunit Gbeta5 deviates significantly from the other four members of Gbeta-subunit family in amino acid sequence and subcellular localization. To detect the protein targets of Gbeta5 in vivo, we have isolated a native Gbeta5 protein complex from the retinal cytosolic fraction and identified the protein tightly associated with Gbeta5 as the regulator of G protein signaling (RGS) protein, RGS7. Here we show that complexes of Gbeta5 with RGS proteins can be formed in vitro from the recombinant proteins. The reconstituted Gbeta5-RGS dimers are similar to the native retinal complex in their behavior on gel-filtration and cation-exchange chromatographies and can be immunoprecipitated with either anti-Gbeta5 or anti-RGS7 antibodies. The specific Gbeta5-RGS7 interaction is determined by a distinct domain in RGS that has a striking homology to Ggamma subunits. Deletion of this domain prevents the RGS7-Gbeta5 binding, although the interaction with Galpha is retained. Substitution of the Ggamma-like domain of RGS7 with a portion of Ggamma1 changes its binding specificity from Gbeta5 to Gbeta1. The interaction of Gbeta5 with RGS7 blocked the binding of RGS7 to the Galpha subunit Galphao, indicating that Gbeta5 is a specific RGS inhibitor.

RGS proteins determine signaling specificity of Gq-coupled receptors

Regulators of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Galpha subunits, thereby attenuating signaling. RGS4 is a GTPase-activating protein for Gi and Gq class alpha subunits. In the present study, we used knockouts of Gq class genes in mice to evaluate the potency and selectivity of RGS4 in modulating Ca2+ signaling transduced by different Gq-coupled receptors. RGS4 inhibited phospholipase C activity and Ca2+ signaling in a receptor-selective manner in both permeabilized cells and cells dialyzed with RGS4 through a patch pipette. Receptor-dependent inhibition of Ca2+ signaling by RGS4 was observed in acini prepared from the rat and mouse pancreas. The response of mouse pancreatic acini to carbachol was about 4- and 33-fold more sensitive to RGS4 than that of bombesin and cholecystokinin (CCK), respectively. RGS1 and RGS16 were also potent inhibitors of Gq-dependent Ca2+ signaling and acted in a receptor-selective manner. RGS1 showed approximately 1000-fold higher potency in inhibiting carbachol than CCK-dependent signaling. RGS16 was as effective as RGS1 in inhibiting carbachol-dependent signaling but only partially inhibited the response to CCK. By contrast, RGS2 inhibited the response to carbachol and CCK with equal potency. The same pattern of receptor-selective inhibition by RGS4 was observed in acinar cells from wild type and several single and double Gq class knockout mice. Thus, these receptors appear to couple Gq class alpha subunit isotypes equally. Difference in receptor selectivity of RGS proteins action indicates that regulatory specificity is conferred by interaction of RGS proteins with receptor complexes.