Cholecystokinin activates Gi1-, Gi2-, Gi3- and several Gs-proteins in rat pancreatic acinar cells.

On separation of rat pancreatic plasma membrane proteins by two-dimensional gel electrophoresis, 15 GTP-binding protein (G-protein) alpha-subunits could be detected immunochemically using an alpha common antibody. These consisted of five 48 kDa proteins (pI 5.70, 5.80, 5.90, 6.10 and 6.25) and five 45 kDa proteins (pI 5.90, 6.05, 6.25, 6.30 and 6.70), presumably corresponding to low- and high-molecular mass forms of the Gs-protein, as well as three 40/41 kDa proteins (pI 5.50, 5.70 and 6.00) and two 39 kDa proteins (pI 5.50 and 6.00). All of these proteins except for the more acidic 39 kDa protein were ADP-ribosylated by cholera toxin (CT). In addition, the three 40/41 kDa proteins and the more alkaline 39 kDa protein were also ADP-ribosylated by pertussis toxin (PT). CT- and PT-induced ADP-ribosylation changed the pI values of G-protein alpha-subunits by 0.2 pI units to more acidic values. Preincubation of isolated pancreatic membranes with cholecystokinin octapeptide (CCK-OP), which stimulates phospholipase C in acinar cells, decreased CT-induced as well as PT-induced ADP-ribosylation of the three 40/41 kDa proteins, whereas CT-induced ADP-ribosylation of one 45 kDa (pI 5.80) and all 48 kDa proteins was enhanced in the presence of CCK. Carbachol, another stimulant of phospholipase C, had no effect. The three 40/41 kDa proteins and one 48 kDa protein could be labelled with the GTP analogue [alpha-32P]GTP-gamma-azidoanilide. CCK, but not carbachol, stimulated incorporation of the GTP analogue into all of these four proteins. Using different anti-peptide antisera specific for alpha-subunits of G-proteins we identified the three 40/41 kDa Gi-proteins as Gi1 (pI 6.00), Gi2 (pI 5.50) and Gi3 (pI 5.70). The Gi3-protein was found to be the major Gi-protein of pancreatic plasma membranes. One of the 39 kDa proteins (pI 6.0) was identified as Go. These results indicate that CCK receptors functionally interact with six Gs-proteins and with Gi1, Gi2 and Gi3-proteins. Since evidence suggests that a 40/41 kDa CT substrate is involved in the stimulation of phospholipase C in pancreatic acinar cells, it is likely that one, two or all three 40/41 kDa Gi-proteins are involved in the coupling of CCK receptors with phospholipase C.     

Effects of Bordetella pertussis toxin on catecholamine inhibition of insulin release from intact and electrically permeabilized rat islets.

Noradrenaline- and clonidine-induced inhibition of insulin release from intact and electrically permeabilized rat islets was markedly relieved by prior exposure to 100 ng of Bordetella pertussis toxin/ml. The reversal of catecholamine inhibition of insulin secretion by this toxin was not associated with a decrease in specific binding of the alpha 2-adrenergic ligand [3H]yohimbine, and could not be fully explained by an increase in intracellular cyclic AMP. Exposure of intact islets to 1 microgram of pertussis toxin/ml for 2 h, followed by electrical permeabilization and incubation with 5 microCi of [alpha-32P]NAD+, resulted in the ADP-ribosylation in situ of a protein of molecular mass approx. 41 kDa. These results suggest that pertussis toxin alleviates catecholamine inhibition of beta-cell secretory responses by ADP-ribosylating at least one protein of molecular mass 41 kDa. In analogous systems the 41 kDa substrate of pertussis toxin has been shown to be the alpha subunit of Gi, but catecholamine-activated G proteins linked to effector systems other than adenylate cyclase might also be modified by this toxin in pancreatic beta-cells.     

Effect of Bordetella pertussis toxin on ADP-ribosylation of membrane proteins, adenylate cyclase activity and insulin release in rat pancreatic islets

Exposure of rat pancreatic islet membranes to [alpha-32P]-NAD+ in the presence of Bordetella Pertussis toxin (islet-activating protein) reveals the ADP-ribosylation of a peptide with a Mr close to 41 kDa, which corresponds to the alpha-subunit of the guanine nucleotide regulatory protein Ni. Islets removed from rats pretreated with the Bordetella Pertussis toxin display a specific increase in adenylate cyclase responsiveness to GTP and are characterized by a resistance to the inhibitory action of alpha2-adrenergic agonists upon either adenylate cyclase activity or glucose-induced insulin release.     

Pertussis toxin catalyzed ADP-ribosylation of a 41 kDa G-protein impairs insulin-stimulated glucose metabolism in BC3H-1 myocytes

In this study, we examined the effects of pertussis toxin (PT) on the ADP-ribosylation of guanine nucleotide binding proteins (G-proteins) and various insulin-stimulated processes in cultured BC3H-1 myocytes. Treatment of intact myocytes with 0.1 microgram/ml PT for 24 hours resulted in the complete ribosylation of a 41 kDa protein. The 41 kDa PT substrate was immunoprecipitated with antibodies directed against a synthetic peptide corresponding to a unique sequence in the alpha subunit of Gi-proteins. PT treatment of intact cells had no effect on insulin receptor binding or internalization. However, PT inhibited insulin-stimulated glucose transport at all insulin-concentrations tested (1-100 ng/ml). Maximally stimulated glucose transport was reduced by 50% +/- 15%. Insulin-stimulated glucose oxidation was also decreased by 31% +/- 8%. The toxin had no significant effect on the basal rates of glucose transport and glucose oxidation. The time course of PT-induced inhibition on glucose transport correlated with the time course of the "in vivo" ADP-ribosylation of the 41 kDa protein. The results suggest that a 41 kDa PT-sensitive G-protein, identical or very similar to Gi, is involved in the regulation of glucose metabolism by insulin in BC3H-1 cells.     

The effects of hydrostatic pressure on pertussis toxin-catalyzed ribosylation of G proteins from deep-living macrourid fishes

To test the effects of hydrostatic pressure on the coupling of receptors to guanyl nucleotide binding reglatory proteins (G proteins) in transmembrane signaling, pertussis toxin (PTX)-catalyzed [32P]ADP-ribosylation was used to probe the guanyl nucleotide-binding proteins Gi and G(o) in brain membranes from four marine teleosts. These macrourids, Coryphaenoides pectoralis, Coryphaenoides cinereus, Coryphaenoides filifer and Coryphaenoides armatus, span depths from 200 to 5400 m. Pertussis toxin specifically labelled proteins of 39-41 kDa. The PTX-catalyzed [32P]ADP-ribosylation reaction was linear for 7 h. Added guanyl nucleotides (guanosine 5'-diphosphate (GDP) and guanosine 5'-O-(3-thiotriphosphate)(GTP[S])) at concentrations up to 1000 microM did not affect ribosylation at atmospheric pressure. Under basal conditions the Gi/G(o) protein population appears to be uncoupled from receptors and bound with GDP. Pressures up to 476 atm were tested in the absence and presence of added guanyl nucleotides, 100 microM GDP and 100 microM GTP[S]. [32P]ADP-ribosylation in brain membranes from the deeper-occurring C. cinereus, C. filifer and C. armatus was not inhibited by increased pressure in the presence of 100 microM GDP. Increasing pressure decreased ribosylation in brain membranes of C. pectoralis. In the presence of 100 microM GTP[S], increased pressure inhibited ribosylation in all species. Pressure appears to enhance the efficacy of GTP[S] in dissociating the heterotrimeric holoprotein.     

Effects of phosphorylation of inhibitory GTP-binding protein by cyclic AMP-dependent protein kinase on its ADP-ribosylation by pertussis toxin, islet-activating protein

Pretreatment of rat cardiac myocytes with the beta-adrenergic agonist, db-cAMP or forskolin decreased ADP-ribosylation of 40-41 kDa protein by islet-activating protein (IAP) in cell membranes. Addition of activated cyclic AMP-dependent protein kinase (protein kinase A) catalytic subunit and MgCl2 also decreased ADP-ribosylation of 40-41 kDa protein by IAP in cell membranes. The alpha- and beta-subunits of partially purified inhibitory GTP-binding protein (Gi) were both phosphorylated by protein kinase A. The amounts of phosphate incorporated into the subunits of Gi were 0.34 and 0.18 mol/mol protein. These show that phosphorylation of Gi by protein kinase A results in a decrease in its ADP-ribosylation by IAP.     

Functional maturation of human T lymphocytes is accompanied by changes in the G-protein pattern

The putative guanine nucleotide binding (G)-protein involved in transduction of signals from the TCR/CD3 complex has not been identified. We have used a UV-photoaffinity labeling technique to covalently attach [alpha-32P]GTP to human lymphocyte and thymocyte membrane proteins. Ten bands specifically labeled with [32P]GTP were detected by SDS-PAGE and autoradiography in T lymphocyte membranes. Among these, a 40-kDa protein was identified by immunoblotting as the alpha-subunit of the adenylate cyclase-inhibiting G-protein, Gi, and two proteins of 44 and 46 kDa were identified as the alpha-subunits of adenylate cyclase stimulating G-protein (Gs). These proteins also served as substrates for ADP-ribosylation by pertussis toxin and cholera toxin, respectively. Comparison of GTP-labeled membrane proteins from immature and more mature thymocytes and blood T lymphocytes, revealed that bands of 26, 30, 34, 40, 44 and 46 kDa were absent or weakly labeled in immature thymocytes, intermediate in mature thymocytes, and strongest in blood T cells. Similar increases were seen in ADP ribosylation of the substrates for pertussis, cholera, and botulinum C3 toxin. However, corresponding quantitative changes in Gi and Gs were not detected by immunoblotting, which suggests that the increased labeling is caused by enhanced affinity of the proteins for GTP rather than by increased amount of protein during thymic maturation. A concomitant maturation of GTP-induced cAMP production was seen in the cell populations, but no such change occurred in direct activation of adenylate cyclase by forskolin. The changes in some (but not all) GTP-binding proteins during acquisition of immunocompetence indicates their importance in T lymphocyte physiology.     

Subcellular distribution and characterization of GTP-binding proteins in human neutrophils

The subcellular distribution of GTP binding proteins in human neutrophils and their functional coupling to the N-formylmethionylleucylphenylalanine (FMLP) receptor was characterized to provide insight into mechanisms of cellular activation. Human neutrophils were nitrogen cavitated and fractionated on discontinuous Percoll gradients. Four subcellular fractions were obtained: cytosol, light membranes enriched for plasma membranes, specific granules and azurophilic granules. ADP-ribosylation catalyzed by pertussis toxin (PT) revealed a major substrate of 40 kDa only in plasma membrane and cytosol, and antiserum specific for Gi alpha confirmed the presence of neutrophil Gi alpha in plasma membrane and cytosol and its absence from specific granules. The cytosolic PT substrate was shown to be mostly in monomeric form by molecular sieve chromatography. The rate of the ribosyltransferase reaction was several-fold lower in cytosol compared to plasma membranes, and the extent of ADP-ribosylation was greatly augmented by supplementation with beta gamma subunits in cytosol. ADP-ribosylation catalyzed by cholera toxin (CT) revealed substrates of 52, 43 and 40 kDa in plasma membrane alone. FMLP receptors in plasma membrane were shown to be coupled to the 40 kDa substrate for CT by ligand-modulation of ADP-ribosylation, while FMLP added to specific granules did not induce ribosylation of this substrate even though FMLP receptors were found in high density in this compartment. Both 24 and 26 kDa [32P]GTP binding proteins were found to codistribute with FMLP receptors in specific granules and plasma membranes. Functional evidence for the coupling of GTP binding proteins to the FMLP receptor in specific granules was obtained by modulating [3H]FMLP binding with GTP gamma S, and by accelerating [35S]GTP gamma S binding with FMLP.     

Photoaffinity labeling with GTP-gamma-azidoanilide of a cholera toxin-sensitive 40 kDa protein from pancreatic acinar cells

In isolated pancreatic acinar plasma membranes a 40 kDa protein was labeled with the photoreactive GTP-analogue [alpha 32P] GTP-gamma-azidoanilide. Increased incorporation of the photolabel into the 40 kDa protein was obtained in the presence of increasing concentrations of cholecystokinin-octapeptide (10(-8) - 10(-5) M) but not with carbachol. Adenylyl cyclase activating hormones such as vasoactive intestinal polypeptide and secretin had no effect. Pretreatment of plasma membranes with cholera toxin reduced incorporation of GTP-gamma-azidoanilide into the 40 kDa protein by about 30%. This reduction was reversed if ADP-ribosylation by cholera toxin was performed in the presence of cholecystokinin, whereas carbachol had no effect. The data indicate that a cholera toxin-sensitive 40 kDa GTP-binding protein is involved in functionally coupling cholecystokinin- but not muscarinic acetylcholine-receptors to phospholipase C.     

Detection of G Proteins in Purified Bovine Brain Myelin

Following a previous report on detection of muscarinic receptors in myelin with the implied presence of G proteins, we now demonstrate by more direct means the presence of such proteins and their quantification. Using [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S) as the binding ligand, purified myelin from bovine brain was found to contain approximately half the binding activity of whole white matter (138 +/- 9 vs. 271 +/- 18 pmol/mg of protein). Scatchard analysis of saturation binding data revealed two slopes, a result suggesting at least two binding populations. This binding was inhibited by GTP and its analog but not by 5'-adenylylimidodiphosphate [App(NH)p], GMP, or UTP. Following sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) of myelin proteins and blotting on nitrocellulose, [alpha-32P]GTP bound to three bands in the 21-27-kDa range in a manner inhibited by GTP and GTP gamma S but not App(NH)p. ADP-ribosylation of myelin with [32P]NAD+ and cholera toxin labeled a protein of 43 kDa, whereas reaction with pertussis toxin labeled two components of 40 kDa. Cholate extract of myelin subjected to chromatography on a column of phenyl-Sepharose gave at least three major peaks of [35S]GTP gamma S binding activity. SDS-PAGE and immunoblot analyses of peak I indicated the presence of Go alpha, Gi alpha, and Gs alpha. Further fractionation of peak II by diethyl-aminoethyl-Sephacel chromatography gave one [35S]GTP gamma S binding peak with the low-molecular-mass (21-27 kDa) proteins and a second showing two major protein bands of 36 and 40 kDa on SDS-PAGE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of guanine nucleotides on cholera toxin catalyzed ADP-ribosylation in rat adipocyte plasma membranes

ADP-ribosylation of rat adipocyte plasma membrane proteins was investigated following incubation of membranes with [alpha-32P]NAD and cholera toxin in the presence and absence of various guanine nucleotides. In membranes incubated without guanine nucleotides, cholera toxin induced incorporation of 32P into three discrete proteins of 48, 45, and 41 kDa. In membranes containing 100 microM GTP or GDP, toxin-catalyzed incorporation of 32P into the 41-kDa protein was inhibited. GMP and Gpp(NH)p (100 microM) allowed moderate incorporation of 32P into the 41-kDa protein. Toxin-catalyzed labeling of all proteins was rapid, reaching maximal levels between 5 and 10 min. Toxin-catalyzed ADP-ribosylation of the 48- and 45-kDa proteins was stimulated by GTP, reaching maximal levels at 10(-5) M GTP. Inhibition of toxin-dependent labeling of the 41-kDa protein required GTP concentrations above 10(-7) M with complete inhibition occurring between 10(-5) and 10(-4) M GTP. Cholera toxin catalyzed ADP-ribosylation was increased up to 2-fold in membranes supplemented with adipocyte cytosol. These results indicate that cholera toxin catalyzes ADP-ribosylation of three distinct adipocyte plasma membrane proteins, each of which is regulated by the amount and type of added guanine nucleotides.     

Quantitative determination of pertussis toxin-sensitive G proteins using [32P]ADP-ribosylation in human platelet membranes: negative correlation with ages.

The optimum condition to quantitate the [32P]ADP-ribosylation catalyzed by pertussis toxin (islet-activating protein, IAP) in human platelet membranes was investigated. Autoradiography indicated the incorporation of 32P into the band corresponding to the molecular weight of 40-41 kDa, which was augmented by the addition of GTP in the presence of 10 mM MgCl2. On the other hand, non-hydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) enhanced the IAP-catalyzed [32P]ADP-ribosylation only in the absence of MgCl2. The amounts of IAP-catalyzed [32P]ADP-ribosylation in the presence of 100 microM GTP and 10 mM MgCl2 were linear in proportion to the protein concentrations within the limited range of protein concentrations, indicating that this simple quantitative method could be adequately used to evaluate the IAP-sensitive G proteins. Data from fifteen healthy volunteers (7 males and 8 females ranging 24 to 60 years old) indicate that the amounts of IAP-sensitive G proteins in platelet membranes are significantly negatively correlated with ages.     

Agonist-dependent, cholera toxin-catalyzed ADP-ribosylation of pertussis toxin-sensitive G-proteins following transfection of the human alpha 2-C10 adrenergic receptor into rat 1 fibroblasts. Evidence for the direct interaction of a single receptor with two pertussis toxin-sensitive G-proteins, Gi2 and Gi3.

A DNA encoding the human alpha 2-C10 adrenergic receptor was transfected into Rat 1 fibroblasts and clones selected on the basis of resistance to G418 sulfate. Two clones, one of which (1C) expressed some 3.5 pmol/mg membrane protein of the receptor as assessed by the specific binding of [3H]yohimbine and one (4D) which did not express detectable amounts of the receptor were selected for further study. When cholera toxin-catalyzed ADP-ribosylation was performed with [32P]NAD on membranes of these cells in the absence of added guanine nucleotides, radioactivity was incorporated into a polypeptide(s) of 40 kDa in addition to the 45- and 42-kDa forms of Gs alpha. Addition of the selective alpha 2 receptor agonist U.K.14304 enhanced markedly, in a dose-dependent manner, the cholera toxin-catalyzed [32P]ADP-ribosylation of the 40-kDa polypeptide(s), but not the 45- or 42-kDa polypeptides, in membranes of the 1C cells. Dose response curves for U.K.14304 enhancement of cholera toxin-labeling of the 40-kDa polypeptide(s) and stimulation of high affinity GTPase activity were identical. By contrast, U.K.14304 was ineffective in either assay in membranes from the 4D cells, demonstrating this effect to be dependent upon receptor activation. Furthermore, the alpha 2 receptor antagonist yohimbine blocked all effects of U.K.14304. The agonist promotion of cholera toxin-catalyzed ADP-ribosylation of Gi was completely blocked by guanine nucleotides. Whether GDP or GDP + fluoroaluminate (as a mimic of GTP) was used, blockade of the agonist effect was complete and indeed both conditions prevented agonist-independent labeling by cholera toxin of the 40-kDa polypeptide(s). Mg2+ produced an agonist-independent cholera toxin-catalyzed [32P]ADP-ribosylation of the 40-kDa polypeptide(s) but even in the presence of [Mg2+], agonist-stimulation of cholera toxin-labeling of the 40-kDa polypeptide(s) was observed and was additive with the effect of [Mg2+]. Agonist stimulation of cholera toxin-catalyzed ADP-ribosylation of Gi was completely attenuated by pretreatment of the cells with pertussis toxin, which prevents contact between receptors and G-proteins which are substrates for this toxin. By contrast, pretreatment of the cells with concentrations of cholera toxin able to "down-regulate" essentially all of the membrane-associated Gs alpha did not prevent agonist stimulation of cholera toxin-catalyzed ADP-ribosylation of Gi.(ABSTRACT TRUNCATED AT 400 WORDS)     

Isoproterenol-stimulated labelling of particulate proteins by using [adenylate-32P]NAD+ independent on a cAMP-dependent protein kinase in parotid acinar cells.

When saponin-permeabilized rat parotid acinar cells were incubated with [adenylate-32P]NAD+, labelling of proteins (33, 27 and 23 kDa) in particulate fractions of the cells was stimulated by isoproterenol. The effect of isoproterenol was completely blocked by a beta-antagonist. Both forskolin or cAMP mimicked the effect of isoproterenol on the labelling. However, an inhibitor of cAMPdPK failed to induce complete inhibition of the effects of isoproterenol, forskolin and cAMP. When the labelled proteins were treated with snake venom phosphodiesterase, neither [32P]5'-AMP nor [32P]phosphoribosyladenosine was released. These results suggest that covalent modification of proteins with NAD+, which is distinct from ADP-ribosylation and cAMPdPK-dependent phosphorylation, is coupled to beta-receptor-cAMP signalling system in rat parotid acinar cells.     

Solubilization of somatostatin receptors in hamster pancreatic beta cells. Characterization as a glycoprotein interacting with a GTP-binding protein

Somatostatin receptors of plasma membranes from beta cells of hamster insulinoma were covalently labelled with 125I-[Leu8,D-Trp22,Tyr25]somatostatin-28 (125I-somatostatin-28) and solubilized with the non-denaturing detergent Triton X-100. Analysis by SDS/PAGE and autoradiography revealed three specific 125I-somatostatin-28 receptor complexes with similar molecular masses (228 kDa, 128 kDa and 45 kDa) to those previously identified [Cotroneo, P., Marie, J.-C. & Rosselin, G. (1988) Eur. J. Biochem. 174, 219-224]. The major labelled complex (128 kDa) was adsorbed to a wheat-germ-agglutinin agarose column and eluted by N-acetylglucosamine. Also, the binding of 125I-somatostatin-28 to plasma membranes was specifically inhibited by the GTP analog, guanosine-5'-O-(3-thiotriphosphate) (GTP[S]) in a dose-dependent manner. Furthermore, when somatostatin-28 receptors were solubilized by Triton X-100 as a reversible complex with 125I-somatostatin-28, GTP[S] specifically dissociated the bound ligand to a larger extent from the soluble receptors than from the plasma-membrane-embedded receptors, the radioactivity remaining bound after 15 min at 37 degrees C being 30% and 83% respectively. After pertussis-toxin-induced [32P]ADP-ribosylation of pancreatic membranes, a 41-kDa [32P]ADP-ribose-labelled inhibitory guanine nucleotide binding protein coeluted with the 128-kDa and 45-kDa receptor complexes. The labelling of both receptor proteins was sensitive to GTP[S]. The labelling of the 228-kDa band was inconsistent. These results support the conclusion that beta cell somatostatin receptors can be solubilized as proteins of 128 kDa and 45 kDa. The major labeled species corresponds to the 128-kDa band and is a glycoprotein. The pancreatic membrane contains a 41-kDa GTP-binding protein that can complex with somatostatin receptors.     

CTP-dependent endogenous ADP-ribosylation of a 38 kDa protein in HL-60 cell membranes

Incubation of membranes of human promyelocytic leukemia HL-60 cells with [32P]NAD led to ADP-ribosylation of several proteins including a 38 kDa protein by endogenous ADP-ribosyltransferases. The ADP-ribosylation of the 38 kDa protein was distinctly different from others on the basis of pH dependency and heat stability at 50 degrees C, suggesting that there are at least two endogenous ADP-ribosyltransferases. It was enhanced by CTP, but not affected by ATP, GTP and UTP, whereas it was inhibited by GTP gamma S. [alpha-32P]CTP bound to the 38 kDa protein immobilized on a nitrocellulose sheet, indicating that the 38 kDa protein which bound CTP is strongly ADP-ribosylated by an endogenous ADP-ribosyltransferase.     

GTP-binding proteins and adenylate cyclase activity in v-Ki-ras transformed NIH/3T3 fibroblast cells

To identify the role of ras oncogene and p21 in the coupling mechanism of GTP-binding proteins to adenylate cyclase, we used v-Ki-ras transformed NIH/3T3 fibroblast cells. In the previous study, we investigated that NaF, cholera toxin and forskolin remarkably enhanced the adenylate cyclase activity in transformed cells compared to normal NIH/3T3 cells. In the present study, adenylate cyclase was more enhanced by GTP gamma S in transformed cells than in normal cells. It was considered that p21 plays enhancing role in coupling of GTP-binding proteins to adenylate cyclase. Further, as measured by the degree of [32P] ADP-ribosylation of GTP-binding proteins by cholera toxin and pertussis toxin respectively, the amount of Gs (46 kDa) was almost equal in both cells, while the amount of Gi (41 kDa) in transformant was about one third of that in normal cells. This difference seems to be reflected in either the biological situations or the quantities of Gi. Our data suggest that v-Ki-ras transformation resulted in the decrease of Gi protein so that the inhibitory regulation on adenylate cyclase relatively becomes low and then stimulatory influence of Gs seems to be enhanced.     

Identification of proteins resembling G-protein alpha subunits in locust muscle

In locust skeletal muscle, FMRFamide-like peptides decrease a K+ conductance. Functional data suggest the involvement of G-proteins. For identification of G-protein alpha-subunits, membranes of locust skeletal muscle were probed with ADP-ribosylating bacterial toxins, the photoreactive GTP analog, [alpha-32P]GTP azidoanilide, and with antibodies against mammalian alpha-subunits. Multiple guanine nucleotide-binding proteins of approximately 24-95 kDa were detected. Pertussis toxin catalyzed the ADP-ribosylation of two proteins comigrating with the ADP-ribosylated alpha-subunits of the mammalian G-proteins Go and Gi. Cholera toxin promoted ADP-ribosylation of a protein comigrating with mammalian cholera toxin substrates (i.e., Gs alpha-subunits). An antibody against mammalian Go alpha-subunits detected a 54-kDa protein. Thus proteins with properties of mammalian G-protein subunits are present in insect muscle.     

Evidence for receptor-mediated inhibition of intrinsic activity of GTP-binding protein, Gi1 and Gi2, but not G0 in reconstitution experiments

The receptor-mediated inhibition of intrinsic activities of GTP-binding proteins (G-proteins) was studied. Pertussis toxin (IAP)-substrate G-protein, Gi1, Gi2 or G0, was prelabeled with [alpha-32P]GDP and reconstituted with synaptic membranes of the guinea pig cerebellum in the presence of 0.02% of Chaps. Intrinsic activities of G-proteins were evaluated by the release of [alpha-32P]GDP in exchange for added GppNHp or GDP in reconstituted preparations. U-50,488H (1 nM-10 microM), a specific kappa-subtype of opioid receptor agonist, inhibited the [alpha-32P]GDP release in exchange for added 1 microM GppNHp in Gi1-reconstituted preparations in a concentration-dependent manner. On the other hand, the kappa-opioid agonist at 10 microM increases the Km values of GppNHp, but not GDP in exchange for [alpha-32P]GDP release in preparations reconstituted with Gi1 or Gi2, but not with G0. These findings indicate that kappa-opioid receptor is coupled to inhibition of intrinsic activities of Gi1 and Gi2, but not G0, in guinea pig cerebellar membranes. In addition, it was revealed that the mode of action is mediated by a decrease in affinity of GTP (or its analog) for G proteins, but not by a change in affinity of GDP.     

Epidermal growth factor inhibits both cholecystokinin octapeptide-induced inositol 1,4,5-trisphosphate production and [CA2+]i increase in rat pancreatic acinar cells

We have studied the effects of epidermal growth factor (EGF) on both cholecystokinin octapeptide (CCK-OP)-induced inositol-1,4,5 trisphosphate (IP3) production and on cytosolic free calcium concentrations [Ca2+]i by fluorescence measurements in fura-2-loaded pancreatic acini. Our data show that EGF inhibits CCK-OP induced IP3 production by 40 +/- 9% and decreases CCK-OP induced rise in cytosolic Ca2+ by 41 +/- 9%. These data indicate that activation of EGF receptors leads to inhibition of CCK-OP induced stimulation of phospholipase C (PLC).