Thrombin and Phorbol Esters Cause the Selective Phosphorylation of a G Protein Other Than Gi in Human Platelets

Ubiquitin proteolysis of a CDK-related kinase regulates titan cell formation and virulence in the fungal pathogen Cryptococcus neoformans

Fungal pathogens often undergo morphological switches, including cell size changes, to adapt to the host environment and cause disease. The pathogenic yeast Cryptococcus neoformans forms so-called 'titan cells' during infection. Titan cells are large, polyploid, display alterations in cell wall and capsule, and are more resistant to phagocytosis and various types of stress. Titan cell formation is regulated by the cAMP/PKA signal pathway, which is stimulated by the protein Gpa1. Here, we show that Gpa1 is activated through phosphorylation by a CDK-related kinase (Crk1), which is targeted for degradation by an E3 ubiquitin ligase (Fbp1). Strains overexpressing CRK1 or an allele lacking a PEST domain exhibit increased production of titan cells similarly to the fbp1∆ mutant. Conversely, CRK1 deletion results in reduced titan cell production, indicating that Crk1 stimulates titan cell formation. Crk1 phosphorylates Gpa1, which then localizes to the plasma membrane and activates the cAMP/PKA signal pathway to induce cell enlargement. Furthermore, titan cell-overproducing strains trigger increased Th1 and Th17 cytokine production in CD4+ T cells and show attenuated virulence in a mouse model of systemic cryptococcosis. Overall, our study provides insights into the regulation of titan cell formation and fungal virulence.

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.

Gpr176 is a Gz-linked orphan G-protein-coupled receptor that sets the pace of circadian behaviour

G-protein-coupled receptors (GPCRs) participate in a broad range of physiological functions. A priority for fundamental and clinical research, therefore, is to decipher the function of over 140 remaining orphan GPCRs. The suprachiasmatic nucleus (SCN), the brain's circadian pacemaker, governs daily rhythms in behaviour and physiology. Here we launch the SCN orphan GPCR project to (i) search for murine orphan GPCRs with enriched expression in the SCN, (ii) generate mutant animals deficient in candidate GPCRs, and (iii) analyse the impact on circadian rhythms. We thereby identify Gpr176 as an SCN-enriched orphan GPCR that sets the pace of circadian behaviour. Gpr176 is expressed in a circadian manner by SCN neurons, and molecular characterization reveals that it represses cAMP signalling in an agonist-independent manner. Gpr176 acts independently of, and in parallel to, the Vipr2 GPCR, not through the canonical Gi, but via the unique G-protein subclass Gz.

The suprachiasmatic nucleus (SCN) is the central regulator of circadian rhythms. Here the authors identify mouse Gpr176 as a pace modulator of this circadian clock and characterize its mode of action as coupling to Gz rather than Gi subunits.

Extracellular loop II modulates GTP sensitivity of the prostaglandin EP3 receptor

Unlike the majority of G protein-coupled receptors, the prostaglandin E(2) (PGE(2)) E-prostanoid 3 (EP3) receptor binds agonist with high affinity that is insensitive to the presence of guanosine 5[prime]-O-(3-thio)triphosphate (GTPγS). We report the identification of mutations that confer GTPγS sensitivity to agonist binding. Seven point mutations were introduced into the conserved motif in the second extracellular loop (ECII) of EP3, resulting in acquisition of GTP-sensitive agonist binding. One receptor mutation W203A was studied in detail. Loss of agonist binding was observed on intact human embryonic kidney 293 cells expressing the W203A receptor, conditions where high GTP levels are present; however, high affinity binding [(3)H]PGE(2) was observed in broken cell preparations washed free of GTP. The [(3)H]PGE(2) binding of W203A in broken cell membrane fractions was inhibited by addition of GTPγS (IC(50) 21 ± 1.8 nM). Taken together, these results suggest that the wild-type EP3 receptor displays unusual characteristics of the complex coupled equilibria between agonist-receptor and receptor-G protein interaction. Moreover, mutation of ECII can alter this coupled equilibrium from GTP-insensitive agonist binding to more conventional GTP-sensitive binding. This suggests that for the mutant receptors, ECII plays a critical role in linking the agonist bound receptor conformation to the G protein nucleotide bound state.

Lithium and valproic acid treatments reduce PKC activation and receptor-G protein coupling in platelets of bipolar manic patients

Dysregulated protein kinase C (PKC) distribution and activation, and abnormal receptor-G protein coupling, have been implicated in the pathophysiology of bipolar affective disorder (BD). The therapeutic effectiveness of lithium has also been correlated with its ability to reduce PKC activation and G protein-mediated signaling. We examine the cellular distribution and activation of PKC and receptor-G protein coupling in blood platelets from normal controls, patients with BD mania or schizophrenia during treatment-free state, and after lithium or valproic acid administration. PKC activity was measured under basal and 50 nM phorbol 12-myristate, 13-acetate (PMA), 1 microM serotonin or 0.5 U/ml thrombin-stimulated conditions. The coupling of G proteins to serotonin or thrombin receptors were assessed by serotonin or thrombin-mediated [35S]GTPgammaS binding to membrane Galpha proteins. The results demonstrate that membrane-associated PKC activity and stimulus-induced PKC translocation are increased in BD manic, whereas stimulus-elicited PKC translocation is attenuated in schizophrenic patients. Lithium and valproic acid treatments attenuated the stimulus-induced PKC translocations to a similar degree and decreased PKC activity in both cytosolic and membranous fractions after two weeks of drug administration. An increase in 5-HT or thrombin stimulated [35S]GTPgammaS binding to Galpha proteins was detected in BD manic but not in schizophrenic patients although basal [35S]GTPgammaS binding was not different across the diagnostic groups. Lithium and valproic acid treatments similarly reduced receptor-G protein coupling with comparable time courses. Thus, increased membrane-associated PKC, cytosol to membrane PKC translocation and receptor-G protein coupling in platelets of BD manic patients were alleviated by lithium or valproic acid treatments.

Regulation of hematopoietic-specific G-protein Galpha15 and Galpha16 by protein kinase C

Heterotrimeric G proteins mediate cell growth and differentiation by coupling cell surface receptors to intracellular effector enzymes. The G-protein alpha subunit, Galpha(16), and its murine homologue Galpha(15), are expressed specifically in hematopoietic cells and their expression is highly regulated during differentiation of normal and leukemic cells. In this study, we examined the phosphorylation of Galpha(15)/Galpha(16) and its role in receptor and effector coupling. We observed a PMA-stimulated intact cell phosphorylation of Galpha(15) in COS7 cells transfected with Galpha(15) and protein kinase Calpha (PKCalpha), and phosphorylation of endogenous Galpha(16) in HL60 cells. We also showed that peptides derived from the two G-proteins were phosphorylated in vitro using purified brain PKC. Furthermore, we identified the putative phosphorylation site and showed that mutation or deletion of this PKC phosphorylation site inhibited phospholipase C (PLC) activation. The behavior of double mutants with the constitutively active G-protein mutation (QL-mutant) and mutation in the putative phosphorylation site suggests that the phosphorylation site of Galpha(15/16) is essential for receptor-coupled activation of PLC, but not for direct interaction of the G-protein with PLC-beta.

G(z alpha) deficient mice: enzyme levels in the autonomic nervous system, neuronal survival and effect of genetic background

Our laboratory has generated a genetically mutant mouse in which the alpha subunit of the heterotrimeric GTP binding protein, G(z) has been made dysfunctional by homologous recombination to determine its in vivo function. These animals show a characteristic failure to thrive phenotype. G(z alpha) is expressed in a variety of nervous system tissues as well as in the adrenal medulla. We therefore examined the autonomic nervous system of the G(z alpha) deficient mouse by measuring the activity of tyrosine hydroxylase and choline acetyltransferase in the superior cervical ganglia, submaxillary gland and the adrenal medulla. Preliminary results using animals of mixed BALB/c and C57BL/6 strains gave inconsistent results. Further experiments demonstrated differences in the activity of tyrosine hydroxylase and choline acetyltransferase between BALB/c and C57BL/6 mouse strains. The analysis of the pure strains showed a reduction in the size and enzyme levels of the adrenal gland and submaxillary glands of the G(z alpha) deficient mouse suggesting a role for adrenal insufficiency and/or nutritional disorders for the failure to thrive phenotype. The survival of sympathetic and sensory neurons was also examined in the G(z alpha) deficient mouse and in the presence of pertussis toxin, sympathetic but not sensory neuronal survival in G(z alpha) deficient mice was significantly attenuated. This suggests that in vivo other pertussis toxin sensitive G proteins may be recruited to compensate for the loss of G(z alpha).

Stimulated D(1) dopamine receptors couple to multiple Galpha proteins in different brain regions

Previous studies have revealed that activation of rat striatal D(1) dopamine receptors stimulates both adenylyl cyclase and phospholipase C via G(s) and G(q), respectively. The differential distribution of these systems in brain supports the existence of distinct receptor systems. The present communication extends the study by examining other brain regions: hippocampus, amygdala, and frontal cortex. In membrane preparations of these brain regions, selective stimulation of D(1) dopamine receptors increases the hydrolysis of phosphatidylinositol/phosphatidylinositol 4,5-biphosphate. In these brain regions, D(1) dopamine receptors couple differentially to multiple Galpha protein subunits. Antisera against Galpha(q) blocks dopamine-stimulated PIP(2) hydrolysis in hippocampal and in striatal membranes. The binding of [(35)S]GTPgammaS or [alpha-(32)P]GTP to Galpha(i) was enhanced in all brain regions. Dopamine also increased the binding of [(35)S]GTPgammaS or [alpha-(32)P]GTP to Galpha(q) in these brain regions: hippocampus = amygdala > frontal cortex. However, dopamine-stimulated binding of [(35)S]GTPgammaS to Galphas only in the frontal cortex and striatum. This differential coupling profile in the brain regions was not related to a differential regional distribution of the Galpha proteins. Dopamine induced increases in GTPgammaS binding to Galpha(s) and Galpha(q) was blocked by the D(1) antagonist SCH23390 but not by D(2) receptor antagonist l-sulpiride, suggesting that D(1) dopamine receptors couple to both Galpha(s) and Galpha(q) proteins. Co-immunoprecipitation of Galpha proteins with receptor-binding sites indicate that in the frontal cortex, D(1) dopamine-binding sites are associated with both Galpha(s) and Galpha(q) and, in hippocampus or amygdala, D(1) dopamine receptors couple solely to Galpha(q). The results indicate that in addition to the D(1)/G(s)/adenylyl cyclase system, brain D(1)-like dopamine receptor sites activate phospholipase C through Galpha(q) protein.

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.

GTPase-activating proteins for heterotrimeric G proteins: regulators of G protein signaling (RGS) and RGS-like proteins

GTPase-activating proteins (GAPs) regulate heterotrimeric G proteins by increasing the rates at which their subunits hydrolyze bound GTP and thus return to the inactive state. G protein GAPs act allosterically on G subunits, in contrast to GAPs for the Ras-like monomeric GTP-binding proteins. Although they do not contribute directly to the chemistry of GTP hydrolysis, G protein GAPs can accelerate hydrolysis >2000-fold. G protein GAPs include both effector proteins (phospholipase C-¿, p115RhoGEF) and a growing family of regulators of G protein signaling (RGS proteins) that are found throughout the animal and fungal kingdoms. GAP activity can sharpen the termination of a signal upon removal of stimulus, attenuate a signal either as a feedback inhibitor or in response to a second input, promote regulatory association of other proteins, or redirect signaling within a G protein signaling network. GAPs are regulated by various controls of their cellular concentrations, by complex interactions with G¿ or with G¿5 through an endogenous G-like domain, and by interaction with multiple other proteins.

Effects of prenatal exposure to cocaine on the developing brain: anatomical, chemical, physiological and behavioral consequences

Earlier studies of human infants and studies employing animal models had indicated that prenatal exposure to cocaine produced developmental changes in the behavior of the offspring. The present paper reports on the results obtained in a rabbit model of in utero exposure to cocaine using intravenous injections (4 mg/kg, twice daily) that mimic the pharmacokinetics of crack cocaine in humans. At this dose, cocaine had no effect on the body weight gain of dams, time to delivery, litter size and body weight or other physical characteristics of the offspring. In spite of an otherwise normal appearance, cocaine-exposed neonates displayed a permanent impairment in signal transduction via the D1 dopamine receptor in caudate nucleus, frontal cortex and cingulate cortex due to an uncoupling of the receptor from its associated Gs protein. This uncoupling in the caudate nucleus was shown to have behavioral consequences in that young or adult rabbits, exposed to cocaine in utero, failed to demonstrate amphetamine-elicited motor responses normally seen after activation of D1 receptors in the caudate. The cocaine progeny also demonstrated permanent morphological abnormalities in the anterior cingulate cortex due to uncoupling of the D1 receptor and the consequent inability of dopamine to regulate neurite outgrowth during neuronal development. Consistent with the known functions of the anterior cingulate cortex, adult cocaine progeny demonstrated deficits in attentional processes. This was reflected by impairment in discrimination learning during classical conditioning that was due to an inability to ignore salient stimuli even when these were not relevant to the task. The impairment in discrimination learning also occurred in an instrumental avoidance task and could be shown to be due to an impairment of cingulothalamic learning-related neuronal coding. It was proposed that the selective loss of D1-related neurotransmission in the anterior cingulate cortex prevented an appropriate activation of GABA neurons and thus a loss of inhibitory regulation that is necessary for processes involved in associative attention. Taken together, these findings suggest that the uncoupling of the D1 receptor from its G protein may be the fundamental source of the anatomic, cognitive and motor disturbances seen in rabbits exposed to cocaine in utero. Moreover, the long-term cognitive and motor deficits observed in the rabbit model are in agreement with the recent reports indicating that persistent attentional and other behavioral deficits may be evident in cocaine-exposed children as they grow older and are challenged to master more complex cognitive tasks.

Epinephrine correction of impaired platelet thromboxane receptor signaling

This study evaluated the mechanism of epinephrine potentiation of platelet secretion induced by thromboxane A(2) (TXA(2)). Dog platelets that do not secrete in response to TXA(2) alone (TXA(2)-) were compared with dog platelets that do secrete (TXA(2)+) and with human platelets. TXA(2)- platelets had impaired TXA(2) receptor (TP receptor)-G protein coupling, indicated by 1) impaired stimulated GTPase activity, 2) elevated basal guanosine 5'-O-(3-thiotriphosphate) binding, and 3) elevated Galpha(q) palmitate turnover that was corrected by preexposure to epinephrine. Kinetic agonist binding studies revealed biphasic dog and human platelet TP receptor association and dissociation. TXA(2)- and TP receptor-desensitized TXA(2)+ dog and human platelets had altered ligand binding parameters compared with untreated TXA(2)+ or human platelets. These parameters were reversed, along with impaired secretion, by epinephrine. Basal phosphorylation of TXA(2)- platelet TP receptors was elevated 60% and was normalized by epinephrine. Epinephrine potentiates platelet secretion stimulated by TXA(2) by reducing basal TP receptor phosphorylation and facilitating TP receptor-G protein coupling in TXA(2)- platelets and, probably, in normal platelets as well.

The alpha subunits of Gz and Gi interact with the eyes absent transcription cofactor Eya2, preventing its interaction with the six class of homeodomain-containing proteins

Yeast two-hybrid techniques were used to identify possible effectors for the heterotrimeric G protein G(z) in human bone marrow cells. Eya2, a human homologue of the Drosophila Eya transcription co-activator, was identified. Eya2 interacts with activated Galpha(z) and at least one other member of the Galpha(i) family, Galpha(i2). Interactions were confirmed in mammalian two-hybrid and glutathione S-transferase fusion protein pull-down assays. Regions of Eya2-mediating interaction were mapped to the C-terminal Eya consensus domain. Eya2 is an intrinsically cytosolic protein that is translocated to the nucleus by members of the Six homeodomain-containing family of proteins. Activated Galpha(z) and Galpha(i2) prevent Eya2 translocation and inhibit Six/Eya2-mediated activation of a reporter gene controlled through the MEF3/TATA promoter. Although G proteins are known to regulate the activity of numerous transcription factors, this regulation is normally achieved indirectly via one or more intermediates. We show here a novel functional regulation of a co-activator directly by G protein subunits.

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.

Potentiation of ganodermic acid S on prostaglandin E(1)-induced cyclic AMP elevation in human platelets

Ganodermic acid S (GAS), isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exhibits inhibitory effects on platelet responses to various aggregating agonists. Our study demonstrated that GAS also participated in potentiating the response of human gel-filtered platelets to prostaglandin (PG) E(1). GAS at <20 microM did not show any significant change of basal cyclic AMP level in gel-filtered platelets. However, GAS potentiated the PGE(1)-evoked cyclic AMP level in a bell-shaped, concentration-dependent manner. The agent at 7.5 microM enhanced the level up to 1.8-fold of that evoked by PGE(1) alone. Collagen did not inhibit the PGE(1)-induced cyclic AMP level in platelets pretreated with GAS at 6 to 7.5 microM. In the presence of 7.5 microM GAS, the agent enhanced the inhibition of PGE(1) on platelet response to collagen in: phosphorylation of myosin light chain and pleckstrin; alpha-granule secretion; cell aggregation and protein-tyrosine phosphorylation. In addition, the agent along with PGE(1) almost abolished the dense-granule secretion and thromboxane (TX) B(2) formation. The results suggest that GAS played an additional role in potentiating the PGE(1)-induced cyclic AMP synthesis. GAS and PGE(1) inhibited additively the platelet response to collagen.

Hypertolerance to morphine in G(z alpha)-deficient mice

Our laboratory has generated a mouse deficient in the alpha (alpha) subunit of the G protein, G(z), (G(z alpha)) gene and we have examined the involvement of G(z alpha) in spinal and supraspinal analgesia and tolerance mechanisms. Spinal analgesia was tested by the response times to heat or cold tail flick times in a water bath at 50 degrees C or -5 degrees C and supraspinal analgesia was tested by the times for paw licking and jumping from a plate at 52 degrees C or 0.5 degrees C. Tolerance to morphine was induced in wild type and G(z alpha)-deficient mice over a 5 day period and the behavioral tests were performed daily. The tail flick reaction times to both hot and cold stimuli did not differ between the wild type and G(z alpha)-deficient mice. Analysis of the reaction times from the hot and cold plate tests showed the G(z alpha)-deficient mice developed tolerance to morphine to a greater degree and at a faster rate than wild type mice. Opioid binding assays were performed on synaptic membranes prepared from naive and morphine tolerant wild type and G(z alpha)-deficient brains. No changes in the affinity of morphine for its receptor or in the density of mu and delta opioid receptors were found between the two groups of mice in the naive or morphine tolerant state. This indicates that the absence of G(z alpha) does not affect opioid receptor affinity or receptor up or down regulation. Our results suggest that the presence of G(z alpha) delays the development of morphine tolerance and represents a possible therapeutic target for improving the clinical use of morphine.

Evidence for transduction of mu but not kappa opioid modulation of extracellular signal-regulated kinase activity by G(z) and G(12) proteins

Chronic treatment with micro or kappa opioid agonists (>/=2 h) inhibits EGF-induced ERK activation in opioid receptor overexpressing COS-7 cells. Although acute mu and kappa opioids activate ERK via a pertussis toxin-sensitive G protein, pertussis toxin insensitivity of the chronic mu (but not kappa) action was observed. Here, we tested several pertussis toxin-insensitive G proteins as candidates to transduce acute and/or chronic opioid modulation of ERK. Overexpressed Galpha(z) (but not Galpha(12)) transduced acute mu (but not kappa) ERK activation in pertussis toxin-treated COS-7 cells. Chronic mu (but not kappa) inhibited EGF stimulation of ERK in pertussis toxin-treated cells overexpressing Galpha(z) or Galpha(12). Transfection of Galpha(13) or Galpha(q) blocked inhibition under the same conditions. Overexpressed interfering and non-interfering Galpha(z) mutants differentially affected mu inhibition of ERK consistent with G(z) transduction. In this and prior studies, Galpha(z) and Galpha(12) immunoreactivity were detected in untransfected COS-7 cells, suggesting that these G proteins may be endogenous mediators of chronic mu inhibitory actions on ERK.

Membrane-associated GAIP is a phosphoprotein and can be phosphorylated by clathrin-coated vesicles

GAIP (G alpha interacting protein) is a member of the RGS (regulators of G protein signaling) family and accelerates the turnover of GTP bound to Galphai, Galphaq, and Galpha13. There are two pools of GAIP-a soluble and a membrane-anchored pool. The membrane-anchored pool is found on clathrin-coated vesicles (CCVs) and pits in rat liver and AtT-20 pituitary cells. By treatment of a GAIP-enriched rat liver fraction with alkaline phosphatase, we found that membrane-bound GAIP is phosphorylated. By immunoprecipitation carried out on [(32)P]orthophosphate-labeled AtT-20 pituitary cells stably expressing GAIP, (32)P-labeling was associated exclusively with the membrane pool of GAIP. Phosphoamino acid analysis revealed that phosphorylation of GAIP occurred largely on serine residues. Recombinant GAIP could be phosphorylated at its N terminus with purified casein kinase 2 (CK2). It could also be phosphorylated by isolated CCVs in vitro. Phosphorylation was Mn(2+)-dependent, using both purified CK2 and CCVs. Ser-24 was identified as one of the phosphorylation sites. Our results establish that GAIP is phosphorylated and that only the membrane pool is phosphorylated, suggesting that GAIP can be regulated by phosphorylation events taking place at the level of clathrin-coated pits and vesicles.

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.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

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.

Stimulation of cAMP synthesis by Gi-coupled receptors upon ablation of distinct Galphai protein expression. Gi subtype specificity of the 5-HT1A receptor

The three Galphai subunits were independently depleted from rat pituitary GH4C1 cells by stable transfection of each Galphai antisense rat cDNA construct. Depletion of any Galphai subunit eliminated receptor-induced inhibition of basal cAMP production, indicating that all Galphai subunits are required for this response. By contrast, receptor-mediated inhibition of vasoactive intestinal peptide (VIP)-stimulated cAMP production was blocked by selective depletions for responses induced by the transfected serotonin 1A (5-HT1A) (Galphai2 or Galphai3) or endogenous muscarinic-M4 (Galphai1 or Galphai2) receptors. Strikingly, receptor activation in Galphai1-depleted clones (for the 5-HT1A receptor) or Galphai3-depleted clones (for the muscarinic receptor) induced a pertussis toxin-sensitive increase in basal cAMP production, whereas the inhibitory action on VIP-stimulated cAMP synthesis remained. Finally, in Galphai2-depleted clones, activation of 5-HT1A receptors increased VIP-stimulated cAMP synthesis. Thus, 5-HT1A and muscarinic M4 receptor may couple dominantly to Galphai1 and Galphai3, respectively, to inhibit cAMP production. Upon removal of these Galphai subunits to reduce inhibitory coupling, stimulatory receptor coupling is revealed that may involve Gbetagamma-induced activation of adenylyl cyclase II, a Gi-stimulated cyclase that is predominantly expressed in GH4C1 cells. Thus Gi-coupled receptor activation involves integration of both inhibitory and stimulatory outputs that can be modulated by specific changes in alphai subunit expression level.

Human Platelets Contain SNARE Proteins and a Sec1p Homologue That Interacts With Syntaxin 4 and Is Phosphorylated After Thrombin Activation: Implications for Platelet Secretion

In response to thrombin and other extracellular activators, platelets secrete molecules from large intracellular vesicles (granules) to initiate thrombosis. Little is known about the molecular machinery responsible for vesicle docking and secretion in platelets and the linkage of that machinery to cell activation. We found that platelet membranes contain a full complement of interacting proteins—VAMP, SNAP-25, and syntaxin 4—that are necessary for vesicle docking and fusion with the plasma membrane. Platelets also contain an uncharacterized homologue of the Sec1p family that appears to regulate vesicle docking through its binding with a cognate syntaxin. This platelet Sec1 protein (PSP) bound to syntaxin 4 and thereby excluded the binding of SNAP-25 with syntaxin 4, an interaction critical to vesicle docking. As predicted by its sequence, PSP was detected predominantly in the platelet cytosol and was phosphorylated in vitro by protein kinase C (PKC), a secretion-linked kinase, incorporating 0.87 ± 0.11 mol of PO4 per mole of protein. PSP was also specifically phosphorylated in permeabilized platelets after cellular stimulation by phorbol esters or thrombin and this phosphorylation was blocked by the PKC inhibitor Ro-31-8220. Phosphorylation by PKC in vitro inhibited PSP from binding to syntaxin 4. Taken together, these studies indicate that platelets, like neurons and other cells capable of regulated secretion, contain a unique complement of interacting vesicle docking proteins and PSP, a putative regulator of vesicle docking. The PKC-dependent phosphorylation of PSP in activated platelets and its inhibitory effects on syntaxin 4 binding provide a novel functional link that may be important in coupling the processes of cell activation, intracellular signaling, and secretion.

Human Platelets Contain SNARE Proteins and a Sec1p Homologue That Interacts With Syntaxin 4 and Is Phosphorylated After Thrombin Activation: Implications for Platelet Secretion

In response to thrombin and other extracellular activators, platelets secrete molecules from large intracellular vesicles (granules) to initiate thrombosis. Little is known about the molecular machinery responsible for vesicle docking and secretion in platelets and the linkage of that machinery to cell activation. We found that platelet membranes contain a full complement of interacting proteins—VAMP, SNAP-25, and syntaxin 4—that are necessary for vesicle docking and fusion with the plasma membrane. Platelets also contain an uncharacterized homologue of the Sec1p family that appears to regulate vesicle docking through its binding with a cognate syntaxin. This platelet Sec1 protein (PSP) bound to syntaxin 4 and thereby excluded the binding of SNAP-25 with syntaxin 4, an interaction critical to vesicle docking. As predicted by its sequence, PSP was detected predominantly in the platelet cytosol and was phosphorylated in vitro by protein kinase C (PKC), a secretion-linked kinase, incorporating 0.87 ± 0.11 mol of PO4 per mole of protein. PSP was also specifically phosphorylated in permeabilized platelets after cellular stimulation by phorbol esters or thrombin and this phosphorylation was blocked by the PKC inhibitor Ro-31-8220. Phosphorylation by PKC in vitro inhibited PSP from binding to syntaxin 4. Taken together, these studies indicate that platelets, like neurons and other cells capable of regulated secretion, contain a unique complement of interacting vesicle docking proteins and PSP, a putative regulator of vesicle docking. The PKC-dependent phosphorylation of PSP in activated platelets and its inhibitory effects on syntaxin 4 binding provide a novel functional link that may be important in coupling the processes of cell activation, intracellular signaling, and secretion.

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.

RGSZ1, a Gz-selective regulator of G protein signaling whose action is sensitive to the phosphorylation state of Gzalpha

Regulators of G protein signaling (RGS) are a family of proteins that attenuate the activity of the trimeric G proteins. RGS proteins act as GTPase-activating proteins (GAPs) for the alpha subunits of several trimeric G proteins, much like the GAPs that regulate the activity of monomeric G proteins such as Ras. RGS proteins have been cloned from many eukaryotes, and those whose biochemical activity has been characterized regulate the members of the Gi family of G proteins; some forms can also act on Gq proteins. In an ongoing effort to elucidate the role of Gzalpha in cell signaling, the yeast two-hybrid system was employed to identify proteins that could interact with a mutationally activated form of Gzalpha. A novel RGS, termed RGSZ1, was identified that is most closely related to two existing RGS proteins termed RetRGS1 and GAIP. Northern blot analysis revealed that expression of RGSZ1 was limited to brain, and expression was particularly high in the caudate nucleus. Biochemical characterization of recombinant RGSZ1 protein revealed that RGSZ1 was indeed a GAP and, most significantly, showed a marked preference for Gzalpha over other members of the Gialpha family. Phosphorylation of Gzalpha by protein kinase C, an event known to occur in cells and that was previously shown to influence alpha-betagamma interactions of Gz, rendered the G protein much less susceptible to RGSZ1 action.

Identification of a GTPase activating protein specific for the heterotrimeric G protein, Gz

Gz is a member of the Gi family of trimeric G proteins whose precise signalling function has not been defined. It can be distinguished from other members of the family by several interesting biochemical properties of its alpha subunit, Gzalpha. One particularly intriguing property is its extremely slow GTPase activity; its kcat for GTP hydrolysis is as much as 200-fold less than other Galpha's. Since there is evidence that cellular factors can accelerate the GTPase activities of Galpha subunits, we have suspected that cells expressing Gzalpha may contain a GTPase-activating protein, or GAP, that would enhance its hydrolytic ability. Using purified Gzalpha-GTP as a substrate, we have identified and characterized such a GAP that acts on Gzalpha, which we have termed Gz-GAP. The protein responsible for this activity is specific for Gzalpha and is found in the membrane fraction of bovine brain and in several other tissues that express Gz. Since G protein effectors are in many cases capable of stimulating the GTPases rate of Galpha subunits, we speculate that a novel effector for Gz is responsible for the activity.

Analysis of GTP-binding proteins, phosphoproteins, and cytosolic calcium in functional heterogeneous human blood platelet subpopulations

The biochemical basis for the functional heterogeneity of human blood platelets was investigated in terms of protein phosphorylation, cytoplasmic calcium ([Ca2+]i), the ratio of 46 and 50 kDa vasodilator-stimulated protein (VASP), and GTP-binding proteins (G-proteins). Platelets were fractionated by density. Comparing resting low-density platelets (LDP) to high-density platelets (HDP) revealed higher phosphorylation of proteins in the 47, 31, and 24 kDa ranges. A higher phosphorylation of the 20 kDa protein in LDP compared to HDP was related to an enhanced [Ca2+]i, an increased ADP-ribosylation of the inhibitory G-protein (G(i alpha1-3)) and rhoA, and a decreased ADP-ribosylation of the stimulatory G-protein (G(s alpha)). The differences in the ribosylation patterns of the subpopulations were not influenced by thrombin stimulation or exposure to prostaglandin E1 (PGE1). An 18 kDa phosphoprotein was more highly phosphorylated in resting HDP than in LDP. Thrombin exposure caused dephosphorylation of the 18 kDa phosphoprotein in the HDP, but generally increased phosphorylation of both HDP and LDP in the 47, 31, 24, and 20 kDa bands. Preincubation with prostaglandin E1 or sodium nitroprusside diminished the subsequent thrombin-induced increase in phosphorylation, particularly in HDP. In unstimulated HDP, the 50 kDa VASP phospho form was enhanced, whereas in unstimulated LDP the 46 kDa VASP dephospho form was increased. Our findings suggest that the functional heterogeneity of platelets is partly derived from differences in signal transduction mechanisms reflected in varying phosphoprotein patterns and G-protein properties of platelet stimulatory and inhibitory pathways.

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.

Mechanism of quenching of phototransduction. Binding competition between arrestin and transducin for phosphorhodopsin

Quenching of phototransduction in retinal rod cells involves phosphorylation of photoactivated rhodopsin by the enzyme rhodopsin kinase followed by binding of the protein arrestin. Although it has been proposed that the mechanism of arrestin quenching of visual transduction is via steric exclusion of transducin binding to phosphorylated light-activated rhodopsin (P-Rh*), direct evidence for this mechanism is lacking. In this study, we investigated both the role of rhodopsin phosphorylation in modulating its interaction with arrestin and transducin and the proposed binding competition between arrestin and transducin for P-Rh*. While the beta-adrenergic receptor kinase promotes significant arrestin binding to rhodopsin at a phosphorylation stoichiometry of >/=2 mol/mol, rhodopsin kinase promotes arrestin binding at a stoichiometry of approximately 0.9 mol/mol. Moreover, while beta-adrenergic receptor kinase phosphorylation of rhodopsin only modestly decreases transducin binding and activation, rhodopsin kinase phosphorylation of rhodopsin significantly decreases transducin binding and activation. Finally, arrestin competes effectively with transducin for binding to P-Rh* (50% inhibition at approximately 1:1 molar ratio of arrestin:transducin) but has no effect on transducin binding to nonphosphorylated light-activated rhodopsin (Rh*), paralleling the functional inhibition by arrestin on P-Rh*-stimulated transducin activation (50% inhibition at approximately 1.7:1 molar ratio of arrestin:transducin). These results demonstrate that a major role of rhodopsin phosphorylation is to promote high-affinity arrestin binding and decrease transducin binding thus allowing arrestin to effectively compete with transducin for binding to photoactivated rhodopsin.

Gx/z is regulated by mu but not delta opioid receptors in the stimulation of the low Km GTPase activity in mouse periaqueductal grey matter

High affinity low K(m) GTPase activity was measured in membrane preparations of adult mouse mesencephalic periaqueductal grey matter (PAG). Opioids displaying selectivity towards mu- or delta-opioid receptors (OR) activated the enzyme in a concentration-dependent manner. Antibodies to mu-OR greatly impaired the potential of mu-agonists, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and morphine, to increase hydrolysis of GTP. The same antibodies had little effect on [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, both agonists at delta-OR. Stimulation of GTPase by DPDPE and [D-Ala2]deltorphin II - but not by morphine or DAMGO - was diminished by antibodies to delta-OR. The blockade of G(i2)alpha subunits by specific antibodies impaired the activation of G alpha-related GTPase by all opioids. Antibodies in vitro, and oligodeoxynucleotides in vivo, prepared against Gx/z alpha subunits, reduced the release of Pi promoted by DAMGO and morphine. The impairment of Gx/z proteins also slightly reduced the effect of the delta2 agonist [D-Ala2]deltorphin II. At delta1 receptors, DPDPE fully expressed its activation of GTPase. These results indicate that in the PAG, mu-OR and delta-OR couple with Gi2 transducer proteins. Notably, mu-OR also regulates the pertussis toxin-insensitive G-protein Gx/z, an effect poorly exhibited by delta-OR in this tissue.

Reconstitution of receptors and GTP-binding regulatory proteins (G proteins) in Sf9 cells. A direct evaluation of selectivity in receptor.G protein coupling

The selectivity in coupling of various receptors to GTP-binding regulatory proteins (G proteins) was examined directly by a novel assay entailing the use of proteins overexpressed in Spodoptera frugiperda (Sf9) cells. Activation of G proteins was monitored in membranes prepared from Sf9 cells co-expressing selected pairs of receptors and G proteins (i.e. alpha, beta1, and gamma2 subunits). Membranes were incubated with [35S]guanosine 5'-(3-O-thio)triphosphate (GTPgammaS) +/- an agonist, and the amount of radiolabel bound to the alpha subunit was quantitated following immunoprecipitation. When expressed without receptor (but with beta1gamma2), the G protein subunits alphaz, alpha12, and alpha13 did not bind appreciable levels of [35S]GTPgammaS, consistent with a minimal level of GDP/[35S]GTPgammaS exchange. In contrast, the subunits alphas and alphaq bound measurable levels of the nucleotide. Co-expression of the 5-hydroxytryptamine1A (5-HT1A) receptor promoted binding of [35S]GTPgammaS to alphaz but not to alpha12, alpha13, or alphas. Binding to alphaz was enhanced by inclusion of serotonin in the assay. Agonist activation of both thrombin and neurokinin-1 receptors promoted a modest increase in [35S]GTPgammaS binding to alphaz and more robust increases in binding to alphaq, alpha12, and alpha13. Binding of [35S]GTPgammaS to alphas was strongly enhanced only by the activated beta1-adrenergic receptor. Our data identify interactions of receptors and G proteins directly, without resort to measurements of effector activity, confirm the coupling of the 5-HT1A receptor to Gz and extend the list of receptors that interact with this unique G protein to the receptors for thrombin and substance P, imply constitutive activity for the 5-HT1A receptor, and demonstrate for the first time that the cloned receptors for thrombin and substance P activate G12 and G13.

Role of chondroitin 4-sulphate as a receptor for polycation induced human platelet aggregation

1. Proteoglycans provide negatively charged sites on the surface of platelets, leukocytes and endothelial cells. Since chondroitin 4-sulphate is the main proteoglycan present on the platelet surface, the role of this molecule in mediating the activation of human platelets by polylysine was studied. 2. Platelets were desensitized with phorbol 12-myristate 13-acetate (PMA, 10 nM) 5 min before the addition of polylysine to platelet-rich plasma (PRP). Changes in the intracellular Ca2+ concentration were measured in fura2-am (2 microM) loaded platelets and protein phosphorylation was assessed by autoradiography of the electrophoretic profile obtained from [32P]-phosphate labelled platelets. The release of dense granule contents was measured in [14C]-5-hydroxytryptamine loaded platelets and the synthesis of thromboxane (TXA2) was assessed by radioimmunoassay. Surface chondroitin 4-sulphate proteoglycan was degraded by incubating platelets with different concentrations of chondroitinase AC (3 min, 37 degrees C). The amount of chondroitin 4-sulphate remaining in the platelets was then quantified after proteolysis and agarose gel electrophoresis. 3. The addition of PMA to PRP before polylysine inhibited the aggregation by 88 +/- 18% (n = 3). Staurosporine (1 microM, 5 min) prevented the PMA-induced inhibition. Chondroitinase AC (4 pu ml-1 to 400 muu ml-1, 3 min) abolished the polylysine-induced aggregation in PRP but caused only a discrete inhibition of ADP-induced aggregation. The concentration of chrondroitin 4-sulphate in PRP (0.96 +/- 0.2 microgram/10(8) platelets, n = 3) and in washed platelets (WP; 0.35 +/- 0.1 microgram/10(8) platelets, n = 3) was significantly reduced following incubation with chondroitinase AC (PRP = 0.63 +/- 0.1 microgram/10(8) platelets and WP = 0.08 +/- 0.06 microgram/10(8) platelets). 4. Washed platelets had a significantly lower concentration of chondroitin 4-sulphate than platelets in PRP. The addition of polylysine to WP induced a rapid increase in light transmission which was not accompanied by TXA2 synthesis or the release of dense granule contents. This effect was not inhibited by sodium nitroprusside (SNP), iloprost, EDTA or the peptide RGDS. This event was accompanied by the discrete phosphorylation of plekstrin and myosin light chain, which were inhibited by staurosporine (10 microM, 10 min). The hydrolysis of platelet surface chondroitin 4-sulphate strongly reduced the polylysine-induced phosphorylation. 5. Our results indicate that polylysine activates platelets through a specific receptor which could be the proteoglycan chondroitin 4-sulphate present on the platelet membrane.

Interaction of transducin with light-activated rhodopsin protects It from proteolytic digestion by trypsin

The tryptic cleavage pattern of transducin (Gt) in solution was compared with that in the presence of phospholipid vesicles, rod outer segment (ROS) membranes kept in the dark, or ROS membranes containing light-activated rhodopsin, metarhodopsin II (Rh*). When Gt was in the high affinity complex with Rh*, the alphat subunit was almost completely protected from proteolysis. The protection of alphat at Arg310 was complete, while Arg204 was substantially protected. The cleavage of alphat at Lys18 was protected in the presence of phospholipid vesicles, ROS membranes kept in the dark, or ROS membranes containing Rh*. The cleavage of betat was slower in the presence of ROS membranes or phospholipid vesicles. When the Rh*. Gt complex was incubated with guanyl-5'-yl thiophosphate, a guanine nucleotide analog known to release the high affinity interaction between Gt and Rh*, the protection at Arg310 and Arg204 was diminished. From our results, we propose that Rh* either physically blocks access of trypsin to Arg204 and Arg310 or maintains the heterotrimer in such a conformation that these cleavage sites are not available. Since Arg204 is involved in the switch interface with betagammat (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319), it may be that betagammat is implicated in protecting this cleavage site in the receptor-bound, stabilized heterotrimer. Arg310 is not near the betagammat subunit, thus we believe that the high affinity binding of Gt to Rh* physically or sterically blocks access of trypsin to this site. Thus, Arg310, only a few angstroms away from the carboxyl terminus of alphat, which is known to directly bind to Rh*, is likely to also be a part of the Rh* binding site. This is in agreement with other studies and has implications for the mechanism by which receptors catalyze GDP release from G proteins. The protection of Lys18 in the presence of phospholipid vesicles suggests that the amino-terminal region is in contact with the membrane, consistent with the crystal structure of the heterotrimer (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319).

Thromboxane A2 agonist modulation of excitatory synaptic transmission in the rat hippocampal slice

1. The effects of the selective thromboxane A2 (TXA2) receptor agonist I-BOP on neuronal excitability and synaptic transmission were studied in the CAl neurones of rat hippocampal slices by an intracellular recording technique. 2. Superfusion of I-BOP (0.5 microM) resulted in a biphasic change of the excitatory postsynaptic potential (e.p.s.p.), which was blocked by pretreatment with SQ 29548, a specific antagonist of TXA2 receptors. The inhibitory phase of I-BOP on the e.p.s.p. was accompanied by a decrease in neuronal membrane input resistance. 3. The sensitivity of postsynaptic neurones to glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA), was unchanged by I-BOP (0.5 microM) pretreatment. 4. Bath application of Ba2+ (0.5 mM) prevented both the I-BOP-induced reduction of the neuronal membrane input resistance and the blockade of e.p.s.p. induced by I-BOP. 5. Intracellular dialysis of the hippocampal CA1 neurones with GDP (10 mM) significantly attenuated the I-BOP inhibition of e.p.s.p. and membrane input resistance. Incubation of the slices with either pertussis toxin (PTX, 5 micrograms ml-1 for 12 h) or cholera toxin (CTX, 5 micrograms ml-1 for 12 h) did not affect the biphasic action of I-BOP on the e.p.s.p. or the reduction of membrane input resistance induced by I-BOP. 6. Pretreatment of the slices with the protein kinase C (PKC) inhibitor, NPC-15437 (20 microM), abolished the biphasic modulation by I-BOP (0.5 microM) of the e.p.s.p. Intracellular application of a specific PKC inhibitor, PKCI 19-36 (20 microM), completely inhibited the I-BOP reduction of e.p.s.p. The specific cyclic AMP-dependent protein kinase (PKA) inhibitor, Rp-cyclic adenosine 3',5'-monophosphate (Rp-cyclic AMPS, 25 microM), had no effect on the I-BOP action. 7. In this study we have demonstrated, for the first time, the existence of functional TXA2 receptors in the hippocampus which mediate the effects of a TXA2 agonist on neuronal excitability and synaptic transmission. Activation of the presynaptic TXA2 receptors may stimulate the release of glutamate. Conversely, activation of postsynaptic TXA2 receptors leads to inhibition of synaptic transmission resulting from a decrease in the membrane input resistance of the neurones. The pre- and postsynaptic actions of the TXA2 agonist are both mediated by PTX- and CTX-insensitive G-protein-coupled activation of PKC pathways.

Effect of prenatal cocaine on dopamine receptor-G protein coupling in mesocortical regions of the rabbit brain

The effects of in utero exposure to cocaine on dopamine receptors in the frontal and cingulate areas of the developing rabbit cortex were assessed by examining receptor-mediated stimulations in GTP binding to alpha-subunits of G proteins. Pregnant Dutch-belted rabbits received intravenous injections of 4 mg/kg of cocaine HCl twice a day on gestational days 8-29, cortical membranes were prepared from their progenies on postnatal days 10-100 and dopamine-stimulated [35S] guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) binding to membrane G alpha proteins was measured. Dopamine increased [35S]GTP gamma S binding to G alpha s and G alpha i. These increases in [35S]GTP gamma S binding reflect the stimulation of D1- and D2-dopamine receptors, respectively. The ability of dopamine to stimulate the binding of [35S]GTP gamma S to G alpha s but not to G alpha i was reduced in both frontal and cingulate cortices obtained from cocaine-exposed animals when examined at 10, 50 or 100 days of age. Prenatal cocaine exposure was also shown to reduce dopamine-stimulated [alpha-32P]GTP binding to G alpha s without influencing binding to G alpha i. The muscarinic cholinergic receptor-evoked increases in [35S]GTP gamma S binding to G alpha i and G alpha o were not altered. Immunoblot analyses revealed no differences in the levels of these alpha subunits in membranes from cocaine-exposed animals vs controls. Furthermore, prenatal cocaine did not affect [3H]8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7- olhemimaleate binding to cortical D1 dopamine receptors. The results suggest that prenatal exposure of rabbits to cocaine selectively uncouples the D1 dopamine receptor from its G protein in mesocortical brain areas and that this change persists through postnatal day 100.

Primary structure of a gamma subunit of G protein, gamma 12, and its phosphorylation by protein kinase C

We have determined the primary structure of a novel gamma subunit (gamma 12, previously designated gamma S1) of G protein purified from bovine spleen. The mature gamma 12 protein composed of 68 amino acids had acetylated serine at the N terminus and geranylgeranylated/carboxylmethylated cysteine at the C terminus. This was consistent with the C-terminal prenylation signal in the amino acid sequence, which was predicted from gamma 12 cDNA isolated from a bovine spleen cDNA library. Western blots with the specific antibody against gamma 12 showed that gamma 12 is present in all tissues examined. Among various gamma subunits (gamma 1, gamma 2, gamma 3, gamma 7, and gamma 12), gamma 12 has a unique property to be phosphorylated by protein kinase C. The phosphorylated amino acid residue was Ser1 (or Ser2). The phosphorylated beta gamma 12 associated with Go alpha more tightly than the unphosphorylated form. Exposure of Swiss 3T3 and aortic smooth muscle cells to phorbol 12-myristate 13-acetate and NaF induced phosphorylation of gamma 12. Stimulation of aortic smooth muscle cells with natural vasoactive agents such as angiotensin II and vasopressin also induced phosphorylation of gamma 12. The extent of phosphorylation of beta gamma 12 in vitro was suppressed by a complex formation with Go alpha, which was relieved by the addition of guanosine 5'-O-(3-thiotriphosphate) or aluminum fluoride. These results strongly suggest that gamma 12 is phosphorylated by protein kinase C during activation of receptor(s) and G protein(s) in living cells.

Phosphorylation of elongation factor 1 (EF-1) by protein kinase C stimulates GDP/GTP-exchange activity

Phosphorylation of the alpha, beta and delta subunits of elongation factor (EF) 1 by protein kinase C results in stimulation of elongation activity up to threefold both in vivo and in vitro [Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 11,993-11,998, Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 12,574-12,580]. The alpha subunit catalyzes the GTP-dependent binding of amino-acyl-tRNA to the ribosome, while the beta gamma and delta subunits of EF-1 catalyze exchange of the residual GDP on EF-1 alpha for GTP. To determine whether the change in elongation rate following phosphorylation by protein kinase C is due to stimulation of GDP/GTP exchange activity, EF-1 and EF-1.valyl-tRNA-synthetase have been purified from rabbit reticulocytes, phosphorylated in vitro by protein kinase C and the effect of phosphorylation on nucleotide-exchange activity analyzed. The alpha, beta and delta subunits are phosphorylated only on serine, and phosphopeptide maps show distinct phosphopeptides for each subunit. Following quantitative phosphorylation of EF-1 by protein kinase C on the alpha, beta, and delta subunits, a twofold enhancement of the rate of nucleotide exchange over the non-phosphorylated controls is observed with EF-1 and EF-1.valyl-tRNA synthetase. Stimulation of nucleotide exchange results in a two-fold increase in the formation of EF-1 alpha.GTP.Phe-tRNA, leading to an increased rate of binding of Phe-tRNA to ribosomes. The magnitude of stimulation of the exchange rate is similar to that reported previously for the rate of elongation following phosphorylation of EF-1 by protein kinase C. Thus, the enhancement of EF-1 activity in response to 4 beta-phorbol 12-myristate 13-acetate appears to be due to stimulation of the rate of GDP/GTP exchange following phosphorylation of EF-1 by protein kinase C.

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.

Characteristics of the beta-adrenergic receptor complex in the epicardial border zone of the 5-day infarcted canine heart

BACKGROUND

The effect of isoproterenol on increasing the peak amplitude of the L-type calcium current is reduced in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart when compared with control cells from noninfarcted hearts. This suggests that specific alterations in the beta-adrenergic receptor complex develop in this setting. The present study is an examination of individual components of the beta-adrenergic receptor complex with the aim of elucidating the biochemical defect(s) that might be responsible for the diminished beta-adrenergic receptor responsiveness in the myocytes that survive in the infarcted heart.

METHODS AND RESULTS

We compared components of the beta-adrenergic receptor signaling pathway in membranes prepared from the EBZ of the 5-day infarcted heart and a remote, noninfarcted region (RZ) of the same ventricle as well as the corresponding regions of noninfarcted ventricles. Defects in multiple components of the beta-adrenergic receptor complex were confined to the EBZ of the 5-day infarcted heart. These include a decrease in beta-adrenergic receptor density; diminished basal, guanine nucleotide-, isoproterenol-, forskolin-, and manganese-dependent adenylyl cyclase activities; an increase in the EC50 for isoproterenol-dependent activation of adenylyl cyclase; a diminished level of the alpha-subunit of the Gs protein. and an elevated level of the alpha-subunit of the Gi protein.

CONCLUSIONS

Defects in multiple components of the membrane beta-adrenergic receptor complex were identified in the EBZ of the 5-day infarcted canine heart. This constellation of abnormalities would be predicted to impair functional beta-adrenergic responsiveness and contribute to the defect in isoproterenol-dependent stimulation of the L-type calcium current in myocytes isolated from this tissue.