RGS7 is palmitoylated and exists as biochemically distinct forms

Palmitoylation and G-protein coupled receptors

More and more it is being appreciated that not all GPCRs are the same, sub-populations of GPCRs exist within a cell and function differently than others. The question is, how does one regulate a given sub-population? One way is through the addition of post-translational modifications to G-protein coupled receptors (GPCR). This process has long been known to occur and play a role in trafficking, pharmacology and ultimately function. This chapter will focus on one particular modification, that of S-palmitoylation, and its impact on GPCR function. We will discuss the history of this modification on these receptors and the connection with disease. We will highlight several examples from the literature of where palmitoylation impacts GPCR function.

The interaction, mechanism and function of GPR158-RGS7 cross-talk

GPR158 is an orphan G protein-coupled receptor (GPCR) that is broadly expressed in the brain and displays unique structural characteristics and signaling mechanisms. GPR158 is a binding partner for the regulator of G protein signaling 7 (RGS7) and augments its expression, subcellular localization, and catalytic activity. Recent cryo-electron microscopy (cryo-EM) studies have revealed the structure of GPR158 alone and in complex with RGS7. The GPR158-RGS7 complex is shown to be regulated by chronic stress exposure and is a modulator of stress-induced depression. This review highlights the signaling mechanism and function of GPR158-RGS7 and provides a context for the unique formation of GPCR-RGS complexes.

14-3-3γ binds regulator of G protein signaling 14 (RGS14) at distinct sites to inhibit the RGS14:Gαi-AlF4- signaling complex and RGS14 nuclear localization

Regulator of G protein signaling 14 (RGS14) is a multifunctional brain scaffolding protein that integrates G protein and Ras/ERK signaling pathways. It is also a nucleocytoplasmic shuttling protein. RGS14 binds active Gα_i/o via its RGS domain, Raf and active H-Ras-GTP via its R1 Ras-binding domain (RBD), and inactive Gα_i1/3 via its G protein regulatory (GPR) domain. RGS14 suppresses long-term potentiation (LTP) in the CA2 region of the hippocampus, thereby regulating hippocampally based learning and memory. The 14-3-3 family of proteins is necessary for hippocampal LTP and associative learning and memory. Here, we show direct interaction between RGS14 and 14-3-3γ at two distinct sties, one phosphorylation-independent and the other phosphorylation-dependent at Ser-218 that is markedly potentiated by signaling downstream of active H-Ras. Using bioluminescence resonance energy transfer (BRET), we show that the pSer-218-dependent RGS14/14-3-3γ interaction inhibits active Gα_i1-AlF₄^- binding to the RGS domain of RGS14 but has no effect on active H-Ras and inactive Gα_i1-GDP binding to RGS14. By contrast, the phosphorylation-independent binding of 14-3-3 has no effect on RGS14/Gα_i interactions but, instead, inhibits (directly or indirectly) RGS14 nuclear import and nucleocytoplasmic shuttling. Together, our findings describe a novel mechanism of negative regulation of RGS14 functions, specifically interactions with active Gα_i and nuclear import, while leaving the function of other RGS14 domains intact. Ongoing studies will further elucidate the physiological function of this interaction between RGS14 and 14-3-3γ, providing insight into the functions of both RGS14 and 14-3-3 in their roles in modulating synaptic plasticity in the hippocampus.

Endogenous RGS14 is a cytoplasmic-nuclear shuttling protein that localizes to juxtanuclear membranes and chromatin-rich regions of the nucleus

Regulator of G protein signaling 14 (RGS14) is a multifunctional scaffolding protein that integrates G protein and H-Ras/MAPkinase signaling pathways to regulate synaptic plasticity important for hippocampal learning and memory. However, to date, little is known about the subcellular distribution and roles of endogenous RGS14 in a neuronal cell line. Most of what is known about RGS14 cellular behavior is based on studies of tagged, recombinant RGS14 ectopically overexpressed in unnatural host cells. Here, we report for the first time a comprehensive assessment of the subcellular distribution and dynamic localization of endogenous RGS14 in rat B35 neuroblastoma cells. Using confocal imaging and 3D-structured illumination microscopy, we find that endogenous RGS14 localizes to subcellular compartments not previously recognized in studies of recombinant RGS14. RGS14 localization was observed most notably at juxtanuclear membranes encircling the nucleus, at nuclear pore complexes (NPC) on both sides of the nuclear envelope and within intranuclear membrane channels, and within both chromatin-poor and chromatin-rich regions of the nucleus in a cell cycle-dependent manner. In addition, a subset of nuclear RGS14 localized adjacent to active RNA polymerase II. Endogenous RGS14 was absent from the plasma membrane in resting cells; however, the protein could be trafficked to the plasma membrane from juxtanuclear membranes in endosomes derived from ER/Golgi, following constitutive activation of endogenous RGS14 G protein binding partners using AlF₄¯. Finally, our findings show that endogenous RGS14 behaves as a cytoplasmic-nuclear shuttling protein confirming what has been shown previously for recombinant RGS14. Taken together, the findings highlight possible cellular roles for RGS14 not previously recognized that are distinct from the regulation of conventional GPCR-G protein signaling, in particular undefined roles for RGS14 in the nucleus.

A central role for R7bp in the regulation of itch sensation

Itch is a protective sensation producing a desire to scratch. Pathologic itch can be a chronic symptom of illnesses such as uremia, cholestatic liver disease, neuropathies and dermatitis, however current therapeutic options are limited. Many types of cell surface receptors, including those present on cells in the skin, on sensory neurons and on neurons in the spinal cord, have been implicated in itch signaling. The role of G protein signaling in the regulation of pruriception is poorly understood. We identify here 2 G protein signaling components whose mutation impairs itch sensation. R7bp (a.k.a. Rgs7bp) is a palmitoylated membrane anchoring protein expressed in neurons that facilitates Gαi/o -directed GTPase activating protein activity mediated by the Gβ5/R7-RGS complex. Knockout of R7bp diminishes scratching responses to multiple cutaneously applied and intrathecally-administered pruritogens in mice. Knock-in to mice of a GTPase activating protein-insensitive mutant of Gαo (Gnao1 G184S/+) produces a similar pruriceptive phenotype. The pruriceptive defect in R7bp knockout mice was rescued in double knockout mice also lacking Oprk1, encoding the G protein-coupled kappa-opioid receptor whose activation is known to inhibit itch sensation. In a model of atopic dermatitis (eczema), R7bp knockout mice showed diminished scratching behavior and enhanced sensitivity to kappa opioid agonists. Taken together, our results indicate that R7bp is a key regulator of itch sensation and suggest the potential targeting of R7bp-dependent GTPase activating protein activity as a novel therapeutic strategy for pathological itch.

Association with the Plasma Membrane Is Sufficient for Potentiating Catalytic Activity of Regulators of G Protein Signaling (RGS) Proteins of the R7 Subfamily

Regulators of G protein Signaling (RGS) promote deactivation of heterotrimeric G proteins thus controlling the magnitude and kinetics of responses mediated by G protein-coupled receptors (GPCR). In the nervous system, RGS7 and RGS9-2 play essential role in vision, reward processing, and movement control. Both RGS7 and RGS9-2 belong to the R7 subfamily of RGS proteins that form macromolecular complexes with R7-binding protein (R7BP). R7BP targets RGS proteins to the plasma membrane and augments their GTPase-accelerating protein (GAP) activity, ultimately accelerating deactivation of G protein signaling. However, it remains unclear if R7BP serves exclusively as a membrane anchoring subunit or further modulates RGS proteins to increase their GAP activity. To directly answer this question, we utilized a rapidly reversible chemically induced protein dimerization system that enabled us to control RGS localization independent from R7BP in living cells. To monitor kinetics of Gα deactivation, we coupled this strategy with measuring changes in the GAP activity by bioluminescence resonance energy transfer-based assay in a cellular system containing μ-opioid receptor. This approach was used to correlate changes in RGS localization and activity in the presence or absence of R7BP. Strikingly, we observed that RGS activity is augmented by membrane recruitment, in an orientation independent manner with no additional contributions provided by R7BP. These findings argue that the association of R7 RGS proteins with the membrane environment provides a major direct contribution to modulation of their GAP activity.

Roles for Regulator of G Protein Signaling Proteins in Synaptic Signaling and Plasticity

The regulator of G protein signaling (RGS) family of proteins serves critical roles in G protein-coupled receptor (GPCR) and heterotrimeric G protein signal transduction. RGS proteins are best understood as negative regulators of GPCR/G protein signaling. They achieve this by acting as GTPase activating proteins (GAPs) for Gα subunits and accelerating the turnoff of G protein signaling. Many RGS proteins also bind additional signaling partners that either regulate their functions or enable them to regulate other important signaling events. At neuronal synapses, GPCRs, G proteins, and RGS proteins work in coordination to regulate key aspects of neurotransmitter release, synaptic transmission, and synaptic plasticity, which are necessary for central nervous system physiology and behavior. Accumulating evidence has revealed key roles for specific RGS proteins in multiple signaling pathways at neuronal synapses, regulating both pre- and postsynaptic signaling events and synaptic plasticity. Here, we review and highlight the current knowledge of specific RGS proteins (RGS2, RGS4, RGS7, RGS9-2, and RGS14) that have been clearly demonstrated to serve critical roles in modulating synaptic signaling and plasticity throughout the brain, and we consider their potential as future therapeutic targets.

Local Palmitoylation Cycles and Specialized Membrane Domain Organization

Palmitoylation is an evolutionally conserved lipid modification of proteins. Dynamic and reversible palmitoylation controls a wide range of molecular and cellular properties of proteins including the protein trafficking, protein function, protein stability, and specialized membrane domain organization. However, technical difficulties in (1) detection of palmitoylated substrate proteins and (2) purification and enzymology of palmitoylating enzymes have prevented the progress in palmitoylation research, compared with that in phosphorylation research. The recent development of proteomic and chemical biology techniques has unexpectedly expanded the known complement of palmitoylated proteins in various species and tissues/cells, and revealed the unique occurrence of palmitoylated proteins in membrane-bound organelles and specific membrane compartments. Furthermore, identification and characterization of DHHC (Asp-His-His-Cys) palmitoylating enzyme-substrate pairs have contributed to elucidating the regulatory mechanisms and pathophysiological significance of protein palmitoylation. Here, we review the recent progress in protein palmitoylation at the molecular, cellular, and in vivo level and discuss how locally regulated palmitoylation machinery works for dynamic nanoscale organization of membrane domains.

Association of Rgs7/Gβ5 complexes with Girk channels and GABAB receptors in hippocampal CA1 pyramidal neurons

In the hippocampus, signaling through G protein-coupled receptors is modulated by Regulators of G protein signaling (Rgs) proteins, which act to stimulate the rate of GTP hydrolysis, and consequently, G protein inactivation. The R7-Rgs subfamily selectively deactivates the G(i/o)-class of Gα subunits that mediate the action of several GPCRs. Here, we used co-immunoprecipitation, electrophysiology and immunoelectron microscopy techniques to investigate the formation of macromolecular complexes and spatial relationship of Rgs7/Gβ5 complexes and its prototypical signaling partners, the GABAB receptor and Girk channel. Co-expression of recombinant GABAB receptors and Girk channels in combination with co-immunoprecipitation experiments established that the Rgs7/Gβ5 forms complexes with GABAB receptors or Girk channels. Using electrophysiological experiments, we found that GABAB -Girk current deactivation kinetics was markedly faster in cells coexpressing Rgs7/Gβ5. At the electron microscopic level, immunolabeling for Rgs7 and Gβ5 proteins was found primarily in the dendritic layers of the hippocampus and showed similar distribution patterns. Immunoreactivity was mostly localized along the extrasynaptic plasma membrane of dendritic shafts and spines of pyramidal cells and, to a lesser extent, to that of presynaptic terminals. Quantitative analysis of immunogold particles for Rgs7 and Gβ5 revealed an enrichment of the two proteins around excitatory synapses on dendritic spines, virtually identical to that of Girk2 and GABAB1 . These data support the existence of macromolecular complexes composed of GABAB receptor-G protein-Rgs7-Girk channels in which Rgs7 and Gβ5 proteins may preferentialy modulate GABAB receptor signaling through the deactivation of Girk channels on dendritic spines. In contrast, Rgs7 and Girk2 were associated but mainly segregated from GABAB1 in dendritic shafts, where Rgs7/Gβ5 signaling complexes might modulate Girk-dependent signaling via a different metabotropic receptor(s).

RGS17: an emerging therapeutic target for lung and prostate cancers

Ligands for G-protein-coupled receptors (GPCRs) represent approximately 50% of currently marketed drugs. RGS proteins modulate heterotrimeric G proteins and, thus, GPCR signaling, by accelerating the intrinsic GTPase activity of the Gα subunit. Given the prevalence of GPCR targeted therapeutics and the role RGS proteins play in G protein signaling, some RGS proteins are emerging as targets in their own right. One such RGS protein is RGS17. Increased RGS17 expression in some prostate and lung cancers has been demonstrated to support cancer progression, while reduced expression of RGS17 can lead to development of chemotherapeutic resistance in ovarian cancer. High-throughput screening is a powerful tool for lead compound identification, and utilization of high-throughput technologies has led to the discovery of several RGS inhibitors, thus far. As screening technologies advance, the identification of novel lead compounds the subsequent development of targeted therapeutics appears promising.

Amino-terminal cysteine residues differentially influence RGS4 protein plasma membrane targeting, intracellular trafficking, and function

Regulator of G-protein signaling (RGS) proteins are potent inhibitors of heterotrimeric G-protein signaling. RGS4 attenuates G-protein activity in several tissues. Previous work demonstrated that cysteine palmitoylation on residues in the amino-terminal (Cys-2 and Cys-12) and core domains (Cys-95) of RGS4 is important for protein stability, plasma membrane targeting, and GTPase activating function. To date Cys-2 has been the priority target for RGS4 regulation by palmitoylation based on its putative role in stabilizing the RGS4 protein. Here, we investigate differences in the contribution of Cys-2 and Cys-12 to the intracellular localization and function of RGS4. Inhibition of RGS4 palmitoylation with 2-bromopalmitate dramatically reduced its localization to the plasma membrane. Similarly, mutation of the RGS4 amphipathic helix (L23D) prevented membrane localization and its G(q) inhibitory function. Together, these data suggest that both RGS4 palmitoylation and the amphipathic helix domain are required for optimal plasma membrane targeting and function of RGS4. Mutation of Cys-12 decreased RGS4 membrane targeting to a similar extent as 2-bromopalmitate, resulting in complete loss of its G(q) inhibitory function. Mutation of Cys-2 did not impair plasma membrane targeting but did partially impair its function as a G(q) inhibitor. Comparison of the endosomal distribution pattern of wild type and mutant RGS4 proteins with TGN38 indicated that palmitoylation of these two cysteines contributes differentially to the intracellular trafficking of RGS4. These data show for the first time that Cys-2 and Cys-12 play markedly different roles in the regulation of RGS4 membrane localization, intracellular trafficking, and G(q) inhibitory function via mechanisms that are unrelated to RGS4 protein stabilization.

Subcellular localization of regulator of G protein signaling RGS7 complex in neurons and transfected cells

The R7 family of regulators of G protein signaling (RGS) is involved in many functions of the nervous system. This family includes RGS6, RGS7, RGS9, and RGS11 gene products and is defined by the presence of the characteristic first found in Disheveled, Egl-10, Pleckstrin (DEP), DEP helical extension (DHEX), Gγ-like, and RGS domains. Herein, we examined the subcellular localization of RGS7, the most broadly expressed R7 member. Our immunofluorescence studies of retinal and dorsal root ganglion neurons showed that RGS7 concentrated at the plasma membrane of cell bodies, in structures resembling lamellipodia or filopodia along the processes, and at the dendritic tips. At the plasma membrane of dorsal root ganglia neurons, RGS7 co-localized with its known binding partners R7 RGS binding protein (R7BP), Gαo, and Gαq. More than 50% of total RGS7-specific immunofluorescence was present in the cytoplasm, primarily within numerous small puncta that did not co-localize with R7BP. No specific RGS7 or R7BP immunoreactivity was detected in the nuclei. In transfected cell lines, ectopic RGS7 had both diffuse cytosolic and punctate localization patterns. RGS7 also localized in centrosomes. Structure-function analysis showed that the punctate localization was mediated by the DEP/DHEX domains, and centrosomal localization was dependent on the DHEX domain.

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.

Regulator of G protein signaling proteins as drug targets: current state and future possibilities

Regulators of G protein signaling (RGS) proteins have emerged in the past two decades as novel drug targets in many areas of research. Their importance in regulating signaling via G protein-coupled receptors has become evident as numerous studies have been published on the structure and function of RGS proteins. A number of genetic models have also been developed, demonstrating the potential clinical importance of RGS proteins in various disease states, including central nervous system disorders, cardiovascular disease, diabetes, and several types of cancer. Apart from their classical mechanism of action as GTPase-activating proteins (GAPs), RGS proteins can also serve other noncanonical functions. This opens up a new approach to targeting RGS proteins in drug discovery as the view on the function of these proteins is constantly evolving. This chapter summarizes the latest development in RGS protein drug discovery with special emphasis on noncanonical functions and regulatory mechanisms of RGS protein expression. As more reports are being published on this group of proteins, it is becoming clear that modulation of GAP activity might not be the only way to therapeutically target RGS proteins.

RSBP-1 is a membrane-targeting subunit required by the Galpha(q)-specific but not the Galpha(o)-specific R7 regulator of G protein signaling in Caenorhabditis elegans

Regulator of G protein signaling (RGS) proteins inhibit G protein signaling by activating Galpha GTPase activity, but the mechanisms that regulate RGS activity are not well understood. The mammalian R7 binding protein (R7BP) can interact with all members of the R7 family of RGS proteins, and palmitoylation of R7BP can target R7 RGS proteins to the plasma membrane in cultured cells. However, whether endogenous R7 RGS proteins in neurons require R7BP or membrane localization for function remains unclear. We have identified and knocked out the only apparent R7BP homolog in Caenorhabditis elegans, RSBP-1. Genetic studies show that loss of RSBP-1 phenocopies loss of the R7 RGS protein EAT-16, but does not disrupt function of the related R7 RGS protein EGL-10. Biochemical analyses find that EAT-16 coimmunoprecipitates with RSBP-1 and is predominantly plasma membrane-associated, whereas EGL-10 does not coimmunoprecipitate with RSBP-1 and is not predominantly membrane-associated. Mutating the conserved membrane-targeting sequence in RSBP-1 disrupts both the membrane association and function of EAT-16, demonstrating that membrane targeting by RSBP-1 is essential for EAT-16 activity. Our analysis of endogenous R7 RGS proteins in C. elegans neurons reveals key differences in the functional requirements for membrane targeting between members of this protein family.

Structure, function, and localization of Gβ5-RGS complexes

Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5-R7 complexes, (2) regional distribution of Gβ5-R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5-R7 complexes, and (4) novel binding partners of Gβ5-R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5-R7 function in vivo.

The R7 RGS protein family: multi-subunit regulators of neuronal G protein signaling

G protein-coupled receptor signaling pathways mediate the transmission of signals from the extracellular environment to the generation of cellular responses, a process that is critically important for neurons and neurotransmitter action. The ability to promptly respond to rapidly changing stimulation requires timely inactivation of G proteins, a process controlled by a family of specialized proteins known as regulators of G protein signaling (RGS). The R7 group of RGS proteins (R7 RGS) has received special attention due to their pivotal roles in the regulation of a range of crucial neuronal processes such as vision, motor control, reward behavior, and nociception in mammals. Four proteins in this group, RGS6, RGS7, RGS9, and RGS11, share a common molecular organization of three modules: (i) the catalytic RGS domain, (ii) a GGL domain that recruits G beta(5), an outlying member of the G protein beta subunit family, and (iii) a DEP/DHEX domain that mediates interactions with the membrane anchor proteins R7BP and R9AP. As heterotrimeric complexes, R7 RGS proteins not only associate with and regulate a number of G protein signaling pathway components, but have also been found to form complexes with proteins that are not traditionally associated with G protein signaling. This review summarizes our current understanding of the biology of the R7 RGS complexes including their structure/functional organization, protein-protein interactions, and physiological roles.

R9AP and R7BP: traffic cops for the RGS7 family in phototransduction and neuronal GPCR signaling

RGS (regulator of G protein signaling) proteins have emerged as crucial regulators, effectors and integrators in G-protein-coupled receptor (GPCR) signaling networks. Many RGS proteins accelerate GTP hydrolysis by Galpha subunits, thereby regulating G protein activity, whereas certain RGS proteins also transduce Galpha signals to downstream targets. Particularly intriguing are members of the RGS7 (R7) family (RGS6, RGS7, RGS9 and RGS11), which heterodimerize with Gbeta5. In Caenorhabditis elegans, R7-Gbeta5 heterodimers regulate synaptic transmission, anesthetic action and behavior. In vertebrates, they regulate vision, postnatal development, working memory and the action of psychostimulants or morphine. Here we highlight R9AP and R7BP, a related pair of recently identified SNARE-like R7-family binding proteins, which regulate intracellular trafficking, expression and function of R7-Gbeta5 heterodimers in retina and brain. Emerging understanding of R7BP and R9AP promises to provide new insights into neuronal GPCR signaling mechanisms relevant to the causes and treatment of neurological disorders.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Postnatal induction and localization of R7BP, a membrane-anchoring protein for regulator of G protein signaling 7 family-Gbeta5 complexes in brain

Members of the regulator of G protein signaling 7 (RGS7) (R7) family and Gbeta5 form obligate heterodimers that are expressed predominantly in the nervous system. R7-Gbeta5 heterodimers are GTPase-activating proteins (GAPs) specific for Gi/o-class Galpha subunits, which mediate phototransduction in retina and the action of many modulatory G protein-coupled receptors (GPCRs) in brain. Here we have focused on the R7-family binding protein (R7BP), a recently identified palmitoylated protein that can bind R7-Gbeta5 complexes and is hypothesized to control the intracellular localization and function of the resultant heterotrimeric complexes. We show that: 1) R7-Gbeta5 complexes are obligate binding partners for R7BP in brain because they co-immunoprecipitate and exhibit similar expression patterns. Furthermore, R7BP and R7 protein accumulation in vivo requires Gbeta5. 2) Expression of R7BP in Neuro2A cells at levels approximating those in brain recruits endogenous RGS7-Gbeta5 complexes to the plasma membrane. 3) R7BP immunoreactivity in brain concentrates in neuronal soma, dendrites, spines or unmyelinated axons, and is absent or low in glia, myelinated axons, or axon terminals. 4) RGS7-Gbeta5-R7BP complexes in brain extracts associate inefficiently with detergent-resistant lipid raft fractions with or without G protein activation. 5) R7BP and Gbeta5 protein levels are upregulated strikingly during the first 2-3 weeks of postnatal brain development. Accordingly, we suggest that R7-Gbeta5-R7BP complexes in the mouse or rat could regulate signaling by modulatory Gi/o-coupled GPCRs in the developing and adult nervous systems.

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

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

R7-binding protein targets the G protein beta 5/R7-regulator of G protein signaling complex to lipid rafts in neuronal cells and brain

Background

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins), composed of Gα, Gβ, and Gγ subunits, are positioned at the inner face of the plasma membrane and relay signals from activated G protein-coupled cell surface receptors to various signaling pathways. Gβ5 is the most structurally divergent Gβ isoform and forms tight heterodimers with regulator of G protein signalling (RGS) proteins of the R7 subfamily (R7-RGS). The subcellular localization of Gβ 5/R7-RGS protein complexes is regulated by the palmitoylation status of the associated R7-binding protein (R7BP), a recently discovered SNARE-like protein. We investigate here whether R7BP controls the targeting of Gβ5/R7-RGS complexes to lipid rafts, cholesterol-rich membrane microdomains where conventional heterotrimeric G proteins and some effector proteins are concentrated in neurons and brain.

Results

We show that endogenous Gβ5/R7-RGS/R7BP protein complexes are present in native neuron-like PC12 cells and that a fraction is targeted to low-density, detergent-resistant membrane lipid rafts. The buoyant density of endogenous raft-associated Gβ5/R7-RGS protein complexes in PC12 cells was similar to that of lipid rafts containing the palmitoylated marker proteins PSD-95 and LAT, but distinct from that of the membrane microdomain where flotillin was localized. Overexpression of wild-type R7BP, but not its palmitoylation-deficient mutant, greatly enriched the fraction of endogenous Gβ5/R7-RGS protein complexes in the lipid rafts. In HEK-293 cells the palmitoylation status of R7BP also regulated the lipid raft targeting of co-expressed Gβ5/R7-RGS/R7BP proteins. A fraction of endogenous Gβ5/R7-RGS/R7BP complexes was also present in lipid rafts in mouse brain.

Conclusion

A fraction of Gβ5/R7-RGS/R7BP protein complexes is targeted to low-density, detergent-resistant membrane lipid rafts in PC12 cells and brain. In cultured cells, the palmitoylation status of R7BP regulated the lipid raft targeting of endogenous or co-expressed Gβ5/R7-RGS proteins. Taken together with recent evidence that the kinetic effects of the Gβ5 complex on GPCR signaling are greatly enhanced by R7BP palmitoylation through a membrane-anchoring mechanism, our data suggest the targeting of the Gβ5/R7-RGS/R7BP complex to lipid rafts in neurons and brain, where G proteins and their effectors are concentrated, may be central to the G protein regulatory function of the complex.

The Importance of N-terminal Polycysteine and Polybasic Sequences for G14α and G16α Palmitoylation, Plasma Membrane Localization, and Signaling Function

Plasma membrane targeting of G protein alpha (Galpha) subunits is essential for competent receptor-to-G protein signaling. Many Galpha are tethered to the plasma membrane by covalent lipid modifications at their N terminus. Additionally, it is hypothesized that Gq family members (Gqalpha,G11alpha,G14alpha, and G16alpha) in particular utilize a polybasic sequence of amino acids in their N terminus to promote membrane attachment and protein palmitoylation. However, this hypothesis has not been tested, and nothing is known about other mechanisms that control subcellular localization and signaling properties of G14alpha and G16alpha. Here we report critical biochemical factors that mediate membrane attachment and signaling function of G14alpha and G16alpha. We find that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences in their N termini and that the polycysteine sequence along with the adjacent polybasic region are both important for G16alpha-mediated signaling at the plasma membrane. Surprisingly, the isolated N termini of G14alpha and G16alpha expressed as peptides fused to enhanced green fluorescent protein each exhibit differential requirements for palmitoylation and membrane targeting; individual cysteine residues, but not the polybasic regions, determine lipid modification and subcellular localization. However, full-length G16alpha, more so than G14alpha, displays a functional dependence on single cysteines for membrane localization and activity, and its full signaling potential depends on the integrity of the polybasic sequence. Together, these findings indicate that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences, and that the adjacent polybasic domain is not required for Galpha palmitoylation but is important for localization and functional activity of heterotrimeric G proteins.

Phosphorylation of Ser166 in RGS5 by protein kinase C causes loss of RGS function

RGS5 is a member of regulators of G protein signaling (RGS) proteins that attenuate heterotrimeric G protein signaling by functioning as GTPase-activating proteins (GAPs). We investigated phosphorylation of RGS5 and the resulting change of its function. In 293T cells, transiently expressed RGS5 was phosphorylated by endogenous protein kinases in the basal state. The phosphorylation was enhanced by phorbol 12-myristate 13-acetate (PMA) and endothelin-1 (ET-1), and suppressed by protein kinase C (PKC) inhibitors, H7, calphostin C and staurosporine. These results suggest involvement of PKC in phosphorylation of RGS5. In in vitro experiments, PKC phosphorylated recombinant RGS5 protein at serine residues. RGS5 protein phosphorylated by PKC showed much lower binding capacity for and GAP activity toward Galpha subunits than did the unphosphorylated RGS5. In cells expressing RGS5, the inhibitory effect of RGS5 on ET-1-induced Ca(2+) responses was enhanced by staurosporine. Mass spectrometric analysis of the phosphorylated RGS5 revealed that Ser166 was one of the predominant phosphorylation sites. Substitution of Ser166 by aspartic acid abolished the binding capacity to Galpha subunits and the GAP activity, and markedly reduced the inhibitory effect on ET-1-induced Ca(2+) responses. These results indicate that phosphorylation at Ser166 of RGS5 by PKC causes loss of the function of RGS5 in G protein signaling. Since this serine residue is conserved in RGS domains of many RGS proteins, the phosphorylation at Ser166 by PKC might act as a molecular switch and have functional significance.

Intramolecular interaction between the DEP domain of RGS7 and the Gbeta5 subunit

The R7 family of RGS proteins (RGS6, -7, -9, -11) is characterized by the presence of three domains: DEP, GGL, and RGS. The RGS domain interacts with Galpha subunits and exhibits GAP activity. The GGL domain permanently associates with Gbeta5. The DEP domain interacts with the membrane anchoring protein, R7BP. Here we provide evidence for a novel interaction within this complex: between the DEP domain and Gbeta5. GST fusion of the RGS7 DEP domain (GST-R7DEP) binds to both native and recombinant Gbeta5-RGS7, recombinant Gbetagamma complexes, and monomeric Gbeta5 and Gbeta1 subunits. Co-immunoprecipitation and FRET assays supported the GST pull-down experiments. GST-R7DEP reduced FRET between CFP-Gbeta5 and YFP-RGS7, indicating that the DEP-Gbeta5 interaction is dynamic. In transfected cells, R7BP had no effect on the Gbeta5/RGS7 pull down by GST-R7DEP. The DEP domain of RGS9 did not bind to Gbeta5. Substitution of RGS7 Glu-73 and Asp-74 for the corresponding Ser and Gly residues (ED/SG mutation) of RGS9 diminished the DEP-Gbeta5 interaction. In the absence of R7BP both the wild-type RGS7 and the ED/SG mutant attenuated muscarinic M3 receptor-mediated Ca2+ mobilization. In the presence of R7BP, wild-type RGS7 lost this inhibitory activity, whereas the ED/SG mutant remained active. Taken together, our results are consistent with the following model. The Gbeta5-RGS7 molecule can exist in two conformations: "closed" and "open", when the DEP domain and Gbeta5 subunit either do or do not interact. The closed conformation appears to be less active with respect to its effect on Gq-mediated signaling than the open conformation.

R7BP augments the function of RGS7*Gbeta5 complexes by a plasma membrane-targeting mechanism

The RGS7 (R7) family of G protein regulators, Gbeta5, and R7BP form heterotrimeric complexes that potently regulate the kinetics of G protein-coupled receptor signaling. Reversible palmitoylation of R7BP regulates plasma membrane/nuclear shuttling of R7*Gbeta5*R7BP heterotrimers. Here we have investigated mechanisms whereby R7BP controls the function of the R7 family. We show that unpalmitoylated R7BP undergoes nuclear/cytoplasmic shuttling and that a C-terminal polybasic motif proximal to the palmitoylation acceptor sites of R7BP mediates nuclear localization, palmitoylation, and plasma membrane targeting. These results suggest a novel mechanism whereby palmitoyltransferases and nuclear import receptors both utilize the C-terminal domain of R7BP to determine the trafficking fate of R7*Gbeta5*R7BP heterotrimers. Analogous mechanisms may regulate other signaling proteins whose distribution between the plasma membrane and nucleus is controlled by palmitoylation. Lastly, we show that cytoplasmic RGS7*Gbeta5*R7BP heterotrimers and RGS7*Gbeta5 heterodimers are equivalently inefficient regulators of G protein-coupled receptor signaling relative to plasma membrane-bound heterotrimers bearing palmitoylated R7BP. Therefore, R7BP augments the function of the complex by a palmitoylation-regulated plasma membrane-targeting mechanism.

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.

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.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

A Key Serine for the GTPase-Activating Protein Function of Regulator of G Protein Signaling Proteins Is Not a General Target for 14-3-3 Interactions

Mammalian regulator of G protein signaling (RGS) proteins are highly conserved within the RGS domain. Of amino acids that are universal, a serine residue at the C terminus of this domain has been described as the binding site in RGS7 for 14-3-3 proteins. However, studies with the related RGS3 indicate that the site of interaction is not within the RGS domain. We confirm that the interaction of RGS3 with 14-3-3tau and 14-3-3zeta requires Ser264 and not the RGS domain and show both that mutation of the conserved RGS domain serine, Ser496 in RGS3, to either alanine or aspartate does not prevent binding of 14-3-3 proteins and that 14-3-3 proteins do not inhibit GTPase-activating protein (GAP) activity against receptor-activated Galpha(o1). However, mutation of Ser496 does directly impair the action of RGS3 as a GAP against receptor-activated Galpha(o1). We mutated the equivalent serine residue in the family B/R4 RGS proteins RGS1 and RGS16. Using two distinct assay formats, conversion to aspartate virtually abolished GAP activity, whereas conversion to alanine decreased potency 20-fold. Neither alteration modulated interactions with 14-3-3tau or 14-3-3zeta, but the 14-3-3 proteins did not modulate the GAP activity of the wild-type or mutant RGS proteins. Although interactions between 14-3-3 proteins and many RGS proteins can be observed, this does not involve this conserved serine and does not inherently modify GAP function.

Multi-tasking RGS proteins in the heart: the next therapeutic target?

Regulator of G-protein-signaling (RGS) proteins play a key role in the regulation of G-protein-coupled receptor (GPCR) signaling. The characteristic hallmark of RGS proteins is a conserved approximately 120-aa RGS region that confers on these proteins the ability to serve as GTPase-activating proteins (GAPs) for G(alpha) proteins. Most RGS proteins can serve as GAPs for multiple isoforms of G(alpha) and therefore have the potential to influence many cellular signaling pathways. However, RGS proteins can be highly regulated and can demonstrate extreme specificity for a particular signaling pathway. RGS proteins can be regulated by altering their GAP activity or subcellular localization; such regulation is achieved by phosphorylation, palmitoylation, and interaction with protein and lipid-binding partners. Many RGS proteins have GAP-independent functions that influence GPCR and non-GPCR-mediated signaling, such as effector regulation or action as an effector. Hence, RGS proteins should be considered multifunctional signaling regulators. GPCR-mediated signaling is critical for normal function in the cardiovascular system and is currently the primary target for the pharmacological treatment of disease. Alterations in RGS protein levels, in particular RGS2 and RGS4, produce cardiovascular phenotypes. Thus, because of the importance of GPCR-signaling pathways and the profound influence of RGS proteins on these pathways, RGS proteins are regulators of cardiovascular physiology and potentially novel drug targets as well.

R7BP: A Surprising New Link Between G Proteins, RGS Proteins, and Nuclear Signaling in the Brain

The regulators of G protein signaling (RGS proteins) bind directly to G protein alpha (Galpha) subunits in brain and other tissues to determine the strength, duration, and fidelity of neurotransmitter receptor signaling. A recent study shows, quite unexpectedly, that one class of RGS proteins [the R7 subfamily bound to Gbeta(5) (R7-Gbeta(5))] shuttles between the plasma membrane and the nucleus with assistance from a novel shuttle protein, R7BP. R7BP binds directly to R7-Gbeta(5) and the protein complex is tethered to the plasma membrane by addition of a lipid, palmitate, on R7BP. Removal of palmitate results in the translocation of the R7BP-R7-Gbeta(5) complex to the nucleus, presumably for nontraditional signaling functions. These findings suggest an entirely novel mechanism for regulating neurotransmitter signaling. That is, R7BP transduces signals directly from receptors and G proteins at the plasma membrane to the nucleus, and this plasma membrane-nuclear shuttling is controlled by reversible palmitoylation of R7BP.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

RATIONALE

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo.

OBJECTIVE

In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids.

RESULTS

RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association.

CONCLUSION

In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Palmitoylation regulates plasma membrane-nuclear shuttling of R7BP, a novel membrane anchor for the RGS7 family

The RGS7 (R7) family of RGS proteins bound to the divergent Gβ subunit Gβ5 is a crucial regulator of G protein–coupled receptor (GPCR) signaling in the visual and nervous systems. Here, we identify R7BP, a novel neuronally expressed protein that binds R7–Gβ5 complexes and shuttles them between the plasma membrane and nucleus. Regional expression of R7BP, Gβ5, and R7 isoforms in brain is highly coincident. R7BP is palmitoylated near its COOH terminus, which targets the protein to the plasma membrane. Depalmitoylation of R7BP translocates R7BP–R7–Gβ5 complexes from the plasma membrane to the nucleus. Compared with nonpalmitoylated R7BP, palmitoylated R7BP greatly augments the ability of RGS7 to attenuate GPCR-mediated G protein–regulated inward rectifying potassium channel activation. Thus, by controlling plasma membrane nuclear–shuttling of R7BP–R7–Gβ5 complexes, reversible palmitoylation of R7BP provides a novel mechanism that regulates GPCR signaling and potentially transduces signals directly from the plasma membrane to the nucleus.

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.

Differential effects of regulator of G protein signaling (RGS) proteins on serotonin 5-HT1A, 5-HT2A, and dopamine D2 receptor-mediated signaling and adenylyl cyclase activity

Regulator of G protein signaling (RGS) proteins function as GTPase accelerating proteins (GAP) for Galpha subunits, attenuating G-protein-coupled receptor signal transduction. The present study tested the ability of members of different subfamilies of RGS proteins to modulate both G-protein-dependent and -independent signaling in mammalian cells. RGS4, RGS10, and RGSZ1 significantly attenuated Galphai-mediated signaling by 5-HT1A, but not by dopamine D2, receptor-expressing cells. Additionally, RGS4 and RGS10 significantly inhibited forskolin-stimulated cAMP production in both cell lines. In contrast, RGS2, RGS7, and RGSZ1 had no effect on forskolin-stimulated cAMP production in these cells. RGS2 and RGS7 significantly decreased Galphaq-mediated signaling by 5-HT2A receptors, confirming that the RGS4 and RGS10 effects on forskolin-stimulated cAMP production were specific, and not simply due to overexpression. Interestingly, similar expression levels of RGS4 protein resulted in greater inhibition of G-protein-independent cAMP production compared to G-protein-dependent GAP activity. Our results suggest specificity and selectivity of RGS proteins on G-protein-dependent and -independent signaling in mammalian cells.

Purification and in vitro functional analysis of R7 subfamily RGS proteins in complex with Gbeta5

The study of purified regulator of G-protein signaling (RGS) proteins in steady-state GTPase assays using reconstituted proteoliposomes is a powerful approach to characterizing the RGS protein-mediated acceleration of intrinsic Galpha subunit GTPase activity in the context of various G-protein and G-protein-coupled receptor (GPCR) combinations. This approach has been applied successfully to the R7 subfamily of RGS proteins, RGS6, -7, -9, and -11, which form heterodimers with Gbeta5 subunits via the G-protein gamma-like domain of R7 proteins. This article describes the purification of heterodimers from Sf9 insect cells following the expression of recombinant R7 protein and histidine-tagged Gbeta5 using affinity and ion-exchange chromatography. The ability of the heterodimers to accelerate the intrinsic GTPase activity of Galpha subunits was assessed in steady-state GTPase assays performed on proteoliposomes consisting of phospholipids, purified G proteins, and purified GPCRs.

Biochemical purification and functional analysis of complexes between the G-protein subunit Gbeta5 and RGS proteins

Regulator of G-protein signaling (RGS) proteins of the R7 subfamily (RGS6, 7, 9, and 11) contain a unique Ggamma-like (GGL) domain that enables their association with the G-protein beta subunit Gbeta5. The existence of these complexes was demonstrated by their purification from native tissues as well as by reconstitution in vitro. According to pulse-chase analysis, Gbeta5 and RGS7 monomers undergo rapid proteolytic degradation in cells, whereas the dimer is stable. Studies of the functional role of Gbeta5-RGS dimers using GTPase activity, ion channel, and calcium mobilization assays showed that, similarly to other RGS proteins, they can negatively regulate G-protein-mediated signal transduction. Protein-protein interactions involving the Gbeta5-RGS7 complex can be studied in cells using fluorescence resonance energy transfer utilizing Gbeta5, RGS, and Galpha subunits fused to the cyan and yellow versions of green fluorescent protein.

Expression of neural RGS-R7 and Gbeta5 Proteins in Response to Acute and Chronic Morphine

The R7 subfamily of regulators of G-protein signaling (RGS) proteins (RGS6, RGS7, RGS9-2, and RGS11), and its binding protein Gbeta5, are found in neural structures of mouse brain. A single intracerebroventricular priming dose of 10 nmol morphine gave rise to acute tolerance to the analgesic effects of successive identical test doses of the opioid. At 2 h after administering the acute opioid, RGS7 mRNA levels in the striatum plus those of RGS9-2 in the striatum and thalamus were increased, whereas RGS9-2 and RGS11 mRNA were reduced in the cortex. Similar but attenuated RGS-R7 mRNA changes persisted 24 h after acute morphine administration. No changes in Gbeta5 mRNA levels were observed. At 2 days after commencing sustained morphine treatment, the levels of mRNA for RGS7, RGS9-2, RGS11, and Gbeta5 increased in most of the brain structures studied (striatum, thalamus, periaqueductal gray matter (PAG), and cortex). In these morphine tolerant-dependent mice, the greater changes were found for RGS9-2 in the thalamus (>500%) and PAG (>200%). In post-dependent mice, the increases in RGS-R7 and Gbeta5 mRNA still persisted in the PAG and striatum at 8 and 16 days after starting the chronic opioid treatment. The raised mRNA levels promoted by chronic, but not by acute, morphine, were accompanied by increases in the encoded proteins. This is probably a result of the costabilization of the RGS-R7 and Gbeta5 proteins forming heterodimers. Opioid-induced adaptations of RGS-R7 and Gbeta5 genes may regulate the severity of morphine-induced tolerance/dependence and the duration of the post-dependent period, helping to recover the normal response.

Role of palmitoylation in RGS protein function

Palmitoylation, the reversible, post-translational addition of palmitate to cysteine residues, occurs on several regulators of G-protein signaling (RGS) proteins. Palmitoylation can occur near the amino terminus, as for RGS4 and RGS16, but can also occur on a cysteine residue in the alpha4 helix of the RGS box, which is conserved in most RGS proteins. For some of the RGS proteins, palmitoylation is required to turn off G-protein signaling by accelerating GTP hydrolysis on the Galpha subunit. This article discusses the role of palmitoylation in RGS function and protocols are given for metabolic and in vitro labeling of RGS proteins with [3H]palmitate and measurement of GTP hydrolysis in membranes.

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.

Assays of Nuclear Localization of R7/Gβ5 Complexes

Heterodimeric complexes between individual members of the R7 subfamily of regulators of G-protein signaling proteins and the Gbeta5 isoform of the heterotrimeric G-protein beta subunit family are strongly expressed in the cell nucleus in neurons and brain, as well as in the cytoplasm and plasma membrane. Native and recombinant Gbeta5 and/or R7 expression have been studied in model systems like rat pheochromocytoma PC12 cells where their nuclear localization can be studied by fluorescence microscopy and/or subcellular fractionation. Nucleic acid counterstains chosen for compatibility with the fluorescent tags on secondary antibodies can facilitate the assay of R7/Gbeta5 nuclear localization by epifluorescence or confocal laser microscopy. Subcellular fractionation allows isolation of a purified nuclear fraction that can be probed for the presence of Gbeta5 and/or R7 subunits by immunoblots or immunoprecipitation and compared to other subcellular fractions. While the function of nuclear R7/Gbeta5 complexes is unknown, comparison with the properties of other RGS proteins that localize to the cell nucleus may suggest modes of action. Models are offered in which the reversible post-translational modification of R7/Gbeta5 complexes regulates their nuclear localization and signaling activity, whether the target of such signaling activity is in the nucleus, at the plasma membrane, or both.

The R7 subfamily of RGS proteins assists tachyphylaxis and acute tolerance at mu-opioid receptors

Members of the R7 subfamily of regulators of G-protein signaling (RGS) proteins (RGS6, RGS7, RGS9-2, and RGS11) are found in the mouse CNS. The expression of these proteins was effectively reduced in different neural structures by blocking their mRNA with antisense oligodeoxynucleotides (ODNs). This was achieved without noticeable changes in the binding characteristics of labeled beta-endorphin to opioid receptors. Knockdown of R7 proteins enhanced the potency of antinociception promoted by morphine and [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO)-both agonists at mu-opioid receptors. The duration of morphine analgesia was greatly increased in RGS9-2 and in RGS11 knockdown mice. The impairment of R7 proteins brought about different changes in the analgesic activity of selective delta agonists. Knockdown of RGS11 reduced [D-Ala(2)]deltorphin II analgesic effects. Those of RGS6 and RGS9-2 proteins caused [D-Ala(2)]deltorphin II to produce a smoothened time-course curve-the peak effect blunted and analgesia extended during the declining phase. RGS9-2 impairment also promoted a similar pattern of change for [D-Pen(2,5)]-enkephalin (DPDPE). RGS7-deficient mice showed an increased response to both [D-Ala(2)]deltorphin II and DPDPE analgesic effects. A single intracerebroventricular (i.c.v.) ED(80) analgesic dose of morphine gave rise to acute tolerance in control mice, but did not promote tolerance in RGS6, RGS7, RGS9-2, or RGS11 knockdown animals. Thus, R7 proteins play a critical role in agonist tachyphylaxis and acute tolerance at mu-opioid receptors, and show differences in their modulation of delta-opioid receptors.

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.

Protein acylation in the inhibition of insulin secretion by norepinephrine, somatostatin, galanin, and PGE2

The major physiological inhibitors of insulin secretion, norepinephrine, somatostatin, galanin, and prostaglandin E2, act via specific receptors that activate pertussis toxin (PTX)-sensitive G proteins. Four inhibitory mechanisms are known: 1) activation of ATP-sensitive K channels and repolarization of the beta-cell; 2) inhibition of L-type Ca2+ channels; 3) decreased activity of adenylyl cyclase; and 4) inhibition of exocytosis at a "distal" site in stimulus-secretion coupling. We have examined the underlying mechanisms of inhibition at this distal site. In rat pancreatic islets, 2-bromopalmitate, cerulenin, and polyunsaturated fatty acids, all of which suppress protein acyltransferase activity, blocked the distal inhibitory effects of norepinephrine in a concentration-dependent manner. In contrast, control compounds such as palmitate, 16-hydroxypalmitate, and etomoxir, which do not block protein acylation, had no effect. Furthermore, 2-bromopalmitate also blocked the distal inhibitory actions of somatostatin, galanin, and prostaglandin E2. Importantly, neither 2-bromopalmitate nor cerulenin affected the action of norepinephrine to decrease cAMP production. We also examined the effects of norepinephrine, 2-bromopalmitate, and cerulenin on palmitate metabolism. Palmitate oxidation and its incorporation into lipids seemed not to contribute to the effects of 2-bromopalmitate and cerulenin on norepinephrine action. These data suggest that protein acylation mediates the distal inhibitory effect on insulin secretion. We propose that the inhibitors of insulin secretion, acting via PTX-sensitive G proteins, activate a specific protein acyltransferase, causing the acylation of a protein or proteins critical to exocytosis. This particular acylation and subsequent disruption of the essential and precise interactions involved in core complex formation would block exocytosis.

Comparative analysis of gene-expression patterns in human and African great ape cultured fibroblasts

Although much is known about genetic variation in human and African great ape (chimpanzee, bonobo, and gorilla) genomes, substantially less is known about variation in gene-expression profiles within and among these species. This information is necessary for defining transcriptional regulatory networks that contribute to complex phenotypes unique to humans or the African great apes. We took a systematic approach to this problem by investigating gene-expression profiles in well-defined cell populations from humans, bonobos, and gorillas. By comparing these profiles from 18 human and 21 African great ape primary fibroblast cell lines, we found that gene-expression patterns could predict the species, but not the age, of the fibroblast donor. Several differentially expressed genes among human and African great ape fibroblasts involved the extracellular matrix, metabolic pathways, signal transduction, stress responses, as well as inherited overgrowth and neurological disorders. These gene-expression patterns could represent molecular adaptations that influenced the development of species-specific traits in humans and the African great apes.

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.