Interaction of atrial muscarinic receptors with three kinds of GTP-binding proteins

Structural and dynamic insights into supra-physiological activation and allosteric modulation of a muscarinic acetylcholine receptor

The M2 muscarinic receptor (M2R) is a prototypical G-protein-coupled receptor (GPCR) that serves as a model system for understanding GPCR regulation by both orthosteric and allosteric ligands. Here, we investigate the mechanisms governing M2R signaling versatility using cryo-electron microscopy (cryo-EM) and NMR spectroscopy, focusing on the physiological agonist acetylcholine and a supra-physiological agonist iperoxo, as well as a positive allosteric modulator LY2119620. These studies reveal that acetylcholine stabilizes a more heterogeneous M2R-G-protein complex than iperoxo, where two conformers with distinctive G-protein orientations were determined. We find that LY2119620 increases the affinity for both agonists, but differentially modulates agonists efficacy in G-protein and β-arrestin pathways. Structural and spectroscopic analysis suggest that LY211620 stabilizes distinct intracellular conformational ensembles from agonist-bound M2R, which may enhance β-arrestin recruitment while impairing G-protein activation. These results highlight the role of conformational dynamics in the complex signaling behavior of GPCRs, and could facilitate design of better drugs.

Interaction of the muscarinic acetylcholine receptor M₂ subtype with G protein Gα(i/o) isotypes and Gβγ subunits as studied with the maltose-binding protein-M₂-Gα(i/o) fusion proteins expressed in Escherichia coli

We expressed the fusion proteins of the muscarinic acetylcholine receptor M2 subtype (M2 receptor) with a maltose-binding protein (MBP) and various G protein α subunits (Gα(i1-i3/o)) at its N- and C-terminals, respectively (MBP-M2-Gα(i/o)), in Escherichia coli, and examined the effect of G protein βγ subunits (Gβγ) on the receptor-Gα interaction as assessed by agonist- and GDP-dependent [(35)S]GTPγS binding of the fusion proteins. We found that (i) Gβγ promoted both the agonist-dependent and -independent [(35)S]GTPγS binding with little effect on the guanine nucleotide-sensitive high-affinity agonist binding, (ii) the specific [(35)S]GTPγS binding activity was much greater for MBP-M2-Gα(oA) than for MBP-M2-Gα(i1-i3) in the absence of Gβγ, whereas Gβγ preferentially promoted the agonist-dependent decrease in the affinity for GDP of MBP-M2-Gα(i1-i3) rather than of MBP-M2-Gα(oA), and (iii) the proportion of agonist-dependent [(35)S]GTPγS binding was roughly 50% irrespective of species of Gα and the presence or absence of Gβγ. These results demonstrate that receptor-Gα fusion proteins expressed in E. coli could be useful for studies of receptor-G interaction.

Short-term desensitization of muscarinic K+ current in the heart

Acetylcholine (ACh) rapidly increases cardiac K(+) currents (IKACh) by activating muscarinic K(+) (KACh) channels followed by a gradual amplitude decrease within seconds. This phenomenon is called short-term desensitization and its precise mechanism and physiological role are still unclear. We constructed a mathematical model for IKACh to examine the conditions required to reconstitute short-term desensitization. Two conditions were crucial: two distinct muscarinic receptors (m2Rs) with different affinities for ACh, which conferred an IKACh response over a wide range of ACh concentrations, and two distinct KACh channels with different affinities for the G-protein βγ subunits, which contributed to reconstitution of the temporal behavior of IKACh. Under these conditions, the model quantitatively reproduced several unique properties of short-term desensitization observed in myocytes: 1), the peak and quasi-steady states with 0.01-100 μM [ACh]; 2), effects of ACh preperfusion; and 3), recovery from short-term desensitization. In the presence of 10 μM ACh, the IKACh model conferred recurring spontaneous firing after asystole of 8.9 s and 10.7 s for the Demir and Kurata sinoatrial node models, respectively. Therefore, two different populations of KACh channels and m2Rs may participate in short-term desensitization of IKACh in native myocytes, and may be responsible for vagal escape at nodal cells.

Molecular properties of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors, which comprise five subtypes (M1-M5 receptors), are expressed in both the CNS and PNS (particularly the target organs of parasympathetic neurons). M1-M5 receptors are integral membrane proteins with seven transmembrane segments, bind with acetylcholine (ACh) in the extracellular phase, and thereafter interact with and activate GTP-binding regulatory proteins (G proteins) in the intracellular phase: M1, M3, and M5 receptors interact with Gq-type G proteins, and M2 and M4 receptors with Gi/Go-type G proteins. Activated G proteins initiate a number of intracellular signal transduction systems. Agonist-bound muscarinic receptors are phosphorylated by G protein-coupled receptor kinases, which initiate their desensitization through uncoupling from G proteins, receptor internalization, and receptor breakdown (down regulation). Recently the crystal structures of M2 and M3 receptors were determined and are expected to contribute to the development of drugs targeted to muscarinic receptors. This paper summarizes the molecular properties of muscarinic receptors with reference to the historical background and bias to studies performed in our laboratories.

Recovery of oligomers and cooperativity when monomers of the M2 muscarinic cholinergic receptor are reconstituted into phospholipid vesicles

FLAG- and HA-tagged M2 muscarinic receptors from coinfected Sf9 cells have been purified in digitonin-cholate and reconstituted into phospholipid vesicles. The purified receptor was predominantly monomeric: it showed no detectable coimmunoprecipitation; it migrated as a monomer during electrophoresis before or after cross-linking with bis(sulfosuccinimidyl)suberate; and it bound agonists and antagonists in a manner indicative of identical and mutually independent sites. Receptor cross-linked after reconstitution or after reconstitution and subsequent solubilization in digitonin-cholate migrated almost exclusively as a tetramer. The binding properties of the reconstituted receptor mimicked those reported previously for cardiac muscarinic receptors. The apparent capacity for N-[3H]methylscopolamine (NMS) was only 60% of that for [3H]quinuclidinylbenzilate (QNB), yet binding at saturating concentrations of [3H]QNB was inhibited fully and in a noncompetitive manner at comparatively low concentrations of unlabeled NMS. Reconstitution of the receptor with a saturating quantity of functional G proteins led to the appearance of three classes of sites for the agonist oxotremorine-M in assays with [3H]QNB; GMP-PNP caused an apparent interconversion from highest to lowest affinity and the concomitant emergence of a fourth class of intermediate affinity. All of the data can be described quantitatively in terms of cooperativity among four interacting sites, presumably within a tetramer; the effect of GMP-PNP can be accommodated as a shift in the distribution of tetramers between two states that differ in their cooperative properties. Monomers of the M2 receptor therefore can be assembled into tetramers with binding properties that closely resemble those of the muscarinic receptor in myocardial preparations.

Activation of nematode G protein GOA-1 by the human muscarinic acetylcholine receptor M2 subtype. Functional coupling of G-protein-coupled receptor and G protein originated from evolutionarily distant animals

Signal transduction mediated by heterotrimeric G proteins regulates a wide variety of physiological functions. We are interested in the manipulation of G-protein-mediating signal transduction using G-protein-coupled receptors, which are derived from evolutionarily distant organisms and recognize unique ligands. As a model, we tested the functionally coupling GOA-1, G alpha(i/o) ortholog in the nematode Caenorhabditis elegans, with the human muscarinic acetylcholine receptor M2 subtype (M2), which is one of the mammalian G alpha(i/o)-coupled receptors. GOA-1 and M2 were prepared as a fusion protein using a baculovirus expression system. The affinity of the fusion protein for GDP was decreased by addition of a muscarinic agonist, carbamylcholine and the guanosine 5'-[3-O-thio]triphosphate ([35S]GTPgammaS) binding was increased with an increase in the carbamylcholine concentrations in a dose-dependent manner. These effects evoked by carbamylcholine were completely abolished by a full antagonist, atropine. In addition, the affinity for carbamylcholine decreased under the presence of GTP as reported for M2-G alpha(i/o) coupling. These results indicate that the M2 activates GOA-1 as well as G alpha(i/o).

Regulation of receptor-coupling to (multiple) G proteins. A challenge for basic research and drug discovery

G protein-coupled receptors induce intracellular signals via interaction of with cytosolic/peripheral membrane proteins, mainly G proteins. There has been much debate about the mode of interaction between the receptors, G proteins and effectors, their mobility and the ways of determining the specificity of interaction. Additional complexity has been added to system upon the discovery of i) coupling of single receptors to several G proteins and ii) active direction of this by different ligands (stimulus trafficking). These data suggest that the most primary unit in the signal transduction is the receptor complexed with a specific G protein, making the investigation of the mechanism of receptor-G protein selection and interaction even more important. In this review, I will summarize the general knowledge of receptor interaction with G proteins and effectors and the ways of investigating this.

Cooperativity and oligomeric status of cardiac muscarinic cholinergic receptors

Muscarinic cholinergic receptors can appear to be more numerous when labeled by [(3)H]quinuclidinylbenzilate (QNB) than by N-[(3)H]methylscopolamine (NMS). The nature of the implied heterogeneity has been studied with M(2) receptors in detergent-solubilized extracts of porcine atria. The relative capacity for [(3)H]NMS and [(3)H]QNB was about 1 in digitonin-cholate, 0.56 in cholate-NaCl, and 0.44 in Lubrol-PX. Adding digitonin to extracts in cholate-NaCl increased the absolute capacity for both radioligands, and the relative capacity increased to near 1. The latency cannot be attributed to a chemically impure radioligand, instability of the receptor, an irreversible effect of NMS, or a failure to reach equilibrium. Binding at near-saturating concentrations of [(3)H]QNB in cholate-NaCl or Lubrol-PX was blocked fully by unlabeled NMS, which therefore appeared to inhibit noncompetitively at sites inaccessible to radiolabeled NMS. Such an effect is inconsistent with the notion of functionally distinct, noninterconverting, and mutually independent sites. Both the noncompetitive effect of NMS on [(3)H]QNB and the shortfall in capacity for [(3)H]NMS can be described quantitatively in terms of cooperative interactions within a receptor that is at least tetravalent; no comparable agreement is possible with a receptor that is only di- or trivalent. The M(2) muscarinic receptor therefore appears to comprise at least four interacting sites, presumably within a tetramer or larger array, and ligands appear to bind in a cooperative manner under at least some conditions.

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.

Effects of an agonist, allosteric modulator, and antagonist on guanosine-gamma-[35S]thiotriphosphate binding to liposomes with varying muscarinic receptor/Go protein stoichiometry

We investigated whether alcuronium, an allosteric modulator of muscarinic acetylcholine receptors, can induce receptor-mediated activation of Go proteins in liposomal membranes incorporating purified M2 receptors and Go proteins and whether its action is affected by the receptor/Go protein (R/Go) ratio. The binding of guanosine-gamma-[35S]thiotriphosphate ([35S]GTPgammaS) served as the indicator of G protein activation. It was stimulated by empty receptors at high receptor densities, and the dose-response curve was shifted to the left by the agonist carbachol and to the right by the antagonist atropine. At an R/Go ratio of 300:100, the rate of [35S]GTPgammaS binding was the same in the presence or absence of 0. 1 mM carbachol. Alcuronium increased the binding of [35S]GTPgammaS at R/Go ratios of <3:100 and diminished it at R/Go ratios of >10:100, similar to previous observations on intact cells expressing muscarinic receptors at different densities. The apparent biphasicity of alcuronium action indicates that the allosteric modulator has at least two effects on muscarinic receptor/G protein interaction but its mechanistic basis is unclear. The "active state" of muscarinic receptors induced by alcuronium probably is different from that induced by carbachol. Changes in the densities of receptors and Go proteins had little effect on the kinetics of [35S]GTPgammaS binding and on receptor affinity for carbachol, provided the R/Go ratio was kept constant. This suggests that the receptors and G proteins are located in microdomains in which their concentrations remain constant, despite variations in the amounts of lipidic membranes in the system.

Rethinking receptor-G protein-effector interactions

Hundreds of different receptors regulate the activity of effector proteins with the assistance of heterotrimeric guanine nucleotide-binding proteins (G proteins). The hypothesis that G protein-coupled receptors (R) govern their effectors (E) indirectly via a shuttling mechanism involving the exchange of heterotrimeric G proteins (G[alpha betagamma]) or parts thereof (G[alpha], G[betagamma]) between ephemeral R-G and G-E complexes has become firmly established. While there is no direct evidence for the cyclical formation and dissociation of these complexes during signalling, experimental changes in second messenger production, GTPase activity, and the binding characteristics of agonists, antagonists, and guanine nucleotides commonly are believed to reflect perturbations in the equilibria between G protein and the other two components. However, a growing body of evidence seems to argue against the shuttling model. The random, transient association of G protein and receptor is largely inconsistent with the binding of agonists to receptors and the allosteric regulation of that binding by guanine nucleotides. Also, the prevailing paradigm does not readily account for receptor-effector coupling specificity, as the promiscuous interaction of most G proteins with both receptors and effectors in vitro is at odds with the general failure of G proteins to be shared among ostensibly congruous signal transduction pathways in vivo. The latter paradox would be obviated by the simultaneous interaction of G protein with both receptor and effector. Indeed, various findings indicate that R-G-E complexes do occur. How and where in the cell such complexes are assembled and disassembled should provide important clues to the true mechanism of G protein-linked transduction.

Cardiac muscarinic receptors. Cooperativity as the basis for multiple states of affinity

Cooperativity has been investigated as the mechanistic basis for effects observed with cardiac muscarinic receptors in washed membranes from Syrian hamsters. Specifically, N-[3H]methylscopolamine labeled only 66-75% of the sites labeled by [3H]quinuclidinylbenzilate at apparently saturating concentrations of each radioligand. Also, receptors labeled by N-[3H]methylscopolamine revealed three states of affinity for agonists, both in native membranes and following irreversible blockade of about 80% of the sites by propylbenzilylcholine mustard; in both preparations, guanylylimidodiphosphate (GMP-PNP) effected an apparent interconversion of sites from higher to lower affinity for agonists and from lower to higher affinity for the antagonist. Excellent and mechanistically consistent descriptions of the data were obtained in terms of a model comprising cooperative and noncooperative forms of the receptor; the former was described by a variant of the Adair equation, and the latter was included to account for low-affinity sites that survived treatment with the mustard. If differences in apparent capacity derive from negative cooperativity in the binding of N-[3H]methylscopolamine, the cooperative form of the receptor was at least trivalent in native membranes; otherwise, constraints imposed by the effects of GMP-PNP at the concentrations of radioligand used in the assays dictate that the cooperative form of the receptor was at least tetravalent. In contrast, a divalent receptor is sufficient with the data from alkylated membranes, in accord with the reduced likelihood of interactions between functional sites within an oligomeric array. A model is presented wherein the receptor interconverts spontaneously between two or more states differing in their cooperative properties. The effects of GMP-PNP can be rationalized as a shift in the equilibrium between the different states.

Cardiac muscarinic receptors. Relationship between the G protein and multiple states of affinity

An expanded version of the mobile receptor model has been assessed in studies on the binding of N-[3H]methylscopolamine and [35S]GTPgammaS to cardiac muscarinic receptors and their attendant G proteins in ventricular membranes from hamster. The model comprises two pools of receptor, one of which lacks G proteins, and a heterogeneous population of G proteins that compete for the receptor within the G protein-containing pool. To guide the formulation of the model itself and to define the various parameters, data were combined from assays performed under various conditions with native membranes and following irreversible blockade of about 80% of the receptors with propylbenzilylcholine mustard. Multiple G proteins are indicated primarily by multiple states of affinity evident in the dose-dependent effect of guanyl nucleotides on the binding of carbachol; G protein-free receptors are indicated by sites of low affinity for carbachol that survive treatment with the mustard. The expanded model generally succeeds where more frugal schemes have been inadequate, but it nevertheless fails to yield a mechanistically consistent description of the data. Guanyl nucleotides and partial alkylation do not affect the inhibitory potency of carbachol in a manner consistent with their supposed effect on the equilibrium between uncoupled and G protein-coupled receptors. As inferred from the model, G proteins are lost upon alkylation of the receptor, and their numbers are regulated by guanyl nucleotides. Parameters estimated via N-[3H]methylscopolamine are wholly inconsistent with the same parameters estimated via [35S]GTPgammaS. The failure of the model suggests that multiple states of affinity may not arise from a ligand-regulated equilibrium between free receptors and G proteins on the one hand and one or more RG complexes on the other.

Cooperativity manifest in the binding properties of purified cardiac muscarinic receptors

Muscarinic receptors were solubilized from porcine atria in digitonin-cholate and were purified by chromatography on DEAE-Sepharose and 3-(2'-aminobenzhydryloxy)tropane-Sepharose. The product identified on Western blots migrated with an apparent molecular mass of 60-75 kDa, with additional bands indicative of homotrimers (190 kDa) and homotetramers (240 kDa). Receptor eluted from the affinity column was accompanied by a mixture of guanyl nucleotide-binding proteins (G-proteins) identified on Western blots as Gi1/2, G(o), Gq/11, and Gs (preparation M2G); the G-proteins were largely removed by further processing on hydroxyapatite (preparation M2). Solubilized purified receptors bound muscarinic ligands in an apparently cooperative manner. In studies at equilibrium, the antagonists [3H]AF-DX 384, N-[3H]methylscopolamine (NMS), and [3H]quinuclidinylbenzilate (QNB) revealed Hill coefficients between about 0.8 and 1.2. Also, the apparent capacity for [3H]QNB exceeded that for [3H]AF-DX 384 and [3H]NMS by about 1.5-fold in M2 and by 2-fold in M2G. Binding to M2G at high concentrations of [3H]QNB was fully inhibited by unlabeled NMS, which therefore affected sites not labeled at similar concentrations of [3H]NMS. Oxotremorine-M displayed a biphasic inhibitory effect on the binding of [3H]AF-DX 384 in M2 and M2G, suggesting that multiple states of affinity are intrinsic to the receptor; 5'-guanylylimidodiphosphate was without appreciable effect in M2 but resulted in a bell-shaped binding profile for the agonist in M2G. All of the data can be described in terms of cooperative interactions within a receptor that is at least tetravalent and presumably an oligomer. In the context of the model, copurifying G-proteins and guanyl nucleotides serve to regulate the degree of cooperativity between successive equivalents of muscarinic ligands.

Endogenous nitric oxide signalling system and the cardiac muscarinic acetylcholine receptor-inotropic response

1. In this paper we have determined the different signalling pathways involved in muscarinic acetylcholine receptor (AChR)-dependent inhibition of contractility in rat isolated atria. 2. Carbachol stimulation of M2 muscarinic AChRs exerts a negative inotropic response, activation of phosphoinositide turnover, stimulation of nitric oxide synthase and increased production of cyclic GMP. 3. Inhibitors of phospholipase C, protein kinase C, calcium/calmodulin, nitric oxide synthase and guanylate cyclase, shifted the dose-response curve of carbachol on contractility to the right. These inhibitors also attenuated the muscarinic receptor-dependent increase in cyclic GMP and activation of nitric oxide synthase. In addition, sodium nitroprusside, isosorbide, or 8-bromo cyclic GMP, induced a negative inotropic effect, increased cyclic GMP and activated nitric oxide synthase. 4. These results suggest that carbachol activation of M2 AChRs, exerts a negative inotropic effect associated with increased production of nitric oxide and cyclic GMP. The mechanism appears to occur secondarily to stimulation of phosphoinositides turnover via phospholipase C activation. This in turn, triggers cascade reactions involving calcium/calmodulin and protein kinase C, leading to activation of nitric oxide synthase and soluble guanylate cyclase.

Activation of a GTP-binding protein and a GTP-binding-protein-coupled receptor kinase (beta-adrenergic-receptor kinase-1) by a muscarinic receptor m2 mutant lacking phosphorylation sites

A mutant of the human muscarinic acetylcholine receptor m2 subtype (m2 receptor), lacking a large part of the third intracellular loop, was expressed and purified using the baculovirus/insect cell culture system. The mutant was not phosphorylated by beta-adrenergic-receptor kinase, as expected from the previous assignment of phosphorylation sites to the central part of the third intracellular loop. However, the m2 receptor mutant was capable of stimulating beta-adrenergic-receptor-kinase-1-mediated phosphorylation of a glutathione S-transferase fusion protein containing the m2 phosphorylation sites in an agonist-dependent manner. Both mutant and wild-type m2 receptors reconstituted with the guanine-nucleotide-binding regulatory proteins (G protein), G(o) and G(i)2, displayed guanine-nucleotide-sensitive high-affinity agonist binding, as assessed by displacement of [3H]quinuclidinyl-benzilate binding with carbamoylcholine, and both stimulated guanosine 5'-3-O-[35S]thiotriphosphate ([35S]GTP[S]) binding in the presence of carbamoylcholine and GDP. The Ki values of carbamoylcholine effects on [3H]quinuclidinyl-benzilate binding were indistinguishable for the mutant and wild-type m2 receptors. Moreover, the phosphorylation of the wild-type m2 receptor by beta-adrenergic-receptor kinase-1 did not affect m2 interaction with G proteins as assessed by the binding of [3H]quinuclidinyl benzilate or [35S]GTP[S]. These results indicate that (a) the m2 receptor serves both as an activator and as a substrate of beta-adrenergic-receptor kinase, and (b) a large part of the third intracellular loop of the m2 receptor does not contribute to interaction with G proteins and its phosphorylation by beta-adrenergic-receptor kinase does not uncouple the receptor and G proteins in reconstituted lipid vesicles.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Effects of adenyl nucleotides and carbachol on cooperative interactions among G proteins

Muscarinic agonists and adenyl nucleotides are noncompetitive modulators of sites labeled by [35S]GTP gamma S in washed cardiac membranes from Syrian golden hamsters. Specific binding of the radioligand and its inhibition by either GTP gamma S or GDP reveals three states of affinity for guanyl nucleotides. In the absence of adenyl nucleotide, carbachol promotes an apparent interconversion of sites from higher to lower affinity for GDP; the effect recalls that of guanyl nucleotides on the binding of agonists to muscarinic receptors. In the presence of 0.1 mM ATP gamma S, the binding of [35S]GTP gamma S is increased at concentrations up to about 50 nM and decreased at higher concentrations. At a radioligand concentration of 160 pM, binding exhibits a bell-shaped dependence on the concentration of both ATP gamma S and AMP-PNP; with ADP and ATP, there is a second increase in bound [35S]GTP gamma S at the highest concentrations of adenyl nucleotide. ATP gamma S and AMP-PNP also modulate the effect of GDP, which itself emerges as a cooperative process: that is, binding of the radioligand in the presence of AMP-PNP exhibits a bell-shaped dependence on the concentration of GDP; moreover, the GDP-dependent increase in bound [35S]GTP gamma S is enhanced by carbachol. The interactions among GDP, GTP gamma S, and carbachol can be rationalized quantitatively in terms of a cooperative model involving two sites tentatively identified as G proteins. Both GTP gamma S and GDP exhibit negative homotropic cooperativity; carbachol enhances the homotropic cooperativity of GDP and induces or enhances positive heterotropic cooperativity between GDP and [35S]GTP gamma S. An analogous mechanism may underlie the guanyl nucleotide-dependent binding of agonists to muscarinic receptors. The data suggest that the binding properties of G proteins and their associated receptors reflect cooperative effects within heterooligomeric arrays; agonist-induced changes in cooperativity may facilitate the exchange of GTP for bound GDP and thereby constitute the mechanism of G protein activation in vivo.

Differential effects of paraoxon on the M3 muscarinic receptor and its effector system in rat submaxillary gland cells

The effects of the organophosphorus anticholinesterase paraoxon on the binding of radioactive ligands to the M3 subtype of the muscarinic receptor and receptor-coupled synthesis of second messengers in intact rat submaxillary gland (SMG) cells were investigated. The binding of [3H]quinuclidinyl benzilate ([3H]QNB) was most sensitive to atropine and the M3-specific antagonist 4-DAMP followed by pirenzepine and least sensitive to the cardioselective M2 antagonist AFDX116. This, and the binding characteristics of [3H]4-DAMP, confirmed that the muscarinic receptors in this preparation are of the M3 subtype. Activation of these muscarinic receptors by carbamylcholine (CBC) produced both stimulation of phosphoinositide (PI) hydrolysis and inhibition of cAMP synthesis, suggesting that this receptor subtype couples to both effector systems. Paraoxon (100 microM) reduced Bmax of [3H]4-DAMP binding from 27 +/- 4 to 13 +/- 3 fmol/mg protein with nonsignificant change in affinity, suggesting noncompetitive inhibition of binding by paraoxon. Like the agonist CBC, paraoxon inhibited the forskolin-induced cAMP formation in SMG cells with an EC50 of 200 nM, but paraoxon was greater than 500 fold more potent than CBC. However, while the inhibition by CBC was counteracted by 2 microM atropine, that by paraoxon was unaffected by up to 100 microM atropine. It suggested that this effect of paraoxon was not via binding to the muscarinic receptor. Paraoxon did not affect beta-adrenoreceptor function in the preparation, since it did not affect the 10 microM isoproterenol-induced cAMP synthesis, which was inhibited totally by 10 microM propranolol and partially by CBC. Paraoxon had a small but significant effect on CBC-stimulated PI metabolism in the SMG cells.(ABSTRACT TRUNCATED AT 250 WORDS)