Inhibition of catalytic unit of adenylate cyclase and activation of GTPase of Ni protein by beta gamma-subunits of GTP-binding proteins

Mechanism and properties of inhibition of purified rat brain adenylate cyclase by G protein beta gamma-subunits

The mode of the inhibition of purified rat brain adenylate cyclase by the beta gamma-subunits of G protein (beta gamma) was studied. These subunits inhibited the catalytic activity of the cyclase with the maximal inhibition of 85% and the half-maximal inhibition at about 0.7 nM beta gamma. The complex of beta gamma and adenylate cyclase isolated by density gradient centrifugation contained 1.8-2.0 mol beta gamma per mol of the cyclase when beta gamma was assayed by immunoblotting and by its inhibitory activity on adenylate cyclase. However, the beta gamma concentration-inhibition curves suggest that one of the two beta gamma molecules bound may be essential for the inhibition. The role for the second beta gamma molecule is unknown. As a tentative estimate, 70% of the adenylate cyclase activity remained inhibited by beta gamma when the complex was isolated. The inhibition was not dependent on G alpha s or calmodulin. Although purified adenylate cyclase contained a protein (0.06-0.08 mol/mol of adenylate cyclase) that reacted with anti-G alpha s antibody, this protein was not liberated from the cyclase when it formed a complex with beta gamma. In addition, guanine nucleotide analogs little affected the cyclase activity or the inhibition by beta gamma. The inhibition by beta gamma was reversed by the dilution of the complex, and the following re-addition of beta gamma suppressed the enzyme activity to about 15% of the initial activity again. These findings provide strong evidence that beta gamma inhibits adenylate cyclase directly and reversibly through the formation of the complex.

Coupling of muscarinic m1, m2 and m3 acetylcholine receptors, expressed in Chinese hamster ovary cells, to pertussis toxin-sensitive/insensitive guanine nucleotide-binding proteins

Chinese hamster ovary (CHO) cells expressing recombinant human m1 (CHO-m1 cells), m2 (CHO-m2 cells), or m3 (CHO-m3 cells) muscarinic receptors were characterised pharmacologically with [3H]N-methylscopolamine. Agonist-stimulated coupling of these receptors with guanine nucleotide-binding proteins (G proteins) was measured by guanine nucleotide- and pertussis toxin-modification of carbachol competition-binding curves, and pertussis toxin-sensitivity of agonist-stimulated [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S) binding, in membrane preparations of the CHO cell clones. High affinity agonist binding and agonist-stimulated [35S]GTP gamma S binding was abolished in CHO-m2 cell membranes (expressing 99 +/- 25 fmol of [3H]N-methylscopolamine binding sites/mg protein) after pertussis toxin pretreatment of cells, suggesting that muscarinic m2 receptors expressed in these cell membranes couple predominantly with pertussis toxin-sensitive G proteins. CHO-m1 (713 +/- 102 fmol/mg protein) and CHO-m3 (1212 +/- 279 fmol/mg protein) cell membranes produced smaller elevations in agonist-stimulated [35S]GTP gamma S binding considering the higher receptor levels, compared with CHO-m2 cell membranes. Pertussis toxin pretreatment of these clones also resulted in a significant attenuation of agonist-stimulated [35S]GTP gamma S binding suggesting that, under these experimental conditions, muscarinic m1 and m3 receptors can couple with both pertussis toxin-sensitive and pertussis toxin-insensitive G proteins. Guanine nucleotide-modification of agonist binding in CHO-m1 and CHO-m3 cell membranes was comparatively smaller than in CHO-m2 cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered adenylyl cyclase activities and G-protein abnormalities in portal hypertensive rabbits

Portal hypertension (PHT) is characterized by splanchnic hyperemia due to a reduction in mesenteric vascular resistance. We hypothesized that alterations in the activity of a guanine-nucleotide regulatory protein (G-protein) might be partially responsible for the marked circulatory disturbances observed in PHT. We, therefore, determined alterations in adenylyl cyclase/cAMP system in prehepatic portal hypertensive rabbits and correlated these changes to the activity of a G-protein. Basal and G-protein-stimulated adenylyl cyclase activities were lower in the PHT superior mesenteric artery (22-26%) and thoracic aorta (31-46%) membranes, but higher (178-321%) in portal vein. The functional activity of Gi alpha proteins (pertussis toxin-catalyzed ADP-dependent ribosylation) increased in the PHT superior mesenteric artery and thoracic aorta, but decreased in portal vein. Immunodetection revealed an increase in the Gi alpha protein subunits (Gi alpha 1/Gi alpha 2 and Gi alpha 3/Go alpha) in PHT thoracic aorta, without any change in Gs alpha proteins; and a decrease in the amount of Gi alpha proteins in PHT portal vein. There was no change in the amount of Gs alpha/Gi alpha in the PHT superior mesenteric artery. We conclude the hemodynamic alterations of PHT are associated with intrinsic alterations in G-protein-enzyme effector systems. These alterations are vessels specific and suggest a possible unique global derangement underlying the vasculopathy of PHT.

Central alpha 2-adrenoceptor-mediated inhibition of gastric motility in rats

We investigated the subtype of alpha-adrenoceptors participating in central noradrenergic inhibition of gastric motility in urethane-anesthetized rats. Noradrenaline at 10 nmole administered intracisternally (i.c.) significantly decreased gastric motility. Yohimbine at 10 nmole, i.c., but not the same dose of prazosin, abolished this noradrenaline-induced decrease in gastric motility. Clonidine at 10 nmole, i.c., but not phenylephrine at 20 nmole, significantly decreased gastric motility. These results suggest that alpha 2-adrenoceptor-mediated mechanisms in the brain stem are involved in noradrenergic inhibitory regulation of gastric motility.

The GppNHp-activated adenylyl cyclase complex from turkey erythrocyte membranes can be isolated with its beta gamma subunits

The adenylyl cyclase complex, derived from turkey erythrocyte membranes, was activated using guanosine 5'-[beta, gamma-imido]triphosphate (Gpp[NH]p) and separated under low-detergent and low-salt conditions using conventional molecular-sieve chromatography followed by high-pressure ion-exchange and molecular-sieve chromatography. Although the complex remains activated with Gpp[NH]p throughout the isolation, the beta gamma subunits copurify with the cyclase. The stoichiometry of the cyclase to the alpha subunit of the stimulatory guanosine-nucleotide-binding regulatory protein (alpha s) to the beta subunit is close to unity, demonstrating that the beta gamma subunits do not dissociate from the Gs.cyclase complex (Gs, guanosine-nucleotide-binding regulatory protein) upon activation of the enzyme. If the final purification step was performed at high-salt concentrations, the beta gamma subunits could be separated from the alpha s.cyclase complex. Previously reported results on bovine brain cyclase also showed that the Gs.cyclase complex remains intact subsequent to activation by hormone and Gpp[NH]p [Marbach, I., Bar-Sinai, A., Minich, M. and Levitzki, A. (1990) J. Biol. Chem. 265, 9999-10,004]. These results, using adenylyl cyclase from two different sources, support our previous kinetic experiments which first suggested that beta gamma subunits are not released from Gs upon cyclase activation. We, therefore, argue that the mode of adenylyl cyclase inhibition by the inhibitory guanosine-nucleotide-binding regulatory protein cannot be via shifting the alpha s to beta gamma equilibrium as is commonly believed, and an alternate hypothesis is proposed.

Signal amplification in HL-60 granulocytes. Evidence that the chemotactic peptide receptor catalytically activates guanine-nucleotide-binding regulatory proteins in native plasma membranes

Receptors for the chemotactic peptide fMet-Leu-Phe (fMet, N-formylmethionine) are present in membranes of myeloid differentiated human leukemia (HL-60) cells and stimulate phospholipase C via a pertussis-toxin-sensitive guanine-nucleotide-binding regulatory protein(s) [G-protein(s)]. We have developed methods for the assessment of formyl-peptide-receptor-stimulated binding of radiolabeled guanosine 5'-[gamma-thio]triphosphate ([35S]GTP[S]) to native HL-60 membranes. Agonist stimulation of [35S]GTP[S] association with the membrane was minimal (less than or equal to 20%) when GTP[S] was the sole nucleotide present in the incubation medium. In contrast, receptor activation led to a marked (up to sixfold) stimulation of [35S]GTP[S] binding when GDP or GTP were present in high (greater than 100-fold) excess of [35S]GTP[S]. The increase in [35S]GTP[S] binding caused by the chemotactic agonist was strictly dependent on the presence of Mg2+ and was significantly increased by Na+. Agonist-independent binding of [35S]GTP[S] and the increase due to the chemotactic agonist were markedly attenuated by both pertussis and cholera toxin. Comparison of the number of chemotactic-peptide-sensitive [35S]GTP[S]-binding sites to the number of chemotactic peptide receptors present in HL-60 membranes provided direct evidence that a single formyl-peptide receptor is capable of catalyzing the binding of [35S]GTP[S] to, and thus the activation of, multiple (up to 20) G-proteins in native plasma membranes.

The regulation of adenylyl cyclase by receptor-operated G proteins

The receptor regulated adenylyl cyclase system is a multiprotein complex which is a member of the family of the receptor-effector systems whose signal is transduced by heterotrimeric GTP-binding proteins. The system consists of stimulatory and inhibitory receptors (Rs and Ri), stimulatory and inhibitory G proteins (Gs and Gi) and the adenylyl cyclase enzyme (C). While quite specific in situ, receptors (stimulatory or inhibitory) from one source can activate the appropriate G protein from other cell types or species which in turn can act on C from other sources. Studies with chimeric proteins have shown that the various specificities (stimulatory or inhibitory) can be mapped to defined domains in both receptors and G proteins. The mechanism by which the heterotrimeric G proteins couple to the stimulatory and inhibitory signals is discussed in detail. Specifically, the data supporting collision coupling vs the shuttle mechanism is reviewed, as well as the role of beta gamma subunits in both the stimulatory and inhibitory signals.

Na+ regulation of formyl peptide receptor-mediated signal transduction in HL 60 cells. Evidence that the cation prevents activation of the G-protein by unoccupied receptors

In neutrophils and several other phagocytes, a pertussis and cholera toxin-sensitive guanine nucleotide-binding protein (G-protein) couples the receptors for formyl methionine-containing chemotactic peptides to stimulation of phospholipase C. We used membranes of myeloid-differentiated HL 60 cells to study the role of Na+ in regulating both the interaction of the formyl peptide receptor with the chemotactic agonist, N-formyl-methionyl-leucyl-phenylalanine (FMLP), and the receptor-mediated activation of the G-protein. Monovalent cations (Na+ greater than Li+ greater than K+ greater than choline+) markedly inhibited the binding of the radiolabeled oligopeptide [3H]FMLP by specifically reducing the number of receptors in the high-affinity state. Half-maximal and maximal inhibition of peptide binding were seen at cation concentrations of approximately 20 and 200 mM, respectively. Inhibition of peptide binding by Na+ was observed in the presence and absence of divalent cations and was strictly additive to inhibition by the poorly hydrolyzable GTP analogue, guanosine-5'-O-(3-thiotriphosphate), or to ADP ribosylation of G-proteins by pertussis toxin. The inhibitory effect of Na+ on peptide binding coincided with a marked reduction of the potency of FMLP to stimulate a high-affinity GTPase. In contrast, the degree of FMLP-stimulated GTPase activity was markedly enhanced in the presence of Na+. This was largely due to the fact that Na+ reduced the agonist-independent basal GTPase activity in the same way but less so than pertussis toxin treatment. The results show that monovalent cations, Na+ in particular, regulate the interaction of the formyl peptide receptor with both the chemotactic agonist and the G-protein by acting on a single site, possibly located on the receptor itself. The observation that basal GTPase activity is markedly reduced by both Na+ and pertussis toxin treatment also suggests (a) that G-proteins interact with and are activated by receptors even in the absence of agonists and (b) that Na+ uncouples unoccupied receptors from G-protein interaction and activation.

Increase of the 40,000-mol wt pertussis toxin substrate (G protein) in the failing human heart

Human heart failure is associated with a diminished contractile response to beta-adrenergic agonists. We hypothesized that alterations in the activity of a guanine nucleotide-binding regulatory protein (G protein) might be partially responsible for this abnormality. We therefore measured the activity of G proteins in failing human myocardium utilizing bacterial toxin-catalyzed ADP ribosylation. The activity of a 40,000-mol wt pertussis toxin substrate (alpha G40) was increased by 36% in failing human hearts when compared with nonfailing controls. In contrast, there was no change in the level of the stimulatory regulatory subunit (Gs). The increased activity in alpha G40 was associated with a 30% decrease in basal as well as 5'-guanylyl imidodiphosphate-stimulated adenylate cyclase activity. These data suggest that increased alpha G40 activity is a new marker for failing myocardium and may account at least in part for the diminished responsiveness to beta 1-adrenergic agonists in the failing human heart.

Activation of rat brain adenylate cyclase by proteases: involvement of distinct protein components in the activation by ?-chymotrypsin and trypsin

Adenylate cyclase in synaptic plasma membranes from rat brain is activated by ?-chymotrypsin or trypsin. These proteases also activate adenylate cyclase reconstituted from the catalytic subunit of adenylate cyclase and the partially purified fraction of the GTP-binding proteins containing both the stimulatory and inhibitory GTP-binding proteins. Properties of the activation of reconstituted adenylate cyclase by the proteases are as follows. (1) The proteases do not directly activate the catalytic subunit. However, the pre-treatment of the partially purified GTP-binding proteins with ?-chymotrypsin (100 ?g/ml) increases the subsequently reconstituted cyclase activity at least 3-fold. Trypsin (10-30 ?g/ml) much more weakly enhances the cyclase activity. (2) ?-Chymotrypsin and trypsin synergistically activate the cyclase. (3) Trypsin but not ?-chymotrypsin no longer activates the cyclase when the purified stimulatory GTP-binding protein (Gs) replaces the partially purified GTP-binding proteins. (4) The stimulatory effects of ?-chymotrypsin and trypsin on the cyclase activity are little or slight unless 5?-guanylylimidodiphosphate (Gpp(NH)p) is present in the reconstitution. (5) The purified ??-subunits of the GTP-binding proteins markedly inhibit adenylate cyclase. This inhibition is nearly completely attenuated by treating the ??-subunits with ?-chymotrypsin (> 10 ?g/ml). (6) Trypsin (1-10 ?g/ml) inactivates the GTPase of the ?-subunit of the inhibitory GTP-binding protein (Gi). This inactivation of the GTPase seems to correlate with the activation of the reconstituted adenylate cyclase by trypsin. We conclude that two distinct protein components are involved in the activation of adenylate cyclase by ?-chymotrypsin and trypsin. One component sensitive to ?-chymotrypsin is probably the ??-subunits of the GTP-binding proteins. The other component sensitive to trypsin may be the ?-subunit of Gi.

Intracellular GTP gamma S restores the ability of morphine to hyperpolarize rat locus coeruleus neurons after blockade by pertussis toxin

The hyperpolarizing effect of morphine on locus coeruleus (LC) neurons, recorded with standard intracellular electrodes, was blocked in brain slices from rats pretreated with pertussis toxin, an inactivator of certain G proteins. In the same slices, when electrodes contained the hydrolysis-resistant GTP analog GTP gamma S, the ability of morphine to rapidly hyperpolarize LC neurons was restored and responses were similar in magnitude to those in control slices. We conclude that there is sufficient residual coupling between opiate receptors and G proteins after pertussis toxin treatment to allow the agonist to be effective when the hydrolysis-resistant GTP analog GTP gamma S is present.

Regulation of signal transfer from beta 1-adrenoceptor to adenylate cyclase by beta gamma subunits in a reconstituted system

The beta gamma subunits of guanine nucleotide binding proteins from bovine brain and bovine rod outer segments have different structural and immunochemical properties. In spite of these structural differences, beta gamma subunits from these sources have been found to be fully interchangeable in terms of their interaction with alpha subunits of pertussis-toxin-sensitive G proteins. In contrast, however, there are striking differences between these beta gamma subunits with regard to their ability to deactivate fluoride-stimulated Gs. These profound differences were also observed when the interaction of the purified components of the adenylate cyclase system was studied after reconstitution into phospholipid vesicles. Addition of beta gamma purified from bovine brain to vesicles containing beta-receptor and Gs results in a biphasic effect on receptor-stimulated GTPase activity, whereas addition of transducin beta gamma was virtually without any effect. Likewise, beta gamma from bovine brain, but not transducin beta gamma, affected adenylate cyclase activity of a reconstituted system consisting of three purified components (R, Gs, C). Thus, the alpha subunit of Gs, but not the alpha subunits of pertussis-toxin-sensitive G proteins discriminate between structurally different beta gamma subunits.

G proteins as regulators of ion channel function

Virtually unknown just a decade ago, GTP-binding proteins (G proteins) have become a major focus of current research. This family of closely related proteins transduce extracellular signals (such as hormones, neurotransmitters and sensory stimuli) into effector responses (1,2). It is now evident that ion channel permeability is one such effector response. In fact, the striking increase in the frequency of reports that demonstrate G protein-regulated ion channel function suggests that channels whose permeability mechanism can be altered by a G protein-mediated process may be more the rule than the exception. It is well-known that the cAMP-dependent modulation of ion channels is under the control of G proteins that regulate adenylate cyclase activity(3,4). However recent studies demonstrate that G proteins also transduce agonist-induced changes in channel activity that do not involve adenylate cyclase. It is on this aspect of G protein signal transduction that this review will focus.