Structural and Functional Analysis of the Regulator of G Protein Signaling 2-Gαq Complex

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Highlights? Two structures of the RGS2-Gα_q complex were determined by X-ray crystallography ? RGS2 binds Gα_q in a manner distinct from how other RGS proteins bind Gα_i/o ? In its distinct pose, RGS2 forms extensive contacts with the α-helical domain of Gα_q ? Helical domain contacts contribute to binding affinity and GAP potency of RGS2     

Integration of G Protein α (Gα) Signaling by the Regulator of G Protein Signaling 14 (RGS14)

RGS14 contains distinct binding sites for both active (GTP-bound) and inactive (GDP-bound) forms of Gα subunits. The N-terminal regulator of G protein signaling (RGS) domain binds active Gα_i/o-GTP, whereas the C-terminal G protein regulatory (GPR) motif binds inactive Gα_i1/3-GDP. The molecular basis for how RGS14 binds different activation states of Gα proteins to integrate G protein signaling is unknown. Here we explored the intramolecular communication between the GPR motif and the RGS domain upon G protein binding and examined whether RGS14 can functionally interact with two distinct forms of Gα subunits simultaneously. Using complementary cellular and biochemical approaches, we demonstrate that RGS14 forms a stable complex with inactive Gα_i1-GDP at the plasma membrane and that free cytosolic RGS14 is recruited to the plasma membrane by activated Gα_o-AlF₄⁻. Bioluminescence resonance energy transfer studies showed that RGS14 adopts different conformations in live cells when bound to Gα in different activation states. Hydrogen/deuterium exchange mass spectrometry revealed that RGS14 is a very dynamic protein that undergoes allosteric conformational changes when inactive Gα_i1-GDP binds the GPR motif. Pure RGS14 forms a ternary complex with Gα_o-AlF₄⁻ and an AlF₄⁻-insensitive mutant (G42R) of Gα_i1-GDP, as observed by size exclusion chromatography and differential hydrogen/deuterium exchange. Finally, a preformed RGS14·Gα_i1-GDP complex exhibits full capacity to stimulate the GTPase activity of Gα_o-GTP, demonstrating that RGS14 can functionally engage two distinct forms of Gα subunits simultaneously. Based on these findings, we propose a working model for how RGS14 integrates multiple G protein signals in host CA2 hippocampal neurons to modulate synaptic plasticity.     

Assembly and Function of the Regulator of G protein Signaling 14 (RGS14)·H-Ras Signaling Complex in Live Cells Are Regulated by Gαi1 and Gαi-linked G Protein-coupled Receptors

Regulator of G protein signaling 14 (RGS14) is a multifunctional scaffolding protein that integrates heterotrimeric G protein and H-Ras signaling pathways. RGS14 possesses an RGS domain that binds active Gα_i/o-GTP subunits to promote GTP hydrolysis and a G protein regulatory (GPR) motif that selectively binds inactive Gα_i1/3-GDP subunits to form a stable heterodimer at cellular membranes. RGS14 also contains two tandem Ras/Rap binding domains (RBDs) that bind H-Ras. Here we show that RGS14 preferentially binds activated H-Ras-GTP in live cells to enhance H-Ras cellular actions and that this interaction is regulated by inactive Gα_i1-GDP and G protein-coupled receptors (GPCRs). Using bioluminescence resonance energy transfer (BRET) in live cells, we show that RGS14-Luciferase and active H-Ras(G/V)-Venus exhibit a robust BRET signal at the plasma membrane that is markedly enhanced in the presence of inactive Gα_i1-GDP but not active Gα_i1-GTP. Active H-Ras(G/V) interacts with a native RGS14·Gα_i1 complex in brain lysates, and co-expression of RGS14 and Gα_i1 in PC12 cells greatly enhances H-Ras(G/V) stimulatory effects on neurite outgrowth. Stimulation of the Gα_i-linked α_2A-adrenergic receptor induces a conformational change in the Gα_i1·RGS14·H-Ras(G/V) complex that may allow subsequent regulation of the complex by other binding partners. Together, these findings indicate that inactive Gα_i1-GDP enhances the affinity of RGS14 for H-Ras-GTP in live cells, resulting in a ternary signaling complex that is further regulated by GPCRs.     

Double Suppression of the Gα Protein Activity by RGS Proteins

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Regulator of G-Protein Signaling – 5 (RGS5) Is a Novel Repressor of Hedgehog Signaling

Hedgehog (Hh) signaling plays fundamental roles in morphogenesis, tissue repair, and human disease. Initiation of Hh signaling is controlled by the interaction of two multipass membrane proteins, patched (Ptc) and smoothened (Smo). Recent studies identify Smo as a G-protein coupled receptor (GPCR)-like protein that signals through large G-protein complexes which contain the Gα_i subunit. We hypothesize Regulator of G-Protein Signaling (RGS) proteins, and specifically RGS5, are endogenous repressors of Hh signaling via their ability to act as GTPase activating proteins (GAPs) for GTP-bound Gα_i, downstream of Smo. In support of this hypothesis, we demonstrate that RGS5 over-expression inhibits sonic hedgehog (Shh)-mediated signaling and osteogenesis in C3H10T1/2 cells. Conversely, signaling is potentiated by siRNA-mediated knock-down of RGS5 expression, but not RGS4 expression. Furthermore, using immuohistochemical analysis and co-immunoprecipitation (Co-IP), we demonstrate that RGS5 is present with Smo in primary cilia. This organelle is required for canonical Hh signaling in mammalian cells, and RGS5 is found in a physical complex with Smo in these cells. We therefore conclude that RGS5 is an endogenous regulator of Hh-mediated signaling and that RGS proteins are potential targets for novel therapeutics in Hh-mediated diseases.     

Splicing Factor Transformer-2β (Tra2β) Regulates the Expression of Regulator of G Protein Signaling 4 (RGS4) Gene and Is Induced by Morphine

Regulator of G protein signaling 4 (RGS4) is a critical modulator of G protein-coupled receptor (GPCR)-mediated signaling and plays important roles in many neural process and diseases. Particularly, drug-induced alteration in RGS4 protein levels is associated with acute and chronic effects of drugs of abuse. However, the precise mechanism underlying the regulation of RGS4 expression is largely unknown. Here, we demonstrated that the expression of RGS4 gene was subject to regulation by alternative splicing of the exon 6. Transformer-2β (Tra2β), an important splicing factor, bound to RGS4 mRNA and increased the relative level of RGS4-1 mRNA isoform by enhancing the inclusion of exon 6. Meanwhile, Tra2β increased the expression of full-length RGS4 protein. In rat brain, Tra2β was co-localized with RGS4 in multiple opioid action-related brain regions. In addition, the acute and chronic morphine treatment induced alteration in the expression level of Tra2β in rat locus coerulus (LC) in parallel to that of RGS4 proteins. It suggests that induction of this splicing factor may contribute to the change of RGS4 level elicited by morphine. Taken together, the results provide the evidence demonstrating the function of Tra2β as a new mediator in opioid-induced signaling pathway via regulating RGS4 expression.     

Chromatin Protein HP1α Interacts with the Mitotic Regulator Borealin Protein and Specifies the Centromere Localization of the Chromosomal Passenger Complex

Accurate mitosis requires the chromosomal passenger protein complex (CPC) containing Aurora B kinase, borealin, INCENP, and survivin, which orchestrates chromosome dynamics. However, the chromatin factors that specify the CPC to the centromere remain elusive. Here we show that borealin interacts directly with heterochromatin protein 1α (HP1α) and that this interaction is mediated by an evolutionarily conserved PXVXL motif in the C-terminal borealin with the chromo shadow domain of HP1α. This borealin-HP1α interaction recruits the CPC to the centromere and governs an activation of Aurora B kinase judged by phosphorylation of Ser-7 in CENP-A, a substrate of Aurora B. Consistently, modulation of the motif PXVXL leads to defects in CPC centromere targeting and aberrant Aurora B activity. On the other hand, the localization of the CPC in the midzone is independent of the borealin-HP1α interaction, demonstrating the spatial requirement of HP1α in CPC localization to the centromere. These findings reveal a previously unrecognized but direct link between HP1α and CPC localization in the centromere and illustrate the critical role of borealin-HP1α interaction in orchestrating an accurate cell division.     

Structural Determinants of G-protein �� Subunit Selectivity by Regulator of G-protein Signaling 2 (RGS2)

“Regulator of G-protein signaling” (RGS) proteins facilitate the termination of G protein-coupled receptor (GPCR) signaling via their ability to increase the intrinsic GTP hydrolysis rate of Gα subunits (known as GTPase-accelerating protein or “GAP” activity). RGS2 is unique in its in vitro potency and selectivity as a GAP for Gα_q subunits. As many vasoconstrictive hormones signal via G_q heterotrimer-coupled receptors, it is perhaps not surprising that RGS2-deficient mice exhibit constitutive hypertension. However, to date the particular structural features within RGS2 determining its selectivity for Gα_q over Gα_i/o substrates have not been completely characterized. Here, we examine a trio of point mutations to RGS2 that elicits Gα_i-directed binding and GAP activities without perturbing its association with Gα_q. Using x-ray crystallography, we determined a model of the triple mutant RGS2 in complex with a transition state mimetic form of Gα_i at 2.8-Å resolution. Structural comparison with unliganded, wild type RGS2 and of other RGS domain/Gα complexes highlighted the roles of these residues in wild type RGS2 that weaken Gα_i subunit association. Moreover, these three amino acids are seen to be evolutionarily conserved among organisms with modern cardiovascular systems, suggesting that RGS2 arose from the R4-subfamily of RGS proteins to have specialized activity as a potent and selective Gα_q GAP that modulates cardiovascular function.G protein-coupled receptors (GPCRs)⁴ form an interface between extracellular and intracellular physiology, as they convert hormonal signals into changes in intracellular metabolism and ultimately cell phenotype and function (1–3). GPCRs are coupled to their underlying second messenger systems by heterotrimeric guanine nucleotide-binding protein (“G-proteins”) composed of three subunits: Gα, Gβ, and Gγ. Four general classes of Gα subunits have been defined based on functional couplings (in the GTP-bound state) to various effector proteins. G_s subfamily Gα subunits are stimulatory to membrane-bound adenylyl cyclases that generate the second messenger 3′,5′-cyclic adenosine monophosphate (cAMP); conversely, G_i subfamily Gα subunits are generally inhibitory to adenylyl cyclases (4). G_12/13 subfamily Gα subunits activate the small G-protein RhoA through stimulation of the GEF subfamily of RGS proteins, namely p115-RhoGEF, LARG, and PDZ-RhoGEF (5). G_q subfamily Gα subunits are potent activators of phospholipase-Cβ enzymes that generate the second messengers diacylglycerol and inositol triphosphate (6); more recently, two additional Gα_q effector proteins have been described: the receptor kinase GRK2 and the RhoA nucleotide exchange factor p63RhoGEF (7, 8).The duration of GPCR signaling is controlled by the time Gα remains bound to GTP before its hydrolysis to GDP. RGS proteins are key modulators of GPCR signaling by virtue of their ability to accelerate the intrinsic GTP hydrolysis activity of Gα subunits (reviewed in Refs. 9 and 10). RGS2/G0S8, one of the first mammalian RGS proteins identified (11) and member of the R4-subfamily (10), has a critical role in the maintenance of normostatic blood pressure both in mouse models (12, 13) and in humans (14, 15); additionally, Rgs2-deficient mice exhibit impaired aggression and increased anxiety (16, 17), behavioral phenotypes with potential human clinical correlates (18, 19).Although many RGS proteins are promiscuous and thus act on multiple Gα substrates in vitro (e.g. Ref. 20), RGS2 exhibits exquisite specificity for Gα_q in biochemical binding assays and single turnover GTPase acceleration assays (20, 21). Consistent with this in vitro selectivity,⁵ mice deficient in RGS2 uniquely exhibit constitutive hypertension and prolonged responses to vasoconstrictors, as would be expected upon loss of a potent negative regulator of Gα_q that mediates signaling from various vasoconstrictive hormones such as angiotensin II, endothelin, thrombin, norepinephrine, and vasopressin (22). In addition, RGS2-deficient mice respond to sustained pressure overload with an accelerated time course of maladaptive cardiac remodeling (23), a pathophysiological response that evokes myocardial hypertrophy known to be critically dependent on Gα_q signaling (24, 25).To gain insight into the structural basis of the unique Gα substrate selectivity exhibited by RGS2, a series of point mutants in RGS2 were evaluated that enable this protein to bind and accelerate GTP hydrolysis by Gα_i; we subsequently delineated the structural determinants of the Gα_i/mutant RGS2 interaction using x-ray crystallography. Three key positions, first identified by Heximer and colleagues (21) and highlighted in our structural studies as key determinants of RGS2 substrate selection, were also found to be conserved throughout the evolution of the RGS2 protein in a manner suggestive of specialization toward cardiovascular signaling modulation.     

Direct inhibition of the cold-activated TRPM8 ion channel by Gα(q)

Activation of the TRPM8 ion channel in sensory nerve endings produces a sensation of pleasant coolness. Here we show that inflammatory mediators such as bradykinin and histamine inhibit TRPM8 in intact sensory nerves, but do not do so through conventional signalling pathways. The G-protein subunit Gα(q) instead binds to TRPM8 and when activated by a Gq-coupled receptor directly inhibits ion channel activity. Deletion of Gα(q) largely abolished inhibition of TRPM8, and inhibition was rescued by a Gα(q) chimaera whose ability to activate downstream signalling pathways was completely ablated. Activated Gα(q) protein, but not Gβγ, potently inhibits TRPM8 in excised patches. We conclude that Gα(q) pre-forms a complex with TRPM8 and inhibits activation of TRPM8, following activation of G-protein-coupled receptors, by a direct action. This signalling mechanism may underlie the abnormal cold sensation caused by inflammation.     

Membrane Anchor R9AP Potentiates GTPase-accelerating Protein Activity of RGS11��G��5 Complex and Accelerates Inactivation of the mGluR6-Go Signaling

The R7 subfamily of RGS proteins critically regulates neuronal G protein-signaling pathways that are essential for vision, nociception, motor coordination, and reward processing. A member of the R7 RGS family, RGS11, is a GTPase-accelerating protein specifically expressed in retinal ON-bipolar cells where it forms complexes with the atypical G protein β subunit, Gβ₅, and transmembrane protein R9AP. Association with R9AP has been shown to be critical for the proteolytic stability of the complex in the retina. In this study we report that R9AP can in addition stimulate the GTPase-accelerating protein activity of the RGS11·Gβ₅ complex at Gα_o. Single turnover GTPase assays reveal that R9AP co-localizes RGS11·Gβ₅ and Gα_o on the membrane and allosterically potentiates the GTPase-accelerating function of RGS11·Gβ₅. Reconstitution of mGluR6-Gα_o signaling in Xenopus oocytes indicates that RGS11·Gβ₅-mediated GTPase acceleration in this system requires co-expression of R9AP. The results provide new insight into the regulation of mGluR6-Gα_o signaling by the RGS11·Gβ₅·R9AP complex and establish R9AP as a general GTPase-accelerating protein activity regulator of R7 RGS complexes.     

Ca2+/Calmodulin-dependent Protein Kinase IIα (αCaMKII) Controls the Activity of the Dopamine Transporter: IMPLICATIONS FOR ANGELMAN SYNDROME

The dopamine transporter (DAT) is a crucial regulator of dopaminergic neurotransmission, controlling the length and brevity of dopaminergic signaling. DAT is also the primary target of psychostimulant drugs such as cocaine and amphetamines. Conversely, methylphenidate and amphetamine are both used clinically in the treatment of attention-deficit hyperactivity disorder and narcolepsy. The action of amphetamines, which induce transport reversal, relies primarily on the ionic composition of the intra- and extracellular milieus. Recent findings suggest that DAT interacting proteins may also play a significant role in the modulation of reverse dopamine transport. The pharmacological inhibition of the serine/threonine kinase αCaMKII attenuates amphetamine-triggered DAT-mediated 1-methyl-4-phenylpyridinium (MPP(+)) efflux. More importantly, αCaMKII has also been shown to bind DAT in vitro and is therefore believed to be an important player within the DAT interactome. Herein, we show that αCaMKII co-immunoprecipitates with DAT in mouse striatal synaptosomes. Mice, which lack αCaMKII or which express a permanently self-inhibited αCaMKII (αCaMKII(T305D)), exhibit significantly reduced amphetamine-triggered DAT-mediated MPP(+) efflux. Additionally, we investigated mice that mimic a neurogenetic disease known as Angelman syndrome. These mice possess reduced αCaMKII activity. Angelman syndrome mice demonstrated an impaired DAT efflux function, which was comparable with that of the αCaMKII mutant mice, indicating that DAT-mediated dopaminergic signaling is affected in Angelman syndrome.     

Cleavage of Signal Regulatory Protein α (SIRPα) Enhances Inflammatory Signaling

Signal regulatory protein α (SIRPα) is a membrane glycoprotein immunoreceptor abundant in cells of monocyte lineage. SIRPα ligation by a broadly expressed transmembrane protein, CD47, results in phosphorylation of the cytoplasmic immunoreceptor tyrosine-based inhibitory motifs, resulting in the inhibition of NF-κB signaling in macrophages. Here we observed that proteolysis of SIRPα during inflammation is regulated by a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10), resulting in the generation of a membrane-associated cleavage fragment in both THP-1 monocytes and human lung epithelia. We mapped a charge-dependent putative cleavage site near the membrane-proximal domain necessary for ADAM10-mediated cleavage. In addition, a secondary proteolytic cleavage within the membrane-associated SIRPα fragment by γ-secretase was identified. Ectopic expression of a SIRPα mutant plasmid encoding a proteolytically resistant form in HeLa cells inhibited activation of the NF-κB pathway and suppressed STAT1 phosphorylation in response to TNFα to a greater extent than expression of wild-type SIRPα. Conversely, overexpression of plasmids encoding the proteolytically cleaved SIRPα fragments in cells resulted in enhanced STAT-1 and NF-κB pathway activation. Thus, the data suggest that combinatorial actions of ADAM10 and γ-secretase on SIRPα cleavage promote inflammatory signaling.     

Regulation of the AGS3·Gαi Signaling Complex by a Seven-transmembrane Span Receptor

G-protein signaling modulators (GPSM) play diverse functional roles through their interaction with G-protein subunits. AGS3 (GPSM1) contains four G-protein regulatory motifs (GPR) that directly bind Gα_i free of Gβγ providing an unusual scaffold for the “G-switch” and signaling complexes, but the mechanism by which signals track into this scaffold are not well understood. We report the regulation of the AGS3·Gα_i signaling module by a cell surface, seven-transmembrane receptor. AGS3 and Gα_i1 tagged with Renilla luciferase or yellow fluorescent protein expressed in mammalian cells exhibited saturable, specific bioluminescence resonance energy transfer indicating complex formation in the cell. Activation of α₂-adrenergic receptors or μ-opioid receptors reduced AGS3-RLuc·Gα_i1-YFP energy transfer by over 30%. The agonist-mediated effects were inhibited by pertussis toxin and co-expression of RGS4, but were not altered by Gβγ sequestration with the carboxyl terminus of GRK2. Gα_i-dependent and agonist-sensitive bioluminescence resonance energy transfer was also observed between AGS3 and cell-surface receptors typically coupled to Gα_i and/or Gα_o indicating that AGS3 is part of a larger signaling complex. Upon receptor activation, AGS3 reversibly dissociates from this complex at the cell cortex. Receptor coupling to both Gαβγ and GPR-Gα_i offer additional flexibility for systems to respond and adapt to challenges and orchestrate complex behaviors.     

Structure,Function, and Dynamics of the Gα Binding Domain of Ric-8A

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Crystal Structure of the TNF-α-Inducing Protein (Tipα) from Helicobacter pylori: Insights into Its DNA-Binding Activity

Helicobacter pylori infection is one of the highest risk factors for gastroduodenal diseases including gastric cancer. Tumor necrosis factor-alpha (TNF-α) is one of the essential cytokines for tumor promotion, and thus, an H. pylori protein that induces TNF-α is believed to play a significant role in gastric cancer development in humans. The HP0596 gene product of H. pylori strain 26695 was identified as the TNF-α-inducing protein (Tipα). Tipα is secreted from H. pylori as dimers and enters the gastric cells. It was shown to have a DNA-binding activity. Here, we have determined the crystal structure of Tipα from H. pylori. Its monomer consists of two structural domains (“mixed domain” and “helical domain”). Tipα exists as a dimer in the crystal, and the dimeric structure represents a novel scaffold for DNA binding. A positively charged surface patch formed across the two monomers of the Tipα dimer by the loop between helices α1 and α2 may be important in DNA binding.     

A Highlights from MBoC Selection: RSBP-1 Is a Membrane-targeting Subunit Required by the Gαq-specific But Not the Gαo-specific R7 Regulator of G protein Signaling in Caenorhabditis elegans

Regulator of G protein signaling (RGS) proteins inhibit G protein signaling by activating Gα 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.     

HOIL-1L Interacting Protein (HOIP) as an NF-κB Regulating Component of the CD40 Signaling Complex

The tumor necrosis factor receptor (TNFR) superfamily mediates signals critical for regulation of the immune system. One family member, CD40, is important for the efficient activation of antibody-producing B cells and other antigen-presenting cells. The molecules and mechanisms that mediate CD40 signaling are only partially characterized. Proteins known to interact with the cytoplasmic domain of CD40 include members of the TNF receptor-associated factor (TRAF) family, which regulate signaling and serve as links to other signaling molecules. To identify additional proteins important for CD40 signaling, we used a combined stimulation/immunoprecipitation procedure to isolate CD40 signaling complexes from B cells and characterized the associated proteins by mass spectrometry. In addition to known CD40-interacting proteins, we detected SMAC/DIABLO, HTRA2/Omi, and HOIP/RNF31/PAUL/ZIBRA. We found that these previously unknown CD40-interacting partners were recruited in a TRAF2-dependent manner. HOIP is a ubiquitin ligase capable of mediating NF-κB activation through the ubiquitin-dependent activation of IKKγ. We found that a mutant HOIP molecule engineered to lack ubiquitin ligase activity inhibited the CD40-mediated activation of NF-κB. Together, our results demonstrate a powerful approach for the identification of signaling molecules associated with cell surface receptors and indicate an important role for the ubiquitin ligase activity of HOIP in proximal CD40 signaling.