Dissociation of guanosine 5'-[gamma-thio]triphosphate from guanine-nucleotide-binding regulatory proteins in native cardiac membranes. Regulation by nucleotides and muscarinic acetylcholine receptors

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

G proteins in reverse mode: receptor-mediated GTP release inhibits G protein and effector function

Active G protein-coupled receptors activate heterotrimeric Galphabetagamma proteins by catalyzing the exchange of GDP by GTP at the Galpha subunit. A paradoxical attenuation of G protein-activated inwardly rectifying potassium channels (GIRK) upon stimulation of native cells with high concentrations of agonist is known. However, a deactivation of activated G proteins by active receptors has not been experimentally studied in intact cells. We monitored GIRK currents and G(o) protein activation by means of fluorescence resonance energy transfer (FRET) in parallel. The results suggested that GIRK currents were paradoxically attenuated due to an inactivation of G(o) proteins by active alpha(2A)-adrenergic receptors. To study the mechanisms, G protein activation and receptor-G protein interactions were analyzed as a function of nucleotide type and nucleotide concentrations by means of FRET, while controlling intracellular nucleotides upon permeabilization of the cell membrane. Results suggested a receptor-catalyzed dissociation of GTP from activated heterotrimeric Galphabetagamma. Consequently, nucleotide-free G proteins were sequestrated in heterotrimeric conformation at the active receptor, thus attenuating downstream signaling in an agonist-dependent manner.

Mechanisms of mu opioid receptor/G-protein desensitization in brain by chronic heroin administration

Previous studies have shown that chronic opiate treatment decreases mu opioid-stimulated [35S]GTPgammaS binding in specific brain regions. To extend these findings, the present study investigated DAMGO-stimulated [35S]GTPgammaS binding in membrane homogenates and coronal sections from rats non-contingently administered heroin. Rats were administered saline or increasing doses of heroin i.v. hourly up to 288 mg/kg/day over 40 days. In brain sections, chronic heroin administration decreased DAMGO-stimulated [35S]GTPgammaS binding in medial thalamus and amygdala, with no effect in cingulate cortex or nucleus accumbens. Chronic heroin administration also reduced [35S]GTPgammaS binding stimulated by the principal metabolite of heroin, 6-monoacetylmorphine. In contrast, no significant changes in mu opioid receptor binding were observed in amygdala or thalamus using [3H]DAMGO autoradiography. In membranes from amygdala and thalamus, chronic heroin treatment decreased the maximal effect of DAMGO in stimulating [35S]GTPgammaS binding, with no effect on DAMGO potency. GTPgammaS saturation analysis showed that chronic heroin treatment decreased the Bmax, and increased the K(D), of DAMGO-stimulated [35S]GTPgammaS binding. These data suggest potential mechanisms by which chronic agonist treatment produces opioid receptor/G-protein desensitization in brain.

The [35S]GTPgammaS binding assay: approaches and applications in pharmacology

Receptors of the of seven transmembrane spanning, heterotrimeric G protein coupled family (GPCR) play crucial roles in regulating physiological functions and consequently are targets for the action of many classes of drugs. Activation of receptor by agonist leads to the dissociation of GDP from Galpha of the Galphabetagamma heterotrimer, followed by the binding of GTP to Galpha and subsequent modulation of downstream effectors. The G protein heterotrimer is reformed by GTPase activity of the Galpha subunit, forming Galpha-GDP and so allowing Galpha and Gbetagamma to recombine. The [35S]GTPgammaS assay measures the level of G protein activation following agonist occupation of a GPCR, by determining the binding of the non-hydrolyzable analog [35S]GTPgammaS to Galpha subunits. Thus, the assay measures a functional consequence of receptor occupancy at one of the earliest receptor-mediated events. The assay allows for traditional pharmacological parameters of potency, efficacy and antagonist affinity, with the advantage that agonist measures are not subjected to amplification or other modulation that may occur when analyzing parameters further downstream of the receptor. In general the assay is experimentally more feasible for receptors coupled to the abundant G(i/o) proteins. Nevertheless, [35S]GTPgammaS binding assays are used with GPCRs that couple to the G(s) and G(q) families of G proteins, especially in artificial expression systems, or using receptor-Galpha constructs or immunoprecipitation of [35S]GTPgammaS-labeled Galpha. The relative simplicity of the assay has made it very popular and its use is providing insights into contemporary pharmacological topics including the roles of accessory proteins in signaling, constitutive activity of receptors and agonist specific signaling.

Functional properties of Pfr(Tic)amide and BIBP3226 at human neuropeptide FF2 receptors

The functional characteristics of two putative neuropeptide FF (NPFF) antagonists, BIBP3226 and PFR(Tic)amide, on the human neuropeptide FF receptor subtype 2 (hNPFF2) were investigated. Surprisingly, PFR(Tic)amide was shown to exhibit agonist properties in the [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding assay. The efficacy of PFR(Tic)amide was significantly greater than that of (1DMe)Y8Fa, a stable analog of NPFF, and PFR(Tic)amide can therefore be classified as a 'super-agonist'. BIBP3226 did act as a reversible competitive antagonist on the hNPFF2 receptor. However, high concentrations of BIBP3226 also non-specifically increased [35S]GTP-gammaS binding. The usefulness of BIBP3226 as an antagonist tool on the NPFF receptor is thus limited.

Characterization of mu opioid receptor binding and G protein coupling in rat hypothalamus, spinal cord, and primary afferent neurons during inflammatory pain

Peripheral analgesic effects of opioids are pronounced under inflammatory conditions, e.g., arthritis; however, little is known about adaptive changes of micro opioid receptor binding and G protein coupling in the peripheral versus central nervous system. The present study investigated the effects of inflammation on mu opioid receptor (MOP receptor) binding and G protein coupling of supraspinal, spinal, and peripheral MOP receptors. In addition, MOP receptors were identified in immunohistochemical experiments in dorsal root ganglia (DRG) of inflamed and noninflamed rats. The number of MOP receptor binding sites decreased from hypothalamus (HT) > spinal cord (SC) > DRG. Unilateral Freund's complete adjuvant inflammation of one hindpaw induced a significant up-regulation of MOP receptor sites only in DRG but not in HT or SC. This up-regulation was time-dependent, restricted to the inflamed side, and showed a peak at 24 h. The full-agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO) induced MOP receptor G protein coupling with decreasing efficacies (E(max)) from HT > SC > DRG. Inflammation resulted in significant increases in MOP receptor G protein coupling only in membranes of DRG, but not in HT, SC, or DRG on the contralateral side of inflammation. This suggests that changes in MOP receptor levels are not related to systemically released mediators. These findings show that inflammation causes changes in MOP receptor binding and G protein coupling after DAMGO stimulation selectively in primary afferent neurons but did not cause any adaptive changes of MOP receptor in HT or SC.

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

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

Relationship between rate and extent of G protein activation: comparison between full and partial opioid agonists

Opioid agonists acting at their receptors alter intracellular events by initiating activation of various types of Gi/Go proteins. This can be measured by the binding of the stable GTP analog [(35)S]guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS). In this study agonist efficacy is defined by the degree to which an opioid stimulates the binding of [(35)S]GTPgammaS. This allows for a definition of full and partial agonists; a full agonist causing a greater stimulation of [(35)S]GTPgammaS binding than a partial agonist. The hypothesis that the rate of agonist-stimulated [(35)S]GTPgammaS binding is dependent upon agonist efficacy was tested using membranes from C6 glioma cells expressing mu- or delta-opioid receptors. At maximal concentrations the rate of agonist-stimulated [(35)S]GTPgammaS binding followed the efficacy of mu-agonists in stimulating [(35)S]GTPgammaS binding, i.e., [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin > morphine > meperidine > butorphanol > nalbuphine. At submaximal concentrations of mu- or delta-full agonists the [(35)S]GTPgammaS association rate was also reduced, such that the rate of [(35)S]GTPgammaS binding correlated with the extent of [(35)S]GTPgammaS bound, whether this binding was stimulated by a full agonist or a partial agonist. Agonists also stimulated [(35)S]GTPgammaS dissociation, showing that binding of this stable nucleotide was reversible. Comparison of the delta-agonists [D-Ser(2),Leu(5)]-enkephalin-Thr and (+/-)-4-((alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxylbenzyl)-N,N-diethylbenzamide, a compound with slow dissociation kinetics, showed the measured rate of G protein activation was not influenced by the agonist switching between receptors. The results are consistent with the idea that the active state(s) of the receptor induced by full or partial agonists is the same, but the number of activated receptors determines the rate of G protein activation.

Constitutive activity at serotonin 5-HT(1D) receptors: detection by homologous GTPgammaS versus [(35)S]-GTPgammaS binding isotherms

Although many G-protein-coupled receptors (GPCRs) may display constitutive activity, their detection has, to date, depended on the use of inverse agonists. The present study exploited a novel procedure to investigate constitutive activity at recombinant human (h) serotonin (5-HT) 5-HT(1D) receptors stably expressed in Chinese hamster ovary (CHO) cells. 5-HT modestly stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]-GTPgammaS) binding to CHO-h5-HT(1D) membranes whereas methiothepin and the 5-HT(1B/1D)-selective ligand, SB224,289, exerted robust inhibition of basal [(35)S]-GTPgammaS binding (inverse agonism). These actions were specific inasmuch as they were reversed by the novel, selective 5-HT(1B/1D) ligand, S18127. Constitutive activity was investigated by homologous inhibition of [(35)S]-GTPgammaS binding to CHO-h5-HT(1D) membranes with unlabelled GTPgammaS. Under 'basal' conditions (absence of receptor ligand), biphasic isotherms were observed. Most (80%) [(35)S]-GTPgammaS binding sites were in the high affinity (HA) versus low affinity (LA) component of the isotherms. HA binding was augmented by 5-HT (to 155%; relative to basal values=100%), but decreased by methiothepin (to 23%) and by SB224,289 (to 67%). In contrast, LA binding was not altered. Further, membranes of untransfected CHO cells exhibited only LA binding sites, indicating that the latter are not related to h5-HT(1D) receptor-G-protein coupling. Thus, at 5-HT(1D) receptors expressed in this CHO cell line, HA binding detected in homologous inhibition experiments (GTPgammaS versus [(35)S]-GTPgammaS) under basal conditions provides a measure of constitutive G-protein activation. Thus, it is suggested that for h5-HT(1D) receptors and, possibly, other GPCRs, inverse agonists will be detectable by [(35)S]-GTPgammaS binding if a HA component is present under basal conditions.

G(q/11) and G(i/o) activation profiles in CHO cells expressing human muscarinic acetylcholine receptors: dependence on agonist as well as receptor-subtype

1. Profiles of G protein activation have been assessed using a [35S]-GTPgammaS binding/immunoprecipitation strategy in Chinese hamster ovary cells expressing either M1, M2, M3 or M4 muscarinic acetylcholine (mACh) receptor subtypes, where expression levels of M1 and M3, or M2 and M4 receptors were approximately equal. 2. Maximal [35S]-GTPgammaS binding to G(q/11)alpha stimulated by M1/M3 receptors, or G(i1-3)alpha stimulated by M2/M4 receptors occurred within approximately 2 min of agonist addition. The increases in G(q/11)alpha-[35S]-GTPgammaS binding after M1 and M3 receptor stimulation differed substantially, with M1 receptors causing a 2-3 fold greater increase in [35S]-GTPgammaS binding and requiring 5 fold lower concentrations of methacholine to stimulate a half-maximal response. 3. Comparison of M2 and M4 receptor-mediated G(i1-3)alpha-[35S]-GTPgammaS binding also revealed differences, with M2 receptors causing a greater increase in G(i1-3)alpha activation and requiring 10 fold lower concentrations of methacholine to stimulate a half-maximal response. 4. Comparison of methacholine- and pilocarpine-mediated effects revealed that the latter partial agonist is more effective in activating G(i3)alpha compared to G(i1/2)alpha for both M2 and M4 receptors. More marked agonist/partial agonist differences were observed with respect to M1/M3-mediated stimulations of G(q/11)alpha- and G(i1-3)alpha-[35S]-GTPgammaS binding. Whereas coupling to these Galpha subclasses decreased proportionately for M1 receptor stimulation by these agonists, pilocarpine possesses a greater intrinsic activity at M3 receptors for G(i)alpha versus G(q/11)alpha activation. 5. These data demonstrate that mACh receptor subtype and the nature of the agonist used govern the repertoire of G proteins activated. They also provide insights into how the diversity of coupling can be pharmacologically exploited, and provide a basis for a better understanding of how multiple receptor subtypes can be differentially regulated.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Chronic delta9-tetrahydrocannabinol treatment produces a time-dependent loss of cannabinoid receptors and cannabinoid receptor-activated G proteins in rat brain

Chronic treatment of rats with delta9-tetrahydrocannabinol (delta9-THC) results in tolerance to its acute behavioral effects. In a previous study, 21-day delta9-THC treatment in rats decreased cannabinoid activation of G proteins in brain, as measured by in vitro autoradiography of guanosine-5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding. The present study investigated the time course of changes in cannabinoid-stimulated [35S]GTPgammaS binding and cannabinoid receptor binding in both brain sections and membranes, following daily delta9-THC treatments for 3, 7, 14, and 21 days. Autoradiographic results showed time-dependent decreases in WIN 55212-2-stimulated [35S]GTPgammaS and [3H]WIN 55212-2 binding in cerebellum, hippocampus, caudate-putamen, and globus pallidus, with regional differences in the rate and magnitude of down-regulation and desensitization. Membrane binding assays in these regions showed qualitatively similar decreases in WIN 55212-2-stimulated [35S]GTPgammaS binding and cannabinoid receptor binding (using [3H]SR141716A), and demonstrated that decreases in ligand binding were due to decreases in maximal binding values, and not ligand affinities. These results demonstrated that chronic exposure to delta9-THC produced time-dependent and region-specific down-regulation and desensitization of brain cannabinoid receptors, which may represent underlying biochemical mechanisms of tolerance to cannabinoids.

Restricting the mobility of Gs alpha: impact on receptor and effector coupling

The alpha-subunit of the stimulatory G protein, Gs, has been shown to dissociate from the plasma membrane into the cytosol following activation by G protein-coupled receptors (GPCR) in some experimental systems. This dissociation may involve depalmitoylation of an amino-terminal cysteine residue. However, the functional significance of this dissociation is not known. To investigate the functional consequence of Gs alpha dissociation, we constructed a membrane-tethered Gs alpha (tetGs alpha), expressed it in Sf9 insect cells, and examined its ability to couple with the beta(2) adrenoceptor and to activate adenylyl cyclase. Compared to wild-type Gs alpha, tetGs alpha coupled much more efficiently to the beta 2 adrenoceptor and the D1 dopamine receptor as determined by agonist-stimulated GTP gamma S binding and GTPase activity. The high coupling efficiency was abolished when Gs )alpha was proteolytically cleaved from the membrane tether. The membrane tether did not prevent the coupling of tetGS alpha to adenylyl cyclase. These results demonstrate that regulating the mobility of Gs alpha relative to the plasma membrane, through fatty acylation or perhaps interactions with cytoskeletal proteins, could have a significant impact on receptor-G protein coupling. Furthermore, by enabling the use of more direct measures of receptor-G protein coupling (GTPase activity, GTP gamma S binding), tetGS alpha can facilitate the study for receptor-G protein interactions.

Effects of guanine, inosine, and xanthine nucleotides on beta(2)-adrenergic receptor/G(s) interactions: evidence for multiple receptor conformations

The aim of our study was to examine the effects of different purine nucleotides [GTP, ITP, and xanthosine 5'-triphosphate (XTP)] on receptor/G protein coupling. As a model system, we used a fusion protein of the beta(2)-adrenergic receptor and the alpha subunit of the G protein G(s). GTP was more potent and efficient than ITP and XTP at inhibiting ternary complex formation and supporting adenylyl cyclase (AC) activation. We also studied the effects of several beta(2)-adrenergic receptor ligands on nucleotide hydrolysis and on AC activity in the presence of GTP, ITP, and XTP. The efficacy of agonists at promoting GTP hydrolysis correlated well with the efficacy of agonists for stimulating AC in the presence of GTP. This was, however, not the case for ITP hydrolysis and AC activity in the presence of ITP. The efficacy of ligands at stimulating AC in the presence of XTP differed considerably from the efficacies of ligands in the presence of GTP and ITP, and there was no evidence for receptor-regulated XTP hydrolysis. Our findings support the concept of multiple ligand-specific receptor conformations and demonstrate the usefulness of purine nucleotides as tools to study conformational states of receptors.

Regulation of guanine nucleotide turnover on Gi/Go by agonist-stimulated and spontaneously active muscarinic receptors in cardiac membranes

Muscarinic receptor regulation of guanine nucleotide turnover on Gi/Go proteins in ventricular sarcolemma was investigated. In the absence of a muscarinic receptor (MR) agonist, GTP bound to background sites with a Kapp value of 60 nM and a Bmax of 50 pmol/mg. The addition of the MR agonist, carbachol, further increased GTP binding by 50 pmol/mg to sites with the same Kapp value of 60 nM. Pertussis toxin treatment reduced GTP binding to carbachol-regulated and background binding sites, thus identifying both sites as Gi/Go. The identity of the carbachol-regulated GTP binding sites was further confirmed by demonstrating that carbachol stimulated GTP binding and inhibited adenylyl cyclase with an EC50 value of 200 nM. Background and carbachol-regulated guanine nucleotide binding sites bound GDP with a Kapp value of 150 nM. However, maximal background GDP binding was 50 pmol/mg, whereas maximal carbachol-regulated GDP binding was only 12-15 pmol/mg. In sarcolemma preloaded with [3H]GDP, carbachol-regulated [3H]GDP release was strictly dependent on the presence of guanine nucleotides. The Kapp values for GTP and GDP to support carbachol-regulated [3H]GDP release were 60 nM and 150 nM, respectively. Guanosine 5'-O-(3-thiotriphosphate) (GDPbetaS) facilitated carbachol-regulated [3H]GDP release with a Kapp value of 2 microM. However, GTP was two times more efficacious than GDP or GDPbetaS in facilitating carbachol-regulated [3H]GDP release. Mn2+ also stimulated [3H]GDP release from carbachol-regulated sites by a mechanism not requiring guanine nucleotides. These studies indicate that two pools of muscarinic receptors, carbachol regulated and spontaneously active, regulate guanine nucleotide turnover on pertussis toxin sensitive Gi/Go. These studies further suggest that guanine nucleotide binding provides the signal to stimulate GDP release from receptor activated Gi/Go proteins. A quaternary mechanism involving G-protein interactions may be necessary to promote guanine nucleotide exchange on Gi/Go.

Cannabinoid receptor agonist efficacy for stimulating [35S]GTPgammaS binding to rat cerebellar membranes correlates with agonist-induced decreases in GDP affinity

The relationship between GDP and cannabinoid-stimulated [35S]guanosine-5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding was investigated in rat cerebellar membranes. Kinetic analyses showed that [35S]GTPgammaS binding reached steady-state levels and that the association rate was increased by the agonist WIN 55212-2 proportional to the concentration of GDP. Dissociation of [35S]GTPgammaS occurred with two rates (t1/2 = 7 and 170 min), and WIN 55212-2 increased the proportion of sites exhibiting the faster rate. Without GDP, [35S]GTPgammaS bound to membranes with high and low affinity, and WIN 55212-2 had no effect. With 30 microM GDP, [35S]GTPgammaS bound to low and intermediate affinity sites, and WIN 55212-2 induced high affinity [35S]GTPgammaS binding without affecting low affinity sites. GDP competed for high affinity [35S]GTPgammaS binding with high and intermediate affinity in the absence of WIN 55212-2 and with high and low affinity in the presence of WIN 55212-2. Cannabinoid ligands displayed differential abilities to maximally stimulate [35S]GTPgammaS binding in the presence of GDP. Efficacy differences among ligands increased with increasing GDP concentrations. GDP competition curves revealed that agonists induced low affinity GDP Ki values that were proportional to agonist Emax values, indicating that agonist efficacy is determined by displacement of GDP from G-proteins.

Autoradiographic characterisation of [35S]GTP gamma S binding sites in rat brain

The binding of [35S]GTP gamma S was characterised with autoradiography in rat brain. The binding was saturable, but the rate of dissociation was very slow. Analysis of binding isotherms revealed one class of binding sites with a Kd of 0.8 microM. The specific binding was 98%. Different guanine nucleotides were all able to compete with [35S]GTP gamma S binding. However, no displacement was seen by the ATP-analogue App[NH]p, indicating that [35S]GTP gamma S does not bind to ATP-sites. Autoradiograms showed a highly homogenous distribution of [35S]GTP gamma S binding, in grey as well as in white matter. However, the pattern changed dramatically in the presence of GTP, which, unlike the non-hydrolysable GTP-analogues Gpp[NH]p and GTP gamma S, did not displace [35S]GTP gamma S binding throughout the brain. In white matter areas the binding was potently displaced, while in many grey matter areas, e.g., the striatum, the binding was seen to increase. This GTP-induced increase in [35S]GTP gamma S binding was strongly Mg(2+)-dependent, with an optimum at 10 mM. This, together with the finding that the regional effects of GTP correspond well to previously reported distribution of low Km GTPase, suggest that the levels of binding of [35S]GTP gamma S in the presence of GTP may reflect functional G-protein activity.

Does subunit dissociation necessarily accompany the activation of all heterotrimeric G proteins?

Heterotrimeric (alpha beta gamma) G proteins mediate a variety of signal transduction events in virtually every cell of every eukaryotic organism. The predominant hypothesis is that dissociation of the alpha-subunit from the G beta gamma-subunit complex necessarily accompanies the activation of these proteins, and that the alpha-subunit is primarily responsible for regulating the response of effector molecules. However, there is increasing evidence that both the alpha-subunit and the beta gamma-subunit complex function in regulating effector activity. Furthermore, data for some G proteins suggest that they function as activated heterotrimers rather than as dissociated subunits.

Receptor-stimulated guanine-nucleotide-triphosphate binding to guanine-nucleotide-binding regulatory proteins. Nucleotide exchange and beta-subunit-mediated phosphotransfer reactions

In order to study whether phosphate transfer reactions are involved in the binding of guanine nucleotide triphosphates to guanine-nucleotide-binding regulatory proteins, binding of the GTP analogues, guanosine 5'-[gamma-thio]triphosphate, GTP[S], and guanosine 5'-[beta, gamma-imino]triphosphate, p[NH]ppG, and the regulation of binding by the formyl-peptide-receptor agonist, fMet-Leu-Phe, were studied in membranes of differentiated HL-60 cells. For fMet-Leu-Phe-stimulated binding of either GTP analogue, a competing nucleotide was required. With GDP as the competing nucleotide, initial rates of fMet-Leu-Phe-stimulated binding of GTP[S] and p[NH]ppG were similar for up to approximately 30 s. Thereafter, receptor-stimulated binding of p[NH]ppG rapidly reached equilibrium, whereas the binding of GTP[S] proceeded further. At equipotent concentrations of p[NH]ppG and GTP[S], maximal fMet-Leu-Phe-stimulated binding of GTP[S] was approximately twofold higher than that of p[NH]ppG. Finally, for half-maximal receptor-stimulated binding of GTP[S], approximately fivefold higher concentrations of both Mg2+ and GDP were required than for p[NH]ppG binding. With p[NH]ppG as the competing nucleotide, the extent of receptor-stimulated binding of GTP[S] as well as its Mg2+ requirement and time course were similar to the receptor-stimulated p[NH]ppG binding observed in the presence of GDP. However, with GTP[S] as the competing nucleotide, fMet-Leu-Phe reduced the binding of p[NH]ppG, a reaction further enhanced when GDP was additionally present. Under similar conditions as used in the binding studies, GTP[S] thiophosphorylated a 35-kDa protein, which is most likely a guanine-nucleotide-binding regulatory protein beta subunit [Wieland, T., Nürnberg, B., Ulibarri, I., Kaldenberg-Stasch, S., Schultz, G. & Jakobs, K. H. (1993) J. Biol. Chem. 268, 18111-18118]. The thiophosphorylation state of this protein was regulated by guanine nucleotides, Mg2+ and, most importantly, by activated formyl-peptide receptors. The data thus provide evidence for an essential difference between GTP[S] and p[NH]ppG binding to guanine-nucleotide-binding regulatory proteins and suggest that, in addition to the nucleotide-exchange reaction, a (thio)phosphate-group-transfer process via guanine-nucleotide-binding regulatory protein beta subunits is involved in the receptor-stimulated binding of guanine nucleotide triphosphates to guanine-nucleotide-binding regulatory proteins.

Pharmacological characterization of acetylcholine-stimulated [35S]-GTP gamma S binding mediated by human muscarinic m1-m4 receptors: antagonist studies

1. We have used dose-ratio analysis to estimate functionally the affinity constants (pKb) and Schild slope factors of a range of selective or atypical antagonists at human muscarinic m1-m4 receptors. 2. The functional response was the stimulation by acetylcholine of [35S]-GTP gamma S binding to membranes from Chinese hamster ovary (CHO) cells stably expressing individual receptor subtypes. 3. A novel experimental design and analysis was used which allowed the estimation of affinity and Schild slope factor from a single antagonist inhibition curve, and the results were compared with other methods of analysis, both theoretically valid and invalid. 4. In general, the affinity estimates were very similar to previously reported values obtained in binding studies with animal tissues and cloned human receptors and the Schild slope factors were close to unity. 5. These results demonstrate the validity of the assay and provide no evidence for species differences in antagonist affinity for muscarinic receptor subtypes. 6. The results confirm both the utility of himbacine in distinguishing between m1 and m4 receptors and a previously reported modest m4-selectivity for tropicamide and secoverine. 7. The cholinesterase inhibitor, tacrine (THA), had a potency profile similar to that of gallamine but with less selectivity. Its affinity could not be determined since it had Schild slope factors of about 2 at all subtypes. 8. o-Methoxy-sila-hexocyclium had only a modest selectivity for the m1 subtype.

Receptor-stimulated dissociation of GTP[S] from Gi-proteins in membranes of HL-60 cells

Formyl peptides stimulate binding of the stable GTP analogue, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]), to G-proteins in membranes of myeloid differentiated human leukaemia (HL-60) cells. On the other hand, agonist-activated formyl peptide receptors can also cause rapid and substantial release of GTP[S] bound to HL-60 membrane G-proteins. For fMet-Leu-Phe-stimulated dissociation of labelled GTP[S], an additional guanine nucleotide, in the potency order, unlabelled GTP[S] >> GTP >> guanosine 5'-[beta,gamma-imino]triphosphate > or = guanosine 5'-O-[beta-thio]diphosphate > or = GDP > GMP = ATP (no effects at 1 mM), was absolutely necessary. While with unlabelled GTP[S] and GTP similar concentrations were required for control and fMet-Leu-Phe-stimulated release, about 50-100-fold higher concentrations of the other nucleotides were necessary for agonist-stimulated than for basal release of bound GTP[S]. The receptor action appeared to be catalytic, required Mg2+ and was pertussis toxin sensitive. The data indicate that binding of GTP[S] to HL-60 membrane G-proteins is reversible and that agonist-activated formyl peptide receptors can interact, either directly or indirectly, with GTP[S]-liganded Gi-proteins, resulting in release of bound GTP[S].

Activation of solubilized G-proteins by muscarinic acetylcholine receptors

Binding of GTP and its analogue, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]) to G-proteins, and release of GTP[S] from G-proteins are stimulated by muscarinic acetylcholine (mACh) receptors in intact cardiac membranes. Upon solubilization of receptors and G-proteins by membrane extraction with the detergent, 3-[(cholamidopropyl)dimethylammonio]-1-propanesulphonate, followed by sucrose density gradient centrifugation, agonist-liganded mACh receptors stimulated binding of GTP[S] and hydrolysis of GTP by G-proteins with similar requirements as in intact membranes. One soluble agonist-activated mACh receptor induced binding of GTP[S] to several (about seven) soluble G-proteins. In contrast to intact membranes, however, agonist activation of mACh receptors did not induce release of GTP[S] from solubilized G-proteins. The data presented indicate that mACh receptors can interact with and efficiently activate G-proteins even in solution, whereas the possible interaction of receptors with GTP[S]-liganded G-proteins observed in intact membranes is lost upon solubilization of these components.