Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Beyond the G protein α subunit: investigating the functional impact of other components of the Gαi3 heterotrimers

BACKGROUND

Specific interactions between G protein-coupled receptors (GPCRs) and G proteins play a key role in mediating signaling events. While there is little doubt regarding receptor preference for Gα subunits, the preferences for specific Gβ and Gγ subunits and the effects of different Gβγ dimer compositions on GPCR signaling are poorly understood. In this study, we aimed to investigate the subcellular localization and functional response of Gαi3-based heterotrimers with different combinations of Gβ and Gγ subunits.

METHODS

Live-cell imaging microscopy and colocalization analysis were used to investigate the subcellular localization of Gαi3 in combination with Gβ1 or Gβ2 heterotrimers, along with representative Gγ subunits. Furthermore, fluorescence lifetime imaging microscopy (FLIM-FRET) was used to investigate the nanoscale distribution of Gαi3-based heterotrimers in the plasma membrane, specifically with the dopamine D2 receptor (D2R). In addition, the functional response of the system was assessed by monitoring intracellular cAMP levels and conducting bioinformatics analysis to further characterize the heterotrimer complexes.

RESULTS

Our results show that Gαi3 heterotrimers mainly localize to the plasma membrane, although the degree of colocalization is influenced by the accompanying Gβ and Gγ subunits. Heterotrimers containing Gβ2 showed slightly lower membrane localization compared to those containing Gβ1, but certain combinations, such as Gαi3β2γ8 and Gαi3β2γ10, deviated from this trend. Examination of the spatial arrangement of Gαi3 in relation to D2R and of changes in intracellular cAMP level showed that the strongest functional response is observed for those trimers for which the distance between the receptor and the Gα subunit is smallest, i.e. complexes containing Gβ1 and Gγ8 or Gγ10 subunit. Deprivation of Gαi3 lipid modifications resulted in a significant decrease in the amount of protein present in the cell membrane, but did not always affect intracellular cAMP levels.

CONCLUSION

Our studies show that the composition of G protein heterotrimers has a significant impact on the strength and specificity of GPCR-mediated signaling. Different heterotrimers may exhibit different conformations, which further affects the interactions of heterotrimers and GPCRs, as well as their interactions with membrane lipids. This study contributes to the understanding of the complex signaling mechanisms underlying GPCR-G-protein interactions and highlights the importance of the diversity of Gβ and Gγ subunits in G-protein signaling pathways. Video Abstract.

N-terminal intrinsic disorder is an ancestral feature of Gγ subunits that influences the balance between different Gβγ signaling axes in yeast

Activated G protein-coupled receptors promote the dissociation of heterotrimeric G proteins into Gα and Gβγ subunits that bind to effector proteins to drive intracellular signaling responses. In yeast, Gβγ subunits coordinate the simultaneous activation of multiple signaling axes in response to mating pheromones, including MAP kinase (MAPK)-dependent transcription, cell polarization, and cell cycle arrest responses. The Gγ subunit in this complex contains an N-terminal intrinsically disordered region that governs Gβγ-dependent signal transduction in yeast and mammals. Here, we demonstrate that N-terminal intrinsic disorder is likely an ancestral feature that has been conserved across different Gγ subtypes and organisms. To understand the functional contribution of structural disorder in this region, we introduced precise point mutations that produce a stepwise disorder-to-order transition in the N-terminal tail of the canonical yeast Gγ subunit, Ste18. Mutant tail structures were confirmed using circular dichroism and molecular dynamics and then substituted for the wildtype gene in yeast. We find that increasing the number of helix-stabilizing mutations, but not isometric mutation controls, has a negative and proteasome-independent effect on Ste18 protein levels as well as a differential effect on pheromone-induced levels of active MAPK/Fus3, but not MAPK/Kss1. When expressed at wildtype levels, we further show that mutants with an alpha-helical N terminus exhibit a counterintuitive shift in Gβγ signaling that reduces active MAPK/Fus3 levels whilst increasing cell polarization and cell cycle arrest. These data reveal a role for Gγ subunit intrinsically disordered regions in governing the balance between multiple Gβγ signaling axes.

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.

Palmitoylation of GNAQ/11 is critical for tumor cell proliferation and survival in GNAQ/11-mutant uveal melanoma

More than 85% of patients with uveal melanoma (UM) carry a GNAQ or GNA11 mutation at a hotspot codon (Q209) that encodes G protein α subunit q/11 polypeptides (Gα_q/11). GNAQ/11 relies on palmitoylation for membrane association and signal transduction. Despite the palmitoylation of GNAQ/11 was discovered long before, its implication in UM remains unclear. Here, results of palmitoylation-targeted mutagenesis and chemical interference approaches revealed that the loss of GNAQ/11 palmitoylation substantially affected tumor cell proliferation and survival in UM cells. Palmitoylation inhibition through the mutation of palmitoylation sites suppressed GNAQ/11^Q209L-induced malignant transformation in NIH3T3 cells. Importantly, the palmitoylation-deficient oncogenic GNAQ/11 failed to rescue the cell death initiated by the knock down of endogenous GNAQ/11 oncogenes in UM cells, which are much more dependent on Gα_q/11 signaling for cell survival and proliferation than other melanoma cells without GNAQ/11 mutations. Furthermore, the palmitoylation inhibitor, 2-bromopalmitate, also specifically disrupted Gα_q/11 downstream signaling by interfering with the MAPK pathway and BCL2 survival pathway in GNAQ/11-mutant UM cells and showed a notable synergistic effect when applied in combination with the BCL2 inhibitor, ABT-199, in vitro. The findings validate that GNAQ/11 palmitoylation plays a critical role in UM and may serve as a promising therapeutic target for GNAQ/11-driven UM.

Rgs4 is a regulator of mTOR activity required for motoneuron axon outgrowth and neuronal development in zebrafish

The Regulator of G protein signaling 4 (Rgs4) is a member of the RGS proteins superfamily that modulates the activity of G-protein coupled receptors. It is mainly expressed in the nervous system and is linked to several neuronal signaling pathways; however, its role in neural development in vivo remains inconclusive. Here, we generated and characterized a rgs4 loss of function model (MZrgs4) in zebrafish. MZrgs4 embryos showed motility defects and presented reduced head and eye sizes, reflecting defective motoneurons axon outgrowth and a significant decrease in the number of neurons in the central and peripheral nervous system. Forcing the expression of Rgs4 specifically within motoneurons rescued their early defective outgrowth in MZrgs4 embryos, indicating an autonomous role for Rgs4 in motoneurons. We also analyzed the role of Akt, Erk and mechanistic target of rapamycin (mTOR) signaling cascades and showed a requirement for these pathways in motoneurons axon outgrowth and neuronal development. Drawing on pharmacological and rescue experiments in MZrgs4, we provide evidence that Rgs4 facilitates signaling mediated by Akt, Erk and mTOR in order to drive axon outgrowth in motoneurons and regulate neuronal numbers.

The role of Gα protein signaling in the membrane estrogen receptor-mediated signaling

Estrogens exert rapid, extranuclear effects by their action on the plasma membrane estrogen receptors (mERs). Gα protein associated with the cell membrane is involved in many important processes regulated by estrogens. However, the Gα's role in the mER-mediated signaling and the signaling pathways involved are poorly understood. This review aims to outline the Gα's role in the mER-mediated signaling. Immunoblotting, immunofluorescence, co-immunoprecipitation, and RNA interference were carried out using vascular endothelial cells (ECs) and human breast carcinoma cell lines as experimental models. Electrophysiology and immunocytochemistry were carried out using guinea pigs as animal models. Recent advances suggest that the signaling of mERα through Gα is required for vascular EC migration or endothelial H₂S release, while Gα₁₃ is involved in estrogen-induced breast cancer cell invasion. Besides, the Gα_q-coupled PLC-PKC-PKA pathway is critical for the neural regulation of energy homeostasis. This review summarizes the contributions of Gα to mER-mediated signaling, including cardiovascular protection, breast cancer metastasis, neural regulation of homeostatic functions, and osteogenesis.

The Gαi protein subclass selectivity to the dopamine D2 receptor is also decided by their location at the cell membrane

Background

G protein-coupled receptor (GPCR) signaling via heterotrimeric G proteins plays an important role in the cellular regulation of responses to external stimuli. Despite intensive structural research, the mechanism underlying the receptor–G protein coupling of closely related subtypes of Gαi remains unclear. In addition to the structural changes of interacting proteins, the interactions between lipids and proteins seem to be crucial in GPCR-dependent cell signaling due to their functional organization in specific membrane domains. In previous works, we found that Gαs and Gαi₃ subunits prefer distinct types of membrane-anchor lipid domains that also modulate the G protein trimer localization. In the present study, we investigated the functional selectivity of dopamine D₂ long receptor isoform (D₂R) toward the Gαi₁, Gαi₂, and Gαi₃ subunits, and analyzed whether the organization of Gαi heterotrimers at the plasma membrane affects the signal transduction.

Methods

We characterized the lateral diffusion and the receptor–G protein spatial distribution in living cells using two assays: fluorescence recovery after photobleaching microscopy and fluorescence resonance energy transfer detected by fluorescence-lifetime imaging microscopy. Depending on distribution of data differences between Gα subunits were investigated using parametric approach–unpaired T-test or nonparametric–Mann–Whitney U test.

Results

Despite the similarities between the examined subunits, the experiments conducted in the study revealed a significantly faster lateral diffusion of the Gαi₂ subunit and the singular distribution of the Gαi₁ subunit in the plasma membrane. The cell membrane partitioning of distinct Gαi heterotrimers with dopamine receptor correlated very well with the efficiency of D₂R-mediated inhibition the formation of cAMP.

Conclusions

This study showed that even closely related subunits of Gαi differ in their membrane-trafficking properties that impact on their signaling. The interactions between lipids and proteins seem to be crucial in GPCR-dependent cell signaling due to their functional organization in specific membrane domains, and should therefore be taken into account as one of the selectivity determinants of G protein coupling.

Gene Expression Profiling of Type 2 Diabetes Mellitus by Bioinformatics Analysis

Objective

The aim of this study was to identify the candidate genes in type 2 diabetes mellitus (T2DM) and explore their potential mechanisms.

Methods

The gene expression profile GSE26168 was downloaded from the Gene Expression Omnibus (GEO) database. The online tool GEO2R was used to obtain differentially expressed genes (DEGs). Gene Ontology (GO) term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed by using Metascape for annotation, visualization, and comprehensive discovery. The protein-protein interaction (PPI) network of DEGs was constructed by using Cytoscape software to find the candidate genes and key pathways.

Results

A total of 981 DEGs were found in T2DM, including 301 upregulated genes and 680 downregulated genes. GO analyses from Metascape revealed that DEGs were significantly enriched in cell differentiation, cell adhesion, intracellular signal transduction, and regulation of protein kinase activity. KEGG pathway analysis revealed that DEGs were mainly enriched in the cAMP signaling pathway, Rap1 signaling pathway, regulation of lipolysis in adipocytes, PI3K-Akt signaling pathway, MAPK signaling pathway, and so on. On the basis of the PPI network of the DEGs, the following 6 candidate genes were identified: PIK3R1, RAC1, GNG3, GNAI1, CDC42, and ITGB1.

Conclusion

Our data provide a comprehensive bioinformatics analysis of genes, functions, and pathways, which may be related to the pathogenesis of T2DM.

Structure, Function, Pharmacology, and Therapeutic Potential of the G Protein, Gα/q,11

G protein coupled receptors (GPCRs) are one of the major classes of cell surface receptors and are associated with a group of G proteins consisting of three subunits termed alpha, beta, and gamma. G proteins are classified into four families according to their α subunit; Gα_i, Gα_s, Gα_12/13, and Gα_q. There are several downstream pathways of Gα_q of which the best known is upon activation via guanosine triphosphate (GTP), Gα_q activates phospholipase Cβ, hydrolyzing phosphatidylinositol 4,5-biphosphate into diacylglycerol and inositol triphosphate and activating protein kinase C and increasing calcium efflux from the endoplasmic reticulum. Although G proteins, in particular, the Gα_q/11 are central elements in GPCR signaling, their actual roles have not yet been thoroughly investigated. The lack of research of the role on Gα_q/11 in cell biology is partially due to the obscure nature of the available pharmacological agents. YM-254890 is the most useful Gα_q-selective inhibitor with antiplatelet, antithrombotic, and thrombolytic effects. YM-254890 inhibits Gα_q signaling pathways by preventing the exchange of guanosine diphosphate for GTP. UBO-QIC is a structurally similar compound to YM-254890, which can inhibit platelet aggregation and cause vasorelaxation in rats. Many agents are available for the study of signaling downstream of Gα_q/11. The role of G proteins could potentially represent a novel therapeutic target. This review will explore the range of pharmacological and molecular tools available for the study of the role of Gα_q/11 in GPCR signaling.

Competition for Gβγ dimers mediates a specific cross-talk between stimulatory and inhibitory G protein α subunits of the adenylyl cyclase in cardiomyocytes

Heterotrimeric G proteins are key regulators of signaling pathways in mammalian cells. Beyond G protein-coupled receptors, the amount and mutual ratio of specific G protein α, β, and γ subunits determine the G protein signaling. However, little is known about mechanisms that regulate the concentration and composition of G protein subunits at the plasma membrane. Here, we show a novel cross-talk between stimulatory and inhibitory G protein α subunits (Gα) that is mediated by G protein βγ dimers and controls the abundance of specific Gα subunits at the plasma membrane. Firstly, we observed in heart tissue from constitutively Gαi2- and Gαi3-deficient mice that the loss of Gαi2 and Gαi3 was accompanied by a slight increase in the protein content of the nontargeted Gαi isoform. Therefore, we analyzed whether overexpression of selected Gα subunits conversely impairs endogenous G protein α and β subunit levels in cardiomyocytes. Integration of overexpressed Gαi2 subunits into heterotrimeric G proteins was verified by co-immunoprecipitation. Adenoviral expression of increasing amounts of Gαi2 led to a reduction of Gαi3 (up to 90 %) and Gαs (up to 75 %) protein levels. Likewise, increasing amounts of adenovirally expressed Gαs resulted in a linear 75 % decrease in both Gαi2 and Gαi3 protein levels. In contrast, overexpression of either Gαi or Gαs isoform did not influence the amount of Gαo and Gαq, both of which are not involved in the regulation of adenylyl cyclase activity. The mRNA expression of the disappearing endogenous Gα subunits was not affected, indicating a posttranslational mechanism. Interestingly, the amount of endogenous G protein βγ dimers was not altered by any Gα overexpression. However, the increase of Gβγ level by adenoviral expression prevented the loss of endogenous Gαs and Gαi3 in Gαi2 overexpressing cardiomyocytes. Thus, our results provide evidence for a novel mechanism cross-regulating adenylyl cyclase-modulating Gαi isoforms and Gαs proteins. The Gα subunits apparently compete for a limited amount of Gβγ dimers, which are required for G protein heterotrimer formation at the plasma membrane.

Misfolded Gβ is recruited to cytoplasmic dynein by Nudel for efficient clearance

The Gβγ heterodimer is an important signal transducer. Gβ, however, is prone to misfolding due to its requirement for Gγ and chaperones for proper folding. How cells dispose of misfolded Gβ (mfGβ) is not clear. Here, we showed that mfGβ was able to be polyubiquitinated and subsequently degraded by the proteasome. It was sequestered in aggresomes after the inhibition of the proteasome activity with MG132. Sustained activation of Gβγ signaling further elevated cellular levels of the ubiquitinated Gβ. Moreover, Nudel, a regulator of cytoplasmic dynein, the microtubule minus end-directed motor, directly interacted with both the unubiquitinated and ubiquitinated mfGβ. Increasing the levels of both mfGβ and Nudel promoted the association of Gβ with both Nudel and dynein, resulting in robust aggresome formation in a dynein-dependent manner. Depletion of Nudel by RNAi reduced the dynein-associated mfGβ, impaired the MG132-induced aggresome formation, and markedly prolonged the half-life of nascent Gβ. Therefore, cytosolic mfGβ is recruited to dynein by Nudel and transported to the centrosome for rapid sequestration and degradation. Such a process not only eliminates mfGβ efficiently for the control of protein quality, but may also help to terminate the Gβγ signaling.

Dopamine D(2) Receptor-Mediated Heterologous Sensitization of AC5 Requires Signalosome Assembly

Chronic dopamine receptor activation is implicated in several central nervous system disorders. Although acute activation of Gα_i-coupled D₂ dopamine receptors inhibits adenylyl cyclase, persistent activation enhances adenylyl cyclase activity, a phenomenon called heterologous sensitization. Previous work revealed a requirement for Gα_s in D₂-induced heterologous sensitization of AC5. To elucidate the mechanism of Gα_s dependency, we expressed Gα_s mutants in Gα_s-deficient Gnas^E2−/E2− cells. Neither Gα_s-palmitoylation nor Gα_s-Gβγ interactions were required for sensitization of AC5. Moreover, we found that coexpressing βARKct-CD8 or Sar1(H79G) blocked heterologous sensitization. These studies are consistent with a role for Gα_s-AC5 interactions in sensitization however, Gβγ appears to have an indirect role in heterologous sensitization of AC5, possibly by promoting proper signalosome assembly.

G protein trafficking

The classical view of heterotrimeric G protein signaling places G -proteins at the cytoplasmic surface of the cell's plasma membrane where they are activated by an appropriate G protein-coupled receptor. Once activated, the GTP-bound Gα and the free Gβγ are able to regulate plasma membrane-localized effectors, such as adenylyl cyclase, phospholipase C-β, RhoGEFs and ion channels. Hydrolysis of GTP by the Gα subunit returns the G protein to the inactive Gαβγ heterotrimer. Although all of these events in the G protein cycle can be restricted to the cytoplasmic surface of the plasma membrane, G protein localization is dynamic. Thus, it has become increasingly clear that G proteins are able to move to diverse subcellular locations where they perform non-canonical signaling functions. This chapter will highlight our current understanding of trafficking pathways that target newly synthesized G proteins to the plasma membrane, activation-induced and reversible translocation of G proteins from the plasma membrane to intracellular locations, and constitutive trafficking of G proteins.

Preferential assembly of G-αβγ complexes directed by the γ subunits

Assembly of the G-αβγ heterotrimer is required for receptor signaling. Although much has been learned about the assembly process itself, the identities of the G-αβγ combinations that actually exist in physiological setting are largely unknown. Moreover, there is uncertainty regarding whether the individual subunits associate by a random process, or combine by a regulated process to form quasi-stable G-αβγ complexes. In this chapter, we will focus on emerging genetic -evidence that supports the latter model. Specifically, we will discuss how use of gene targeted mice has revealed preferential assembly of the striatal-specific Gα(olf)β(2)γ(7) complex occurs by a sequential process that is directed by the γ(7) subunit. The existence of specific G-αβγ complexes responsible for transducing the signals from different receptors may have profound implications by providing a possible explanation for biased agonism.

Synthesis and assembly of G protein βγ dimers: comparison of in vitro and in vivo studies

The heterotrimeric GTP-binding proteins (G proteins) are the canonical cellular machinery used with the approximately 700 G protein-coupled receptors (GPCRs) in the human genome to transduce extracellular signals across the plasma membrane. The synthesis of the constituent G protein subunits, and their assembly into Gβγ dimers and G protein heterotrimers, determines the signaling repertoire for G-protein/GPCR signaling in cells. These synthesis/assembly -processes are intimately related to two other overlapping events in the intricate pathway leading to formation of G protein signaling complexes, posttranslational modification and intracellular trafficking of G proteins. The assembly of the Gβγ dimer is a complex process involving multiple accessory proteins and organelles. The mechanisms involved are becoming increasingly appreciated, but are still incompletely understood. In vitro and in vivo (cellular) studies provide different perspectives of these processes, and a comparison of them can provide insight into both our current level of understanding and directions to be taken in future investigations.

Diverse β subunits of heterotrimeric G proteins are present in thyroid plasma membranes

The functioning of heterotrimeric G protein α subunits in the transduction of hormonal signals to appropriate intracellular responses is well recognized. Much less is known about the distribution of isoforms and functions of G protein β subunits. Here, using specific antibodies, we documented that in plasma membranes of the thyroid cell line Nthy-ori 3-1 all Gβ isoforms-Gβ(1), Gβ(2), Gβ(3), Gβ(4) and Gβ(5) are present, while the Gβ(3) occurs in minute amount. In plasma membrane fraction isolated from pooled postoperative thyroids of patients with nodular goiter and Graves' disease, the Gβ(1), Gβ(2), Gβ(4) and Gβ(5) subunits were found, whereas Gβ(3) could not be detected. Competition studies revealed that the Gβ(2) is the principal Gβ subunit in membranes from cultured thyroid cells, originated from normal thyroid, as well as in membranes from patients' thyroids. This suggests that Gβ(2) subunit cooperates with Gα(s) subunit, the most active of the Gα variants, during stimulation of adenylate cyclase which constitutes the main route of physiological thyroid stimulation.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

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.

Regulation of constitutive cargo transport from the trans-Golgi network to plasma membrane by Golgi-localized G protein betagamma subunits

Observations of Golgi fragmentation upon introduction of G protein βγ (Gβγ) subunits into cells have implicated Gβγ in a pathway controlling the fission at the trans-Golgi network (TGN) of plasma membrane (PM)-destined transport carriers. However, the subcellular location where Gβγ acts to provoke Golgi fragmentation is not known. Additionally, a role for Gβγ in regulating TGN-to-PM transport has not been demonstrated. Here we report that constitutive or inducible targeting of Gβγ to the Golgi, but not other subcellular locations, causes phospholipase C- and protein kinase D-dependent vesiculation of the Golgi in HeLa cells; Golgi-targeted β(1)γ(2) also activates protein kinase D. Moreover, the novel Gβγ inhibitor, gallein, and the Gβγ-sequestering protein, GRK2ct, reveal that Gβγ is required for the constitutive PM transport of two model cargo proteins, VSV-G and ss-HRP. Importantly, Golgi-targeted GRK2ct, but not a PM-targeted GRK2ct, also blocks protein transport to the PM. To further support a role for Golgi-localized Gβγ, endogenous Gβ was detected at the Golgi in HeLa cells. These results are the first to establish a role for Golgi-localized Gβγ in regulating protein transport from the TGN to the cell surface.

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.

Caveolin-1 and lipid microdomains regulate Gs trafficking and attenuate Gs/adenylyl cyclase signaling

Lipid rafts and caveolae are specialized membrane microdomains implicated in regulating G protein-coupled receptor signaling cascades. Previous studies have suggested that rafts/caveolae may regulate beta-adrenergic receptor/Galpha(s) signaling, but underlying molecular mechanisms are largely undefined. Using a simplified model system in C6 glioma cells, this study disrupts rafts/caveolae using both pharmacological and genetic approaches to test whether caveolin-1 and lipid microdomains regulate G(s) trafficking and signaling. Lipid rafts/caveolae were disrupted in C6 cells by either short-term cholesterol chelation using methyl-beta-cyclodextrin or by stable knockdown of caveolin-1 and -2 by RNA interference. In imaging studies examining Galpha(s)-GFP during signaling, stimulation with the betaAR agonist isoproterenol resulted in internalization of Galpha(s)-GFP; however, this trafficking was blocked by methyl-beta-cyclodextrin or by caveolin knockdown. Caveolin knockdown significantly decreased Galpha(s) localization in detergent insoluble lipid raft/caveolae membrane fractions, suggesting that caveolin localizes a portion of Galpha(s) to these membrane microdomains. Methyl-beta-cyclodextrin or caveolin knockdown significantly increased isoproterenol or thyrotropin-stimulated cAMP accumulation. Furthermore, forskolin- and aluminum tetrafluoride-stimulated adenylyl cyclase activity was significantly increased by caveolin knockdown in cells or in brain membranes obtained from caveolin-1 knockout mice, indicating that caveolin attenuates signaling at the level of Galpha(s)/adenylyl cyclase and distal to GPCRs. Taken together, these results demonstrate that caveolin-1 and lipid microdomains exert a major effect on Galpha(s) trafficking and signaling. It is suggested that lipid rafts/caveolae are sites that remove Galpha(s) from membrane signaling cascades and caveolins might dampen globally Galpha(s)/adenylyl cyclase/cAMP signaling.

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.

G protein-coupled receptor-promoted trafficking of Gbeta1gamma2 leads to AKT activation at endosomes via a mechanism mediated by Gbeta1gamma2-Rab11a interaction

G-protein coupled receptors activate heterotrimeric G proteins at the plasma membrane in which most of their effectors are intrinsically located or transiently associated as the external signal is being transduced. This paradigm has been extended to the intracellular compartments by studies in yeast showing that trafficking of Galpha activates phosphatidylinositol 3-kinase (PI3K) at endosomal compartments, suggesting that vesicle trafficking regulates potential actions of Galpha and possibly Gbetagamma at the level of endosomes. Here, we show that Gbetagamma interacts with Rab11a and that the two proteins colocalize at early and recycling endosomes in response to activation of lysophosphatidic acid (LPA) receptors. This agonist-dependent association of Gbetagamma to Rab11a-positive endosomes contributes to the recruitment of PI3K and phosphorylation of AKT at this intracellular compartment. These events are sensitive to the expression of a dominant-negative Rab11a mutant or treatment with wortmannin, suggesting that Rab11a-dependent Gbetagamma trafficking promotes the activation of the PI3K/AKT signaling pathway associated with endosomal compartments. In addition, RNA interference-mediated Rab11a depletion, or expression of a dominant-negative Rab11a mutant attenuated LPA-dependent cell survival and proliferation, suggesting that endosomal activation of the PI3K/AKT signaling pathway in response to Gbetagamma trafficking, via its interaction with Rab11, is a relevant step in the mechanism controlling these fundamental events.

Abscisic acid regulation of guard-cell K+ and anion channels in Gbeta- and RGS-deficient Arabidopsis lines

In mammals, basal currents through G protein-coupled inwardly rectifying K(+) (GIRK) channels are repressed by Galpha(i/o)GDP, and the channels are activated by direct binding of free Gbetagamma subunits released upon stimulation of Galpha(i/o)-coupled receptors. However, essentially all information on G protein regulation of GIRK electrophysiology has been gained on the basis of coexpression studies in heterologous systems. A major advantage of the model organism, Arabidopsis thaliana, is the ease with which knockout mutants can be obtained. We evaluated plants harboring mutations in the sole Arabidopsis Galpha (AtGPA1), Gbeta (AGB1), and Regulator of G protein Signaling (AtRGS1) genes for impacts on ion channel regulation. In guard cells, where K(+) fluxes are integral to cellular regulation of stomatal apertures, inhibition of inward K(+) (K(in)) currents and stomatal opening by the phytohormone abscisic acid (ABA) was equally impaired in Atgpa1 and agb1 single mutants and the Atgpa1 agb1 double mutant. AGB1 overexpressing lines maintained a wild-type phenotype. The Atrgs1 mutation did not affect K(in) current magnitude or ABA sensitivity, but K(in) voltage-activation kinetics were altered. Thus, Arabidopsis cells differ from mammalian cells in that they uniquely use the Galpha subunit or regulation of the heterotrimer to mediate K(in) channel modulation after ligand perception. In contrast, outwardly rectifying (K(out)) currents were unaltered in the mutants, and ABA activation of slow anion currents was conditionally disrupted in conjunction with cytosolic pH clamp. Our studies highlight unique aspects of ion channel regulation by heterotrimeric G proteins and relate these aspects to stomatal aperture control, a key determinant of plant biomass acquisition and drought tolerance.

G protein betagamma dimer expression in cardiomyocytes: developmental acquisition of Gbeta3

Heterotrimeric G proteins are comprised of a guanine nucleotide binding Galpha subunit and the Gbetagamma dimers that link G protein-coupled receptors (GPCRs) to effectors. This study focuses on the expression and localization patterns for certain Gbeta and Ggamma subunits in neonatal and adult cardiomyocytes. We identify developmental downregulation of Gbeta1, Gbeta2, and Ggamma2, and a switch in the molecular form of Ggamma3, in cardiomyocytes. Gbeta1 is highly localized to caveolae membranes, whereas Gbeta2 is identified in caveolae and other membrane fractions. Gbeta3 is not detected in neonatal cardiomyocytes, but rather Gbeta3 is upregulated in adult cardiomyocytes and detected in the caveolae and soluble fractions. The observation that cardiomyocytes co-express multiple Gbeta and Ggamma subunits in a developmentally regulated manner, and that these Gbeta and Ggamma subunits assume distinct subcellular localization patterns, provides for a high level of signaling specificity in the heart.

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.

Dopamine Receptor-interacting Protein 78 Acts as a Molecular Chaperone for Gγ Subunits before Assembly with Gβ

Heterotrimeric G proteins play a central role in intracellular communication mediated by extracellular signals, and both Galpha and Gbetagamma subunits regulate effectors downstream of activated receptors. The particular constituents of the G protein heterotrimer affect both specificity and efficiency of signal transduction. However, little is known about mechanistic aspects of G protein assembly in the cell that would certainly contribute to formation of heterotrimers of specific composition. It was recently shown that phosducin-like protein (PhLP) modulated both Gbetagamma expression and subsequent signaling by chaperoning nascent Gbeta and facilitating heterodimer formation with Ggamma subunits (Lukov, G. L., Hu, T., McLaughlin, J. N., Hamm, H. E., and Willardson, B. M. (2005) EMBO J. 24, 1965-1975; Humrich, J., Bermel, C., Bunemann, M., Harmark, L., Frost, R., Quitterer, U., and Lohse, M. J. (2005) J. Biol. Chem. 280, 20042-20050). Here we demonstrate using a variety of techniques that DRiP78, an endoplasmic reticulum resident protein known to regulate the trafficking of several seven transmembrane receptors, interacts specifically with the Ggamma subunit but not Gbeta or Galpha subunits. Furthermore, we demonstrate that DRiP78 and the Gbeta subunit can compete for the Ggamma subunit. DRiP78 also protects Ggamma from degradation until a stable partner such as Gbeta is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific Gbetagamma heterodimers, as selectivity was observed among Ggamma isoforms for interaction with DRiP78 depending on the presence of particular Gbeta subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of Gbetagamma by linking Ggamma to PhLP.Gbeta complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of Gbetagamma subunits of the G protein.

Regulation of cardiac cAMP synthesis and contractility by nucleoside diphosphate kinase B/G protein beta gamma dimer complexes

Heterotrimeric G proteins are pivotal regulators of myocardial contractility. In addition to the receptor-induced GDP/GTP exchange, G protein alpha subunits can be activated by a phosphate transfer via a plasma membrane-associated complex of nucleoside diphosphate kinase B (NDPK B) and G protein betagamma-dimers (Gbetagamma). To investigate the physiological role of this phosphate transfer in cardiomyocytes, we generated a Gbeta1gamma2-dimer carrying a single amino acid exchange at the intermediately phosphorylated His-266 in the beta1 subunit (Gbeta1H266Lgamma2). Recombinantly expressed Gbeta1H266Lgamma2 were integrated into heterotrimeric G proteins in rat cardiomyocytes but were deficient in intermediate Gbeta phosphorylation. Compared with wild-type Gbeta1gamma2 (Gbeta1WTgamma2), overexpression of Gbeta1H266Lgamma2 suppressed basal cAMP formation up to 55%. A similar decrease in basal cAMP production occurred when the formation of NDPK B/Gbetagamma complexes was attenuated by siRNA-mediated NDPK B knockdown. In adult rat cardiomyocytes expressing Gbeta1H266Lgamma2, the basal contractility was suppressed by approximately 50% which correlated to similarly reduced basal cAMP levels and reduced Ser16-phosphorylation of phospholamban. In the presence of the beta-adrenoceptor agonist isoproterenol, the total cAMP formation and contractility were significantly lower in Gbeta1H266Lgamma2 than in Gbeta1WTgamma2 expressing cardiomyocytes. However, the relative isoproterenol-induced increased was not affected by Gbeta1H266Lgamma2. We conclude that the receptor-independent activation of G proteins via NDPK B/Gbetagamma complexes requires the intermediate phosphorylation of G protein beta subunits at His-266. Our results highlight the histidine kinase activity of NDPK B for Gbeta and demonstrate its contribution to the receptor-independent regulation of cAMP synthesis and contractility in intact cardiomyocytes.

Platelets possess and require an active protein palmitoylation pathway for agonist-mediated activation and in vivo thrombus formation

OBJECTIVE

Several platelet proteins are palmitoylated, but whether protein palmitoylation functions in platelet activation is unknown. We sought to determine the role of platelet protein palmitoylation in platelet activation and thrombus formation.

METHODS AND RESULTS

Platelet proteins were depalmitoylated by infusing acyl-protein thioesterase 1 into permeabilized platelets. In intact platelets, platelet protein palmitoylation was blocked using the protein palmitoylation inhibitor cerulein. The effects of inhibiting platelet protein palmitoylation on platelet function and on thrombus formation in vivo were evaluated. When infused into permeabilized platelets, acyl-protein thioesterase 1 reduced total platelet protein palmitoylation and inhibited protease-activated receptor-1-mediated alpha-granule secretion with an IC50 of 175 nmol/L and maximal inhibition of > or = 90%. G(alpha q) and SNAP-23, membrane-associated proteins that are constitutively palmitoylated, translocated to the cytosol when permeabilized platelets were exposed to recombinant acyl-protein thioesterase 1. The protein palmitoylation inhibitor cerulein also inhibited platelet granule secretion and aggregation. Studies using intravital microscopy showed that incubation with cerulein decreased the rate of platelet accumulation into thrombi formed after laser-induced injury of mouse arterioles and inhibited maximal platelet accumulation by >60%.

CONCLUSION

These studies show that platelets possess a protein palmitoylation machinery that is required for both platelet activation and platelet accumulation into thrombi. These studies show that inhibition of platelet protein palmitoylation blocks platelet aggregation and granule secretion. In a murine model of thrombus formation, inhibition of protein palmitoylation markedly inhibits platelet accumulation into thrombi at sites of vascular injury.

Plant G protein heterotrimers require dual lipidation motifs of Gα and Gγ and do not dissociate upon activation

In plants one bona fide Gα subunit has been identified, as well as a single Gβ and two Gγ subunits. To study the roles of lipidation motifs in the regulation of subcellular location and heterotrimer formation in living plant cells, GFP-tagged versions of the Arabidopsis thaliana heterotrimeric G protein subunits were constructed. Mutational analysis showed that the Arabidopsis Gα subunit, GPα1, contains two lipidation motifs that were essential for plasma membrane localization. The Arabidopsis Gβ subunit, AGβ1, and the Gγ subunit, AGG1, were dependent upon each other for tethering to the plasma membrane. The second Gγ subunit, AGG2, did not require AGβ1 for localization to the plasma membrane. Like AGG1, AGG2 contains two putative lipidation motifs, both of which were necessary for membrane localization. Interaction between the subunits was studied using fluorescence resonance energy transfer (FRET) imaging by means of fluorescence lifetime imaging microscopy (FLIM). The results suggest that AGβ1 and AGG1 or AGβ1 and AGG2 can form heterodimers independent of lipidation. In addition, FLIM-FRET revealed the existence of GPα1-AGβ1-AGG1 heterotrimers at the plasma membrane. Importantly, rendering GPα1 constitutively active did not cause a FRET decrease in the heterotrimer, suggesting no dissociation upon GPα1 activation.

Palmitoylation of ligands, receptors, and intracellular signaling molecules

Palmitate, a 16-carbon saturated fatty acid, is attached to more than 100 proteins. Modification of proteins by palmitate has pleiotropic effects on protein function. Palmitoylation can influence membrane binding and membrane targeting of the modified proteins. In particular, many palmitoylated proteins concentrate in lipid rafts, and enrichment in rafts is required for efficient signal transduction. This Review focuses on the multiple effects of palmitoylation on the localization and function of ligands, receptors, and intracellular signaling proteins. Palmitoylation regulates the trafficking and function of transmembrane proteins such as ion channels, neurotransmitter receptors, heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors, and integrins. In addition, immune receptor signaling relies on protein palmitoylation at many levels, including palmitoylated co-receptors, Src family kinases, and adaptor or scaffolding proteins. The localization and signaling capacities of Ras and G proteins are modulated by dynamic protein palmitoylation. Cycles of palmitoylation and depalmitoylation allow H-Ras and G protein alpha subunits to reversibly bind to and signal from different intracellular cell membranes. Moreover, secreted ligands such as Hedgehog, Wingless, and Spitz use palmitoylation to regulate the extent of long- or short-range signaling. Finally, palmitoylation can alter signaling protein function by direct effects on enzymatic activity and substrate specificity. The identification of the palmitoyl acyltransferases has provided new insights into the biochemistry of this posttranslational process and permitted new substrates to be identified.

Analysis of G protein betagamma dimer formation in live cells using multicolor bimolecular fluorescence complementation demonstrates preferences of beta1 for particular gamma subunits

The specificity of G protein betagamma signaling demonstrated by in vivo knockouts is greater than expected based on in vitro assays of betagamma function. In this study, we investigated the basis for this discrepancy by comparing the abilities of seven beta1gamma complexes containing gamma1, gamma2, gamma5, gamma7, gamma10, gamma11, or gamma12 to interact with alphas and of these gamma subunits to compete for interaction with beta1 in live human embryonic kidney (HEK) 293 cells. betagamma complexes were imaged using bimolecular fluorescence complementation, in which fluorescence is produced by two nonfluorescent fragments (N and C) of cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP) when brought together by proteins fused to each fragment. Plasma membrane targeting of alphas-CFP varied inversely with its expression level, and the abilities of YFP-N-beta1YFP-C-gamma complexes to increase this targeting varied by 2-fold or less. However, there were larger differences in the abilities of the CFP-N-gamma subunits to compete for association with CFP-C-beta1. When the intensities of coexpressed CFP-C-beta1CFP-N-gamma (cyan) and CFP-C-beta1YFP-N-gamma2 (yellow) complexes were compared under conditions in which CFP-C-beta1 was limiting, the CFP-N-gamma subunits exhibited a 4.5-fold range in their abilities to compete with YFP-N-gamma2 for association with CFP-C-beta1. CFP-N-gamma12 and CFP-N-gamma1 were the strongest and weakest competitors, respectively. Taken together with previous demonstrations of a role for betagamma in the specificity of receptor signaling, these results suggest that differences in the association preferences of coexpressed beta and gamma subunits for each other can determine which complexes predominate and participate in signaling pathways in intact cells.

Light-regulated translocation of signaling proteins in Drosophila photoreceptors

Illumination of Drosophila photoreceptor cells induces multi-facet responses, which include generation of the photoreceptor potential, screening pigment migration and translocation of signaling proteins which is the focus of recent extensive research. Translocation of three signaling molecules is covered in this review: (1) Light-dependent translocation of arrestin from the cytosol to the signaling membrane, the rhabdomere, determines the lifetime of activated rhodopsin. Arrestin translocates in PIP3 and NINAC myosin III dependent manner, and specific mutations which disrupt the interaction between arrestin and PIP3 or NINAC also impair the light-dependent translocation of arrestin and the termination of the response to light. (2) Activation of Drosophila visual G protein, DGq, causes a massive and reversible, translocation of the alpha subunit from the signaling membrane to the cytosol, accompanied by activity-dependent architectural changes. Analysis of the translocation and the recovery kinetics of DGq(alpha) in wild-type flies and specific visual mutants indicated that DGq(alpha) is necessary but not sufficient for the architectural changes. (3) The TRP-like (TRPL) but not TRP channels translocate in a light-dependent manner between the rhabdomere and the cell body. As a physiological consequence of this light-dependent modulation of the TRP/TRPL ratio, the photoreceptors of dark-adapted flies operate at a wider dynamic range, which allows the photoreceptors enriched with TRPL to function better in darkness and dim background illumination. Altogether, signal-dependent movement of signaling proteins plays a major role in the maintenance and function of photoreceptor cells.

Proteasome involvement in the degradation of the G(q) family of Galpha subunits

Metabolically unstable proteins are involved in a multitude of regulatory networks, including those that control cell signaling, the cell cycle and in many responses to physiological stress. In the present study, we have determined the stability and characterized the degradation process of some members of the G(q) class of heterotrimeric G proteins. Pulse-chase experiments in HEK293 cells indicated a rapid turnover of endogenously expressed Galpha(q) and overexpressed Galpha(q) and Galpha(16) subunits. Pretreatment with proteasome inhibitors attenuated the degradation of both G alpha subunits. In contrast, pretreatment of cells with inhibitors of lysosomal proteases and nonproteasomal cysteine proteases had very little effect on the stability of the proteins. Significantly, the turnover of these proteins is not affected by transient activation of their associated receptors. Fractionation studies showed that the rates of Galpha(q) and Galpha16 degradation are accelerated in the cytosol. In fact, we show that a mutant Galpha(q) which lacks its palmitoyl modification site, and which is localized almost entirely in the cytoplasm, has a marked increase in the rate of degradation. Taken together, these results suggest that the G(q) class proteins are degraded through the proteasome pathway and that cellular localization and/or other protein interactions determine their stability.

Functional interactions between the alpha1b-adrenoceptor and Galpha11 are compromised by de-palmitoylation of the G protein but not of the receptor

Both the alpha1b-adrenoceptor and Galpha11 are targets for post-translational thio-acylation that is regulated by agonist occupancy of the receptor [P.A. Stevens, J. Pediani, J.J. Carrillo, G. Milligan, J. Biol. Chem. 276 (2001) 35883]. In co-expression studies mutation of the sites of thio-acylation in the G protein or treatment of cell membranes with hydroxylamine greatly reduced agonist stimulation of guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTPgammaS) binding. In alpha1b-adrenoceptor-Galpha11 fusion proteins mutation of thio-acylation sites in receptor or G protein did not alter the binding affinity of the antagonist [3H]prazosin or the agonist phenylephrine. Although the potency of phenylephrine to stimulate binding of [35S]GTPgammaS to alpha1b-adrenoceptor-Galpha11 fusion proteins was unaffected by the thio-acylation potential of either element, the maximal effect was reduced by some 50% when the G protein but not the receptor was mutated to prevent thio-acylation. This reflected lack of thio-acylation of the G protein rather than mutation of Cys9 and Cys10 to Ser because treatment with hydroxylamine mimicked this in fusions containing the wild type G protein but was without effect in those mutated to prevent thio-acylation. Mutation to reduce binding of beta/gamma to Galpha11 markedly reduced phenylephrine stimulation of [35S]GTPgammaS binding. Combination of mutations to prevent thio-acylation and beta/gamma binding did not, however, have an additive effect on [35S]GTPgammaS binding. These results indicate that the thio-acylation status of the alpha1b-adrenoceptor does not regulate G protein activation whereas thio-acylation of Galpha11 plays a key role in activation by the receptor beyond providing membrane association and proximity.

Excess of Gbetae over Gqalphae in vivo prevents dark, spontaneous activity of Drosophila photoreceptors

Drosophila melanogaster photoreceptor cells are capable of detecting single photons. This utmost sensitivity is critically dependent on the maintenance of an exceedingly low, dark, spontaneous activity of photoreceptor cells. However, the underlying mechanisms of this hallmark of phototransduction are not fully understood. An analysis of the Drosophila visual heterotrimeric (alphabetagamma) Gq protein revealed that wild-type Drosophila flies have about a twofold excess of Gbeta over Galpha subunits of the visual Gq protein. Studies of Gbetae mutants in which the excess of Gbeta was genetically eliminated showed dramatic dark, spontaneous activity of the photoreceptor cells, whereas concurrent genetic reduction of the Galpha subunit, which restored the excess of Gbeta, abolished this effect. These results indicate that an excess of Gbeta over Galpha is a strategy used in vivo for the suppression of spontaneous activity, thereby yielding a high signal to noise ratio, which is characteristic of the photoreceptor light response. This mechanism could be relevant to the regulation of G protein signaling in general.

The heterotrimeric G-protein subunits GNG-1 and GNB-1 form a Gbetagamma dimer required for normal female fertility, asexual development, and galpha protein levels in Neurospora crassa

We have identified a gene encoding a heterotrimeric G protein gamma subunit, gng-1, from the filamentous fungus Neurospora crassa. gng-1 possesses a gene structure similar to that of mammalian Ggamma genes, consisting of three exons and two introns, with introns present in both the open reading frame and 5'-untranslated region. The GNG-1 amino acid sequence displays high identity to predicted Ggamma subunits from other filamentous fungi, including Giberella zeae, Cryphonectria parasitica, Trichoderma harzianum, and Magnaporthe grisea. Deletion of gng-1 leads to developmental defects similar to those previously characterized for Deltagnb-1 (Gbeta) mutants. Deltagng-1, Deltagnb-1, and Deltagng-1 Deltagnb-1 strains conidiate inappropriately in submerged cultures and are female sterile, producing aberrant female reproductive structures. Similar to previous results obtained with Deltagnb-1 mutants, loss of gng-1 negatively influences levels of Galpha proteins (GNA-1, GNA-2, and GNA-3) in plasma membrane fractions isolated from various tissues of N. crassa and leads to a significant reduction in the amount of intracellular cyclic AMP. In addition, we show that GNB-1 is essential for maintenance of normal steady-state levels of GNG-1, suggesting a functional interaction between GNB-1 and GNG-1. Direct evidence for a physical association between GNB-1 and GNG-1 in vivo was provided by coimmunoprecipitation.

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.

G Protein βγ Dimer Formation: Gβ and Gγ Differentially Determine Efficiency of in Vitro Dimer Formation

The Gbeta and Ggamma subunit of the heterotrimeric G proteins form a functional dimer that is stable once assembled in vivo or in vitro. The requirements, mechanism, and specificity of dimer formation are still incompletely understood, but represent important biochemical processes involved in the specificity of cellular signaling through G proteins. Here, seven Gbeta and 12 FLAG-epitope-tagged Ggamma subunits were separately synthesized in vitro using a rabbit reticulocyte lysate expression system. The translation products were combined and dimers isolated by immunoprecipitation. Gbeta1 and Gbeta4 formed dimers with all Ggamma subunit isoforms, generally with Gbeta/Ggamma stoichiometries between 0.2:1 and 0.5:1. Gbeta5, Gbeta5L, and Gbeta3s did not form significant amounts of dimer with any of the gamma subunit isoforms. Gbeta2 and Gbeta3 formed dimers with selected Ggamma isoforms to levels intermediate between that of Gbeta1/Gbeta4 and Gbeta3s/Gbeta5/Gbeta5L. We also expressed selected Gbetagamma in HEK293 cells and measured PLCbeta2 activity. Gbetagamma dimer-dependent increases in IP3 production were seen with most Gbeta1, Gbeta2, and Gbeta5 combinations, indicating functional dimer expression in intact cells. These results define the complete set of G protein betagamma dimers that are formed using a single biochemical assay method and suggest that there are Gbeta isoform-specific factors in rabbit reticulocyte lysates that determine the efficacy of Gbetagamma dimer formation.

Silencing the expression of multiple Gbeta-subunits eliminates signaling mediated by all four families of G proteins

The Gbetagamma-subunit complex derived from heterotrimeric G proteins can act to regulate the function of a variety of protein targets. We established lentiviral-based RNA interference in J774A.1 mouse macrophages to characterize the role of Gbeta in G protein-coupled receptor signaling. The expression of Gbeta1 and Gbeta2, the major subtypes present in J774A.1 cells, was eliminated by sequential treatment with small hairpin RNA expressing lentivirus. These betagamma complex-deficient cells lost the ability to respond to G protein-mediated signals. Chemotaxis and the phosphorylation of Akt in response to C5a were both blocked. Similarly, C5a-mediated actin polymerization, C5a- and UTP-stimulated intracellular calcium mobilization, and the stimulation of cAMP formation by isoproterenol were all eliminated in the absence of the Gbeta-subunits. In addition, stabilization and membrane localization of several Galpha- and Ggamma-subunit proteins was strongly effected. Furthermore, in DNA microarray analysis, regulation of gene expression stimulated by prostaglandin E2 and UTP was not observed in cells lacking Gbeta-subunits. In contrast, phagocytotic activity, serum-dependent cell growth and the patterns of gene expression induced by stimulating the Toll receptors with LPS were similar in wild-type cells and small hairpin RNA-containing cells. Thus, ablation of the Gbeta-subunits destabilized Galpha- and Ggamma-subunits and effectively eliminated G protein-mediated signaling responses. Unrelated ligand regulated pathways remained intact. These cells provide a system that can be used to study signaling in the absence of most G protein-mediated functions.

Activation of an alpha2A-adrenoceptor-Galphao1 fusion protein dynamically regulates the palmitoylation status of the G protein but not of the receptor

Post-translational thio-acylation of a fusion protein between the alpha2A-adrenoceptor and the alpha subunit of the G protein G(o1) is both dynamic and regulated by agonist binding. Incorporation of [3H]palmitate into the fusion protein was reduced substantially in the presence of the agonist adrenaline. This was dependent on the concentration of adrenaline and correlated with occupancy of the ligand binding site. Both the receptor and G-protein elements of the fusion construct incorporated [3H]palmitate but this occurred more rapidly for the G-protein element and regulation of acylation by the agonist occurred only for the G protein. The kinetics of de-palmitoylation of the alpha2A-adrenoceptor-Galpha(o1) fusion were accelerated markedly by agonist. Again, this reflected modulation of the G protein but not of the receptor. Agonist-induced regulation of the kinetics of thio-acylation of the G protein was abolished, however, in a mutant unable to bind guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTP[S]) in response to adrenaline. Despite the dynamic nature of the post-translational acylation and its regulation by agonist, the ability of adrenaline to activate the G protein, monitored by stimulation of the binding of [35S]GTP[S] to such fusion constructs, was unaffected by the palmitoylation potential of either the receptor or G-protein element.

G-protein signaling: back to the future

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

Real-time monitoring of receptor and G-protein interactions in living cells

G protein-coupled receptors (GPCRs) represent the largest family of proteins involved in signal transduction. Here we present a bioluminescence resonance energy transfer (BRET) assay that directly monitors in real time the early interactions between human GPCRs and their cognate G-protein subunits in living human cells. In addition to detecting basal precoupling of the receptors to Galpha-, Gbeta- and Ggamma-subunits, BRET measured very rapid ligand-induced increases in the interaction between receptor and Galphabetagamma-complexes (t(1/2) approximately 300 ms) followed by a slower (several minutes) decrease, reflecting receptor desensitization. The agonist-promoted increase in GPCR-Gbetagamma interaction was highly dependent on the identity of the Galpha-subunit present in the complex. Therefore, this G protein-activity biosensor provides a novel tool to directly probe the dynamics and selectivity of receptor-mediated, G-protein activation-deactivation cycles that could be advantageously used to identify ligands for orphan GPCRs.

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

The G protein subunit, betagamma, plays an important role in targeting alpha subunits to the plasma membrane and is essential for binding and activation of the heterotrimer by heptahelical receptors. Mutation of residues in the N-terminal alpha-helix of alpha s and alpha q that contact betagamma in the crystal structure of alpha i reduces binding between alpha and betagamma, inhibits plasma membrane targeting and palmitoylation of the alpha subunit, and results in G proteins that fail to couple receptor activation to stimulation of effector. Overexpression of betagamma can recover this loss of signaling through Gs but not Gq. In fact, a single mutation (I25A) in alpha q can block alpha q-mediated generation of inositol phosphates. Function is not recovered by betagamma overexpression nor myristoylation directed plasma membrane localization. Introduction of a Q209L activating mutation with I25A results in a constitutively active alpha q as expected, but surprisingly a R183C activating mutation does not result in constitutive activity when present with I25A. Examination of binding between alpha and betagamma via a pull down assay shows that the N-terminal betagamma-binding mutations inhibit alpha-betagamma binding significantly more than the R183C or Q209L activating mutations do. Moreover, introduction of the I25A mutation into alpha q RC disrupts co-immunoprecipitation with PLCbeta1. Taken together, results presented here suggest that alpha-betagamma binding is necessary at a point downstream from receptor activation of the heterotrimeric G protein for signal transduction by alpha q.

Mice with deficiency of G protein gamma3 are lean and have seizures

Emerging evidence suggests that the gamma subunit composition of an individual G protein contributes to the specificity of the hundreds of known receptor signaling pathways. Among the twelve gamma subtypes, gamma3 is abundantly and widely expressed in the brain. To identify specific functions and associations for gamma3, a gene-targeting approach was used to produce mice lacking the Gng3 gene (Gng3-/-). Confirming the efficacy and specificity of gene targeting, Gng3-/- mice show no detectable expression of the Gng3 gene, but expression of the divergently transcribed Bscl2 gene is not affected. Suggesting unique roles for gamma3 in the brain, Gng3-/- mice display increased susceptibility to seizures, reduced body weights, and decreased adiposity compared to their wild-type littermates. Predicting possible associations for gamma3, these phenotypic changes are associated with significant reductions in beta2 and alphai3 subunit levels in certain regions of the brain. The finding that the Gng3-/- mice and the previously reported Gng7-/- mice display distinct phenotypes and different alphabetagamma subunit associations supports the notion that even closely related gamma subtypes, such as gamma3 and gamma7, perform unique functions in the context of the organism.

Live cell imaging of Gs and the beta2-adrenergic receptor demonstrates that both alphas and beta1gamma7 internalize upon stimulation and exhibit similar trafficking patterns that differ from that of the beta2-adrenergic receptor

To visualize and investigate the regulation of the localization patterns of Gs and an associated receptor during cell signaling, we produced functional fluorescent fusion proteins and imaged them in HEK-293 cells. alphas-CFP, with cyan fluorescent protein (CFP) inserted into an internal loop of alphas, localized to the plasma membrane and exhibited similar receptor-mediated activity to that of alphas. Functional fluorescent beta1gamma7 dimers were produced by fusing an amino-terminal yellow fluorescent protein (YFP) fragment to beta1 (YFP-N-beta1) and a carboxyl-terminal YFP fragment to gamma7 (YFP-C-gamma7). When expressed together, YFP-N-beta1 and YFP-C-gamma7 produced fluorescent signals in the plasma membrane that were not seen when the subunits were expressed separately. Isoproterenol stimulation of cells co-expressing alphas-CFP, YFP-N-beta1/YFP-C-gamma7, and the beta2-adrenergic receptor (beta2AR) resulted in internalization of both fluorescent signals from the plasma membrane. Initially, alphas-CFP and YFP-N-beta1/YFP-C-gamma7 stained the cytoplasm diffusely, and subsequently they co-localized on vesicles that exhibited minimal overlap with beta2AR-labeled vesicles. Moreover, internalization of beta2AR-GFP, but not alphas-CFP or YFP-N-beta1/YFP-C-gamma7, was inhibited by a fluorescent dominant negative dynamin 1 mutant, Dyn1(K44A)-mRFP, indicating that the Gs subunits and beta2AR utilize different internalization mechanisms. Subsequent trafficking of the Gs subunits and beta2AR also differed in that vesicles labeled with the Gs subunits exhibited less overlap with RhoB-labeled endosomes and greater overlap with Rab11-labeled endosomes. Because Rab11 regulates traffic through recycling endosomes, co-localization of alphas and beta1gamma7 on these endosomes may indicate a means of recycling specific alphasbetagamma combinations to the plasma membrane.