Solubilization of the bombesin receptor from Swiss 3T3 cell membranes. Functional association to a guanine nucleotide regulatory protein

Bombesin and structurally related peptides including gastrin releasing peptide (GRP) are potent mitogens for Swiss 3T3 cells. Here we attempted to solubilize bombesin receptors under conditions in which the ligand (125I-labelled GRP) was prebound to the receptor prior to detergent extraction. We found that 125I-GRP-receptor complexes were solubilized from Swiss 3T3 cell membranes by using the detergents taurodeoxycholate or deoxycholate. These detergents promoted ligand-receptor solubilization in a dose-dependent manner. In contrast, a variety of other detergents including Triton X-100, octylglycoside, CHAPS, digitonin, cholic acid and n-dodecyl-beta-D-maltoside, were much less effective. Addition of guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) to ligand-receptor complexes isolated by gel filtration enhanced the rate of ligand dissociation in a concentration-dependent and nucleotide-specific manner. Our results demonstrate for the first time the successful solubilization of 125I-GRP-receptor complexes from Swiss 3T3 cell membranes and provide evidence for the physical association between the ligand-receptor complex and a guanine nucleotide binding protein(s).     

Functional reconstitution of the alpha 2-adrenergic receptor with guanine nucleotide regulatory proteins in phospholipid vesicles

We describe the successful reconstitution of functional interactions between an inhibitory adenylate cyclase-coupled receptor and various nucleotide-binding regulatory proteins in phospholipid vesicles. The receptor is the alpha 2-adrenergic receptor (alpha 2AR) which has been partially purified (approximately 500-5000-fold) from human platelet membranes. The nucleotide-binding regulatory proteins include purified preparations of human erythrocyte Ni and Ns, bovine retinal transducin and the recently discovered bovine brain No. Addition of the physiologic ligand, epinephrine, to vesicles containing the alpha 2AR and Ni results in stimulation of the GTPase activity in Ni. This stimulation of GTPase activity by epinephrine is prevented in the presence of the alpha-adrenergic antagonist, phentolamine, which indicates that a functional reconstitution of the alpha 2AR and Ni has been established. The maximum turnover number for the alpha 2AR-mediated epinephrine-stimulated GTPase activity in Ni is similar to the maximal turnover numbers obtained for the beta-adrenergic receptor-mediated isoproterenol-stimulated GTPase activity in Ns and the rhodopsin-mediated light-stimulated GTPase activity in transducin (0.5-1.5 mol of Pi released per min per mol of nucleotide regulatory protein). Functional similarities between the alpha 2AR and rhodopsin are observed in their interactions with the various nucleotide-binding regulatory proteins. Thus, both of these receptor proteins are capable of promoting the maximal activation of Ni and No while being much less effective in promoting the activation of Ns. However, there are differences between the alpha 2AR and rhodopsin in their interactions with transducin. Specifically, while rhodopsin will maximally activate transducin, the alpha 2AR is much less effective in promoting this activation (i.e. approximately 20% as effective as rhodopsin). Overall, these results suggest the following specificities of interaction: for rhodopsin, transducin approximately equal to Ni approximately equal to No much greater than Ns; while for alpha 2AR, Ni approximately equal to No greater than transducin greater than or equal to Ns.     

A prostaglandin E receptor coupled to a pertussis toxin-sensitive guanine nucleotide regulatory protein in rabbit cortical collecting tubule cells

At different concentrations, prostaglandin E2 (PGE2) can either stimulate or inhibit cAMP formation in freshly isolated rabbit cortical collecting tubule (RCCT) cells, but in cultured RCCT cells PGE2 can only stimulate cAMP synthesis (Sonnenburg, W. K., and Smith W. L. (1989) J. Biol. Chem. 263, 6155-6160). Here, we report characteristics of [3H]PGE2 binding to membrane receptor preparations from both freshly isolated and cultured RCCT cells. [3H]PGE2 binding to membranes from freshly isolated RCCT cells was saturable and partially reversible. Equilibrium binding analyses indicated that in the absence of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) there is a single class of PGE2 binding sites (KD = 4.2 +/- 0.4 nM; Bmax = 583 +/- 28 fmol/mg); in the presence of 100 microM GTP gamma S, there is also only one class of binding sites but with a somewhat lower KD = 1.2 +/- 0.5 nM (Bmax = 370 +/- 40 fmol/mg). This stimulatory effect of GTP gamma S was blocked by pretreatment of the freshly isolated RCCT cells with pertussis toxin. The relative affinities of prostanoids for the [3H]PGE2-binding site were determined to be 17,18,19,20-tetranor-16-phenoxy-PGE2-methylsulfonylamide (sulprostone) approximately PGE2 approximately PGE1 approximately 16,16-dimethyl-PGE2 greater than carbacyclin approximately PGF2 alpha greater than PGD2. This is the order of potency with which prostaglandins inhibit arginine vasopressin-induced cAMP formation in fresh RCCT cells. Interestingly, [3H]PGE2 binding to membranes from cultured cells, which, unlike fresh cells, fail to show an inhibitory response to PGE2, was only 10-20% of that observed with membranes from fresh cells; moreover, binding of [3H]PGE2 to membranes from cultured cells was neither stimulated by GTP gamma S nor inhibited by sulprostone. The prostanoid binding specificities and the unusual pertussis toxin-sensitive, stimulatory effect of GTP gamma S on binding of [3H]PGE2 to membranes from freshly isolated RCCT cells are characteristics shared by a Gi-linked PGE receptor from renal medulla (Watanabe, T., Umegaki, K., and Smith, W. L. (1986) J. Biol. Chem. 261, 14340-14349). Our results suggest that the [3H]PGE2 binding site of freshly isolated RCCT cells is the PGE receptor which is coupled to a Gi to attenuate arginine vasopressin-induced cAMP synthesis in the renal collecting tubule.     

The epidermal growth factor receptor is coupled to a pertussis toxin-sensitive guanine nucleotide regulatory protein in rat hepatocytes.

Activation of epidermal growth factor (EGF) receptors stimulates inositol phosphate production in rat hepatocytes via a pertussis toxin-sensitive mechanism, suggesting the involvement of a G protein in the process. Since the first event after receptor-G protein interaction is exchange of GTP for GDP on the G protein, the effect of EGF was measured on the initial rates of guanosine 5'-O-(3-[35S]thiotriphosphate) [( 35S]GTP gamma S) association and [alpha-32P]GDP dissociation in rat hepatocyte membranes. The initial rate of [35S]GTP gamma S binding was stimulated by EGF, with a maximal effect observed at 8 nM EGF. EGF also increased the initial rate of [alpha-32P]GDP dissociation. The effect of EGF on [35S]GTP gamma S association was blocked by boiling the peptide for 5 min in 5 mM dithiothreitol or by incubation of the membranes with guanosine 5'-O-(2-thiodiphosphate) (GDP beta S). EGF-stimulated [35S]GTP gamma S binding was completely abolished in hepatocyte membranes prepared from pertussis toxin-treated rats and was inhibited in hepatocyte membranes that were treated directly with the resolved A-subunit of pertussis toxin. The amount of guanine nucleotide binding affected by occupation of the EGF receptor was approximately 6 pmol/mg of membrane protein. Occupation of angiotensin II receptors, which are known to couple to G proteins in hepatic membranes, also stimulated [35S]GTP gamma S association with and [alpha-32P]GDP dissociation from the membranes. The effect of angiotensin II on [alpha-32P]GDP dissociation was blocked by the angiotensin II receptor antagonist [Sar1,Ile8]angiotensin II, demonstrating that the guanine nucleotide binding was receptor-mediated. In A431 human epidermoid carcinoma cells, EGF stimulates inositol lipid breakdown, but the effect is not blocked by treatment of the cells with pertussis toxin. In these cells, EGF had no effect on [35S]GTP gamma S binding. Occupation of the beta-adrenergic receptor in A431 cell membranes with isoproterenol did stimulate [35S] GTP gamma S binding, and the effect could be completely blocked by l-propranolol. These results support the concept that in hepatocyte membranes, EGF receptors interact with a pertussis toxin-sensitive G protein via a mechanism similar to other hormone receptor-G protein interactions, but that in A431 human epidermoid carcinoma cells, EGF may activate phospholipase C via different mechanisms.     

A novel guanine nucleotide-binding protein coupled to the alpha 1-adrenergic receptor. II. Purification, characterization, and reconstitution

In the previous paper, we reported the identification of a 74-kDa G-protein that co-purifies with the alpha 1-adrenergic receptor following ternary complex formation. We report here on the purification and characterization of this 74-kDa G-protein (termed Gh) isolated de novo from rat liver membranes. After solubilization of rat liver membranes with the detergent sucrose monolaurate, Gh was isolated by sequential chromatography using heparin-agarose, Ultrogel AcA 34, hydroxylapatite, and heptylamine-Sepharose columns. The protein, thus isolated, is not a substrate for cholera or pertussis toxin but displays GTPase activity (turnover number, 3-5 min-1) and high-affinity guanosine 5'-O-3-thiotriphosphate (GTP gamma S) binding (half-maximal binding = 0.25-0.3 microM), which is Mg2(+)-dependent and saturable. The relative order of nucleotide binding by Gh is GTP gamma S greater than GTP greater than GDP greater than ITP much much greater than ATP greater than or equal to adenyl-5'-yl imidodiphosphate, which is similar to that observed for other heterotrimeric G-proteins involved in receptor signaling. Moreover, specific alpha 1-agonist-stimulated GTPase (turnover number, 10-15 min-1) and GTP gamma S binding activity could be demonstrated after reconstitution of purified Gh with partially purified alpha 1-adrenergic receptor into phospholipid vesicles. The alpha 1-agonist stimulation of GTP gamma S binding and GTPase activity was inhibited by the alpha-antagonist phentolamine. A 50-kDa protein co-purifies with the 74-kDa G-protein. This protein does not bind guanine nucleotides and may be a subunit (beta-subunit) of Gh. These findings indicate that Gh is a G-protein that functionally couples to the alpha 1-adrenergic receptor.     

A reconstitution assay for the guanine nucleotide regulatory protein of the adenylate cyclase system using turkey erythrocyte membranes as the acceptor preparation

The turkey erythrocyte beta-adrenergic receptor-adenylate cyclase system has the unusual property that neither GTP nor Gpp(NH)p are effective in activating adenylate cyclase unless a beta-agonist is present simultaneously. This property results in essentially no basal activity and the inability of GTP or Gpp(NH)p alone to activate the catalytic moiety. In this study, we have exploited these characteristics to utilize turkey erythrocyte membranes as the acceptor preparation in a reconstitution assay. Rat reticulocyte or turkey erythrocyte membranes that have been activated with isoproterenol and Gpp(NH)p followed by solubilization with sodium cholate serve as the donor source of the guanine nucleotide regulatory protein (N). By reconstituting this Gpp(NH)p-activated N protein, it has been found that: (1) exogenous Gpp(NH)p-associated N could activate the catalytic unit of adenylate cyclase in turkey erythrocyte membranes; (2) this system can be used to assay N protein activity; (3) the endogenous pathway for activation of turkey erythrocyte membrane adenylate cyclase by hormones and fluoride remains qualitatively functional; and (4) the effects of combined activation via the endogenous and exogenous pathways are additive and saturable.     

Restoration of guanine nucleotide- and fluoride-stimulated activity to an adenylate cyclase-deficient cell line with affinity-purified guanine nucleotide regulatory protein.

A guanine nucleotide-binding protein purified from turkey erythrocytes by affinity chromatography confers both F-- and guanine nucleotide-stimulation of adenylate cyclase to membranes from CYC- cells, a mutant cell line deficient in these responses. Interaction of turkey erythrocyte membranes with beta-adrenergic agonists before affinity chromatography, which is essential for binding of the guanine nucleotide regulatory protein to the affinity matrix, was also required for recovery of F--stimulation restoring activity in the affinity eluate.     

Functional reconstitution of prostaglandin E receptor from bovine adrenal medulla with guanine nucleotide binding proteins

Prostaglandin E2 (PGE2) was found to bind specifically to a 100,000 x g pellet prepared from bovine adrenal medulla. The PGE receptor was associated with a GTP-binding protein (G-protein) and could be covalently cross-linked with this G-protein by dithiobis(succinimidyl propionate) in the 100,000 x g pellet (Negishi, M., Ito, S., Tanaka, T., Yokohama, H., Hayashi, H., Katada, T., Ui, M., and Hayaishi, O. (1987) J. Biol. Chem. 262, 12077-12084). In order to characterize the G-protein associated with the PGE receptor and reconstitute these proteins in phospholipid vesicles, we purified the G-protein to apparent homogeneity from the 100,000 x g pellet. The G-protein served as a substrate of pertussis toxin but differed in its alpha subunit from two known pertussis toxin substrate G-proteins (Gi and Go) purified from bovine brain. The molecular weight of the alpha subunit was 40,000, which is between those of Gi and Go. The purified protein was also distinguished immunologically from Gi and Go and was referred to as Gam. PGE receptor was solubilized by 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid and freed from G-proteins by wheat germ agglutinin column chromatography. Reconstitution of the PGE receptor with pure Gam, Gi, or Go in phospholipid vesicles resulted in a remarkable restoration of [3H]PGE2 binding activity in a GTP-dependent manner. The efficiency of these three G-proteins in this capacity was roughly equal. When pertussis toxin- or N-ethylmaleimide-treated G-proteins, instead of the native ones, were reconstituted into vesicles, the restoration of binding activity was no longer observed. The displacement of [3H]PGE2 binding was specific for PGE1 and PGE2. Furthermore, addition of PGE2 stimulated the GTPase activity of the G-proteins in reconstituted vesicles. These results indicate that the PGE receptor can couple functionally with Gam, Gi, or Go in phospholipid vesicles and suggest that Gam may be involved in signal transduction of the PGE receptor in bovine adrenal medulla.     

Lipid requirements for reconstitution of the delipidated beta-adrenergic receptor and the regulatory protein

The role of lipids in the interaction of the beta-adrenergic receptor (R) with the regulatory protein (Gs) was investigated. Solubilized preparations of R and of Gs from turkey erythrocytes were delipidated by gel filtration. They were subsequently combined and reconstituted by the addition of various lipids. When reconstitution was carried out in the presence of soybean lipids, Gs could be fully activated via R by addition of hormone plus GTP gamma S. In contrast, purified phospholipids or a phospholipid fraction from soybean failed to produce an active system. Fractionation of soybean lipids revealed that acetone-soluble neutral lipids are essential for the reconstitution of a hormone responsive system. The acetone fraction could be replaced by specific neutral lipids such as alpha-tocopherol or cholesteryl arachidonate while a mixture of phosphatidylethanolamine, -choline and -serine satisfied the phospholipid requirement of the system.     

Functional differences in the beta gamma complexes of transducin and the inhibitory guanine nucleotide regulatory protein

We have examined the mechanism of inhibition of adenylate cyclase using the purified alpha and beta gamma subunits of bovine brain inhibitory guanine nucleotide regulatory protein (Ni) (i.e., alpha i and beta gamma N) and bovine retinal transducin (alpha T and beta gamma T) in reconstituted phospholipid vesicle systems. The addition of beta gamma N or beta gamma T to lipid vesicles containing the pure stimulatory guanine nucleotide regulatory protein (Ns) from human erythrocytes as well as a resolved preparation of the catalytic moiety (C) of bovine caudate adenylate cyclase results in significant inhibition of guanine nucleotide stimulated cyclase activity (80-90%). The inhibition by these beta gamma subunit complexes appears to fully account for the inhibitory effects observed with holo-Ni or holotransducin. A variety of structure-function comparisons of the beta gamma N and beta gamma T complexes were performed in order to further probe the molecular mechanisms involved in the inhibitory pathway. Whereas the beta subunits of beta gamma N and beta gamma T appear to be very similar, if not identical, on the basis of comparisons of their gel electrophoretic mobility and immunological cross-reactivity, clear differences exist in the apparent structures of gamma N and gamma T. Interestingly, functional differences are observed in the effectiveness of these two beta gamma complexes to inhibit adenylate cyclase activity. Specifically, while both beta gamma N and beta gamma T are capable of effecting significant levels of inhibition of the guanine nucleotide stimulated activities, the beta gamma N complex is consistently more potent than beta gamma T in inhibiting these activities.(ABSTRACT TRUNCATED AT 250 WORDS)     

Deficient activity of guanine nucleotide regulatory protein in erythrocytes from patients with pseudohypoparathyroidism

The activity in two assay systems of guanine nucleotide regulatory protein was significantly lower in red cells from pseudohypoparathyroid patients compared to that of normal subjects. The results support the hypothesis that deficient activity of the guanine nucleotide regulatory protein is the molecular basis for hormone resistance in this inherited disorder.     

Association of a solubilized prostaglandin E2 receptor from renal medulla with a pertussis toxin-reactive guanine nucleotide regulatory protein

Prostaglandin E2 (PGE2) was found to bind specifically, reversibly, and in a protein-dependent manner to a single class of high affinity (KD approximately equal to 20 nM) binding sites in membranes prepared from canine renal outer medulla. PGE2 binding activity was solubilized from these membranes in a stable form (t1/2 greater than 14 days) in the absence of ligand in 75% yields using digitonin. The characteristics of PGE2 binding to membranes and solubilized protein were similar with respect to pH dependence, KD for PGE2, and order of potency of prostaglandins (PGE2 approximately PGE1 greater than PGF2 alpha greater than PGD2) in inhibiting the binding of [3H]PGE2. Importantly, the extents of binding of PGE2 to membranes and to a solubilized preparation partially purified by chromatography on wheat germ agglutinin-Affi-Gel 10 were both increased about 2-fold by GDP and GTP and its analogs. Treatment of the digitonin-solubilized PGE2 binding activity with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS) rendered the binding activity insensitive to stimulation by GTP and decreased the apparent molecular weight of the peak of PGE2 binding activity from about 175,000 to about 65,000. These results suggest that the PGE2 binding activity resides in a protein which is tightly associated with, but distinct from, a guanine nucleotide regulatory (N) protein. PGE2 (greater than or equal to 10 nM) was found to stimulate GTPase activity of renal outer medullary membranes, and this stimulation was eliminated by pretreatment of membranes with pertussis toxin and NAD, but not cholera toxin and NAD. Treatment of both particulate and solubilized preparations of PGE2 binding activity with pertussis toxin plus NAD also eliminated the ability of GTP to stimulate PGE2 binding. This evidence indicates that it is the inhibitory guanine nucleotide regulatory protein, Ni, with which the PGE2 binding activity is associated. Thus, this PGE2 binding activity is an inhibitory PGE2 receptor, quite possibly one that mediates inhibition of vasopressin-induced cAMP formation in the medullary thick ascending limb and/or collecting tubule of the kidney.     

Functional modification of the guanine nucleotide regulatory protein after desensitization of turkey erythrocytes by catecholamines

Densensitization of turkey erythrocytes by exposure to the beta-adrenergic agonist (-)isoproterenol leads to decreased activation of adenylate cyclase by agonist, NaF, and guanyl-5'-yl imido diphosphate, with no reduction in the number of beta-adrenergic receptors. Interactions between the receptor and the guanine nucleotide regulatory protein (N protein) also seem to be impaired. These observations suggest that a component distal to the beta-adrenergic receptor may be a locus of modification. Accordingly we examined the N protein to determine whether it was altered by desensitization. The rate at which (-)isoproterenol stimulated the release of [3H]GDP from the N protein was substantially lower in membranes prepared from desensitized cells, providing further evidence for uncoupling of the receptor and the N protein. The amount of N protein in membranes from control and desensitized cells was compared by labeling the 42,000 Mr component of the N protein with [32P]NAD+ and cholera toxin; no significant difference was found. However, significantly more N protein (p less than .001) was solubilized by cholate extraction of desensitized membranes, suggesting an altered association of the N protein with the membrane after desensitization. The functional activity of the N protein was measured by reconstitution of cholate extracts of turkey erythrocyte membranes into S49 lymphoma cyc- membranes. Reconstitution of (-)isoproterenol stimulation of adenylate cyclase activity was reduced significantly (p less than .05) after desensitization. These observations suggest that desensitization of the turkey erythrocyte by (-)isoproterenol results in functional modifications of the guanine nucleotide regulatory protein, leading to impaired interactions with the beta-adrenergic receptor and reduced activation of adenylate cyclase.     

Genetically acquired diabetes: adipocyte guanine nucleotide regulatory protein expression and adenylate cyclase regulation

Adipocyte membranes from diabetic (db/db) animals showed marked elevations in the levels of α-subunits for G_i-1 which were almost twice those in membranes from their normal, lean littermates. In contrast, no apparent differences were noted for levels of the α-subunits of G_i-2 and G_i-3, and 42 and 45 kDa forms of G_s and for G-protein β-subunits. Adenylate cyclase specific activity was similar in membranes from both normal and diabetic animals under basal conditions and also when stimulated by optimal concentrations of either NaF or forsckolin. In contrast, the ability of isoprenaline, glucagon and secretin to stimulate adenylate cyclase activity was greater in membranes from normal animals compared with membranes from diabetic animals. Receptor-mediated inhibition of adenylate cyclase, as assessed using PGE₁ and nicotinate, was similar using membranes from both sources, but PIA (phenylisopropyladenosine) was a slightly more effective inhibitor in membranes from diabetic animals. A doubling in the expression of G₁-1 thus appears to have little discernible effect upon the inhibitory regulation of adenylate cyclase.     

Cholecystokinin induces the interaction of its receptor with a guanine nucleotide binding protein

To evaluate the relation between the pancreatic cholecystokinin (CCK) receptor and guanine nucleotide-binding protein(s) we studied the effects of nucleotides on 125I-CCK binding to pancreatic acinar plasma membranes, 125I-CCK binding to solubilized 125I-CCK receptors, and the stability of the solubilized 125I-CCK-receptor complex. In plasma membranes, guanine nucleotides both inhibited CCK binding and increased the dissociation of CCK from its receptor. The potency of the nucleotides studied was GTP gamma S = GMP-PNP greater than GTP much greater than ATP. When membranes were solubilized with digitonin, subsequent binding of CCK was insensitive to guanine nucleotides including GTP, GMP-PNP and GTP gamma S. However, if CCK binding occurred before solubilization of the membranes, guanine nucleotides increased dissociation at concentrations and with a specificity similar to that observed for effects on intact pancreatic membranes. It is concluded that guanine nucleotides act via a protein which is separable from the receptor to induce dissociation of bound CCK. Moreover, CCK binding induces an association in the plasma membrane of the CCK receptor with this guanine nucleotide binding protein.     

Multiple expressions of the activity of guanine nucleotide regulatory protein in human thyroid adenylate cyclase

Adenylate cyclase (ATP pyrophosphate-lyase, EC 4.6.1.1) in plasma membranes from human thyroid was highly responsive to thyrotropin. Pretreatment of thyroid plasma membranes with 5′-guanylylimidodiphosphate (Gpp(NH)p) in the presence of Mg²⁺ led to a temperature-dependent activation, which was seen neither in the absence of Mg²⁺ nor at 4 °C. By contrast, thyrotropin bound to its receptors regardless of the temperature and produced its maximal effect after 2 min of preincubation in the absence or presence of Mg²⁺. Furthermore, activation was seen after treatment with thyrotropin and Gpp(NH)p even carried out in the absence of Mg²⁺ or at 4 °C. However, the full activation by Gpp(NH)p required Mg²⁺, hormone, and elevated temperature. These observations suggest that there appears to be two types of nucleotide interaction responsible for the Gpp(NH)p activation in human thyroid membrane; one type seen in the absence of hormone may represent the system uncoupled from hormone receptor, while the fully coupled hormone-sensitive adenylate cyclase accounts for the second type of interaction which requires the presence of hormone.     

The insulin receptor tyrosyl kinase phosphorylates holomeric forms of the guanine nucleotide regulatory proteins Gi and Go

An affinity purified human insulin receptor preparation was shown to phosphorylate the alpha- and beta-subunits of the guanine nucleotide-regulatory proteins Gi and Go, derived from bovine brain. The presence of insulin stimulated the rate of their phosphorylation some 2-fold. The presence of Gi and Go did not affect the degree of autophosphorylation of the beta-subunit of the insulin receptor. Under conditions known to cause the dissociation of Gi and Go into their constituent subunits then phosphorylation of Gi and Go by the insulin receptor was abolished. The alpha-subunits of Gi and Go could be selectively phosphorylated by the insulin receptor tyrosyl kinase using appropriate concentrations of Mg2+ and GTP-gamma-S.     

Quantitation of a guanine nucleotide binding regulatory protein by an enzyme-linked immunosorbent competition assay

We have developed a new method using a competitive enzyme-linked immunosorbent assay, ELISA, for the determination of levels of the stimulatory guanine nucleotide binding protein, Gs, in membrane extracts. The method is based on the use of antipeptide antibodies generated in rabbits directed against amino acids 28-42 in the alpha-subunit, alpha s, of Gs. The peptide is utilized as the stationary phase in the ELISA and anti-alpha s antibody bound to the microtiter plate is assessed by a peroxidase-coupled anti-rabbit immunoglobulin G antibody that yields detectable color development at 490 nm. Gs purified from rabbit liver is utilized as the standard to assess the ability of Gs present in cholate extracts of membrane samples to compete with bound peptides for primary antibody. This assay provides a direct means to quantify changes in levels of native Gs in membranes and cells.