Loss of association between activated Galpha q and Gbetagamma disrupts receptor-dependent and receptor-independent signaling

The guanine nucleotide exchange factor Ric-8A regulates the sensitivity of constitutively active Gαq to the inhibitor YM-254890

Heterotrimeric G proteins are stimulated under normal circumstances by G protein-coupled receptors to promote downstream intracellular signaling. Mutations can occur in αq at glutamine 209 (Q209) that cause constitutive, G protein-coupled receptor independent signaling due to disruption of GTPase activity. Specifically, Q209L/P mutations are oncogenic drivers of uveal melanoma. YM-254890 (YM) has been shown to selectively inhibit both WT and constitutively active (CA) αqQ209L/P by preventing the release of GDP and exchange for GTP, thereby halting downstream signaling. Because αqQL/P are thought to be primarily GTP-bound and GTPase deficient, the current mechanistic understanding of YM inhibition needs further investigation to clarify how a GDP-dissociation inhibitor could potently inhibit these oncogenic mutants. Here, we expand on the current knowledge of CA αq cellular regulation by demonstrating a direct role for the αq chaperone and guanine nucleotide exchange factor Ric-8A in YM sensitivity. Through signaling assays in RIC-8A KO cells, we found that myristoylated αqQL/P mutants (αqAG-QL/P), previously demonstrated to be YM-resistant, became YM-sensitive, and this was reversed by reintroduction of Ric-8A. Additionally, αqQL demonstrated increased YM sensitivity in the absence of Ric-8A, which was directly altered by the reintroduction of Ric-8A. Pull-down and BRET assays with the RGS-homology domain of GRK2, which can only bind activated αq, further demonstrated that Ric-8A expression enhances activation of αq, its ability to bind effectors, and therefore its ability to signal. With the understanding of YM acting as a GDP-dissociation inhibitor, we propose that Ric-8A hinders YM inhibitory effects by promoting GTP-bound, activated αqQL/P.

Going Rogue: Mechanisms, Regulation, and Roles of Mutationally Activated Gα in Human Cancer

G protein-coupled receptors (GPCRs) couple to heterotrimeric G proteins, comprised of α and βγ subunits, to convert extracellular signals into activation of intracellular signaling pathways. Canonically, GPCR-mediated activation results in the exchange of GDP for GTP on G protein α subunits (Gα) and the dissociation of Gα-GTP and G protein βγ subunits (Gβγ), both of which can regulate a variety of signaling pathways. Hydrolysis of bound GTP by Gα returns the protein to Gα-GDP and allows reassociation with Gβγ to reform the inactive heterotrimer. Naturally occurring mutations in Gα have been found at conserved glutamine and arginine amino acids that disrupt the canonical G protein cycle by inhibiting GTP hydrolysis, rendering these mutants constitutively active. Interestingly, these dysregulated Gα mutants are found in many different cancers due to their ability to sustain aberrant signaling without a need for activation by GPCRs. This review will highlight an increased recognition of the prevalence of such constitutively activating Gα mutations in cancers and the signaling pathways activated. In addition, we will discuss new knowledge regarding how these constitutively active Gα are regulated, how different mutations are biochemically distinct, and how mutationally activated Gα are unique compared with GPCR-activated Gα Lastly, we will discuss recent progress in developing inhibitors directly targeting constitutively active Gα mutants. SIGNIFICANCE STATEMENT: Constitutively activating mutations in G protein α subunits (Gα) widely occur in and contribute to the development of many human cancers. To develop ways to inhibit dysregulated, oncogenic signaling by these mutant Gα, it is crucial to better understand mechanisms that lead to constitutive Gα activation and unique mechanisms that regulate mutationally activated Gα in cells. The prevalence of activating mutations in Gα in various cancers makes Gα proteins compelling targets for the development of therapeutics.

Inhibition of adenylyl cyclase by GTPase-deficient Gαi is mechanistically different from that mediated by receptor-activated Gαi

Signal transduction through G protein-coupled receptors (GPCRs) has been a major focus in cell biology for decades. Numerous disorders are associated with GPCRs that utilize Gi proteins to inhibit adenylyl cyclase (AC) as well as regulate other effectors. Several early studies have successfully defined the AC-interacting domains of several members of Gαi by measuring the loss of activity upon homologous replacements of putative regions of constitutive active Gαi mutants. However, whether such findings can indeed be translated into the context of a receptor-activated Gαi have not been rigorously verified. To address this issue, an array of known and new chimeric mutations was introduced into GTPase-deficient Q204L (QL) and R178C (RC) mutants of Gαi1, followed by examinations on their ability to inhibit AC. Surprisingly, most chimeras failed to abolish the constitutive activity brought on by the QL mutation, while some were able to eliminate the inhibitory activity of RC mutants. Receptor-mediated inhibition of AC was similarly observed in the same chimeric constructs harbouring the pertussis toxin (PTX)-resistant C351I mutation. Moreover, RC-bearing loss-of-function chimeras appeared to be hyper-deactivated by endogenous RGS protein. Molecular docking revealed a potential interaction between AC and the α3/β5 loop of Gαi1. Subsequent cAMP assays support a cooperative action of the α3/β5 loop, the α4 helix, and the α4/β6 loop in mediating AC inhibition by Gαi1-i3. Our results unveiled a notable functional divergence between constitutively active mutants and receptor-activated Gαi1 to inhibit AC, and identified a previously unknown AC-interacting domain of Gαi subunits. These results collectively provide valuable insights on the mechanism of AC inhibition in the cellular environment.

Disruption of the interaction between mutationally activated Gαq and Gβγ attenuates aberrant signaling

Heterotrimeric G protein stimulation via G protein-coupled receptors promotes downstream proliferative signaling. Mutations can occur in Gα proteins which prevent GTP hydrolysis; this allows the G proteins to signal independently of G protein-coupled receptors and can result in various cancers, such as uveal melanoma (UM). Most UM cases harbor Q209L, Q209P, or R183C mutations in Gα_q/11 proteins, rendering the proteins constitutively active (CA). Although it is generally thought that active, GTP-bound Gα subunits are dissociated from and signal independently of Gβγ, accumulating evidence indicates that some CA Gα mutants, such as Gα_q/11, retain binding to Gβγ, and this interaction is necessary for signaling. Here, we demonstrate that disrupting the interaction between Gβγ and Gα_q is sufficient to inhibit aberrant signaling driven by CA Gα_q. Introduction of the I25A point mutation in the N-terminal α helical domain of CA Gα_q to inhibit Gβγ binding, overexpression of the G protein Gα_o to sequester Gβγ, and siRNA depletion of Gβ subunits inhibited or abolished CA Gα_q signaling to the MAPK and YAP pathways. Moreover, in HEK 293 cells and in UM cell lines, we show that Gα_q-Q209P and Gα_q-R183C are more sensitive to the loss of Gβγ interaction than Gα_q-Q209L. Our study challenges the idea that CA Gα_q/11 signals independently of Gβγ and demonstrates differential sensitivity between the Gα_q-Q209L, Gα_q-Q209P, and Gα_q-R183C mutants.

Enhanced membrane binding of oncogenic G protein αqQ209L confers resistance to inhibitor YM-254890

Heterotrimeric G proteins couple activated G protein-coupled receptors (GPCRs) to intracellular signaling pathways. They can also function independently of GPCR activation upon acquiring mutations that prevent GTPase activity and result in constitutive signaling, as occurs with the αqQ209L mutation in uveal melanoma. YM-254890 (YM) can inhibit signaling by both GPCR-activated WT αq and GPCR-independent αqQ209L. Although YM inhibits WT αq by binding to αq-GDP and preventing GDP/GTP exchange, the mechanism of YM inhibition of cellular αqQ209L remains to be fully understood. Here, we show that YM promotes a subcellular redistribution of αqQ209L from the plasma membrane (PM) to the cytoplasm. To test if this loss of PM localization could contribute to the mechanism of inhibition of αqQ209L by YM, we developed and examined N-terminal mutants of αqQ209L, termed PM-restricted αqQ209L, in which the addition of membrane-binding motifs enhanced PM localization and prevented YM-promoted redistribution. Treatment of cells with YM failed to inhibit signaling by these PM-restricted αqQ209L. Additionally, pull-down experiments demonstrated that YM promotes similar conformational changes in both αqQ209L and PM-restricted αqQ209L, resulting in increased binding to βγ and decreased binding to regulator RGS2, and effectors p63RhoGEF-DH/PH and phospholipase C-β. GPCR-dependent signaling by PM-restricted WT αq is strongly inhibited by YM, demonstrating that resistance to YM inhibition by membrane-binding mutants is specific to constitutively active αqQ209L. Together, these results indicate that changes in membrane binding impact the ability of YM to inhibit αqQ209L and suggest that YM contributes to inhibition of αqQ209L by promoting its relocalization.

Real-time visualization of heterotrimeric G protein Gq activation in living cells

Background

Gq is a heterotrimeric G protein that plays an important role in numerous physiological processes. To delineate the molecular mechanisms and kinetics of signalling through this protein, its activation should be measurable in single living cells. Recently, fluorescence resonance energy transfer (FRET) sensors have been developed for this purpose.

Results

In this paper, we describe the development of an improved FRET-based Gq activity sensor that consists of a yellow fluorescent protein (YFP)-tagged Gγ2 subunit and a Gαq subunit with an inserted monomeric Turquoise (mTurquoise), the best cyan fluorescent protein variant currently available. This sensor enabled us to determine, for the first time, the k_on (2/s) of Gq activation. In addition, we found that the guanine nucleotide exchange factor p63RhoGEF has a profound effect on the number of Gq proteins that become active upon stimulation of endogenous histamine H1 receptors. The sensor was also used to measure ligand-independent activation of the histamine H1 receptor (H1R) upon addition of a hypotonic stimulus.

Conclusions

Our observations reveal that the application of a truncated mTurquoise as donor and a YFP-tagged Gγ2 as acceptor in FRET-based Gq activity sensors substantially improves their dynamic range. This optimization enables the real-time single cell quantification of Gq signalling dynamics, the influence of accessory proteins and allows future drug screening applications by virtue of its sensitivity.

Real-time BRET assays to measure G protein/effector interactions

Advances in imaging assays based on resonance energy transfer (RET) have made it possible to study protein/protein interactions in living cells under physiological conditions. It is now possible to measure the kinetics of changes in these interactions in response to ligand stimulation in real time. Here we describe protocols for these assays focusing on the basal and ligand-stimulated interaction between tagged Gβγ subunits and adenylyl cyclase II. We describe relevant positive and negative controls and various experimental considerations for optimization of these experiments.

An N-terminal polybasic motif of Gαq is required for signaling and influences membrane nanodomain distribution

Regions of basic amino acids in proteins can promote membrane localization through electrostatic interactions with negatively charged membrane lipid head groups. Previous work showed that the heterotrimeric G protein subunit α(q) contains a polybasic region in its N terminus that contributes to plasma membrane localization. Here, the role of the N-terminal polybasic region of α(q) in signaling was addressed. For α(q) mutants, loss of plasma membrane localization correlated with loss of signaling function, as measured by the ability to couple activated G protein-coupled receptors (GPCRs) to stimulation of inositol phosphate production. However, recovery of plasma membrane localization of α(q) polybasic mutants by introduction of a site for myristoylation or by coexpression of βγ failed to recover signaling, suggesting a role for N-terminal basic amino acids of α(q) beyond simple plasma membrane localization. It is noteworthy that an α(q)4Q mutant, containing glutamine substitutions at arginines 27, 30, 31, and 34, was identified that failed to mediate signaling yet retained plasma membrane localization. Although α(q)4Q failed to couple activated receptors to inositol phosphate production, it was able to bind βγ, bind RGS4 in an activation-dependent manner, stimulate inositol phosphate production in a receptor-independent manner, and productively interact with a GPCR in isolated membranes. It is noteworthy that α(q)4Q showed a differing localization to plasma membrane nanodomains compared with wild-type α(q). Thus, basic amino acids in the N terminus of α(q) can affect its lateral segregation on plasma membranes, and changes in such lateral segregation may be responsible for the observed signaling defects of α(q)4Q.

Combining resonance energy transfer methods reveals a complex between the alpha2A-adrenergic receptor, Galphai1beta1gamma2, and GRK2

Traditionally, G-protein-coupled receptor (GPCR) interactions with their G proteins and regulatory proteins, GPCR kinases (GRKs) and arrestins, are described as sequential events involving rapid assemblies/disassemblies. To directly monitor the dynamics of these interactions in living cells, we combined two spectrally resolved bioluminescence and one fluorescence resonance energy transfer (RET) methods. The RET combination analysis revealed that stimulation of the α(2A)-adrenergic receptor (α(2A)AR) leads to the recruitment of GRK2 at a receptor still associated with the Gα(i1)β(1)γ(2) complex. The interaction kinetics of GRKs with Gγ(2) (2.8 ± 0.4 s) and α(2A)AR (5.2 ± 0.5 s) were similar to that of the receptor-promoted change in RET between Gα(i1) and Gγ(2) (5.2 ± 1.2 s), and persisted until the translocation of βarrestin2 to the receptor, indicating that GRK2 remains associated to the receptor/G-protein complex for longer periods than anticipated. Moreover, GRK2 or a kinase-deficient GRK2 mutant, but not GRK5, potentiated the receptor-promoted changes in RET between Gα(i1) and Gγ(2) and abrogated the α(2A)AR-stimulated calcium response, suggesting that the recruitment of GRK2 to the complex contributes to the structural rearrangement and functional regulation of the signaling unit, independently of the kinase activity. RET combination analysis revealed unanticipated dynamics in GPCR signaling and will be applicable to many biological systems.

Fluorescence changes reveal kinetic steps of muscarinic receptor-mediated modulation of phosphoinositides and Kv7.2/7.3 K+ channels

G protein–coupled receptors initiate signaling cascades. M₁ muscarinic receptor (M₁R) activation couples through Gα_q to stimulate phospholipase C (PLC), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂). Depletion of PIP₂ closes PIP₂-requiring Kv7.2/7.3 potassium channels (M current), thereby increasing neuronal excitability. This modulation of M current is relatively slow (6.4 s to reach within 1/e of the steady-state value). To identify the rate-limiting steps, we investigated the kinetics of each step using pairwise optical interactions likely to represent fluorescence resonance energy transfer for M₁R activation, M₁R/Gβ interaction, Gα_q/Gβ separation, Gα_q/PLC interaction, and PIP₂ hydrolysis. Electrophysiology was used to monitor channel closure. Time constants for M₁R activation (<100 ms) and M₁R/Gβ interaction (200 ms) are both fast, suggesting that neither of them is rate limiting during muscarinic suppression of M current. Gα_q/Gβ separation and Gα_q/PLC interaction have intermediate 1/e times (2.9 and 1.7 s, respectively), and PIP₂ hydrolysis (6.7 s) occurs on the timescale of M current suppression. Overexpression of PLC accelerates the rate of M current suppression threefold (to 2.0 s) to become nearly contemporaneous with Gα_q/PLC interaction. Evidently, channel release of PIP₂ and closure are rapid, and the availability of active PLC limits the rate of M current suppression.

G protein subunit dissociation and translocation regulate cellular response to receptor stimulation

We examined the role of G proteins in modulating the response of living cells to receptor activation. The response of an effector, phospholipase C-β to M3 muscarinic receptor activation was measured using sensors that detect the generation of inositol triphosphate or diacylglycerol. The recently discovered translocation of Gβγ from plasma membrane to endomembranes on receptor activation attenuated this response. A FRET based G protein sensor suggested that in contrast to translocating Gβγ, non-translocating Gβγ subunits do not dissociate from the αq subunit on receptor activation leading to prolonged retention of the heterotrimer state and an accentuated response. M3 receptors with tethered αq induced differential responses to receptor activation in cells with or without an endogenous translocation capable γ subunit. G protein heterotrimer dissociation and βγ translocation are thus unanticipated modulators of the intensity of a cell's response to an extracellular signal.

ADP-ribosylation factors regulate the development of CT signaling in immature human enterocytes

Diarrheal disease is a major cause of morbidity and mortality in infants and children worldwide. Evidence suggests that the interaction of immature human enterocytes with bacteria and their enterotoxins may account for the increased susceptibility of neonates to diarrheal diseases. However, the precise mechanisms that contribute to the excessive response to cholera toxin by the immature gut are largely unknown. Our aim was to characterize the cellular/molecular changes in Gs(alpha) during gut development. In this study, a colonic human epithelial cell line (T84) was used as representative of a mature enterocyte and a human fetal primary small intestinal cell line (H4) as representative of an immature enterocyte. Using our cell culture model of human intestinal development, we provide consistent evidence that cholera toxin (CT)-mediated Gs(alpha) activation in fetal enterocytes differs from that of mature enterocytes, and the difference may be related to ADP-ribosylation factor (ARF) interaction with the CT-signaling process. Here we demonstrated that ARF1 may play a critical role in clathrin-mediated CT trafficking through the endoplasmic reticulum and Golgi and that ARF6 may facilitate clathrin-mediated CT endocytosis that leads to enhanced Gs(alpha) activation by CT. Collectively, these findings support our hypothesis that there is a developmentally regulated intestinal cellular response to bacterial exotoxins involving complex cellular events that accounts for the increased incidence and severity of toxogenic diarrhea during infancy.

The role of Gbetagamma subunits in the organization, assembly, and function of GPCR signaling complexes

The role of Gbetagamma subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Galpha subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gbetagamma subunits interact with G protein-coupled receptors, Galpha subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gbetagamma assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gbetagamma subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

N-terminal polybasic motifs are required for plasma membrane localization of Galpha(s) and Galpha(q)

Heterotrimeric G proteins typically localize at the cytoplasmic face of the plasma membrane where they interact with heptahelical receptors. For G protein alpha subunits, multiple membrane targeting signals, including myristoylation, palmitoylation, and interaction with betagamma subunits, facilitate membrane localization. Here we show that an additional membrane targeting signal, an N-terminal polybasic region, plays a key role in plasma membrane localization of non-myristoylated alpha subunits. Mutations of N-terminal basic residues in alpha(s) and alpha(q) caused defects in plasma membrane localization, as assessed through immunofluorescence microscopy and biochemical fractionations. In alpha(s), mutation of four basic residues to glutamine was sufficient to cause a defect, whereas in alpha(q) a defect in membrane localization was not observed unless nine basic residues were mutated to glutamine or if three basic residues were mutated to glutamic acid. betagamma co-expression only partially rescued the membrane localization defects; thus, the polybasic region is also important in the context of the heterotrimer. Introduction of a site for myristoylation into the polybasic mutants of alpha(s) and alpha(q) recovered strong plasma membrane localization, indicating that myristoylation and polybasic motifs may have complementary roles as membrane targeting signals. Loss of plasma membrane localization coincided with defects in palmitoylation. The polybasic mutants of alpha(s) and alpha(q) were still capable of assuming activated conformations and stimulating second messenger production, as demonstrated through GST-RGS4 interaction assays, cAMP assays, and inositol phosphate assays. Electrostatic interactions with membrane lipids have been found to be important in plasma membrane targeting of many proteins, and these results provide evidence that basic residues play a role in localization of G protein alpha subunits.

Dissociation of heterotrimeric g proteins in cells

Heterotrimeric G proteins dissociate into their component Galpha and Gbetagamma subunits when these proteins are activated in solution. Until recently, it has not been known if subunit dissociation also occurs in cells. The development of optical methods to study G protein activation in live cells has made it possible to demonstrate heterotrimer dissociation at the plasma membrane. However, subunit dissociation is far from complete, and many active [guanosine triphosphate (GTP)-bound] heterotrimers are intact in a steady state. This unexpectedly reluctant dissociation calls for inclusion of a GTP-bound heterotrimeric state in models of the G protein cycle and places renewed emphasis on the relation between subunit dissociation and effector activation.

Caveolin-1 alters Ca(2+) signal duration through specific interaction with the G alpha q family of G proteins

Caveolae are membrane domains having caveolin-1 (Cav1) as their main structural component. Here, we determined whether Cav1 affects Ca(2+) signaling through the Galpha(q)-phospholipase-Cbeta (PLCbeta) pathway using Fischer rat thyroid cells that lack Cav1 (FRTcav(-)) and a sister line that forms caveolae-like domains due to stable transfection with Cav1 (FRTcav(+)). In the resting state, we found that eCFP-Gbetagamma and Galpha(q)-eYFP are similarly associated in both cell lines by Forster resonance energy transfer (FRET). Upon stimulation, the amount of FRET between Galpha(q)-eYFP and eCFP-Gbetagamma remains high in FRTcav(-) cells, but decreases almost completely in FRTcav(+) cells, suggesting that Cav1 is increasing the separation between Galpha(q)-Gbetagamma subunits. In FRTcav(-) cells overexpressing PLCbeta, a rapid recovery of Ca(2+) is observed after stimulation. However, FRTcav(+) cells show a sustained level of elevated Ca(2+). FRET and colocalization show specific interactions between Galpha(q) and Cav1 that increase upon stimulation. Fluorescence correlation spectroscopy studies show that the mobility of Galpha(q)-eGFP is unaffected by activation in either cell type. The mobility of eGFP-Gbetagamma remains slow in FRTcav(-) cells but increases in FRTcav(+) cells. Together, our data suggest that, upon stimulation, Galpha(q)(GTP) switches from having strong interactions with Gbetagamma to Cav1, thereby releasing Gbetagamma. This prolongs the recombination time for the heterotrimer, thus causing a sustained Ca(2+) signal.

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

Pertussis toxin induces parallel loss of neuropeptide Y Y1 receptor dimers and Gi alpha subunit function in CHO cells

Treatment with pertussis toxin in addition to a stable inhibition of G(i)alpha subunits of G-proteins also strongly reduced human neuropeptide Y Y(1) receptors expressed in Chinese hamster ovary (CHO) cells. This was reflected in abolition of the inhibition by Y(1) agonists of forskolin-stimulated adenylyl cyclase in intact cells, and of Y(1) agonist stimulation of GTPgammaS binding to particulates from disrupted cells. The loss of both receptor and G(i)alpha subunit function was attenuated by ammonium chloride, an inhibitor of acid proteinases, pointing to a chaperoning co-protection of active pertussis toxin-sensitive Galpha subunits and Y(1) receptors. The surface complement of the Y(1) receptor was changed a little in conditions of approximately 85% decrease of the Y(1) population, but the rate of the Y(1) receptor-linked internalization of agonist peptides was reduced about 70%. The preserved receptor fraction consisted of monomers significantly coupled to G(q)alpha subunits. The persistent pertussis toxin-insensitive internalization of agonists with the Y(1) receptor may reflect a rescue or alternative switching that could be important for cell functioning in neuropeptide Y-rich environments. The results are compatible with a loss, due to G(i)alpha subunit inactivation by the toxin, of a large Y(1) receptor reserve constituted of oligomers associating with heterotrimeric G-proteins.

Agonist induced-phosphorylation of Galpha11 protein reduces coupling to 5-HT2A receptors

We previously demonstrated that 24-h treatment with (-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) causes phosphorylation of Galpha11 protein at serine 154 and that this phosphorylation causes desensitization of serotonin (5-HT) 2A receptor signaling in A1A1v cells (Shi et al., 2007). We now report that treatment of A1A1v cells with DOI for 24 h produces a greater reduction in the Bmax of [125I](+/-)-DOI-labeled high-affinity binding sites (46%) than the reduction of [3H]ketanserin binding sites (25%). Although the KD values are not altered, there is a smaller amount of GTPgammaS [guanosine 5'-3-O-(thio)triphosphate]-sensitive [125I](+/-)-DOI binding in DOI-treated cells. These results suggest that DOI treatment causes down-regulation of 5-HT2A receptors and reductions in G protein-coupled 5-HT2A receptors. In contrast, in cells transfected with the phosphorylation state mimic G(alpha11)S154D, GTPgammaS-sensitive [125I](+/-)-DOI binding was decreased by 48%; however, there was no significant difference in the KD and Bmax values of [125I](+/-)-DOI-labeled receptors. The receptor binding experiments suggest that phosphorylation of Galpha11 on serine 154 reduces coupling of 5-HT2A receptors, whereas DOI causes down-regulation of 5-HT2A receptors in addition to the phosphorylation-induced uncoupling of Galpha11 to 5-HT2A receptors. To determine whether DOI increases phosphorylation of Galphaq/11 protein in vivo, rats were treated with 1 mg/kg/day DOI or saline for 1 to 7 days. Seven days of DOI treatment significantly decreased phospholipase C activity stimulated by an Emax concentration of 5-HT by 40% and increased phosphorylation of Galphaq/11 proteins by 51% in the frontal cortex. These data suggest that DOI causes phosphorylation of Galphaq/11 in vivo and could thereby contribute to the desensitization of 5-HT2A receptors.

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.

Assembly and trafficking of heterotrimeric G proteins

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This current topic will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.

Signalling complexes associated with adenylyl cyclase II are assembled during their biosynthesis

We have previously demonstrated that adenylyl cyclase II (ACII) interacts with beta2-adrenergic receptors and heterotrimeric G proteins as part of a pre-assembled signalling complex. In this study, we further show that AC interacts with these proteins before it is targetted to the cell surface. Using a combination of approaches including bioluminescence resonance energy transfer (BRET) in concert with subcellular fractionation, we show that ACII and beta2AR initially interact in the ER. Further, dominant-negative Rab1 and Sar1 GTPases which block anterograde trafficking out of the ER have no effect on either ACII/receptor or ACII/Gbetagamma protein interactions. However, DN Rab1 and Sar1 constructs (but not DN Rabs 2, 6, 8 or 11) prevent the inclusion of Galpha subunits in ACII signalling complexes suggesting it assembles into the complex at a slightly later stage. Thus, like Kir3.1 inwardly rectifying potassium channels, signalosomes containing ACII are formed during their biosynthesis and not in response to agonist at the cell surface.

Some G protein heterotrimers physically dissociate in living cells

Heterotrimeric G proteins mediate physiological processes ranging from phototransduction to cell migration. In the accepted model of G protein signaling, Galphabetagamma heterotrimers physically dissociate after activation, liberating free Galpha subunits and Gbetagamma dimers. This model is supported by evidence obtained in vitro with purified proteins, but its relevance in vivo has been questioned. Here, we show that at least some heterotrimeric G protein isoforms physically dissociate after activation in living cells. Galpha subunits extended with a transmembrane (TM) domain and cyan fluorescent protein (CFP) were immobilized in the plasma membrane by biotinylation and cross-linking with avidin. Immobile CFP-TM-Galpha greatly decreased the lateral mobility of intracellular Gbeta1gamma2-YFP, indicating the formation of stable heterotrimers. A GTPase-deficient (constitutively active) mutant of CFP-TM-GalphaoA lost the ability to restrict Gbeta1gamma2-YFP mobility, whereas GTPase-deficient mutants of CFP-TM-Galphai3 and CFP-TM-Galphas retained this ability. Activation of cognate G protein-coupled receptors partially relieved the constraint on Gbeta1gamma2-YFP mobility induced by immobile CFP-TM-GalphaoA and CFP-TM-Galphai3 but had no effect on the constraint induced by CFP-TM-Galphas. These results demonstrate the physical dissociation of heterotrimers containing GalphaoA and Galphai3 subunits in living cells, supporting the subunit dissociation model of G protein signaling for these subunits. However, these results are also consistent with the suggestion that G protein heterotrimers (e.g., Galphas) may signal without physically dissociating.

Stable Association between Gαq and Phospholipase Cβ1 in Living Cells

Signal transduction through G alpha(q) involves stimulation of phospholipase C beta (PLC beta) that results in increased intracellular Ca2+ and activation of protein kinase C. We have measured complex formation between G alpha(q) and PLC beta1 in vitro and in living PC12 and HEK293 cells by fluorescence resonance energy transfer. In vitro measurements show that PLC beta1 will bind to G alpha(q)(guanosine 5'-3-O-(thio)triphosphate) and also to G alpha(q)(GDP), and the latter association has a different protein-protein orientation. In cells, image analysis of fluorescent-tagged proteins shows that G alpha(q) is localized almost entirely to the plasma membrane, whereas PLC beta1 has a significant cytosolic population. By using fluorescence resonance energy transfer, we found that these proteins are pre-associated in the unstimulated state in PC12 and HEK293 cells. By determining the cellular levels of the two proteins in transfected versus nontransfected cells, we found that under our conditions overexpression should not significantly promote complex formation. G alpha(q)-PLC beta1 complexes are observed in both single cell measurements and measurements of a large (i.e. 10(6)) cell suspension. The high level (approximately 40% maximum) of FRET is surprising considering that G alpha(q) is more highly expressed than PLC beta1 and that not all PLC beta1 is plasma membrane-localized. Our measurements suggest a model in which G proteins and effectors can exist in stable complexes prior to activation and that activation is achieved through changes in intermolecular interactions rather than diffusion and association. These pre-formed complexes in turn give rise to rapid, localized signals.

Heterotrimeric G proteins form stable complexes with adenylyl cyclase and Kir3.1 channels in living cells

Bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several G-protein subunits (Gαs, Gαi, Gβ1 and Gγ2) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically Gβ1-YFP1-158 and Gγ2-YFP159-238, which heterodimerize to produce fluorescent YFP-Gβ1γ2). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-Gγ2, GFP-Gβ1 or YFP-Gβ1γ2. Gα subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and Gβγ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.