Analysis of Gz alpha by site-directed mutagenesis. Sites and specificity of protein kinase C-dependent phosphorylation

A Differential Hypofunctionality of Gαi Proteins Occurs in Adolescent Idiopathic Scoliosis and Correlates with the Risk of Disease Progression

Adolescent idiopathic scoliosis is the most prevalent spine deformity and the molecular mechanisms underlying its pathophysiology remain poorly understood. We have previously found a differential impairment of melatonin receptor signaling in AIS osteoblasts allowing the classification of patients into three biological endophenotypes or functional groups (FG1, FG2 and FG3). Here, we provide evidence that the defect characterizing each endophenotype lies at the level of Gαi proteins leading to a systemic and generalized differential impairment of Gi-coupled receptor signaling. The three Gαi isoforms exhibited a selective serine phosphorylation patterns for each AIS endophenotype resulting in a differential reduction in Gαi protein activity as determined by cellular dielectric spectroscopy and small interfering RNA methods. We found that one endophenotype (FG2) with phosphorylated Gαi₁ and Gαi₂ was consistently associated with a significantly high risk of spinal deformity progression when compared to the other two endophenotypes (FG1 and FG3). We further demonstrated that each endophenotype is conserved among affected family members. This study expands our understanding of the mechanism underlying the Gi-coupled receptor signaling dysfunction occurring in AIS and provides the first evidence for its hereditary nature. Collectively, our findings offers a new perspective on Gαi hypofunctionality in a human disease by revealing specific serine phosphorylation signatures of Gαi isoforms that may facilitate the identification of AIS patients at risk of spinal deformity progression.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

Regulation of Gβγi-dependent PLC-β3 activity in smooth muscle: inhibitory phosphorylation of PLC-β3 by PKA and PKG and stimulatory phosphorylation of Gαi-GTPase-activating protein RGS2 by PKG

In gastrointestinal smooth muscle, agonists that bind to Gi-coupled receptors activate preferentially PLC-β3 via Gβγ to stimulate phosphoinositide (PI) hydrolysis and generate inositol 1,4,5-trisphosphate (IP3) leading to IP3-dependent Ca(2+) release and muscle contraction. In the present study, we identified the mechanism of inhibition of PLC-β3-dependent PI hydrolysis by cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG). Cyclopentyl adenosine (CPA), an adenosine A1 receptor agonist, caused an increase in PI hydrolysis in a concentration-dependent fashion; stimulation was blocked by expression of the carboxyl-terminal sequence of GRK2(495-689), a Gβγ-scavenging peptide, or Gαi minigene but not Gαq minigene. Isoproterenol and S-nitrosoglutathione (GSNO) induced phosphorylation of PLC-β3 and inhibited CPA-induced PI hydrolysis, Ca(2+) release, and muscle contraction. The effect of isoproterenol on all three responses was inhibited by PKA inhibitor, myristoylated PKI, or AKAP inhibitor, Ht-31, whereas the effect of GSNO was selectively inhibited by PKG inhibitor, Rp-cGMPS. GSNO, but not isoproterenol, also phosphorylated Gαi-GTPase-activating protein, RGS2, and enhanced association of Gαi3-GTP and RGS2. The effect of GSNO on PI hydrolysis was partly reversed in cells (i) expressing constitutively active GTPase-resistant Gαi mutant (Q204L), (ii) phosphorylation-site-deficient RGS2 mutant (S46A/S64A), or (iii) siRNA for RGS2. We conclude that PKA and PKG inhibit Gβγi-dependent PLC-β3 activity by direct phosphorylation of PLC-β3. PKG, but not PKA, also inhibits PI hydrolysis indirectly by a mechanism involving phosphorylation of RGS2 and its association with Gαi-GTP. This allows RGS2 to accelerate Gαi-GTPase activity, enhance Gαβγi trimer formation, and inhibit Gβγi-dependent PLC-β3 activity.

G-protein αq participates in the steroid hormone 20-hydroxyecdysone nongenomic signal transduction

The nuclear receptor-mediated genomic pathways of the animal steroid hormones are well known. However, the cell membrane receptor-mediated nongenomic pathways of the animal steroid hormones are little understood. In this study, we report the participation of a G-protein alpha q (Gαq)(1) subunit in the 20E nongenomic pathway in the cell membrane and regulating gene expression during molting and metamorphosis in a lepidopteran insect, Helicoverpa armigera. 20E-induced phosphorylation of Gαq was detected using two-dimensional electrophoresis techniques. Knockdown of Gαq by injecting double-stranded RNA suppressed the development of larvae, delayed metamorphosis, and inhibited 20E-induced gene expression. Gαq was distributed throughout the cell, and migrated toward the plasma membrane upon 20E induction. Gαq was necessary in the 20E-induced intracellular Ca(2+) release and extracellular Ca(2+) influx. The protein kinase C (PKC) inhibitor could repress 20E-induced phosphorylation of cyclin-dependent kinase 10 (CDK10) and transcription factor ultraspiracle (USP1). PKC inhibitor could repress the Gαq phosphorylation and membrane trafficking. These results suggest that Gαq participates in 20E signaling in the cell membrane at the pre-genomic stage by modulating the increase of the intracellular Ca(2+) and phosphorylation of CDK10 and USP1 in 20E transcription complex to regulate gene transcription.

Ion transport regulated by protease-activated receptor 2 in human airway Calu-3 epithelia

We examined the mechanisms underlying anion secretion mediated by protease-activated receptor 2 (PAR2) and its role in the regulation of ion transport, using polarized human airway Calu-3 cells. PAR2 stimulation by trypsin and a PAR2-activating peptide (PAR2AP), especially from the basolateral aspect, caused transient Cl(-) secretion due to cytosolic Ca(2+) mobilization. Antagonists of PI-PLC (U73122, ET-18-OCH(3)) and inositol 1,4,5-triphosphate (xestospongin C (Xest C)) were without effect on the PAR2AP-mediated Cl(-) secretion, whereas it was attenuated by D609 (a PC-PLC inhibitor) and phorbol 12-myristate 13 acetate (PMA, a PKC activator). Even 30 min after removal of PAR2AP after a 10-min-exposure, cells were still poorly responsive to PAR2 stimulation, but the reduced responsiveness was upregulated by a PKC inhibitor, GF109203X (GFX). Pretreatment with PAR2AP did not affect responses to anion secretagogues, such as isoproterenol, forskolin, thapsigargin, 1-ethyl-2-benzimdazolinone, and adenosine, but ATP-induced responses were significantly reduced. Nystatin permeabilization studies revealed that the presence of PAR2AP prevented ATP-induced increments in basolateral membrane K(+) conductance without affecting apical membrane Cl(-) conductance. ATP-elicited Ca(2+) mobilization, which was sensitive to D609 and PMA, was inhibited by the pretreatment with PAR2AP, and this inhibition was blunted by the presence of GFX. Collectively, stimulation of PAR2 generates a brief response of Cl(-) secretion through PC-PLC-mediated pathway, followed by not only auto-desensitization of PAR2 itself but also cross-desensitization of a PC-PLC-coupled purinoceptor. The two types of desensitization seem likely to have PKC-mediated downregulation of PC-PLC in common.

RGSZ1 and GAIP regulate mu- but not delta-opioid receptors in mouse CNS: role in tachyphylaxis and acute tolerance

In the CNS, the regulators of G-protein signaling (RGS) proteins belonging to the Rz subfamily, RGS19 (G(alpha) interacting protein (GAIP)) and RGS20 (Z1), control the activity of opioid agonists at mu but not at delta receptors. Rz proteins show high selectivity in deactivating G(alpha)z-GTP subunits. After reducing the expression of RGSZ1 with antisense oligodeoxynucleotides (ODN), the supraspinal antinociception produced by morphine, heroin, DAMGO ([D-Ala2, N-MePhe4,Gly-ol5]-enkephalin), and endomorphin-1 was notably increased. No change was observed in the effect of endomorphin-2. This agrees with the proposed existence of different mu receptors for the endomorphins. The activities of DPDPE ([D-Pen2,5]-enkephalin) and [D-Ala2] deltorphin II, agonists at delta receptors, were also unchanged. Knockdown of GAIP and of the GAIP interacting protein C-terminus (GIPC) led to changes in agonist effects at mu but not at delta receptors. The impairment of RGSZ1 extended the duration of morphine analgesia by at least 1 h beyond that observed in control animals. CTOP (Cys2, Tyr3, Orn5, Pen7-amide) antagonized morphine analgesia when given during the period in which the effect of morphine was enhanced by RGSZ1 knockdown. Thus, in naive mice, morphine tachyphylaxis originated in the presence of the opioid agonist and during the analgesia time course. The knockdown of RGSZ1 facilitated the development of tolerance to a single dose of morphine and accelerated tolerance to continuous delivery of the opioid. These results indicate that mu but not delta receptors are linked to Rz regulation. The mu receptor-mediated activation of Gz proteins is effective at recruiting the adaptive mechanisms leading to the development of opioid desensitization.

Protein kinase A-mediated phosphorylation of the Galpha13 switch I region alters the Galphabetagamma13-G protein-coupled receptor complex and inhibits Rho activation

The present studies mapped the protein kinase A (PKA) phosphorylation site of Galpha(13) and studied the consequences of its phosphorylation. Initial experiments using purified human Galpha(13) and the PKA catalytic subunit established that PKA directly phosphorylates Galpha(13). The location of this phosphorylation site was next investigated with a new synthetic peptide (G(13)SRI(pep)) containing the PKA consensus sequence (Arg-Arg-Pro-Thr(203)) within the switch I region of Galpha(13). G(13)SRI(pep) produced a dose-dependent inhibition of PKA-mediated Galpha(13) phosphorylation. On the other hand, the Thr-phosphorylated derivative of G(13)SRI(pep) possessed no inhibitory activity, suggesting that Galpha(13) Thr(203) may represent the phosphorylation site. Confirmation of this notion was obtained by showing that the Galpha(13)-T203A mutant (in COS-7 cells) could not be phosphorylated by PKA. Additional studies using co-elution affinity chromatography and co-immunoprecipitation demonstrated that Galpha(13) phosphorylation stabilized coupling of Galpha(13) with platelet thromboxane A(2) receptors but destabilized coupling of Galpha(13) to its betagamma subunits. In order to determine the functional consequences of this phosphorylation on Galpha(13) signaling, activation of the Rho pathway was investigated. Specifically, Chinese hamster ovary cells overexpressing human Galpha(13) wild type (Galpha(13)-WT) or Galpha(13)-T203A mutant were generated and assayed for Rho activation. It was found that 8-bromo-cyclic AMP caused a significant decrease (50%; p < 0.002) of Rho activation in Galpha(13) wild type cells but produced no change of basal Rho activation levels in the mutant (p > 0.4). These results therefore suggest that PKA blocks Rho activation by phosphorylation of Galpha(13) Thr(203).

Identity of adenylyl cyclase isoform determines the G protein mediating chronic opioid-induced adenylyl cyclase supersensitivity

To determine the intracellular signal transduction pathway responsible for the development of tolerance/dependence, the ability of Gzalpha to substitute for pertussis toxin (PTX)-sensitive G proteins in mediating adenylyl cyclase (AC) supersensitivity was examined in the presence of defined AC isoforms. In transiently micro-opioid receptor (OR) transfected COS-7 cells (endogenous inhibitory G proteins: Gialpha2, Gialpha3 and Gzalpha), neither acute (1 micro mol/L) nor chronic morphine treatment (1 micromol/L; 18 h) influenced intracellular cAMP production. Coexpression of the micro -OR together with AC type V and VI fully restored the ability of morphine to acutely inhibit cAMP generation. Chronic morphine treatment further resulted in the development of tolerance/dependence, as assessed by desensitization of the acute inhibitory opioid effect (tolerance) as well as the induction of AC supersensitivity after drug withdrawal (dependence). Specific direction of micro -OR signalling via Gzalpha by both PTX treatment and Gzalpha over-expression had no effect on chronic morphine regulation of AC type V, but completely abolished the development of tolerance/dependence with AC type VI. Similar results were obtained in stably micro -OR-expressing HEK293 cells transiently cotransfected with Gzalpha and either AC type V or VI. Coprecipitation studies further verified that Gzalpha specifically binds to AC type V but not type VI. Taken together, these results demonstrate that in principle each of the OR-activated G proteins per se is able to mediate AC supersensitivity. However, they also indicate that it is the molecular nature of AC isoform that selects and determines the OR-activated G protein mediating tolerance/dependence.

G protein coupling to M1 and M3 muscarinic receptors in sublingual glands

Rat sublingual gland M2 and M3 muscarinic receptors each directly activate exocrine secretion. To investigate the functional role of coreceptor expression, we determined receptor-G protein coupling. Although membrane proteins of 40 and 41 kDa are ADP-ribosylated by pertussis toxin (PTX), and 44 kDa proteins by cholera toxin (CTX), both carbachol-stimulated high-affinity GTPase activity and the GTP-induced shift in agonist binding are insensitive to CTX or PTX. Carbachol enhances photoaffinity labeling ([alpha-(32)P]GTP-azidoaniline) of only 42-kDa proteins that are subsequently tractable to immunoprecipitation by antibodies specific for Galpha(q) or Galpha(11) but not Galpha(12) or Galpha(13). Carbachol-stimulated photoaffinity labeling as well as phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis is reduced 55% and 60%, respectively, by M1 receptor blockade with m1-toxin. Galpha(q/11)-specific antibody blocks carbachol-stimulated PIP2 hydrolysis. We also provide estimates of the molar ratios of receptors to Galpha(q) and Galpha(11). Although simultaneous activation of M1 and M3 receptors is required for a maximal response, both receptor subtypes are coupled to Galpha(q) and Galpha(11) to stimulate exocrine secretion via redundant mechanisms.

G(z) signaling: emerging divergence from G(i) signaling

A large variety of neurotransmitters, hormones, and chemokines regulate cellular functions via cell surface receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins) belonging to the G(i) subfamily. All members of the G(i) subfamily, with the sole exception of G(z), are substrates for the pertussis toxin ADP-ribosyl transferase. G(z) also exhibits unique biochemical and regulatory properties. Initial portrayals of the cellular functions of G(z) bear high resemblance to those of other G(i) proteins both in terms of the receptors and effectors linked to G(z). However, recent discoveries have begun to insinuate a distinct role for G(z) in cellular communication. Functional interactions of the alpha subunit of G(z) (Galpha(z)) with the NKR-P1 receptor, Galpha(z)-specific regulator of G protein signaling, p21-activated kinase, G protein-regulated inducers of neurite outgrowth, and the Eya2 transcription cofactor have been demonstrated. These findings provide possible links for G(z) to participate in cellular development, survival, proliferation, differentiation and even apoptosis. In this review, we have drawn a sketch of a signaling network with G(z) as the centerpiece. The emerging picture is one that distinguishes G(z) from other members of the G(i) subfamily.

Regulation of G proteins by covalent modification

Heterotrimeric G protein alpha,beta, and gamma subunits are subject to several kinds of co- and post-translational covalent modifications. Among those relevant to G protein-coupled receptor signaling in normal cell function are lipid modifications and phosphorylation. N-myristoylation is a co-translational modification occurring for members of the G(i) family of Galpha subunits, while palmitoylation is a post-translational modification that occurs for these and most other Galpha subunits. One or both modifications are required for plasma membrane targeting and contribute to regulating strength of interaction with the Gbetagamma heterodimer, effectors, and regulators of G protein signaling (RGS proteins). Galpha subunits, including those with transforming activity, are often inactive when unable to be modified with lipids. The reversible nature of palmitoylation is intriguing in this regard, as it lends itself to a regulation integrated with the activation state of the G protein. Several Galpha subunits are substrates for phosphorylation by protein kinase C and at least one is a substrate for phosphorylation by the p21-activated protein kinase. Phosphorylation in both instances inhibits the interactions of these subunits with the Gbetagamma heterodimer and RGS proteins. Several Galpha subunits are also substrates for tyrosine phosphorylation. A Ggamma subunit is phosphorylated by protein kinase C, with the consequence that it interacts more tightly with a Galpha subunit but less well with an effector.

Heterologous desensitization of response mediated by selective PKC-dependent phosphorylation of G(i-1) and G(i-2)

This study examined the ability of protein kinase C (PKC) to induce heterologous desensitization by targeting specific G proteins and limiting their ability to transduce signals in smooth muscle. Activation of PKC by pretreatment of intestinal smooth muscle cells with phorbol 12-myristate 13-acetate, cholecystokinin octapeptide, or the phosphatase 1 and phosphatase 2A inhibitor, calyculin A, selectively phosphorylated Galpha(i-1) and Galpha(i-2), but not Galpha(i-3) or Galpha(o), and blocked inhibition of adenylyl cyclase mediated by somatostatin receptors coupled to G(i-1) and opioid receptors coupled to G(i-2), but not by muscarinic M(2) and adenosine A(1) receptors coupled to G(i-3). Phosphorylation of Galpha(i-1) and Galpha(i-2) and blockade of cyclase inhibition were reversed by calphostin C and bisindolylmaleimide, and additively by selective inhibitors of PKCalpha and PKCepsilon. Blockade of inhibition was prevented by downregulation of PKC. Phosphorylation of Galpha-subunits by PKC also affected responses mediated by betagamma-subunits. Pretreatment of muscle cells with cANP-(4-23), a selective agonist of the natriuretic peptide clearance receptor, NPR-C, which activates phospholipase C (PLC)-beta3 via the betagamma-subunits of G(i-1) and G(i-2), inhibited the PLC-beta response to somatostatin and [D-Pen(2,5)]enkephalin. The inhibition was partly reversed by calphostin C. Short-term activation of PKC had no effect on receptor binding or effector enzyme (adenylyl cyclase or PLC-beta) activity. We conclude that selective phosphorylation of Galpha(i-1) and Galpha(i-2) by PKC partly accounts for heterologous desensitization of responses mediated by the alpha- and betagamma-subunits of both G proteins. The desensitization reflects a decrease in reassociation and thus availability of heterotrimeric G proteins.

Loss of signaling through the G protein, Gz, results in abnormal platelet activation and altered responses to psychoactive drugs

Heterotrimeric G proteins mediate the earliest step in cell responses to external events by linking cell surface receptors to intracellular signaling pathways. G(z) is a member of the G(i) family of G proteins that is prominently expressed in platelets and brain. Here, we show that deletion of the alpha subunit of G(z) in mice: (i) impairs platelet aggregation by preventing the inhibition of cAMP formation normally seen at physiologic concentrations of epinephrine, and (ii) causes the mice to be more resistant to fatal thromboembolism. Loss of G(zalpha) also results in greatly exaggerated responses to cocaine, reduces the analgesic effects of morphine, and abolishes the effects of widely used antidepressant drugs that act as catecholamine reuptake inhibitors. These changes occur despite the presence of other G(ialpha) family members in the same cells and are not accompanied by detectable compensatory changes in the level of expression of other G protein subunits. Therefore, these results provide insights into receptor selectivity among G proteins and a model for understanding platelet function and the effects of psychoactive drugs.

N-Myristoylation and betagamma play roles beyond anchorage in the palmitoylation of the G protein alpha(o) subunit

Many of the alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated, a modification proposed to play a key role in the stable anchorage of the subunits to the plasma membrane. Palmitoylation of alpha subunits from the G(i) family is preceded by N-myristoylation, which alone or together with betagamma probably supports a reversible interaction of the alpha subunit with membrane as a prerequisite to the eventual incorporation of palmitate. Previous studies have not addressed, however, the question of whether membrane association alone, carried out through N-myristoylation, interaction with betagamma, or other events, is sufficient for palmitoylation. We report here for alpha(o) that it is not. We found that N-myristoylation is required for palmitoylation at least in part because it supports events subsequent to membrane attachment. Mutants of alpha(o) designed to target the subunit to membrane without an N-myristoyl group are unable to be palmitoylated as evaluated by incorporation of [(3)H]palmitate. Mutants of alpha(o) unable to interact normally with betagamma yet still attach to membrane demonstrate that betagamma, in contrast, is not required for palmitoylation. betagamma becomes necessary, however, when the N-myristoyl group is absent. Our results suggest that N-myristoylation and betagamma, while almost certainly relevant to the reversible interaction of alpha(o) with membrane, also play at least partly overlapping, post-anchorage roles in palmitoylation.

Functional interaction between Galpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

G(z) is a member of the G(i) family of trimeric G proteins whose primary role in cell physiology is still unknown. In an ongoing effort to elucidate the cellular functions of G(z), the yeast two-hybrid system was employed to identify proteins that specifically interact with a mutationally activated form of Galpha(z). One of the molecules uncovered in this screen was Rap1GAP, a previously identified protein that specifically stimulates GTP hydrolytic activity of the monomeric G protein Rap1 and thus is believed to function as a down-regulator of Rap1 signaling. Like G(z), the precise role of Rap1 in cell physiology is poorly understood. Biochemical analysis using purified recombinant proteins revealed that the physical interaction between Galpha(z) and Rap1GAP blocks the ability of RGSs (regulators of G protein signaling) to stimulate GTP hydrolysis of the alpha subunit, and also attenuates the ability of activated Galpha(z) to inhibit adenylyl cyclase. Structure-function analyses indicate that the first 74 amino-terminal residues of Rap1GAP, a region distinct from the catalytic core domain responsible for the GAP activity toward Rap1, is required for this interaction. Co-precipitation assays revealed that Galpha(z), Rap1GAP, and Rap1 can form a stable complex. These data suggest that Rap1GAP acts as a signal integrator to somehow coordinate and/or integrate G(z) signaling and Rap1 signaling in cells.

Reciprocal signaling between heterotrimeric G proteins and the p21-stimulated protein kinase

p21-activated protein kinase (PAK)-1 phosphorylated Galpha(z), a member of the Galpha(i) family that is found in the brain, platelets, and adrenal medulla. Phosphorylation approached 1 mol of phosphate/mol of Galpha(z) in vitro. In transfected cells, Galpha(z) was phosphorylated both by wild-type PAK1 when stimulated by the GTP-binding protein Rac1 and by constitutively active PAK1 mutants. In vitro, phosphorylation occurred only at Ser(16), one of two Ser residues that are the major substrate sites for protein kinase C (PKC). PAK1 did not phosphorylate other Galpha subunits (i1, i2, i3, o, s, or q). PAK1-phosphorylated Galpha(z) was resistant both to RGSZ1, a G(z)-selective GTPase-activating protein (GAP), and to RGS4, a relatively nonselective GAP for the G(i) and G(q) families of G proteins. Phosphorylation of Ser(27) by PKC did not alter sensitivity to either GAP. The previously described inhibition of G(z) GAPs by PKC is therefore mediated by phosphorylation of Ser(16). Phosphorylation of either Ser(16) by PAK1 or Ser(27) by PKC decreased the affinity of Galpha(z) for Gbetagamma; phosphorylation of both residues by PKC caused no further effect. PAK1 thus regulates Galpha(z) function by attenuating the inhibitory effects of both GAPs and Gbetagamma. In this context, the kinase activity of PAK1 toward several protein substrates was directly inhibited by Gbetagamma, suggesting that PAK1 acts as a Gbetagamma-regulated effector protein. This inhibition of mammalian PAK1 by Gbetagamma contrasts with the stimulation of the PAK homolog Ste20p in Saccharomyces cerevisiae by the Gbetagamma homolog Ste4p/Ste18p.

Cyclic AMP-dependent phosphorylation of thromboxane A(2) receptor-associated Galpha(13)

Although it is well established that cAMP inhibits platelet activation induced by all agonists, the thromboxane A(2) signal transduction pathway was found to be particularly sensitive to such inhibition. Therefore, we examined whether cAMP-dependent kinase mediates phosphorylation of the thromboxane A(2) receptor-G-protein complex. It was found that cAMP induces protein kinase A-dependent [gamma-(32)P]ATP labeling of solubilized membrane proteins in the region of Galpha subunits, i.e. 38-45 kDa. Moreover, ligand affinity chromatography purification of thromboxane A(2) receptor-G-protein complexes from these membranes revealed that 38-45-kDa phosphoproteins co-purify with thromboxane A(2) receptors. Immunoprecipitation of the affinity column eluate with a Galpha(13) antibody demonstrated that 8-Br-cAMP increased phosphorylation of thromboxane A(2) receptor-associated Galpha(13) by 87 +/- 27%. In separate experiments, immunopurification of Galpha(13) on microtiter wells coated with a different Galpha(13) antibody revealed that 8-Br-cAMP increased Galpha(13) phosphorylation by 53 +/- 19%. Finally, treatment of (32)P-labeled whole platelets with prostacyclin resulted in a 90 +/- 14% increase in phosphorylated Galpha(13) that was abolished by pretreatment with the adenylate cyclase inhibitor MDL-12. These results provide the first evidence that protein kinase A mediates phosphorylation of Galpha(13) both in vitro and in vivo and provides a basis for the preferential inhibition of thromboxane A(2)-mediated signaling in platelets by cAMP.

Protein kinase C-promoted inhibition of Galpha(11)-stimulated phospholipase C-beta activity

The effects of protein kinase C (PKC) activation on inositol lipid signaling were examined. Using the turkey erythrocyte model of receptor-regulated phosphoinositide hydrolysis, we developed a membrane reconstitution assay to study directly the effects of activation of PKC on the activities of Galpha(11), independent of potential effects on the receptor or on PLC-beta. Membranes isolated from erythrocytes pretreated with 4beta-phorbol-12beta-myristate-13alpha-acetate (PMA) exhibited a decreased capacity for Galpha(11)-mediated activation of purified, reconstituted PLC-beta1. This inhibitory effect was dependent on both the time and concentration of PMA incubation and occurred as a decrease in the efficacy of GTPgammaS for activation of PLC-beta1, both in the presence and absence of agonist; no change in the apparent affinity for the guanine nucleotide occurred. Similar inhibitory effects were observed after treatment with the PKC activator phorbol-12,13-dibutyrate but not after treatment with an inactive phorbol ester. The inhibitory effects of PMA were prevented by coaddition of the PKC inhibitor bisindolylmaleimide. Although the effects of PKC could be localized to the membrane, no phosphorylation of Galpha(11) occurred either in vitro in the presence of purified PKC or in intact erythrocytes after PMA treatment. These results support the hypothesis that a signaling protein other than Galpha(11) is the target for PKC and that PKC-promoted phosphorylation of this protein results in a phosphorylation-dependent suppression of Galpha(11)-mediated PLC-beta1 activation.

Functional regulation of Galpha16 by protein kinase C

Recent evidence demonstrates that the alpha subunits of some heterotrimeric GTP-binding proteins (G proteins) are subject to modification by protein kinase C (PKC). For the family of G proteins that activate the phospholipase C/inositol trisphosphate/calcium/PKC pathway, such modification could result in G protein autoregulation. To examine the potential regulation of members of the Galphaq family by PKC phosphorylation, we expressed the thyrotropin-releasing hormone (TRH) receptor in combination with Galphaq, Galpha11, Galpha14, Galpha15, or Galpha16 in Xenopus oocytes and examined the regulation of signaling by PKC activators and inhibitors. For Galpha16 and Galpha15, the two family members of hematopoietic lineage, PKC activators reduce both the magnitude and the time course of TRH-mediated responses; PKC inhibitors have the opposite effect. The PKC-mediated effects are evident in measurements of GTPase activity, suggesting that the regulation is occurring early in the signaling pathway. In vivo phosphorylation experiments demonstrate that Galpha16 is a substrate for PKC modification. By comparison, Galphaq is not phosphorylated by PKC in vivo, and oocytes expressing Galphaq are not functionally modulated by PKC. Repeated TRH stimulation of oocytes expressing Galpha16 mimics the effects of PKC activators, and this functional regulation is correlated with an increase in Galpha16 phosphorylation. A mutant Galpha16 with four consensus PKC phosphorylation sites removed is not phosphorylated in vivo, and TRH responses mediated through the mutant are not regulated by PKC. These results demonstrate that signaling involving hematopoietic G proteins is subject to PKC-mediated autoregulation, at least in part, by phosphorylation of the G protein alpha subunit.

Galphai is not required for chemotaxis mediated by Gi-coupled receptors

Pertussis toxin inhibits chemotaxis of neutrophils by preventing chemoattractant receptors from activating trimeric G proteins in the Gi subfamily. In HEK293 cells expressing recombinant receptors, directional migration toward appropriate agonist ligands requires release of free G protein betagamma subunits and can be triggered by agonists for receptors coupled to Gi but not by agonists for receptors coupled to two other G proteins, Gs and Gq. Because activation of any G protein presumably releases free Gbetagamma, we tested the hypothesis that chemotaxis also requires activated alpha subunits (Galphai) of Gi proteins. HEK293 cells were stably cotransfected with the Gi-coupled receptor for interleukin-8, CXCR1, and with a chimeric Galpha, Galphaqz5, which resembles Galphai in susceptibility to activation by Gi-coupled receptors but cannot regulate the Galphai effector, adenylyl cyclase. These cells, unlike cells expressing CXCR1 alone, migrated toward interleukin-8 even after treatment with pertussis toxin, which prevents activation of endogenous Galphai but not that of Galphaqz5. We infer that chemotaxis does not require activation of Galphai. Because chemotaxis is mediated by Gbetagamma subunits released when Gi-coupled receptors activate Galphaqz5, but not when Gq- or Gs-coupled receptors activate their respective G proteins, we propose that Gi-coupled receptors transmit a necessary chemotactic signal that is independent of Galphai.

An EGF receptor-mediated signal attenuates the inhibitory effect of LPA on an adenylate cyclase activity

A tyrosine kinase receptor-mediated and a heterotrimeric G protein-coupled receptor-mediated signals have been shown to evoke distinct intracellular signaling events. There has been increasing evidence that cross-talk exists between a tyrosine kinase receptor-mediated and a heterotrimeric G protein-coupled receptor-mediated signal transduction pathways. In the present study, we have studied effects of EGF receptor activation on activities of inhibitory G protein (Gi). We show that the amounts of Gi/Go ADP-ribosylated by islet-activating protein (IAP) increased by 30-40% in the membranes of Rat 1 fibroblast cells pretreated with EGF compared with those without pretreatment. When an effect of lysophosphatidic acid (LPA) stimulation on an adenylate cyclase activity was examined, LPA partly attenuated forskolin-stimulated adenylate cyclase activity via Gi because IAP pretreatment blocked the inhibitory effect of LPA. Pretreatment with EGF reduced the ability of LPA to inhibit the forskolin-stimulated adenylate cyclase activity, while the pretreatment did not have any effects on the forskolin-stimulated activity. Thus, the EGF receptor-mediated signal appears to cause the impairment of Gi function in Rat 1 fibroblast cells.

G protein alpha subunit G alpha z couples neurotransmitter receptors to ion channels in sympathetic neurons

The functional roles subserved by G(alpha)z, a G protein alpha subunit found predominantly in neuronal tissues, have remained largely undefined. Here, we report that G(alpha)z coupled neurotransmitter receptors to N-type Ca2+ channels when transiently overexpressed in rat sympathetic neurons. The G(alpha)z-mediated inhibition was voltage dependent and PTX insensitive. Recovery from G(alpha)z-mediated inhibition was extremely slow but accelerated by coexpression with RGS proteins. G(alpha)z selectively interacted with a subset of receptors that ordinarily couple to N-type Ca2+ channels via PTX-sensitive Go/i proteins. In addition, G(alpha)z rescued the activation of heterologously expressed GIRK channels in PTX-treated neurons. These results suggest that G(alpha)z is capable of coupling receptors to ion channels and might underlie PTX-insensitive ion channel modulation observed in neurons under physiological and pathological conditions.

RGSZ1, a Gz-selective RGS protein in brain. Structure, membrane association, regulation by Galphaz phosphorylation, and relationship to a Gz gtpase-activating protein subfamily

We cloned the cDNA for human RGSZ1, the major Gz-selective GTPase-activating protein (GAP) in brain (Wang, J., Tu, Y., Woodson, J., Song, X., and Ross, E. M. (1997) J. Biol. Chem. 272, 5732-5740) and a member of the RGS family of G protein GAPs. Its sequence is 83% identical to RET-RGS1 (except its N-terminal extension) and 56% identical to GAIP. Purified, recombinant RGSZ1, RET-RGS1, and GAIP each accelerated the hydrolysis of Galphaz-GTP over 400-fold with Km values of approximately 2 nM. RGSZ1 was 100-fold selective for Galphaz over Galphai, unusually specific among RGS proteins. Other enzymological properties of RGSZ1, brain Gz GAP, and RET-RGS1 were identical; GAIP differed only in Mg2+ dependence and in its slightly lower selectivity for Galphaz. RGSZ1, RET-RGS1, and GAIP thus define a subfamily of Gz GAPs within the RGS proteins. RGSZ1 has no obvious membrane-spanning region but is tightly membrane-bound in brain. Its regulatory activity in membranes depends on stable bilayer association. When co-reconstituted into phospholipid vesicles with Gz and m2 muscarinic receptors, RGSZ1 increased agonist-stimulated GTPase >15-fold with EC50 <12 nM, but RGSZ1 added to the vesicle suspension was <0.1% as active. RGSZ1, RET-RGS1, and GAIP share a cysteine string sequence, perhaps targeting them to secretory vesicles and allowing them to participate in the proposed control of secretion by Gz. Phosphorylation of Galphaz by protein kinase C inhibited the GAP activity of RGSZ1 and other RGS proteins, providing a mechanism for potentiation of Gz signaling by protein kinase C.

Heterotrimeric G-protein candidates for Ge in the ACTH secretory pathway

The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used to identify candidate heterotrimeric G-proteins for G-exocytosis (Ge) which mediates calcium ion-stimulated adrenocorticotrophin (ACTH) secretion in this cell line. AtT-20 cells express several heterotrimeric G-protein alpha subunits; Gs alpha, Gt alpha, Gq alpha, G11alpha, G12alpha, G13alpha, G14alpha, G15alpha, Gz alpha, Gi2alpha, Gi3alpha, and Go alpha and so heterotrimeric G-protein selective agents were used to differentiate between these candidates. Agents which stimulate ACTH secretion via Ge were not pertussis toxin (PTX)-sensitive nor was cholera toxin (CTX) able to stimulate ACTH secretion from permeabilised cells in the absence of calcium. G-protein antagonists which inhibit activation of Gs, Gi, and Gq subfamilies did not attenuate Ge-stimulated ACTH secretion from permeabilised AtT-20 cells. In AtT-20 cells the stimulatory G-protein involved in the late stages of the ACTH secretory pathway does not belong to the Gs, Gi (with the exception of Gz) or Gq subfamilies of heterotrimeric G-proteins leaving Gz, G12 or G13 as the strongest candidates for Ge.

Phosphorylation of serine 1105 by protein kinase A inhibits phospholipase Cbeta3 stimulation by Galphaq

The mechanism by which protein kinase A (PKA) inhibits Galphaq -stimulated phospholipase C activity of the beta subclass (PLCbeta ) is unknown. We present evidence that phosphorylation of PLCbeta3 by PKA results in inhibition of Galphaq -stimulated PLCbeta3 activity, and we identify the site of phosphorylation. Two-dimensional phosphoamino acid analysis of in vitro phosphorylated PLCbeta3 revealed a single phosphoserine as the putative PKA site, and peptide mapping yielded one phosphopeptide. The residue was identified as Ser1105 by direct sequencing of reverse-phase high pressure liquid chromatography-isolated phosphopeptide and by site-directed mutagenesis. Overexpression of Galphaq with PLCbeta3 or PLCbeta (Ser1105--> Ala) mutant in COSM6 cells resulted in a 5-fold increase in [3H]phosphatidylinositol 1,4,5-trisphosphate formation compared with expression of Galphaq, PLCbeta3, or PLCbeta3 (Ser1105 --> Ala mutant alone. Whereas Galpha1-stimulated PLCbeta3, activity was inhibited by 58-71% by overexpression of PKA catalytic subunit, Galphaq-stimulated PLCbeta3 (Ser1105 --> Ala) mutant activity was not affected. Furthermore, phosphatidylinositide turnover stimulated by presumably Galpha1-coupled M1 muscarinic and oxytocin receptors was completely inhibited by pretreating cells with 8-[4-chlorophenythio]-cAMP in RBL-2H3 cells expressing only PLCbeta3. These data establish that direct phosphorylation by PKA of Ser1105 in the putative G-box of PLCbeta3 inhibits Galphaq-stimulated PLCbeta3 activity. This can at least partially explain the inhibitory effect of PKA on Galphaq-stimulated phosphatidylinositide turnover observed in a variety of cells and tissues.

Coupling of D1 and D5 dopamine receptors to multiple G proteins: Implications for understanding the diversity in receptor-G protein coupling

Dopamine receptors are a subclass of the super family of G protein-coupled receptors, that transduce their effects by coupling to specific G proteins. Within the dopamine receptor family, the adenylyl cyclase stimulatory receptors include the D1 and D5 subtypes. The D1 and D5 dopamine receptors are genetically distinct, sharing >80% sequence homology within the highly conserved seven transmembrane spanning domains, but displaying only 50% overall homology at the amino acid level. When expressed in transfected GH4C1 rat pituitary cells, both D1 and D5 receptors stimulate adenylyl cyclase and have identical affinities toward dopaminergic agonists and antagonists. In order to analyze specific signaling pathways mediated by activation of either D1 or D5 receptors, we have identified the G proteins that are coupled to these receptors. Through functional analyses and competition binding studies, and from immunoprecipitation techniques, using antisera against the various alpha subunits of G proteins, we have established that both D1 and D5 receptors couple to G(s)alpha. In addition, D1 receptors are also coupled to G(o)alpha. Since G(o)alpha has been implicated in the regulation of Ca2+, K+, and Na+ channels, this finding would suggest that D1 receptors can mediate the functional activity of these ion channels. There is also evidence to indicate that D5 receptors couple to G(z)alpha, a novel G protein abundantly expressed in neurons. Thus, despite similar pharmacological properties, such differential coupling of D1 and D5 receptors to G proteins other than G(s)alpha, indicates that dopamine can transduce varied signaling responses upon the simultaneous stimulation of both these receptors.

Platelet signal transduction defect with Galpha subunit dysfunction and diminished Galphaq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of alpha-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[gammaS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Galphas) and its inhibition (mediated by Galphai) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of Galpha subunits in the patient's platelet membranes showed a decrease in Galphaq (<50%) but not Galphai, Galphaz, Galpha12, and Galpha13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Galphaq in thrombin-induced responses.

Galpha12 and galpha13 are phosphorylated during platelet activation

The ubiquitously expressed G-proteins G12 and G13 whose function is currently not clear have been shown to be activated in platelet membranes through receptors that stimulate platelet aggregation. We used intact human platelets to determine whether alpha subunits of both G-proteins can be phosphorylated under physiological conditions. Activation of human platelets by thrombin and the thromboxane A2 receptor agonist U46619 lead to phosphorylation of Galpha12 and Galpha13. Phosphorylation occurred rapidly after addition of thrombin and was not mediated by glycoprotein IIb/IIIa (integrin alphaIIbbeta3) activation. Phosphorylation of Galpha12 and Galpha13 could be mimicked by phorbol 12-myristate 13-acetate, and thrombin-induced phosphorylation was inhibited by the protein kinase C inhibitor calphostin C indicating an involvement of protein kinase C in Galpha12/13 phosphorylation induced by thrombin in human platelets. The phosphorylation of both G protein alpha subunits was reconstituted in COS-7 cells cotransfected with Galpha12 or Galpha13 and different protein kinase C isoforms. Among the protein knase C isoforms tested, protein kinase C beta, delta, and epsilon were most effective in promoting phosphorylation of Galpha12 and Galpha13 in a phorbol 12-myristate 13-acetate-dependent manner. These data demonstrate that Galpha12 and Galpha13 are phosphorylated under in vivo conditions and that this phosphorylation involves protein kinase C.

Mechanisms of desensitization and resensitization of proteinase-activated receptor-2

Proteinase-activated receptor-2 (PAR-2) is a G-protein-coupled receptor that is expressed by intestinal epithelial cells, which are episodically exposed to pancreatic trypsin in the intestinal lumen. Trypsin cleaves PAR-2 to expose a tethered ligand, which irreversibly activates the receptor. Thus, PAR-2 may desensitize and resensitize by novel mechanisms. We examined these mechanisms in kidney epithelial cells, stably expressing human PAR-2, and intestinal epithelial cells, which naturally express PAR-2. Trypsin stimulated a prompt increase in [Ca2+]i, due to mobilization of intracellular Ca2+, followed by a sustained plateau, due to influx of extracellular Ca2+. Repeated application of trypsin caused marked desensitization of this response, which is due in part to (a) irreversible cleavage of the receptor by trypsin and (b) protein kinase C-mediated termination of signaling. Trypsin exposure resulted in internalization of PAR-2 into early endosomes and then lysosomes; but endocytosis was not the mechanism of rapid desensitization. Thus, activated PAR-2 is endocytosed and degraded. The Ca2+ response to trypsin resensitized by 60-90 min. Brefeldin A, which disrupted Golgi stores of PAR-2, and cycloheximide, which inhibited protein synthesis, markedly attenuated resensitization. Thus, PAR-2-mediated Ca2+ mobilization desensitizes by irreversible receptor cleavage, protein kinase C-mediated termination of signaling, and PAR-2 targeting to lysosomes. It resensitizes by mobilization of large Golgi stores and synthesis of new receptors.

Asymmetrical distribution of G proteins in syncytiotrophoblastic brush-border and basal-plasma membranes of human term placenta

In human placental syncytiotrophoblast brush-border (BBM, facing the mother) and basal-plasma membranes (BPM, facing to fetus) we have recently demonstrated the presence of calcaemic hormone-specific receptors for parathyroid hormone and calcitonin, which could be implicated in calcium transport from the mother to the fetus. It is well recognized that signal transducing G proteins (guanosinc nucleotide-binding proteins) can associate with various transmembrane receptors and effector proteins, and regulate a variety of second-messenger systems and ion channels. In this present paper, we investigated the presence of a variety of alpha and beta subunits of G proteins in both syncytiotrophoblast, BBM and BPM by Western blot technique. For the first time, we were able to demonstrate the presence of G proteins in the bipolar syncytiotrophoblast membranes, which were evaluated by immunoblotting using affinity purified antiserum raised against the alpha subunits of Gi1, Gi1/i2, Gi3, G0, Gq, Gs, G7 and against the beta subunits. In BBM, we identified the alpha subunits of Gi1, Gi3, G0, Gq, Gs (42, 46 kDa), Gz and beta subunits. The same alpha subunits of G proteins were found in BPM, although alpha subunits of Gi1, Gq, Gs (46 kDa) were located predominantly in the BBM, and the alpha subunit of G0 was found preferentially in BPM. Moreover, in BBM and BPM, a purified antisera raised against the alpha subunits of Gi1 and Gs, detected a 105 kDa protein and a 67 kDa protein, respectively. Interestingly, the 67 kDa protein was preferentially located in BBM, and none of these proteins were detectable in membranes prepared from brain (control). The asymmetrical distribution of the alpha subunits of G proteins among the two different placental bipolar membranes might reflect the very specialized function of these syncytiotrophoblast membranes in ions and nutrients transport from the mother to the fetus.

Protein kinase C phosphorylates G12 alpha and inhibits its interaction with G beta gamma

Of nine G protein alpha subunits examined, only alpha 12 and alpha z served as substrates for phosphorylation by various isoforms of protein kinase C in vitro. A close homolog of alpha 12, alpha 12 was not phosphorylated. Exposure of NIH 3T3 cells that stably express alpha 12 to phorbol 12-myristate 13-acetate also resulted in phosphorylation of the protein. Phosphorylation in vitro occurred near the amino terminus (probably Ser38), and approximately 1 mol of phosphate was incorporated per mol of alpha 12. Although G protein heterotrimers containing either alpha 12 or a z were poor substrates for phosphorylation, the isolated alpha subunits were phosphorylated equally well in their GDP- or GTP gamma S-bound forms. The guanine nucleotide binding properties of purified alpha 12 and alpha z were unaltered by phosphorylation, as was the capacity of alpha z to inhibit type V adenylyl cyclase. However, phosphorylation of either protein greatly reduced its affinity for G protein beta gamma subunits, consistent with the newly determined crystal structure of a G protein heterotrimer. We suggest that protein kinase C regulates alpha 12- and alpha z-mediated signaling pathways by preventing their association with beta gamma.

Phosphorylation of Gz alpha by protein kinase C blocks interaction with the beta gamma complex

Gz alpha is a G protein alpha subunit with biochemical properties that distinguish it from other members of the G protein alpha subunit family. One such property is its ability to be stoichiometrically phosphorylated by protein kinase C (PKC), both in vitro and in intact cells. The site of this phosphorylation has been mapped to a region near the N terminus of Gz alpha, but no functional significance of the modification has been established. To investigate this question, we have developed a baculovirus/Sf9 cell expression system to produce Gz alpha. The protein purified from Sf9 cells is functional as assessed by its ability both to bind guanine nucleotide in a Mg(2+)-sensitive fashion and to serve as a substrate for phosphorylation by PKC. Furthermore, addition of the G protein beta gamma complex purified from bovine brain inhibits phosphorylation of Gz alpha in a dose-dependent manner. Conversely, phosphorylation of Gz alpha inhibits its ability to interact with beta gamma subunits. These results establish a functional consequence for PKC-catalyzed phosphorylation of Gz alpha and suggest a mechanism for regulation of signaling through Gz by preventing reassociation of its subunits.

Expression of the human 5-hydroxytryptamine1A receptor in Sf9 cells. Reconstitution of a coupled phenotype by co-expression of mammalian G protein subunits

The possibility that Spodoptera frugiperda (Sf9) cells can provide an intact cell setting for reconstitution of the human 5-hydroxytryptamine1A (5-HT1A) receptor with mammalian G protein subunits was explored. The 5-HT1A receptor was found to assume an uncoupled phenotype when expressed alone in Sf9 cells at relatively high levels (5-34 pmol of receptor/mg of membrane protein), i.e. agonist-binding to the receptor was characterized by a relatively high Kd and an insensitivity to GTP. Co-expression of the receptor with members of the alpha i "family" together with various combinations of beta 1 and gamma subunits increased the affinity for agonists to that observed for the coupled form of receptor in mammalian cells, concomitant with conferrance of guanosine 5'-(beta,gamma-imino)triphosphate sensitivity. The agonists employed were [3H]8-hydroxy-N,N-dipropyl-2-aminotetralin ([3H]8-OH-DPAT) and [125I]R(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl) amino]tetralin ([125I]8-OH-PIPAT). The binding of an antagonist, [125I]4-(2'-methoxyphenyl)-1-[2'-[N-(2"- pyridinyl)-p-iodobenzamido]ethyl]piperazine ([125I]p-MPPI), was unaffected by co-expression of G protein subunits. Both alpha and beta gamma subunits were required for optimal coupling. No differences were evident among alpha i1, alpha i2, alpha i3, alpha o, and alpha z when expressed with beta 1 gamma 2 in this regard, nor among most permutations of beta 1 gamma subunits when expressed with alpha i1 (beta 1 gamma 2 approximately beta 1 gamma 3 approximately beta 1 gamma 5 > beta 1 gamma 1). Alpha s and alpha q expressed with beta 1 gamma 2 did not participate in coupling. These data support the conclusion that normal interactions between a mammalian receptor and a select array of G proteins can be established in intact Sf9 cells, and extend previous observations of 5-HT1A receptor coupling to G(o) and the pertussis toxin-insensitive G protein Gz.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.

Muscarinic regulation of Alzheimer's disease amyloid precursor protein secretion and amyloid beta-protein production in human neuronal NT2N cells

The Alzheimer amyloid precursor protein (APP) undergoes complex processing resulting in the production of a 4-kDa amyloid peptide (A beta) which has been implicated in the pathogenesis of Alzheimer's disease. Recent studies have shown that cells can secrete carboxyl terminus truncated APP derivatives (APP-S) in response to physiological stimulus. We have used human central nervous system neurons (NT2N) derived from a teratocarcinoma cell line (NT2) to study the signal transduction pathways involved in APP-S secretion and A beta production. Muscarinic receptors (m2 and m3) as well as the heterotrimeric GTP-binding protein Gq and the beta 1 isoform of phospholipase C were present in NT2N neurons. Stimulation of the muscarinic receptor with carbachol resulted in phospholipase C activation as shown by a transient increase in the second messengers 1,2-diacyl-sn-glycerol and inositol 1,4,5-trisphosphate. Carbachol also caused an increase in intracellular Ca2+ levels measured in single NT2N neurons. Under these conditions, carbachol caused a time-dependent 2-fold increase in APP-S secretion into the medium. In contrast, prolonged treatment with carbachol caused a decrease in A beta production into the medium. These results suggest that APP-S secretion and A beta production in NT2N neurons are regulated by the muscarinic/phospholipase C signal transduction pathway. Furthermore, activation of this pathway results in dissociation of APP-S secretion and A beta production.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Activation of the cloned platelet thrombin receptor decreases the pertussis-toxin-dependent ADP-ribosylation of the membrane and soluble inhibitory guanine-nucleotide-binding-alpha proteins. Inhibition by the prostacyclin analog, iloprost

Thrombin binds at least to two sites of the platelet surface; to the recently cloned thrombin receptor [Vu, T. K., Hung, D. T., Wheaton, V. I. & Coughlin, S. R. (1991) Cell 64, 1057-1068] and to glycoprotein Ib. In the present study, the decrease of pertussis-toxin-dependent ADP-ribosylation of membrane and soluble inhibitory guanine-nucleotide-binding alpha (Gi alpha) proteins was measured after platelet stimulation with a thrombin-receptor-activating peptide (TRAP), and compared to stimulation with thrombin. Stimulation of intact platelets with TRAP decreased the pertussis-toxin-dependent ADP-ribosylation of the major membrane 41-kDa Gi alpha protein and the minor soluble 40 kDa Gi alpha protein recently described in platelets [Gennity, J. M. & Siess, W. (1991) Biochem. J. 279, 643-650]. The kinetics and extent of the decrease of pertussis-toxin-dependent ADP-ribosylation after stimulation of TRAP were similar to the effect of thrombin. The decrease of pertussis-toxin-dependent ADP-ribosylation of the soluble Gi alpha protein was more pronounced and observed at lower agonist concentrations than the decrease of the membrane Gi alpha protein. Desensitization of the thrombin receptor by incubating platelets with a low concentration of TRAP reduced the subsequent decrease of pertussis-toxin-dependent ADP-ribosylation of Gi alpha proteins, evoked by TRAP or thrombin. Platelet stimulation with gamma-thrombin that does not bind to glycoprotein Ib also showed a decrease in the pertussis-toxin-dependent ADP-ribosylation of the soluble and membrane Gi alpha proteins. Treatment of platelets with the stable prostacyclin analog, iloprost, reduced the decrease of pertussis-toxin-dependent ADP-ribosylation of Gi alpha proteins induced by TRAP or thrombin. Among other platelet stimuli tested (endoperoxide/thromboxane analog U44619, collagen, ADP, vasopressin), only U44619 decreased the pertussis-toxin-dependent ADP-ribosylation of the soluble and membrane Gi alpha proteins to a degree comparable to TRAP. It is concluded that the thrombin-induced activation of both the membrane and soluble Gi alpha proteins in platelets occurs via stimulation of the recently cloned thrombin receptor and is independent of the binding of thrombin to glycoprotein Ib. Furthermore, the coupling thrombin receptor/Gi protein is reduced by intracellular cAMP.