Persistent membrane association of activated and depalmitoylated G protein alpha subunits

Rab3gap1 palmitoylation cycling modulates cardiomyocyte exocytosis and atrial natriuretic peptide release

Rab3 GTPase-activating protein 1 (Rab3gap1) hydrolyzes GTP on Rab3 to inactivate it and reinitiate the Rab3 cycle, which regulates exocytic release of neuropeptides and hormones from neuroendocrine cells and atrial natriuretic peptide (ANP) secretion by cardiomyocytes. Cysteine palmitoylation of Rab3gap1 by the Golgi-localized S-acyltransferase zDHHC9 was recently shown to hinder ANP release by impairing Rab3gap1-mediated nucleotide cycling on Rab3a. Here, we interrogate the cysteine residues of Rab3gap1 modified by palmitoylation and impacts on ANP secretion in cardiomyocytes. Although mutation of the previously identified cysteine (Cys)-678 site of Rab3gap1 alone was insufficient to elicit complete loss of Rab3gap1 palmitoylation in cardiomyocytes, combinatorial mutation of Cys-509, 510, 521, 522, and 678 (Rab3gap15CS) dramatically reduced Rab3gap1 palmitoylation. Notably, total cellular GTPase-activating protein (GAP) activity in cardiomyocytes was maintained with mutation of the Rab3gap1 palmitoylation sites as the Rab3gap15CS mutant substantially reduced steady-state Rab3a-GTP levels in cardiomyocytes similar to wild-type Rab3gap1. However, although expression of wild-type Rab3gap1 induced robust secretion of ANP and greatly enhanced phenylephrine-stimulated ANP release, the Rab3gap15CS palmitoylation-deficient mutant was incapable of promoting exocytosis and ANP release by cardiomyocytes. These data suggest Rab3gap1 cysteine palmitoylation may target Rab3gap1 to Rab3a for regulated GAP-mediated inactivation at specific intracellular membrane domains to modulate the Rab3 cycle and exocytosis. Collectively, these data support a role for Rab3gap1 palmitoylation cycling in spatiotemporal control of the Rab3 cycle to regulate exocytosis and ANP secretion by cardiomyocytes.

Overexpressed Gα13 activates serum response factor through stoichiometric imbalance with Gβγ and mislocalization to the cytoplasm

Gα13, a heterotrimeric G protein α subunit of the G12/13 subfamily, is an oncogenic driver in multiple cancer types. Unlike other G protein subfamilies that contribute to cancer progression via amino acid substitutions that abolish their deactivating, intrinsic GTPase activity, Gα13 rarely harbors such mutations in tumors and instead appears to stimulate aberrant cell growth via overexpression as a wildtype form. It is not known why this effect is exclusive to the G12/13 subfamily, nor has a mechanism been elucidated for overexpressed Gα13 promoting tumor progression. Using a reporter gene assay for serum response factor (SRF)-mediated transcription in HEK293 cells, we found that transiently expressed, wildtype Gα13 generates a robust SRF signal, approximately half the amplitude observed for GTPase-defective Gα13. When epitope-tagged, wildtype Gα13 was titrated upward in cells, a sharp increase in SRF stimulation was observed coincident with a "spillover" of Gα13 from membrane-associated to a soluble fraction. Overexpressing G protein β and γ subunits caused both a decrease in this signal and a shift of wildtype Gα13 back to the membranous fraction, suggesting that stoichiometric imbalance in the αβγ heterotrimer results in aberrant subcellular localization and signalling by overexpressed Gα13. We also examined the acylation requirements of wildtype Gα13 for signalling to SRF. Similar to GTPase-defective Gα13, S-palmitoylation of the wildtype α subunit was necessary for SRF activation but could be replaced functionally by an engineered site for N-terminal myristoylation. However, a key difference was observed between wildtype and GTPase-defective Gα13: whereas the latter protein lacking palmitoylation sites was rescued in its SRF signalling by either an engineered polybasic sequence or a C-terminal isoprenylation site, these motifs failed to restore signalling by wildtype, non-palmitoylated Gα13. These findings illuminate several components of the mechanism in which overexpressed, wildtype Gα13 contributes to growth and tumorigenic signalling, and reveal greater stringency in its requirements for post-translational modification in comparison to GTPase-defective Gα13.

A network of Gαi signaling partners is revealed by proximity labeling proteomics analysis and includes PDZ-RhoGEF

G protein-coupled receptors (GPCRs) that couple to the Gα_i family of G proteins are key regulators of cell and tissue physiology. Our previous work has revealed new roles for Gα_i in regulating the migration of neutrophils and fibrosarcoma cells downstream of activated chemoattractant receptors. Here, we used an intact cell proximity-based labeling coupled to tandem mass tag (TMT)-based quantitative proteomics analysis to identify proteins that selectively interacted with the GTP-bound form of Gα_i1. Multiple targets were identified and validated with a BioID2-tagged, constitutively active Gα_i1 mutant, suggesting a network of interactions for activated Gα_I proteins in intact cells. We showed that active Gα_i1, but not Gα_i2, stimulated one candidate protein, PDZ-RhoGEF (PRG), despite more than 85% sequence identity between the G proteins. We also demonstrated in primary human neutrophils that active Gα_i likely regulated the polarization of phosphorylated myosin light chain, a process critical for migration, through the activation of PRG. The identification and characterization of new targets directly or indirectly regulated by Gα_i will aid in the investigation of the functional roles of Gα_i-coupled GPCRs in multiple biological processes.

A short C-terminal peptide in Gγ regulates Gβγ signaling efficacy

G protein beta-gamma (Gβγ) subunits anchor to the plasma membrane (PM) through the carboxy-terminal (CT) prenyl group in Gγ. This interaction is crucial for the PM localization and functioning of Gβγ, allowing GPCR-G protein signaling to proceed. The diverse Gγ family has 12 members, and we have recently shown that the signaling efficacies of major Gβγ effectors are Gγ-type dependent. This dependency is due to the distinct series of membrane-interacting abilities of Gγ. However, the molecular process allowing for Gβγ subunits to exhibit a discrete and diverse range of Gγ-type-dependent membrane affinities is unclear and cannot be explained using only the type of prenylation. The present work explores the unique designs of membrane-interacting CT residues in Gγ as a major source for this Gγ-type-dependent Gβγ signaling. Despite the type of prenylation, the results show signaling efficacy at the PM, and associated cell behaviors of Gβγ are governed by crucially located specific amino acids in the five to six residue preprenylation region of Gγ. The provided molecular picture of Gγ-membrane interactions may explain how cells gain Gγ-type-dependent G protein-GPCR signaling as well as how Gβγ elicits selective signaling at various subcellular compartments.

Control of protein palmitoylation by regulating substrate recruitment to a zDHHC-protein acyltransferase

Although palmitoylation regulates numerous cellular processes, as yet efforts to manipulate this post-translational modification for therapeutic gain have proved unsuccessful. The Na-pump accessory sub-unit phospholemman (PLM) is palmitoylated by zDHHC5. Here, we show that PLM palmitoylation is facilitated by recruitment of the Na-pump α sub-unit to a specific site on zDHHC5 that contains a juxtamembrane amphipathic helix. Site-specific palmitoylation and GlcNAcylation of this helix increased binding between the Na-pump and zDHHC5, promoting PLM palmitoylation. In contrast, disruption of the zDHHC5-Na-pump interaction with a cell penetrating peptide reduced PLM palmitoylation. Our results suggest that by manipulating the recruitment of specific substrates to particular zDHHC-palmitoyl acyl transferases, the palmitoylation status of individual proteins can be selectively altered, thus opening the door to the development of molecular modulators of protein palmitoylation for the treatment of disease.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

G protein-membrane interactions I: Gαi1 myristoyl and palmitoyl modifications in protein-lipid interactions and its implications in membrane microdomain localization

G proteins are fundamental elements in signal transduction involved in key cell responses, and their interactions with cell membrane lipids are critical events whose nature is not fully understood. Here, we have studied how the presence of myristic and palmitic acid moieties affects the interaction of the Gαi1 protein with model and biological membranes. For this purpose, we quantified the binding of purified Gαi1 protein and Gαi1 protein acylation mutants to model membranes, with lipid compositions that resemble different membrane microdomains. We observed that myristic and palmitic acids not only act as membrane anchors but also regulate Gαi1 subunit interaction with lipids characteristics of certain membrane microdomains. Thus, when the Gαi1 subunit contains both fatty acids it prefers raft-like lamellar membranes, with a high sphingomyelin and cholesterol content and little phosphatidylserine and phosphatidylethanolamine. By contrast, the myristoylated and non-palmitoylated Gαi1 subunit prefers other types of ordered lipid microdomains with higher phosphatidylserine content. These results in part explain the mobility of Gαi1 protein upon reversible palmitoylation to meet one or another type of signaling protein partner. These results also serve as an example of how membrane lipid alterations can change membrane signaling or how membrane lipid therapy can regulate the cell's physiology.

Tubulin, actin and heterotrimeric G proteins: coordination of signaling and structure

G proteins mediate signals from membrane G protein coupled receptors to the cell interior, evoking significant regulation of cell physiology. The cytoskeleton contributes to cell morphology, motility, division, and transport functions. This review will discuss the interplay between heterotrimeric G protein signaling and elements of the cytoskeleton. Also described and discussed will be the interplay between tubulin and G proteins that results in atypical modulation of signaling pathways and cytoskeletal dynamics. This will be extended to describe how tubulin and G proteins act in concert to influence various aspects of cellular behavior. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Protein palmitoylation in signal transduction of hematopoietic cells

The covalent attachment of palmitate to proteins can alter protein-lipid and protein-protein interactions thereby influencing protein function. Palmitoylation is a reversible post-translational modification. Thus, like protein phosphorylation, protein palmitoylation can function in activation-dependent signaling pathways. This review will provide an overview of the mechanisms and regulation of protein palmitoylation and focus on the role of palmitoylation in signal transduction pathways of lymphocytes and platelets.

Protein palmitoylation and pathogenesis in apicomplexan parasites

Apicomplexan parasites comprise a broad variety of protozoan parasites, including Toxoplasma gondii, Plasmodium, Eimeria, and Cryptosporidium species. Being intracellular parasites, the success in establishing pathogenesis relies in their ability to infect a host-cell and replicate within it. Protein palmitoylation is known to affect many aspects of cell biology. Furthermore, palmitoylation has recently been shown to affect important processes in T. gondii such as replication, invasion, and gliding. Thus, this paper focuses on the importance of protein palmitoylation in the pathogenesis of apicomplexan parasites.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Identification and subcellular localization of molecular complexes of Gq/11α protein in HEK293 cells

Heterotrimeric G-proteins localized in the plasma membrane convey the signals from G-protein-coupled receptors (GPCRs) to different effectors. At least some types of G-protein α subunits have been shown to be partly released from plasma membranes and to move into the cytosol after receptor activation by the agonists. However, the mechanism underlying subcellular redistribution of trimeric G-proteins is not well understood and no definitive conclusions have been reached regarding the translocation of Gα subunits between membranes and cytosol. Here we used subcellular fractionation and clear-native polyacrylamide gel electrophoresis to identify molecular complexes of G(q/11)α protein and to determine their localization in isolated fractions and stability in naïve and thyrotropin-releasing hormone (TRH)-treated HEK293 cells expressing high levels of TRH receptor and G(11)α protein. We identified two high-molecular-weight complexes of 300 and 140 kDa in size comprising the G(q/11) protein, which were found to be membrane-bound. Both of these complexes dissociated after prolonged treatment with TRH. Still other G(q/11)α protein complexes of lower molecular weight were determined in the cytosol. These 70 kDa protein complexes were barely detectable under control conditions but their levels markedly increased after prolonged (4-16 h) hormone treatment. These results support the notion that a portion of G(q/11)α can undergo translocation from the membrane fraction into soluble fraction after a long-term activation of TRH receptor. At the same time, these findings indicate that the redistribution of G(q/11)α is brought about by the dissociation of high-molecular-weight complexes and concomitant formation of low-molecular-weight complexes containing the G(q/11)α protein.

The inhibitory effect of phospholemman on the sodium pump requires its palmitoylation

Phospholemman (PLM), the principal sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. We investigated post-translational modifications of PLM additional to phosphorylation in adult rat ventricular myocytes (ARVM). LC-MS/MS of tryptically digested PLM immunoprecipitated from ARVM identified cysteine 40 as palmitoylated in some peptides, but no information was obtained regarding the palmitoylation status of cysteine 42. PLM palmitoylation was confirmed by immunoprecipitating PLM from ARVM loaded with [(3)H]palmitic acid and immunoblotting following streptavidin affinity purification from ARVM lysates subjected to fatty acyl biotin exchange. Mutagenesis identified both Cys-40 and Cys-42 of PLM as palmitoylated. Phosphorylation of PLM at serine 68 by PKA in ARVM or transiently transfected HEK cells increased its palmitoylation, but PKA activation did not increase the palmitoylation of S68A PLM-YFP in HEK cells. Wild type and unpalmitoylatable PLM-YFP were all correctly targeted to the cell surface membrane, but the half-life of unpalmitoylatable PLM was reduced compared with wild type. In cells stably expressing inducible PLM, PLM expression inhibited the sodium pump, but PLM did not inhibit the sodium pump when palmitoylation was inhibited. Hence, palmitoylation of PLM controls its turnover, and palmitoylated PLM inhibits the sodium pump. Surprisingly, phosphorylation of PLM enhances its palmitoylation, probably through the enhanced mobility of the phosphorylated intracellular domain increasing the accessibility of cysteines for the palmitoylating enzyme, with interesting theoretical implications. All FXYD proteins have conserved intracellular cysteines, so FXYD protein palmitoylation may be a universal means to regulate the sodium pump.

Subcellular localization of mu-opioid receptor G(s) signaling

In membranes obtained from mu-opioid receptor (MOR) expressing Chinese hamster ovary (CHO) cells (MOR-CHO), the MOR-selective agonist sufentanil produced a concentration-dependent stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate binding to G(s)alpha that was abolished by blocking MOR with naloxone. This unequivocally demonstrates the long-debated functionality of the previously described association of MOR with G(s)alpha. Several complementary observations indicate the relevance of caveolae to MOR-coupled G(s)alpha signaling. 1) In MOR-CHO membranes, sufentanil stimulated the translocation of G(s)alpha into Triton-insoluble membrane compartments. 2) Sufentanil enhanced the coimmunoprecipitation (co-IP) of G(s)alpha and adenylyl cyclase (AC) with caveolin-1 (a marker for caveolae) from the Triton-insoluble membrane fraction of spinal cord and MOR-CHO. 3) MOR blockade (via naloxone) or G(s) inactivation (via cholera toxin) abolished both the increased trafficking of G(s)alpha into the Triton-insoluble membrane fraction of MOR-CHO and the augmented co-IP from spinal cord membranes of G(s)alpha and AC with caveolin-1. This indicates that these events occurred subsequent to activation of MOR and G(s)alpha. Strikingly, lesser-phosphorylated G(s)alpha, which preferentially couple to MOR (Mol Brain Res 135:217-224, 2005; Mol Pharmacol 72:753-760, 2007; Mol Pharmacol 73:868-879, 2008), are concentrated in caveolae, underscoring their relevance to MOR G(s)alpha signaling. MOR-stimulated trafficking of G(s)alpha and AC into caveolae and the likelihood of increased MOR G(s)alpha coupling within caveolae could suggest that they contain the downstream effectors for MOR G(s)alpha AC signaling.

Shuttling and translocation of heterotrimeric G proteins and Ras

Heterotrimeric G proteins (alphabetagamma) and Ras proteins are activated by cell-surface receptors that sense extracellular signals. Both sets of proteins were traditionally thought to be constrained to the plasma membrane and some intracellular membranes. Live-cell-imaging experiments have now shown that these proteins are mobile inside a cell, shuttling continually between the plasma membrane and intracellular membranes in the basal state, maintaining these proteins in dynamic equilibrium in different membrane compartments. Furthermore, on receptor activation, a family of G protein betagamma subunits translocates rapidly and reversibly to the Golgi and endoplasmic reticulum enabling direct communication between the extracellular signal and intracellular membranes. A member of the Ras family has similarly been shown to translocate on activation. Although the impact of this unexpected intracellular movement of signaling proteins on cell physiology is likely to be distinct, there are striking similarities in the properties of these two families of signal-transducing proteins.

Coupling mode of receptors and G proteins

Signaling via G-protein-coupled receptors (GPCRs) is crucial to many physiological and pathophysiological processes in multicellular organisms, and GPCRs themselves are targets for important drugs. Classical cell supplementation experiments suggest a collision coupling model, in which receptors and G proteins diffuse randomly within the cell membrane and interact only if receptors are activated. This model is also backed by kinetic and live cell imaging data. According to the challenging theory, receptors and G proteins are precoupled--meaning they are forming stable complexes in the absence of agonist, which prevail during signaling. This model has been favored on the basis of copurification and coimmunoprecipitation of inactive receptors with G proteins and more recently by some approaches measuring energy transfer between labeled receptors and G proteins. This article reviews key findings regarding the receptor/G protein coupling mode, including most recent findings obtained by optical techniques.

Palmitoylation participates in G protein coupled signal transduction by affecting its oligomerization

Much in vivo and in vitro evidence has shown that the alpha subunits of heterotrimeric GTP-binding proteins (G proteins) exist as oligomers in their base state and disaggregate when being activated. In this article, the influence of palmitoylation modification of Galpha(o) on its oligomerization was explored extensively. Galpha(o) protein was expressed and purified from Escherichia coli strain JM109 cotransformed with pQE60(Galpha(o)) and pBB131(N-myristoyltransferase). Non-denaturing gel electrophoresis analysis revealed that Galpha(o) existed to a small extent as monomers but mostly as oligomers including dimers, trimers, tetramers and pentamers which could disaggregate completely into monomers by GTPgammaS stimulation. Palmitoylated Galpha(o), on the other hand, only present as oligomers that were difficult to disaggregate into monomers. The effect of palmitoylation on oligomerization of Galpha(o) was further investigated by several other biochemical and biophysical methods including gel filtration chromatography, analytical ultracentrifugation and atomic force microscopy analysis. The results consistently demonstrated that palmitoylation facilitated oligomerization of the Galpha(o) protein. Autoradiography indicated that [(14)C]-palmitoylated Galpha(o) would in no case disaggregate into monomers after treatment with GTPgammaS. [(35)S]-GTPgammaS binding activity assay showed that palmitoylated Galpha(o) was saturated at only 7.8 nmol/mg compared to 21.8 nmol/mg for non-palmitoylated Galpha(o). Fluorescent quenching studies using BODIPY FL-GTPgammaS as a probe showed that the conformation of GTP-binding domain of Galpha(o) tended to become more compact after palmitoylation. These results implied that palmitoylation may regulate the GDP/GTP exchange of Galpha(o) by influencing the oligomerization state of Galpha(o) and thereby modulate the on-off switch of the G protein in G protein-coupled signal transduction.

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.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

Translocation of activated heterotrimeric G protein Galpha(o) to ganglioside-enriched detergent-resistant membrane rafts in developing cerebellum

The association of gangliosides with specific proteins in the central nervous system was examined by co-immunoprecipitation with an anti-ganglioside antibody. The monoclonal antibody to the ganglioside GD3 immunoprecipitated phosphoproteins of 40, 53, 56, and 80 kDa from the rat cerebellum. Of these proteins, the 40-kDa protein was identified as the alpha-subunit of a heterotrimeric G protein, G(o) (Galpha(o)). Using sucrose density gradient analysis of cerebellar membranes, Galpha(o), but not Gbetagamma, was observed in detergent-resistant membrane (DRM) raft fractions in which GD3 was abundant after the addition of guanosine 5'-O-(thiotriphosphate) (GTPgammaS), which stabilizes G(o) in its active form. On the other hand, both Galpha(o) and Gbetagamma were excluded from the DRM raft fractions in the presence of guanyl-5'-yl thiophosphate, which stabilizes G(o) in its inactive form. Only Galpha(o) was observed in the DRM fractions from the cerebellum on postnatal day 7, but not from that in adult. After pertussis toxin treatment, Galpha(o) was not observed in the DRM fractions, even from the cerebellum on postnatal day 7. These results indicate the activation-dependent translocation of Galpha(o) into the DRM rafts. Furthermore, Galpha(o) was concentrated in the neuronal growth cones. Treatment with stromal cell-derived factor-1alpha, a physiological ligand for the G protein-coupled receptor, stimulated [(35)S]GTPgammaS binding to Galpha(o) and caused Galpha(o) translocation to the DRM fractions and RhoA translocation to the membrane fraction, leading to the growth cone collapse of cerebellar granule neurons. The collapse was partly prevented by pretreatment with the cholesterol-sequestering and raft-disrupting agent methyl-beta-cyclodextrin. These results demonstrate the involvement of signal-dependent Galpha(o) translocation to the DRM in the growth cone behavior of cerebellar granule neurons.

Trafficking and signaling by fatty-acylated and prenylated proteins

A wide variety of signaling proteins are modified by covalently linked fatty acids and/or prenyl groups. These hydrophobic moieties, which include myristate, palmitate, farnesyl and geranylgeranyl, are more than just fat: they provide distinct information that modulates the specificity and efficiency of signal transduction. Recent studies show that lipid modification influences the movement of a signaling protein within the cell and its final destination. Protein lipidation can also confer reversible association with membranes and other signaling proteins. These findings provide new insights into the biochemical and biophysical mechanisms that regulate membrane targeting, trafficking and signaling by lipid-modified proteins.

Differential modulation of type 1 and type 2 cannabinoid receptors along the neuroimmune axis

Endocannabinoid-signaling chains have been implicated in a variety of pathophysiological functions, including memory, coordination, vasoregulation, reproduction, neurodegeneration, and inflammation. These activities were thought to be mediated by the activation of two G-protein-coupled receptors (GPCRs), type 1 and type 2 cannabinoid receptors (CB(1)R and CB(2)R). These two CBR subtypes share common agonists and trigger similar signaling pathways, yet they present several important differences in structure and cell distribution. In particular, recent research has shown that the CB(1)R and CB(2)R are differentially linked to lipid rafts, specialized microdomains of the plasma membrane involved in the signaling of many other GPCRs. We present an overview of the current literature on the effects that lipid raft perturbation have on CBRs activities, and provide a mechanistic model to interpret these data in terms of structural and functional aspects. These findings may also have important implications for the development of new therapeutic approaches, including lipid raft perturbing drugs, aimed to selectively modulate CB(1)R signaling in a variety of pathological conditions.

Mechanisms of dominant negative G-protein α subunits

G-protein-coupled receptors (GPCRs) represent the largest class of membrane proteins and are the targets of 25-50% of drugs currently on the market. Dominant negative mutant Galpha subunits of heterotrimeric G-proteins have been extensively utilized to delineate G-protein signaling pathways and represent a promising new tool to study GPCR-dependent signaling in the CNS. There are different regions in various types of Galpha subunits in which mutations can give rise to a dominant negative phenotype. Such a mutant Galpha would compete with wild-type Galpha for binding to other proteins involved in the G-protein cycle and either block or reduce the response caused by wild-type Galpha. To date, there are three different mechanisms described for dominant negative Galpha subunits: sequestration of the Gbetagamma subunits, sequestration of the activated GPCR by the heterotrimeric complex, and sequestration of the activated GPCR by nucleotide-free Galpha. This review focuses on the development of dominant negative Galpha subunits, the different mechanisms used by various mutant Galpha subunits, and potential structural changes underlying the dominant negative effects.

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.

Higher-order organization and regulation of adenylyl cyclases

There is increasing awareness of the compartmentalization of cAMP signalling--the means by which cAMP levels change in discrete domains of the cell with discrete local consequences. Current developments in understanding the organization of adenylyl cyclases in the plasma membrane are illuminating how the earliest part of cAMP compartmentalization could occur. This review focuses on recent findings regarding three levels of adenylyl cyclase organization--oligomerization, positioning to lipid rafts and participation in multiprotein signalling complexes. This organization, coupled with the role of scaffolding proteins in arranging the downstream effectors of cAMP, helps to identify complexes that greatly facilitate the translation of enzyme activation into local consequences.

Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein

The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G(s) and an orphan member of the G protein-coupled receptor family, GPR3. To determine whether GPR3 activates G(s), the localization of Galpha(s) in follicle-enclosed oocytes from Gpr3(+/+) and Gpr3(-/-) mice was compared by using immunofluorescence and Galpha(s)GFP. GPR3 decreased the ratio of Galpha(s) in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Galpha(s) in the oocyte. Both of these properties indicate that GPR3 activates G(s). The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G(s) activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.

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.

Lipids, lipid rafts and caveolae: Their importance for GPCR signaling and their centrality to the endocannabinoid system

Scientific views of cell membrane organization are presently changing. Rather than serving only as the medium through which membrane proteins diffuse, lipid bilayers have now been shown to form compartmentalized domains with different biophysical properties (rafts/caveolae). For membrane proteins such as the G protein coupled receptors (GPCRs), a raft domain provides a platform for the assembly of signaling complexes and prevents cross-talk between pathways. Lipid composition also has a strong influence on the conformational activity of GPCRs. For certain GPCRs, such as the cannabinoid receptors, the lipid bilayer has additional significance. Endocannabinoids such as anandamide (AEA) are created in a lipid bilayer from lipid and act at the membrane embedded CB1 receptor. Endocannabinoids exiting the CB1 receptor are transported either by a carrier-mediated or a simple diffusion process to the membrane of the postsynaptic cell. Following cellular uptake, perhaps via caveolae/lipid raft-related endocytosis, AEA is rapidly metabolized by a membrane-associated enzyme, fatty acid amide hydrolase (FAAH) located in the endoplasmic reticulum. The entry point for AEA into FAAH appears to be from the lipid bilayer. This review explores the importance of lipid composition and lipid rafts to GPCR signaling and then focuses on the intimate relationship that exists between the lipid environment and the endocannabinoid system.

Dominant Portion of Thyrotropin-Releasing Hormone Receptor Is Excluded from Lipid Domains. Detergent-Resistant and Detergent-Sensitive Pools of TRH Receptor and Gqα/G11α Protein

Some G protein-coupled receptors might be spacially targetted to discrete domains within the plasma membrane. Here we assessed the localization in membrane domains of the epitope-tagged, fluorescent version of thyrotropin-releasing hormone receptor (VSV-TRH-R-GFP) expressed in HEK293 cells. Our comparison of three different methods of cell fractionation (detergent extraction, alkaline treatment/sonication and mechanical homogenization) indicated that the dominant portion of plasma membrane pool of the receptor was totally solubilized by Triton X-100 and its distribution was similar to that of transmembrane plasma membrane proteins (glycosylated and non-glycosylated forms of CD147, MHCI, CD29, CD44, transmembrane form of CD58, Tapa1 and Na,K-ATPase). As expected, caveolin and GPI-bound proteins CD55, CD59 and GPI-bound form of CD58 were preferentially localized in detergent-resistant membrane domains (DRMs). Trimeric G proteins G(q)alpha/G(11)alpha, G(i)alpha1/G(i)alpha2, G(s)alphaL/G(s)alphaS and Gbeta were distributed almost equally between detergent-resistant and detergent-solubilized pools. In contrast, VSV-TRH-R-GFP, Galpha, Gbeta and caveolin were localized massively only in low-density membrane fragments of plasma membranes, which were generated by alkaline treatment/sonication or by mechanical homogenization of cells. These data indicate that VSV-TRH-R-GFP as well as other transmembrane markers of plasma membranes are excluded from TX-100-resistant, caveolin-enriched membrane domains. Trimeric G protein G(q)alpha/G(11)alpha occurs in both DRMs and in the bulk of plasma membranes, which is totally solubilized by TX-100.

Palmitoylation regulates GDP/GTP exchange of G protein by affecting the GTP-binding activity of Goalpha

The effect of palmitoylation on the GTP-binding activity and conformation of Goalpha protein in hydrophobic and hydrophilic environments was studied. The binding assay was performed with an isotope labeled analog of GTP, GTP-gamma-35S, and its fluorescent analog, BODIPY FL-GTPgammaS was used to detect conformational change in the GTP-binding domain of Goalpha. Investigation of the GTP-gamma-35S binding activity of Goalpha shows that in a hydrophobic environment, mimicked by the presence of detergent, the apparent dissociation constant for palmitoylated Goalpha (K(D)=25.5x10(-9)+/-1.7x10(-9)M) increased threefold compared with that of non-palmitoylated Goalpha (K(D)=9.9x10(-9)+/-0.8x10(-9)M), while in an aqueous environment without detergent there is no significant difference between palmitoylated (K(D)=50.1 x 10(-9)+/-5.2x10(-9)M) and non-palmitoylated (K(D)=65.5x10(-9)+/-7.6x10(-9)M) Go(. This indicates that in a membrane environment palmitoylation may weaken the GTPgammaS binding ability of Go(. Fluorescent quenching studies using BODIPY FL-GTPgammaS as a probe showed that the conformation of the GTP-binding domain of Go( tends to become more compact after palmitoylation. These results imply that palmitoylation may regulate the GTP/GDP exchange of Goalpha by influencing the GTP-binding activity of Goalpha and facilitating the on-off switch function of the G protein in G protein-coupled signal transduction.

Beta-adrenergic receptor stimulation promotes G alpha s internalization through lipid rafts: a study in living cells

Upon binding hormones or drugs, many G protein-coupled receptors are internalized, leading to receptor recycling, receptor desensitization, and down-regulation. Much less understood is whether heterotrimeric G proteins also undergo agonist-induced endocytosis. To investigate the intracellular trafficking of G alpha s, we developed a functional G alpha s-green fluorescent protein (GFP) fusion protein that can be visualized in living cells during signal transduction. C6 and MCF-7 cells expressing G alpha s-GFP were treated with 10 microM isoproterenol, and trafficking was assessed with fluorescence microscopy. Upon isoproterenol stimulation, G alpha s-GFP was removed from the plasma membrane and internalized into vesicles. Vesicles containing G alpha s-GFP did not colocalize with markers for early endosomes or late endosomes/lysosomes, revealing that G alpha s does not traffic through common endocytic pathways. Furthermore, G alpha s-GFP did not colocalize with internalized beta2-adrenergic receptors, suggesting that G alpha s and receptors are removed from the plasma membrane by distinct endocytic pathways. Nonetheless, activated G alpha s-GFP did colocalize in vesicles labeled with fluorescent cholera toxin B, a lipid raft marker. Agonist significantly increased G alpha s protein in Triton X-100 -insoluble membrane fractions, suggesting that G alpha s moves into lipid rafts/caveolae after activation. Disruption of rafts/caveolae by treatment with cyclodextrin prevented agonist-induced internalization of G alpha s-GFP, as did overexpression of a dominant-negative dynamin. Taken together, these results suggest that receptor-activated G alpha s moves into lipid rafts and is internalized from these membrane microdomains. It is suggested that agonist-induced internalization of G alpha s plays a specific role in G protein-coupled receptor-mediated signaling and could enable G alpha s to traffic into the cellular interior to regulate effectors at multiple cellular sites.

Palmitoylation of intracellular signaling proteins: regulation and function

Protein S-palmitoylation is the thioester linkage of long-chain fatty acids to cysteine residues in proteins. Addition of palmitate to proteins facilitates their membrane interactions and trafficking, and it modulates protein-protein interactions and enzyme activity. The reversibility of palmitoylation makes it an attractive mechanism for regulating protein activity, and this feature has generated intensive investigation of this modification. The regulation of palmitoylation occurs through the actions of protein acyltransferases and protein acylthioesterases. Identification of the protein acyltransferases Erf2/Erf4 and Akr1 in yeast has provided new insight into the palmitoylation reaction. These molecules work in concert with thioesterases, such as acyl-protein thioesterase 1, to regulate the palmitoylation status of numerous signaling molecules, ultimately influencing their function. This review discusses the function and regulation of protein palmitoylation, focusing on intracellular proteins that participate in cell signaling or protein trafficking.

Ethanol inhibits palmitoylation of G protein Gαs

Neurobiological actions of ethanol have been linked to perturbations in cyclic AMP (cAMP)-dependent signaling processes. Chronic ethanol exposure leads to desensitization of cAMP production in response to physiological ligands (heterologous desensitization). Ethanol-induced alterations in neuronal expression of G proteins G(s) and G(i) have been invoked as a cause of heterologous desensitization. However, effects of ethanol on G protein expression vary considerably among different experimental protocols, various brain regions and diverse neuronal cell types. Dynamic palmitoylation of G protein alpha subunits is critical for membrane localization and protein-protein interactions, and represents a regulatory feature of G protein function. We studied the effect of ethanol on G alpha(s) palmitoylation. In NG108-15 rat neuroblastoma x glioma hybrid cells, acute exposure to pharmacologically relevant concentrations of ethanol (25-100 mm) inhibited basal and prostaglandin E1-stimulated incorporation of palmitate into G alpha(s). Exposure of NG108-15 cells to ethanol for 72 h induced a shift in G alpha(s) to its non-palmitoylated state, coincident with an inhibition of prostaglandin E1-induced cAMP production. Both parameters were restored following 24 h of ethanol withdrawal. Chronic ethanol exposure also induced the depalmitoylation of G alpha(s) in human embryonic kidney (HEK)293 cells that overexpress wild-type G alpha(s) and caused heterologous desensitization of adenylyl cyclase. By contrast, HEK293 cells that express a non-palmitoylated mutant of G alpha(s) were insensitive to heterologous desensitization after chronic ethanol exposure. In summary, the findings identify a novel effect of ethanol on post-translational lipid modification of G alpha(s), and represent a mechanism by which ethanol might affect adenylyl cyclase activity.

Differentially Regulated Expression of Endogenous RGS4 and RGS7

Regulators of G protein signaling (RGS proteins) constitute a family of newly appreciated components of G protein-mediated signal transduction. With few exceptions, most information available on mammalian RGS proteins was gained by transfection/overexpression or in vitro experiments, with relatively little known about the endogenous counterparts. Transfection studies, typically of tagged RGS proteins, have been conducted to overcome the low natural abundance of endogenous RGS proteins. Because transfection studies can lead to imprecise or erroneous conclusions, we have developed antibodies of high specificity and sensitivity to focus study on endogenous proteins. Expression of both RGS4 and RGS7 was detected in rat brain tissue and cultured PC12 and AtT-20 cells. Endogenous RGS4 presented as a single 27-28-kDa protein. By contrast, cultured cells transfected with a plasmid encoding RGS4 expressed two observable forms of the protein, apparently due to utilization of distinct sites of initiation of protein synthesis. Subcellular localization of endogenous RGS4 revealed predominant association with membrane fractions, rather than in cytosolic fractions, where most heterologously expressed RGS4 has been found. Endogenous levels of RGS7 exceeded RGS4 by 30-40-fold, and studies of cultured cells revealed regulatory differences between the two proteins. We observed that RGS4 mRNA and protein were concomitantly augmented with increased cell density and decreased by exposure of PC12M cells to nerve growth factor, whereas RGS7 was unaffected. Endogenous RGS7 was relatively stable, whereas proteolysis of endogenous RGS4 was a strong determinant of its lower level expression and short half-life. Although we searched without finding evidence for regulation of RGS4 proteolysis, the possibility remains that alterations in the degradation of this protein could provide a means to promptly alter patterns of signal transduction.

Palmitoylation is required for membrane association of activated but not inactive invertebrate visual Gqalpha

The invertebrate visual G protein, iGqalpha plays a central role in invertebrate phototransduction by relaying signals from rhodopsin to phospholipase C leading to membrane depolarization. Previous studies have shown reversible association of iGqalpha with rhabdomeric membranes regulated by light. To address the mechanism of membrane association we cloned iGqalpha from a Loligo pealei photoreceptor cDNA library and expressed it in HEK293T cells. Mutations were introduced to eliminate putative sites for palmitoylation at cysteines in positions 3 and 4. Membrane and soluble fractions were prepared from cells where iGqalpha was either activated or maintained in the GDP-bound form, followed by identification of iGqalpha through immunoblot analysis. The wild-type iGqalpha was entirely membrane-bound and shown to be post-translationally modified by palmitoylation. The mutant iGqalpha (C3,4A) was not palmitoylated yet it was found to be membrane-associated in the inactive state, however, approximately half of the protein became soluble when activated. These results suggest that palmitoylation is not required for membrane association of iGqalpha in the inactive state but is important in maintaining the stable membrane association of activated iGqalpha-GTP. The mechanism by which iGqalpha moves away from the membrane into the cytosol in response to prolonged light-stimulation in the native squid eye appears, therefore, to involve both activation and depalmitoylation processes.

Distinct rates of palmitate turnover on membrane-bound cellular and oncogenic H-ras

H-Ras displays dynamic cycles of GTP binding and palmitate turnover. GTP binding is clearly coupled to activation, but whether the palmitoylated COOH terminus participates in signaling, especially when constrained by membrane tethering, is unknown. As a way to compare COOH termini of membrane-bound, lipid-modified H-Ras, palmitate removal rates were measured for various forms of H-Ras in NIH 3T3 cells. Depalmitoylation occurred slowly (t(1/2) approximately 2.4 h) in cellular (H-RasWT) or dominant negative (H-Ras17N) forms and more rapidly (t(1/2) approximately 1 h) in oncogenic H-Ras61L or H-RasR12,T59. Combining this data with GTP binding measurements, the palmitate half-life of H-Ras in the fully GTP-bound state was estimated to be less than 10 min. Slow palmitate removal from cellular H-Ras was not explained by sequestration in caveolae, as neither cellular nor oncogenic H-Ras showed alignment with caveolin by immunofluorescence. Conversely, although it had faster palmitate removal, oncogenic H-Ras was located in the same fractions as H-RasWT on four types of density gradients, and remained fully membrane-bound. Thus the different rates of deacylation occurred even though oncogenic and cellular H-Ras appeared to be in similar locations. Instead, these results suggest that acylprotein thioesterases access oncogenic H-Ras more easily because the conformation of its COOH terminus against the membrane is altered. This previously undetected difference could help produce distinctive effector interactions and signaling of oncogenic H-Ras.

delta-Opioid receptors exhibit high efficiency when activating trimeric G proteins in membrane domains

Low-density membrane fragments (domains) were separated from the bulk of plasma membranes of human embryonic kidney (HEK)293 cells expressing a delta-opioid (DOP) receptor-Gi1alpha fusion protein by drastic homogenization and flotation on equilibrium sucrose density gradients. The functional activity of trimeric G proteins and capacity of the DOP receptor to stimulate both the fusion protein-linked Gi1alpha and endogenous pertussis-toxin sensitive G proteins was measured as d-Ala2, d-Leu5-enkephalin stimulated high-affinity GTPase or guanosine-5'-[gamma-35S]triphosphate ([35S]GTPgammaS) binding. The maximum d-Ala2-d-Leu5 enkephalin (DADLE)-stimulated GTPase was two times higher in low-density membrane fragments than in bulk of plasma membranes; 58 and 27 pmol/mg/min, respectively. The same difference was obtained for [35S]GTPgammaS binding. Contrarily, the low-density domains contained no more than half the DOP receptor binding sites (Bmax = 6.6 pmol/mg versus 13.6 pmol/mg). Thus, when corrected for expression levels of the receptor, low-density domains exhibited four times higher agonist-stimulated GTPase and [35S]GTPgammaS binding than the bulk plasma membranes. The regulator of G protein signaling RGS1, enhanced further the G protein functional activity but did not remove the difference between domain-bound and plasma membrane pools of G protein. The potency of the agonist in functional studies and the affinity of specific [3H]DADLE binding to the receptor were, however, the same in both types of membranes - EC50 = 4.5 +/- 0.1 x 10(-8) and 3.2 +/- 1.4 x 10(-8) m for GTPase; Kd = 1.2 +/- 0.1 and 1.3 +/- 0.1 nm for [3H]DADLE radioligand binding assay. Similar results were obtained when sodium bicarbonate was used for alkaline isolation of membrane domains. By contrast, detergent-insensitive membrane domains isolated following treatment of cells with Triton X100 exhibited no DADLE-stimulated GTPase or GTPgammaS binding. Functional coupling between the DOP receptor and cognate G proteins was also blocked by high-energy ultrasound and repeated freezing-thawing. Our data indicate, for the first time, that membrane domains isolated using 'detergent-free' procedures exhibit higher efficiency of coupling between a G protein-coupled receptor and its corresponding G protein(s) than bulk plasma membranes. Detergent-extraction diminishes these interactions, even when the receptor and G proteins are physically tethered together.

Regulation of light-dependent Gqalpha translocation and morphological changes in fly photoreceptors

Heterotrimeric G-proteins relay signals between membrane-bound receptors and downstream effectors. Little is known, however, about the regulation of Galpha subunit localization within the natural endogenous environment of a specialized signaling cell. Here we show, using live Drosophila flies, that light causes massive and reversible translocation of the visual Gqalpha to the cytosol, associated with marked architectural changes in the signaling compartment. Molecular genetic dissection together with detailed kinetic analysis enabled us to characterize the translocation cycle and to unravel how signaling molecules that interact with Gqalpha affect these processes. Epistatic analysis showed that Gqalpha is necessary but not sufficient to bring about the morphological changes in the signaling organelle. Furthermore, mutant analysis indicated that Gqbeta is essential for targeting of Gqalpha to the membrane and suggested that Gqbeta is also needed for efficient activation of Gqalpha by rhodopsin. Our results support the 'two-signal model' hypothesis for membrane targeting in a living organism and characterize the regulation of both the activity-dependent Gq localization and the cellular architectural changes in Drosophila photoreceptors.

Dependence of the coupling of dopamine receptors to G proteins on the protein redox state in the neural plasma membranes of pond snail

Binding analysis using [3H]dopamine has shown that reduction of protein thiol groups with dithiothreitol (DTT) led to a dual effect on the receptors. First, the amount of dopamine-binding sites on the membranes and their affinity to the ligand were decreased. Second, the affinity of the receptors to [3H]dopamine was enhanced in the presence of GDP. Binding of D(1) antagonist [3H]SCH23390 to dopamine receptors increased following DTT treatment, opposite to the case with D(1) agonist [3H]SKF38393. The displacement of [3H]GDP by GTPgammaS was depressed by dopamine. Stimulation of [3H]GDP binding by dopamine was potentiated after incubation with DTT. Membrane nitrosylation eliminated the reciprocal dependence of GDP and dopamine binding to the membranes. It is suggested that binding of dopamine to the receptors can lead to both stimulation and inhibition of G protein activity, and the ratio of these effects depends on the reduction and oxidation of sulfhydryl groups of membrane proteins. Thiol reduction potentiated inhibitory action of dopamine receptors on coupled G proteins, and nitrosylation led to their uncoupling.

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

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

Characterization of Saccharomyces cerevisiae acyl-protein thioesterase 1, the enzyme responsible for G protein alpha subunit deacylation in vivo

Thioacylation is a reversible lipid modification of proteins that plays a role in the regulation of signal transduction. Acyl-protein thioesterase 1 (APT1) was identified as an enzyme capable of deacylating some thioacylated proteins in vitro. Saccharomyces cerevisiae open reading frame YLR118c encodes an enzyme homologous to Rattus norvegicus APT1. We demonstrate that the catalytic activity of the protein encoded by the yeast open reading frame is similar to that of rat APT1, and we designate the protein S. cerevisiae Apt1p. Yeasts bearing a disruption of the APT1 gene lack significant biochemically detectable acyl-protein thioesterase activity. They also fail to deacylate Gpa1p, the yeast G alpha subunit, in metabolic radiolabeling studies. We conclude that native APT1 is the enzyme responsible for G alpha subunit deacylation in S. cerevisiae and presumably other eukaryotes as well.

Real-time visualization of a fluorescent G(alpha)(s): dissociation of the activated G protein from plasma membrane

To study behavior of activated G(alpha)(s) in living cells, green fluorescent protein (GFP) was inserted within the internal amino acid sequence of G(alpha)(s) to generate a G(alpha)(s)-GFP fusion protein. The fusion protein maintained a bright green fluorescence and was identified by immunoblotting with antibodies against G(alpha)(s) or GFP. The cellular distribution of G(alpha)(s)-GFP was similar to that of endogenous G(alpha)(s). G(alpha)(s)-GFP was tightly coupled to the beta adrenergic receptor to activate the G(alpha)(s) effector, adenylyl cyclase. Activation of G(alpha)(s)-GFP by cholera toxin caused a gradual displacement of the fusion protein from the plasma membrane throughout the cytoplasm in living cells. Unlike the slow release of G(alpha)(s)-GFP from the membrane induced by cholera toxin, the beta-adrenergic agonist isoproterenol caused a rapid partial release of the fusion protein into the cytoplasm. At 1 min after treatment with isoproterenol, the extent of G(alpha)(s)-GFP release from plasma membrane sites was maximal; however, insertion of G(alpha)(s)-GFP at other membrane sites occurred during the same time period. Translocation of G(alpha)(s)-GFP fusion protein induced by isoproterenol suggested that the internalization of G(alpha)(s) might play a role in signal transduction by interacting with effector molecules and cytoskeletal elements at multiple cellular sites.

Mu and Delta opioid receptors activate the same G proteins in human neuroblastoma SH-SY5Y cells

1. There is evidence for interactions between mu and delta opioid systems both in vitro and in vivo. This work examines the hypothesis that interaction between these two receptors can occur intracellularly at the level of G protein in human neuroblastoma SH-SY5Y cells. 2. The [(35)S]GTP gamma S binding assay was used to measure G protein activation following agonist occupation of opioid receptors. The agonists DAMGO (EC(50), 45 nM) and SNC80 (EC(50), 32 nM) were found to be completely selective for stimulation of [(35)S]-GTP gamma S binding through mu and delta opioid receptors respectively. Maximal stimulation of [(35)S]-GTP gamma S binding produced by SNC80 was 57% of that seen with DAMGO. When combined with a maximally effective concentration of DAMGO, SNC80 caused no additional [(35)S]-GTP gamma S binding. This effect was also seen when measured at the level of adenylyl cyclase. 3. Receptor activation increased the dissociation of pre-bound [(35)S]-GTP gamma S. In addition, the delta agonist SNC80 promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using the mu agonist DAMGO. Conversely, DAMGO promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using SNC80. 4. Tolerance to DAMGO and SNC80 in membranes from cells exposed to agonist for 18 h was homologous and there was no evidence for alteration in G protein activity. 5. The findings support the hypothesis that mu- and delta-opioid receptors share a common G protein pool, possibly through a close organization of the two receptors and G protein at the plasma membrane.

Enrichment of G-protein palmitoyltransferase activity in low density membranes: in vitro reconstitution of Galphai to these domains requires palmitoyltransferase activity

Many signaling proteins are targeted to low density, sphingomyelin- and cholesterol-enriched membranes, also called lipid rafts. These domains organize receptor-mediated signaling events at the plasma membrane. Fatty acylation is one mechanism for targeting proteins to rafts. It was therefore of interest to determine if protein palmitoyltransferase activity is also present in these domains. In this study, protein palmitoyltransferase activity, assayed using G-protein alpha subunits as a substrate, was found to be highly enriched in low density membranes derived from cells that express caveolin as well as those that do not. Depletion of cellular cholesterol with the drug methyl-beta-cyclodextrin resulted in inhibition of palmitoyltransferase activity and a redistribution of the remaining activity to membranes of higher density. This effect was reversed by adding cholesterol to cyclodextrin-treated cells. When reconstituted into cell membranes, the population of purified recombinant G(alphai) that was palmitoylated was highly enriched in the low density membrane fractions, whereas the bulk unmodified G(alphai)-protein was largely excluded. This effect required palmitoyltransferase activity and was abolished if the palmitoylated cysteine was mutated. Thus, palmitoyltransferase facilitates the enrichment of fatty acylated signaling molecules in plasma membrane subdomains.

Maturation of rat brain is accompanied by differential expression of the long and short splice variants of G(s)alpha protein: identification of cytosolic forms of G(s)alpha

Distribution of the alpha subunit of the stimulatory G protein (G(s)alpha) was analyzed in membrane and cytosolic (supernatant 200 000 g) fractions from rat cortex, thalamus and hippocampus during the course of post-natal development. In parallel, changes in beta-adrenoceptor density and adenylyl cyclase activity were determined. Long (G(s)alphaL) and short (G(s)alphaS) variants of G(s)alpha were assessed by immunoblotting using specific polyclonal antisera reacting with both G(s)alpha isoforms. Post-natal development was associated with an increase in the total amount of brain G(s)alpha. G(s)alphaL was the dominant isoform of G(s)alpha in the membrane fractions of all studied brain regions and its amount increased markedly between post-natal day (PD) 1 and 90. The level of membrane-bound G(s)alphaS also elevated during post-natal development, but more pronounced changes were found in cytosolic G(s)alphaS. Although only a small amount of G(s)alphaS (much smaller than G(s)alphaL) was detected among soluble proteins shortly after birth, G(s)alphaS prevailed over G(s)alphaL at PD90. The G(s)alphaL/G(s)alphaS ratio decreased, respectively, from 3.2 to 1.2 and from 5.0 to 1.5 in the membrane fractions of cortex and hippocampus, but remained almost constant in thalamus between PD1 and 90. More dramatic changes were found in the cytosolic fractions of all studied brain regions: the G(s)alphaL/G(s)alphaS ratio decreased sharply in cortex (from 14.1 to 0.9), hippocampus (from 3.7 to 0.8), and also in thalamus (from 9.5 to 0.5). These results demonstrate that the membrane-cytosol balance of G(s)alpha proteins alters dramatically during the course of brain development. Both G(s)alphaL and G(s)alphaS were expressed in a region- and age-specific manner, which suggests different roles in the maturation of the brain tissue. A cyc(-) reconstitutive assay of cytosolic G(s)alpha indicated that only approximately 20% of this protein was functional, compared with membrane-bound G(s)alpha, and its ability to reconstitute adenylyl cyclase activity increased during the course of maturation. The number of beta-adrenoceptors increased sharply during early post-natal development but only slightly in adulthood, and both GTP- and isoproterenol-stimulated adenylate cyclase activity reached peak values around PD12.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

Homologous and heterologous uncoupling of muscarinic M(3) and alpha(1B) adrenoceptors to Galpha(q/11) in SH-SY5Y human neuroblastoma cells

1. The present study employed a [(35)S]-GTPgammaS binding protocol in conjunction with immunoprecipitation (IP) of the Galpha subunits to investigate the desensitization of G(q/11)-coupled receptors at the level of the G-protein activation. Membranes from SH-SY5Y cells expressing the recombinant human alpha(1B)-adrenoceptor (alpha(1B)-AR) (and endogenously expressing the M(3) muscarinic acetylcholine receptor (M(3)-AChR)) exhibited G(q/11) activation in a concentration-dependent manner in response to noradrenaline or methacholine. 2. Pre-treatment of intact cells with agonist prior to membrane preparation and use in the [(35)S]-GTPgammaS IP assay demonstrated that both receptors were homologously desensitized by pre-treatment with agonist since the G(q/11) activation in response to a secondary challenge with agonist was markedly reduced. Stimulation of alpha(1B)-AR was effective at heterologously desensitizing the M(3)-AChR. The PKC inhibitor, Ro-31-8220 (10 microM) was ineffective at preventing the agonist-mediated receptor desensitization. 3. [(32)P]P(i)-labelled cells allowed the detection of increases in receptor phosphorylation. Phorbol 12,13 dibutyrate (PDBu) (1 microM) was effective at producing a Ro-31-8220 (10 microM)-sensitive, detectable increase in alpha(1B)-AR but not M(3)-AChR phosphorylation. Noradrenaline (30 microM) stimulated alpha(1B)-AR phosphorylation, which could be partially inhibited by Ro-31-8220 (10 microM). The phosphorylation of M(3)-AChR was increased by methacholine (100 microM) incubation and this effect appeared to be insensitive to Ro-31-8220 (10 microM). 4. These findings demonstrate that [(35)S]-GTPgammaS-Galpha-subunit IP can be used to estimate receptor desensitization as a decline in receptor-G-protein coupling. Both the alpha(1B)-AR and M(3)-AChR undergo rapid homologous desensitization that is associated with an increase in receptor phosphorylation. The heterologous desensitization of M(3)-AChR produced by alpha(1B)-AR stimulation is not associated with a detectable increase in M(3)-AChR phosphorylation, suggesting that receptor phosphorylation is not necessarily a prerequisite for desensitization.