A cysteine-3 to serine mutation of the G-protein Gi1 alpha abrogates functional activation by the alpha 2A-adrenoceptor but not interactions with the beta gamma complex

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.

Evidence for Noncanonical Neurotransmitter Activation: Norepinephrine as a Dopamine D2-Like Receptor Agonist

The Gαi/o-coupled dopamine D2-like receptor family comprises three subtypes: the D2 receptor (D2R), with short and long isoform variants (D2SR and D2LR), D3 receptor (D3R), and D4 receptor (D4R), with several polymorphic variants. The common overlap of norepinephrine innervation and D2-like receptor expression patterns prompts the question of a possible noncanonical action by norepinephrine. In fact, previous studies have suggested that norepinephrine can functionally interact with D4R. To our knowledge, significant interactions between norepinephrine and D2R or D3R receptors have not been demonstrated. By using radioligand binding and bioluminescent resonance energy transfer (BRET) assays in transfected cells, the present study attempted a careful comparison between dopamine and norepinephrine in their possible activation of all D2-like receptors, including the two D2R isoforms and the most common D4R polymorphic variants. Functional BRET assays included activation of G proteins with all Gαi/o subunits, adenylyl cyclase inhibition, and β arrestin recruitment. Norepinephrine acted as a potent agonist for all D2-like receptor subtypes, with the general rank order of potency of D3R > D4R ≥ D2SR ≥ D2L. However, for both dopamine and norepinephrine, differences depended on the Gαi/o protein subunit involved. The most striking differences were observed with Gαi2, where the rank order of potencies for both dopamine and norepinephrine were D4R > D2SR = D2LR >> D3R. Furthermore the results do not support the existence of differences in the ability of dopamine and norepinephrine to activate different human D4R variants. The potency of norepinephrine for adrenergic α2A receptor was only about 20-fold higher compared with D3R and D4R across the three functional assays.

Galpha subunit Gpa2 recruits kelch repeat subunits that inhibit receptor-G protein coupling during cAMP-induced dimorphic transitions in Saccharomyces cerevisiae

All eukaryotic cells sense extracellular stimuli and activate intracellular signaling cascades via G protein-coupled receptors (GPCR) and associated heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 and associated Galpha subunit Gpa2 sense extracellular carbon sources (including glucose) to govern filamentous growth. In contrast to conventional Galpha subunits, Gpa2 forms an atypical G protein complex with the kelch repeat Gbeta mimic proteins Gpb1 and Gpb2. Gpb1/2 negatively regulate cAMP signaling by inhibiting Gpa2 and an as yet unidentified target. Here we show that Gpa2 requires lipid modifications of its N-terminus for membrane localization but association with the Gpr1 receptor or Gpb1/2 subunits is dispensable for membrane targeting. Instead, Gpa2 promotes membrane localization of its associated Gbeta mimic subunit Gpb2. We also show that the Gpa2 N-terminus binds both to Gpb2 and to the C-terminal tail of the Gpr1 receptor and that Gpb1/2 binding interferes with Gpr1 receptor coupling to Gpa2. Our studies invoke novel mechanisms involving GPCR-G protein modules that may be conserved in multicellular eukaryotes.

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.

PTH-dependent adenylyl cyclase activation in SaOS-2 cells: passage dependent effects on G protein interactions

Parathyroid hormone (PTH) sensitive adenylyl cyclase activity (ACA) in SaOS-2 cells varies as a function of cell passage. In early passage (EP) cells (< 6), ACA in response to PTH and forskolin (FOR) was relatively low and equivalent, whereas in late passage (LP) cells (> 22), PTH exceeded FOR dependent ACA. Potential biochemical mechanisms for this passage dependent change in ACA were considered. In EP, prolonged exposure to pertussis toxin (PT) markedly enhanced ACA activity in response to PTH, Isoproterenol and Gpp(NH)p, whereas ACA in response to FOR was decreased. In contrast, the identical treatment of LP with PT diminished all ACA in response to PTH, Gpp(NH)p, and FOR. The dose dependent effects of PT on subsequent [(32)P]ADP-ribosylation of its substrates, GTPase activity, as well as FOR-dependent ACA, were equivalent in EP and LP. The relative amounts of G(alpha)i and G(alpha)s proteins, as determined both by Western blot, PT and cholera toxin (CT) dependent [(32)P]ADP-ribosylation, were quantitatively similar in EP and LP. Western blot levels of G(alpha)s and G(alpha)i proteins were not influenced by prior exposure to PT. Both PT and CT dependent [(32)P]ADP-ribosylation were dose-dependently decreased following exposure to PT. However, the PT-dependent decline in CT-dependent [(32)P]ADP-ribosylation occurred with enhanced sensitivity in LP. The protein synthesis inhibitor cycloheximide partially reversed the PT associated decrease in FOR dependent ACA in EP. In contrast, cycloheximide completely reversed the PT associated decrease in FOR and as well as PTH dependent ACA in LP. G(alpha)s activity, revealed by cyc(-) reconstitution, was not altered either by cell passage or exposure to PT. The results suggest that the coupling between the components of the complex may be pivotally important in the differential responsiveness of early and late passage SaOS-2 cells to PTH.

Coordinated agonist regulation of receptor and G protein palmitoylation and functional rescue of palmitoylation-deficient mutants of the G protein G11alpha following fusion to the alpha1b-adrenoreceptor: palmitoylation of G11alpha is not required for interaction with beta*gamma complex

Transfection of either the alpha(1b)-adrenoreceptor or Galpha(11) into a fibroblast cell line derived from a Galpha(q)/Galpha(11) double knockout mouse failed to produce elevation of intracellular [Ca(2+)] upon the addition of agonist. Co-expression of these two polypeptides, however, produced a significant stimulation. Co-transfection of the alpha(1b)-adrenoreceptor with the palmitoylation-resistant C9S,C10S Galpha(11) also failed to produce a signal, and much reduced and kinetically delayed signals were obtained using either C9S Galpha(11) or C10S Galpha(11). Expression of a fusion protein between the alpha(1b)-adrenoreceptor and Galpha(11) allowed [Ca(2+)](i) elevation, and this was also true for a fusion protein between the alpha(1b)-adrenoreceptor and C9S,C10S Galpha(11), since this strategy ensures proximity of the two polypeptides at the cell membrane. For both fusion proteins, co-expression of transducin alpha, as a beta.gamma-sequestering agent, fully attenuated the Ca(2+) signal. Both of these fusion proteins and one in which an acylation-resistant form of the receptor was linked to wild type Galpha(11) were also targets for agonist-regulated [(3)H]palmitoylation and bound [(35)S]guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) in an agonist concentration-dependent manner. The potency of agonist to stimulate [(35)S]GTPgammaS binding was unaffected by the palmitoylation potential of either receptor or G protein. These studies provide clear evidence for coordinated, agonist-mediated regulation of the post-translational acylation of both a receptor and partner G protein and demonstrate the capacity of such fusions to bind and then release beta.gamma complex upon agonist stimulation whether or not the G protein can be palmitoylated. They also demonstrate that Ca(2+) signaling in EF88 cells by such fusion proteins is mediated via release of the G protein beta.gamma complex.

Thioacylation is required for targeting G-protein subunit G(o1alpha) to detergent-insoluble caveolin-containing membrane domains

alpha-Subunits of heterotrimeric G(i)-like proteins (alpha(i), alpha(o) and alpha(z)) associate with the cytoplasmic leaflet of the plasma membrane by means of N-terminally linked myristic acid and palmitic acid. An additional role for palmitate has been recently suggested by the observation that fusion with the palmitoylated N-terminus of alpha(i1) relocalizes cytosolic green-fluorescent-protein reporter to low buoyancy, Triton-insoluble membrane domains (TIFF; Triton-insoluble floating fraction), enriched with caveolin-1 [Galbiati, Volonté, Meani, Milligan, Lublin, Lisanti and Parenti (1999) J. Biol. Chem 274, 5843-5850]. Here we show that, upon transient expression in transfected COS-7 cells, myristoylated and palmitoylated alpha(o) (alpha(o)wt, where wt is wild-type) is exclusively found in TIFF, from where non-palmitoylated alpha(o)wt and alpha(o)C3S (Cys(3)-->Ser) mutant are excluded. Moreover, alpha(o) fused to N-terminally truncated human vasopressin V2 receptor (V2TR-alpha(o)), lacking myristate and palmitate, still localizes at the plasma membrane by means of first transmembrane helix of V2R, but is excluded from TIFF. Likewise, alpha(o)C3S does not partition into TIFF, even when its membrane avidity is enhanced by co-expression of betagamma-subunits. Thus membrane association, in the absence of added palmitate, is not sufficient to confer partitioning of alpha(o) within TIFF, suggesting that palmitoylation is a signal for membrane compartmentalization of dually acylated alpha-subunits.

Regulation of galpha i palmitoylation by activation of the 5-hydroxytryptamine-1A receptor

Nearly all alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated at cysteine residues near the N terminus. A regulated cycle of palmitoylation could provide a mechanism for modulating G protein signaling by affecting protein interactions and localization of the subunit. In the present studies we utilized both [(3)H]palmitate incorporation and pulse-chase techniques to address the dynamics of alpha(i) palmitoylation in Chinese hamster ovary cells. Both techniques demonstrated a dose- and time-dependent change in [(3)H]palmitate labeling of alpha(i) upon activation of stably expressed 5-hydroxytryptamine-1A receptors by the agonist (+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (DPAT), with an EC(50) of approximately 10 nm. For the incorporation assay, DPAT elicited an approximate doubling in labeling at the earliest time point measured. For the pulse-chase assay, DPAT promoted a significant loss of radiolabel almost equally as fast. These data demonstrate that the exchange of palmitate on alpha(i) is increased upon stimulation of 5-hydroxytryptamine-1A receptors through the combined processes of depalmitoylation and palmitoylation. These results provide the basis for extending the concept of regulated exchange of palmitate beyond G(s) and provide a framework for exploring the specific functional attributes of the palmitoylated and depalmitoylated forms of subunit.

Lipid-linked proteins of plants

Increasing numbers of plant proteins are being shown to have posttranslationally-attached lipids. The modifications include N-myristoylation, S-palmitoylation, prenylation by farnesyl or geranylgeranyl moieties, or attachment of glycosylphosphatidylinositol anchors. This report summarizes recent findings regarding the structure, metabolism and physiological functions of these important protein-linked lipids.

The membrane association domain of RGS16 contains unique amphipathic features that are conserved in RGS4 and RGS5

Regulators of G protein signaling (RGS proteins) modulate G protein-mediated signaling pathways by acting as GTPase-activating proteins for Gi, Gq, and G12 alpha-subunits of heterotrimeric G proteins. Although it is known that membrane association is critical for the biological activities of many RGS proteins, the mechanism underlying this requirement remains unclear. We reported recently that the NH2 terminus of RGS16 is required for its function in vivo. In this study, we show that RGS16 lacking the NH2 terminus is no longer localized to the plasma membrane as is the wild type protein, suggesting that membrane association is important for biological function. The region of amino acids 7-32 is sufficient to confer the membrane-targeting activity, of which amino acids 12-30 are predicted to adopt an amphipathic alpha-helix. Site-directed mutagenesis experiments showed that the hydrophobic residues of the nonpolar face of the helix and the strips of positively charged side chains positioned along the polar/nonpolar interface of the helix are crucial for membrane association. Subcellular fractionation by differential centrifugation followed by conditions that distinguish peripheral membrane proteins from integral ones indicate that RGS16 is a peripheral membrane protein. We show further that RGS16 membrane association does not require palmitoylation. Our results, together with other recent findings, have defined a unique membrane association domain with amphipathic features. We believe that these structural features and the mechanism of membrane association of RGS16 are likely to apply to the homologous domains in RGS4 and RGS5.

betagamma dimers derived from Go and Gi proteins contribute different components of adrenergic inhibition of Ca2+ channels in rat sympathetic neurones

1. Using perforated-patch recordings, we have examined the part played by endogenous G-protein subunits in the alpha2-adrenoceptor-mediated inhibition of N-type Ca2+ currents in sympathetic neurones. 2. Two components of ICa inhibition by noradrenaline were recorded: a prominent, high affinity and voltage-dependent pertussis toxin (PTX)-sensitive pathway and a minor, low affinity and mostly voltage-insensitive PTX-resistant pathway. 3. PTX-sensitive inhibition was reduced by microinjection of antibodies against either GalphaoA,B or Galphai1,2. The voltage-dependent fraction of inhibition was reduced by anti-Galphao but not by anti-Galphai antibody. 4. Antisense depletion of GalphaoA led to a marked reduction of noradrenaline-induced inhibition and voltage dependence. By contrast, Galphai depletion attenuated noradrenergic modulation without affecting the voltage dependence. 5. Expression of the betagamma-binding agents beta-adrenergic receptor kinase 1 (C-terminus, betaARK1C-ter) or Galphai1 with a Cys3 to Ser mutation partially prevented noradrenergic inhibition while alpha-transducin abolished it. Residual inhibition was mostly voltage independent in cells expressing betaARK1C-ter but was strongly reversed by depolarization in Galphai1 Cys3Ser-expressing cells. 6. Expression of the PTX-resistant Galphai1 Cys351Ile mutant in cells treated with PTX restored alpha2-adrenoceptor inhibition. This restored inhibition was weakly reversed by depolarization. Both the degree and voltage dependence of inhibition were correlated with the level of expression of the Galphai1 Cys351Ile subunit. 7. Our findings identify betagamma dimers associated with GalphaoA and Galphai as mediators of the PTX-sensitive alpha2-adrenoceptor-mediated inhibition of N-type Ca2+ channels. Different betagamma combinations may account for the differential voltage-dependent effects of Go and Gi on ICa.

The dually acylated NH2-terminal domain of gi1alpha is sufficient to target a green fluorescent protein reporter to caveolin-enriched plasma membrane domains. Palmitoylation of caveolin-1 is required for the recognition of dually acylated g-protein alpha subunits in vivo

Here we investigate the molecular mechanisms that govern the targeting of G-protein alpha subunits to the plasma membrane. For this purpose, we used Gi1alpha as a model dually acylated G-protein. We fused full-length Gi1alpha or its extreme NH2-terminal domain (residues 1-32 or 1-122) to green fluorescent protein (GFP) and analyzed the subcellular localization of these fusion proteins. We show that the first 32 amino acids of Gi1alpha are sufficient to target GFP to caveolin-enriched domains of the plasma membrane in vivo, as demonstrated by co-fractionation and co-immunoprecipitation with caveolin-1. Interestingly, when dual acylation of this 32-amino acid domain was blocked by specific point mutations (G2A or C3S), the resulting GFP fusion proteins were localized to the cytoplasm and excluded from caveolin-rich regions. The myristoylated but nonpalmitoylated (C3S) chimera only partially partitioned into caveolin-containing fractions. However, both nonacylated GFP fusions (G2A and C3S) no longer co-immunoprecipitated with caveolin-1. Taken together, these results indicate that lipid modification of the NH2-terminal of Gi1alpha is essential for targeting to its correct destination and interaction with caveolin-1. Also, a caveolin-1 mutant lacking all three palmitoylation sites (C133S, C143S, and C156S) was unable to co-immunoprecipitate these dually acylated GFP-G-protein fusions. Thus, dual acylation of the NH2-terminal domain of Gi1alpha and palmitoylation of caveolin-1 are both required to stabilize and perhaps regulate this reciprocal interaction at the plasma membrane in vivo. Our results provide the first demonstration of a functional role for caveolin-1 palmitoylation in its interaction with signaling molecules.

Signalling functions of protein palmitoylation

Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.

Quantitative analysis of a cysteine351glycine mutation in the G protein Gi1alpha: effect on alpha2A-adrenoceptor-Gi1alpha fusion protein activation

Fusion proteins were constructed between the porcine alpha2A-adrenoceptor and either wild-type (Cys351) or a pertussis toxin-resistant (Gly351) form of the G protein Gi1alpha. Addition of adrenaline to membranes expressing the fusion proteins resulted in concentration-dependent stimulation of their high affinity GTPase activity. The alpha2A-adrenoceptor-wild type Gi1alpha fusion protein produced substantially higher maximal stimulation of GTPase activity in response to adrenaline than that containing Gly351 Gi1alpha. Treatment of the fusion proteins as agonist-regulated enzymes allowed measurement of Vmax and turnover number for adrenaline-stimulation of the GTPase activity of each fusion construct. The turnover number of the alpha2A-adrenoceptor-Cys351 Gly Gi1alpha fusion protein was only 44'S, of that for the alpha2A-adrenoceptor-wild type Gi1alpha fusion protein. These data provide the first direct quantitative evaluation of the effects of a mutation of a G protein on the capacity of an agonist-occupied receptor to activate the mutant.

New aspects of G-protein-coupled receptor signalling and regulation

Research on the structure, regulation and signalling properties of the family of seven-transmembrane-helix, heterotrimeric guanine nucleotide-binding protein (G-protein)-coupled receptors (GPCRs) continues at a frantic pace. This reflects their central role in transmission of hormone- and neurotransmitter-encoded information across the plasma membrane of cells. The location of the ligand-binding sites on the extracellular face of the membrane has made them obvious targets for therapeutic intervention in a wide range of conditions resulting from endocrine imbalance. Furthermore, based on the identification of many novel GPCR sequences emerging from expressed sequence tags (ESTs) and other DNA sequencing programmes, it has become clear that the GPCR family is likely to be considerably larger than appreciated in even the recent past. Although neither the natural ligands nor synthetic pharmaceuticals have yet been identified for these so-called ;orphan' GPCRs, they offer the potential for a plethora of new therapeutic targets. Within a short review, it is impossible to cover all the current developments in this field and the topics selected represent a personal view of recent highlights of areas that provide both novel and general insights into the function and regulation of GPCRs.