Affinity chromatography of the bovine cerebral cortex A1 adenosine receptor

An approximate 140-fold purification of the A1 adenosine receptor of bovine cerebral cortex has been obtained via affinity chromatography. The affinity column consists of Affi-Gel 10 coupled through an amide linkage to XAC, a high-affinity A1 adenosine receptor antagonist. As assessed by [3H]XAC binding, bovine brain membranes solubilized with the detergent CHAPS had a specific binding activity of 1.1 pmol/mg protein. Interaction of solubilized A1 adenosine receptors with the XAC-Affi-Gel was biospecific and 30% of the receptor activity was bound by the gel. Demonstration of [3H]XAC binding in the material eluted from the column with R-PIA required insertion of receptor into phospholipid vesicles. The specific activity of the affinity column purified receptor was 146 +/- 22 pmol/mg protein with typically 5-15% of the bound receptor recovered. The purified receptor displayed high-affinity antagonist binding and bound agonists with the potency order expected of the bovine brain A1 adenosine receptor: R-PIA greater than S-PIA greater than NECA. In purified preparations, the photoaffinity probe [125I]PAPAXAC-SANPAH specifically labelled a protein of molecular mass 38,000 which has previously been shown to be the A1 adenosine receptor binding subunit.     

Solubilization and reconstitution of dopamine D1 receptor from bovine striatal membranes: Effects of agonist and antagonist pretreatment

The bovine striatal dopamine D₁ receptor was solubilized with a combination of sodium cholate and NaCl in the presence of phospholipids, following treatment of membranes with a dopaminergic agonist (SKF-82526-J) or antagonist (SCH-23390). The solubilized receptors were subsequently reconstituted into lipid vesicles by gel-filtration. A comparison of ligand-binding properties shows that the solubilized and reconstituted receptors bound [³H]SCH-23390 to a homogeneous site in a saturable, stereospecific and reversible manner with a K_d of 0.95 and 1.1 nM and a B_max of 918 and 885 fmol/mg protein respectively for agonist- and antagonist-pretreated preparations. These values are very similar to those obtained for membrane-bound receptors. The competition of antagonists for [³H]SCH-23390 binding exhibited a clear D₁ dopaminergic order in the reconstituted preparation obtained from either agonist or antagonist-pretreated membranes, except that (+)butaclamol was about four-fold more potent thancis-flupentixol in displacing [³H]SCH-23390 binding in preparation obtained from agonist-pretreated membranes compared to antagonist-pretreated membranes. The agonist/[³H]SCH-23390 competition studies revealed the presence of a highaffinity component of agonist binding in both the reconstituted receptor preparations. The number of high-affinity agonist binding sites, however, is 40–80% higher in reconstituted preparation obtained from antagonist-treated membrane compared to that obrained from the agonist-treated membrane. In both the preparations, 100 M guanylylimidodiphosphate (Gpp(NH)p) completely abolished the high-affinity component of agonist binding compared to partial abolition in the native membranes, indicating a close association of a G-protein with the solubilized receptors. Whether the receptor was solubilized following agonist or antagonist preincubation of the membranes, the receptor-detergent complex eluted from a steric-exclusion HPLC column with an apparent molecular size of 360,000. Preincubation of the solubilized preparations with Gpp(NH)p had virtually no effect on the elution profile suggesting a lack of guanine nucleotide-dependent dissociation of G-protein receptor complex.     

Human heat shock gene expression and the modulation of plasma membrane Na+, K+-ATPase activity

Benzodiazepine receptor solubilized from bovine cortical membranes was bound to a new benzodiazepine affinity column, the synthesis of which is described. Bio-specific elution with the benzodiazepine compound chlorazepate resulted in the elution of fractions highly enriched in specific binding for the GABA receptor agonist muscimol. Specific activity for [³H]muscimol binding was >1.3 nmol/mg protein. It is shown that [³H]flunitrazepam binding activity can be recovered by removal of chlorazepate from the purified fraction. These results strongly support a model which suggests that the 2 binding sites reside on the same physical entity.     

Solubilization and purification of rat pituitary gonadotropin-releasing hormone receptor

Gonadotropin-releasing hormone (GnRH) receptors were solubilized from rat pituitary membrane preparations in an active form by using the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid). The solubilized receptor exhibits high affinity, saturability, and specificity. The soluble supernatant retained 100% of the original binding activity when stored at 4 or -20 degrees C in the presence of 10% glycerol. The receptors were resolved into two components on the basis of chromatography on wheat germ agglutinin-agarose. Homogeneous receptor preparation was obtained by two cycles of affinity chromatography on immobilized avidin column coupled to [biotinyl-D-Lys6]GnRH. The overall recovery of the purified receptor was 4-10% of the initial activity in the CHAPS extract, and the calculated purification -fold was approximately 10,000 to 15,000. Analysis of iodinated purified GnRH receptors by autoradiography indicated the presence of two bands, Mr = 59,000 and 57,000. This was confirmed by photoaffinity labeling of the partially purified receptors and suggests that both components can specifically bind the hormone.     

Solubilization and assay of a colony-stimulating factor receptor from murine macrophages

The colony-stimulating factor, CSF-1, selectively stimulates the survival, proliferation, and differentiation of mononuclear phagocytes. The solubilization, assay, and characteristics of the CSF-1 receptor from the J774.2 murine macrophage cell line are described. The recovery of cell-surface receptor in the postnuclear supernatant membrane fraction of hypotonically disrupted cells was 76%. Recovery of the ligand binding activity of the receptor after solubilization of this fraction with 1% Triton X-100 was approximately 150%. The binding of 125I-CSF-1 to intact cells and membrane preparations was consistent with the existence of a single class of high-affinity receptor sites. In contrast, the equilibrium binding of 125I-CSF-1 to the solubilized postnuclear fraction indicated the existence of two distinct classes of binding site (apparent Kds 0.15 nM and 10 nM). A rapid assay was developed for the high-affinity sites, which were shown to be associated with the CSF-1 receptor. The function of the low-affinity sites, which have not been demonstrated on intact cells or cell membranes and which are 13 times more abundant than the high-affinity sites, is unknown. The solubilized high-affinity receptor-CSF-1 complex was stable on storage at 0 degrees C and -70 degrees C but dissociated at 37 degrees C. Dissociation also occurred at 0 degrees C in buffers of low pH (4.0) or high ionic strength (0.7 M NaCl).     

Solubilization and partial purification of GABAB receptor from bovine brain

gamma-Aminobutyric acid (GABA)B receptor has been solubilized and partially purified by an affinity column chromatography. GABAB receptor was solubilized by 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) in the presence of asolectin. The solubilized GABAB receptor was adsorbed on baclofen-coupled epoxy-activated Sepharose 6B. The affinity matrix adsorbed 80% of the solubilized [3H]GABA binding activity to GABAB receptor, and approximately 75% of the adsorbed activity could be eluted with 1 M KC1. GABAB receptor binding in the fraction eluted from affinity column was displaced by GABA, baclofen and 2-hydroxy saclofen in a dose-dependent manner. Furthermore, the purified GABAB receptor showed approximately 2800-fold purification as compared with the original solubilized fraction and possessed the specific binding activity of 17.68 p mol/mg of protein. This binding consisted of a single binding site with a dissociation constant of 64.4 nM. The present results indicate that affinity column chromatographic procedures using baclofen-coupled epoxy-activated Sepharose 6B are suitable for the partial purification of GABAB receptor from cerebral tissues.     

Solubilization of human placental tumor necrosis factor receptors as a complex with a guanine nucleotide-binding protein.

Human placental membranes exhibited high-affinity receptors for tumor necrosis factor (TNF) (Kd = 5.6 x 10(-10) M) with a density of 1.2-1.7 x 10(10) sites/mg protein. The receptors were solubilized from these membranes with 1% Nonidet P-40, and the solubilized receptor was adsorbed to Con A-Sepharose and wheat germ agglutinin agarose columns, indicating that the TNF receptor derived from human placenta contains carbohydrate chains recognized by these lectins. TNF binding activity was eluted from a column of Sephacryl S-300 as a single peak of Mr 300 kDa. The solubilized receptor was further purified by TNF-Sepharose prepared by coupling of TNF to tresyl-activated Sepharose 4B. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the purified sample resolved five major bands of Mr 90, 78, 41, 35, and 11 kDa, suggesting that these polypeptides constitute a multimeric complex with a molecular mass of 300 kDa, as observed in gel filtration study. Furthermore, the TNF-Sepharose-bound fraction demonstrated GTP gamma S binding and GTPase activity. Immunoblot analysis showed that the 41- and 35-kDa polypeptides were recognized by antisera against alpha subunits and beta subunit of GTP-binding proteins, respectively. These results suggest that the native TNF receptor couples to a guanine nucleotide-binding protein to form a large complex structure in human placental membranes.     

Biochemical characterization of the BETA-adrenergic receptor of the frog erythrocyte

Summary The beta-adrenergic receptor which is coupled to adenylate cyclase in the frog erythrocycte plasma membrane provides a convenient model system for probing the molecular characteristics of an adenylate cyclase coupled hormone receptor. Direct radioligand binding studies with beta-adrenergic agonists and antagonists such as [³H]hydroxybenzylisoproterenol and [³H]dihydroalprenolol have shed new light on the biochemical properties of the receptor as well as on its mode of interaction with other components of the adenylate cyclase system. Agonist binding to the receptor induces a high affinity state of the receptor which can be selectively reverted to a low agonist affinity state by guanyl nucleotides. This agonist-induced high affinity state of the receptor appears to correspond to a receptor moiety which has larger apparent molecular weight and which is probably a complex of the beta-adrenergic receptor and nucleotide regulatory binding protein. Antagonists do not appear capable of inducing or stabilizing the formation of this high affinity receptor-nucleotide site complex.The beta-adrenergic receptors have been solubilized using the plant glycoside digitonin as the detergent and have been highly purified by biospecific affinity chromatography on an alprenolol-agarose affinity support. These highly purified receptor preparations retain all of the binding characteristics observed in the unpurified soluble receptor preparations.Remarkably, antibodies raised in rabbits against affinity chromatography purified preparations of the receptor, themselves bind beta-adrenergic ligands with typical beta-adrenergic specificity. Such antibodies which possess binding sites similar to those of physiological receptors provide useful model systems for further probing the molecular characteristics of beta-adrenergic binding sites.     

Reconstitution of High-Affinity Binding of a β-Scorpion Toxin to Neurotoxin Receptor Site 4 on Purified Sodium Channels

Abstract: Reconstitution of purified sodium channels into phospholipid vesicles restores many aspects of sodium channel function including high-affinity neurotoxin binding and action at neurotoxin receptor sites 1–3 and 5, but neurotoxin binding and action at receptor site 4 has not previously been demonstrated in purified and reconstituted preparations. Toxin IV from the venom of the American scorpion Centruroides suffusus suffusus (Css IV), a β-scorpion toxin, shifts the voltage dependence of sodium channel activation by binding with high affinity to neurotoxin receptor site 4. Sodium channels were purified from rat brain and reconstituted into phospholipid vesicles composed of phosphatidylcholine and phosphatidylethanolamine (65:35). ¹²⁵I-Css IV, purified by reversed-phase HPLC, bound rapidly and specifically to reconstituted sodium channels. Dissociation of the bound toxin was biphasic with half-times of 0.22 min^?1 and 0.015 min^?1. At equilibrium, the toxin bound to two classes of specific high-affinity sites, a variable minor class with K_D of ～0.1 nM and a major class with a K_D of ～5 nM. Approximately 0.8 mol ¹²⁵I-Css IV was bound per mole of reconstituted, right-side-out sodium channels, as assessed from comparison of binding of saxitoxin and Css IV. Binding of Css IV was unaffected by membrane potential or by neurotoxins that bind at sites 1–3 or 5, consistent with the characteristics of binding of β-scorpion toxins to sodium channels in cells and membrane preparations. Our results show that specific, high-affinity binding at neurotoxin receptor site 4 on purified sodium channels can be restored by reconstitution into phospholipid vesicles and provide an experimental approach to analysis of the peptide components of the toxin receptor site.     

Solubilization and characterization of [3H]Imipramine and [3H]Paroxetine binding sites from calf striatum

The serotonin (5-HT) transporter from calf striatum cerebral membranes was solubilized with digitonin and characterized by gel exclusion chromatography. [³H]Imipramine and [³H]paroxetine were utilized as markers for labeling it.³H-imipramine labels a high- and a low-affinity site on striaturn membranes, whereas it binds to a single high-affinity site on the solubilized fraction. [³H]Paroxetine binds with the same affinity to a single site on both membranes and solubilized preparations. After gel exclusion chromatography of the solubilizate both [³H]imipramine and [³H]paroxetine bind on an identical fraction of 205 kDa molecular weight, with a similar maximum number of binding sites (Bmax). Our results suggest that both³H-imipramine and [³H]paroxetine bind to a common site on the 5-HT transporter.     

Analogues of the Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) Antagonist Ned-19 Indicate Two Binding Sites on the NAADP Receptor

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a Ca²⁺-releasing messenger. Biological data suggest that its receptor has two binding sites: one high-affinity locking site and one low-affinity opening site. To directly address the presence and function of these putative binding sites, we synthesized and tested analogues of the NAADP antagonist Ned-19. Ned-19 itself inhibits both NAADP-mediated Ca²⁺ release and NAADP binding. A fluorometry bioassay was used to assess NAADP-mediated Ca²⁺ release, whereas a radioreceptor assay was used to assess binding to the NAADP receptor (only at the high-affinity site). In Ned-20, the fluorine is para rather than ortho as in Ned-19. Ned-20 does not inhibit NAADP-mediated Ca²⁺ release but inhibits NAADP binding. Conversely, Ned-19.4 (a methyl ester of Ned-19) inhibits NAADP-mediated Ca²⁺ release but cannot inhibit NAADP binding. Furthermore, Ned-20 prevents the self-desensitization response characteristic of NAADP in sea urchin eggs, confirming that this response is mediated by a high-affinity allosteric site to which NAADP binds in the radioreceptor assay. Collectively, these data provide the first direct evidence for two binding sites (one high- and one low-affinity) on the NAADP receptor.     

Solubilization of the opiate receptor

The opiate receptor is solubilized from rat neural membranes by treating the membranes with Triton X-100, followed by centrifugation. Removal of the Triton X-100 was accomplished with Bio-beads SM-2, and the resulting supernatant was capable of stereospecifically binding opiates at 10^?13 moles/mg protein under saturating conditions. Stereospecific binding was measured by equilibrium dialysis and gel filtration using a Sephadex G-25 column, equilibrated with [³H] -ligand and either dextrorphan or levorphanol. The solubilized receptor has affinities for the opiates similar to those observed in membrane preparations and $\text{in vivo}$ experiments. The addition of phosphatidylserine to the supernatant enhances stereospecific binding of etorphine slightly. Phospholipase A₂, trypsin and chymotrypsin completely inhibit binding. The addition of albumin prevents, but does not reverse the inhibition caused by low concentrations of phospholipase A₂. Phosphatidylserine decarboxylase inhibits stereospecific binding by 95%, despite the fact only 10% of the phosphatidylserine present in the supernatant is converted to phosphatidylethanolamine. The solubilized opiate receptor, like the receptor in neural membranes, appears to consist of both protein and lipid moieties.     

Reconstitution of affinity-purified dopamine D2 receptor binding activities by specific lipids

The role of lipids in maintaining ligand binding properties of affinity-purified bovine striatal dopamine D₂ receptor was investigated in detail. The receptor, purified on a haloperidol-linked Sepharose CL6B affinity column, exhibited low [³H]spiroperidol binding unless reconstituted with soybean phospholipids. In order to understand the role of individual phospholipids in maintaining the receptor binding activity, the purified preparation was reconstituted separately with individual phospholipids and assayed for [³H]spiroperidol binding. Except for phosphatidylcholine and phosphatidylethanolamine, that respectively restored 30 and 20% binding as compared to that obtained with soybean lipids, reconstitution with other lipids had very little effect. When various combinations of phospholipids were used for reconstitution, a phosphatidylcholine and phosphatidylserine mixture seemed to almost fully restore the receptor binding. A mixture of phosphatidylcholine and phosphatidylethanolamine was as effective as phosphatidylcholine alone in reconstituting ligand binding; however, when phosphatidylserine was also included in the mixture, there was a pronounced increase in binding (about 2-fold compared to the soybean lipids and about 6-fold compared to the phosphatidylcholine-phosphatidylethanolamine mixture). Substitution of other phospholipids or cholesterol for phosphatidylserine in phosphatidylcholine and phosphatidylethanolamine mixture had little effect. Maximal reconstitution of [³H]spiroperidol binding was obtained with phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine mixture (2:2:1, w/w) at a concentration of 0.5 mg/ml. The reconstituted receptor exhibited high affinity binding for [³H]spiroperidol which was comparable to that obtained with membrane or solubilized preparations. Various dopaminergic antagonists and agonists showed appropriate order of potency for the reconstituted receptor. The presently described reconstitution data suggest a role of specific phospholipids in preserving the binding properties of dopamine D₂ receptor and should prove useful in studies on functional reconstitution of the receptor.     

Solubilization of benzodiazepine binding site from rat cortex.

The high-affinity binding site for [³H] diazepam has been solubilized from rat brain using 0.5% Lubrol-PX. Using a polyethylene glycol (PEG)-γ-globulin assay, it has been possible to demonstrate solubilization of about 60% of the binding sites in a single step. The solubilized binding site possesses a K_D of 11 nM for [³H] diazepam compared to approximately 4 nM for the membrane-bound form, and binding is to a single class of sites. The order of potency of benzodiazepines is identical for the solubilized receptor and the membrane-bound form. Binding of [³H] diazepam is temperature dependent and higher at 4° than 37°C. Both urea and guanidine-HC1 were capable of totally inhibiting binding, and this inhibition was partly reversible; neither sulfhydryl groups nor carbohydrate moieties seem to be important for binding. γ-Aminobutyric acid which enhanced [³H] diazepam binding to membrane fractions was without effect on the solubilized binding site.     

Solubilization of γ-aminobutyric acid receptor from rat brain

Gamma-aminobutyric acid (GABA) binding sites were solubilized from rat brain synaptosomal fractions by extraction with a combination of sodium deoxycholate and potassium chloride. Specific ³H-GABA binding to the solubilized fraction was saturable with the apparent dissociation constant, Kd = 23.4 ± 0.2 nM. GABA agonists and an antagonist inhibited the binding. The relative potencies of these drugs in competing for ³H-GABA binding to the solubilized fraction are in good agreement with findings with the membrane fraction, suggesting that the binding sites in the solubilized fraction retain the characteristics of membrane-bound GABA receptor. The sedimentation coefficient value of ³H-GABA binding site was estimated to be 11.3S by sucrose density gradient centrifugation, and this value was identical with that of ³H-flunitrazepam binding site in the same solubilized fraction.     

Assay and purification of a solubilized membrane receptor that binds the lysosomal enzyme α-l-iduronidase

A receptor that binds the lysosomal enzyme α-l-iduronidase via phosphorylated mannose residues on the enzyme has been solubilized from Swarm rat chondrosarcoma membranes using a pH 9.5 buffer containing 0.1% Triton X-100. Detergent-solubilized receptor in crude and purified preparations was measured by assay of bound α-l-iduronidase after adsorbing the receptor-enzyme complex onto insoluble phospholipid vesicles (liposomes). Binding of α-l-iduronidase to the liposomes required receptor and was completely inhibited by mannose 6-phosphate but not glucose 6-phosphate, indicating that the receptor maintained specificity following solubilization. Receptors from rat chondrosarcoma and human diploid fibroblasts were purified to apparent homogeneity using a phosphomannan-Sepharose affinity column. Both had identical molecular weights in polyacrylamide gels containing sodium dodecyl sulfate (M_r = 215,000). Amino acid analysis and two-dimensional gel electrophoresis was carried out on the purified rat chondrosarcoma receptor. Two forms of the receptor with different pI's were observed (pI 5.5 and 6.2). One form (pI 5.5) was made more basic (pI 5.8) by treatment with neuraminidase.     

Characterization of High-Affinity Dopamine D2 Receptors and Modulation of Affinity States by Guanine Nucleotides in Cholate-Solubilized Bovine Striatal Preparations

3,4-Dihydroxyphenylethylamine (dopamine) D2 receptors, solubilized from bovine striatal membranes using a cholic acid-NaCl combination, exhibited the typical pharmacological characteristics of both agonist and antagonist binding. The rank order potency of the agonists and antagonists to displace [3H]spiroperidol binding was the same as that observed with membrane-bound receptors. Computer-assisted analysis of the [3H]spiroperidol/agonist competition curves revealed the retention of high- and low-affinity states of the D2 receptor in the solubilized preparations and the proportions of receptor subpopulations in the two affinity states were similar to those reported in membrane. Guanine nucleotide almost completely converted the high-affinity sites to low-affinity sites for the agonists. The binding of the high-affinity agonist [3H]N-n-propylnorapomorphine ([3H]NPA) was clearly demonstrated in the solubilized preparations for the first time. Addition of guanylyl-imidodiphosphate completely abolished the [3H]NPA binding. When the solubilized receptors were subjected to diethylaminoethyl-Sephacel chromatography, the dopaminergic binding sites eluted in two distinct peaks, showing six- to sevenfold purification of the receptors in the major peak. Binding studies performed on both peaks indicated that the receptor subpopulation present in the first peak may have a larger proportion of high-affinity binding sites than the second peak. The solubilized preparation also showed high-affinity binding of [35S]guanosine-5'-(gamma-thio)triphosphate, a result suggesting the presence of guanine nucleotide binding sites, which may interact with the solubilized D2 receptors. These data are consistent with the retention of the D2 receptor-guanine nucleotide regulatory protein complex in the solubilized preparations and should provide a suitable model system to study the receptor-effector interactions.     

Sedimentation behavior of solubilized gonadotropin receptor from plasma membranes of bovine corpus luteum

The gonadotropin receptors associated with plasma membrane fractions were solubilized by detergents, including Triton X-100, Lubrol WX, Lubrol PX and sodium deoxycholate before and after equilibration with ¹²⁵I-labelled human chorionic gonadotropin. The binding activity remained in solution even after centrifugation at 300 000 × g for 3 h. The solubilized gonadotropin receptor or gonadotropin receptor complex was characterized by gel filtration and sucrose density gradient centrifugation. Sucrose density gradient centrifugation of solubilized gonadotropin-receptor complex in the presence of Triton X-100 had a sedimentation coefficient of 6.5 S whereas the solubilized uncomplexed receptor had a sedimentation coefficient of 5.1 S. In the absence of the detergent, solubilized hormone receptor complex from plasma membrane fractions I and II sedimented with a apparent sedimentation coefficient of 6.6 S and 7.4 S, respectively. Similary, the free receptor also showed higher sedimentation profile with a apparent sedimentation coefficient of 6.7 S for fraction I and 7.2 S for fraction II. Treatment of plasma membranes with phospholipase A and C inhibited the binding of ¹²⁵I-labelled human chorionic gonadotropin in a dose dependent manner, whereas phospholipase D was without any effect. Doses of 1.4 mI.U. of phospholipase A or 0.6 mI.U. of phospholipase C were required to produce 50% inhibition of the binding activity. These phospholipases had no effect on the performed ¹²⁵I-labelled human chorionic gonadotropin-receptor complex nor on the sedimentation profile of solubilized gonadotropin receptor complex.     

Studies of nicotinic acetylcholine receptor protein from rat brain: II. Partial purification

The pharmacological specificity of the binding of ¹²⁵I-labeled α-bungarotoxin to a 1% Emulphogene BC-720 extract of a rat brain particulate fraction has been investigated. The extract contains a component which possesses the binding characteristics of a nicotinic acetylcholine receptor protein. The crude soluble acetylcholine receptor protein was purified by affinity chromatography utilizing the α-neurotoxin of Naja naja siamensis as ligand and 1.0 M carbamylcholine chloride as eluant. A single, batch-wise, affinity chromatography procedure yields an average purification of 510-fold. When this purified material is treated a second time by affinity chromatography, purification as high as 12 600-fold has been obtained. Binding of ¹²⁵I-labeled α-bungarotoxin to this purified acetylcholine receptor protein is saturable with a K_d of 1·10^?8 M. Nicotine and acetylcholine iodide at concentrations of 10^?5 M inhibit ¹²⁵I-labeled toxin-acetylcholine receptor protein complex formation by 41 and 61% respectively. At 10^?4 M, carbamylcholine chloride and (+)-tubocurarine chloride give respectively 52 and 82% inhibition. Eserine sulfate and atropine sulfate have no effect on complex formation at a concentration of 10^?4 M. These data support the isolation of partially purified nicotinic acetylcholine receptor protein.     

Solubilization and reconstitution of the formylmethionylleucylphenylalanine receptor coupled to guanine nucleotide regulatory protein

We describe the solubilization, resolution, and reconstitution of the formylmethionylleucylphenylalanine (fMet-Leu-Phe) receptor and guanine nucleotide regulatory proteins (G-proteins). The receptor was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Guanine nucleotides decreased the number of high-affinity binding sites and accelerated the rate of dissociation of the receptor-ligand complex, suggesting that the solubilized receptor remained coupled to endogenous G-proteins. The solubilized receptor was resolved from endogenous G-proteins by fractionation on a wheat germ agglutinin (WGA)-Sepharose 4B column. High-affinity [3H]fMet-Leu-Phe binding to the WGA-purified receptor was diminished and exhibited reduced guanine nucleotide sensitivity. High-affinity [3H]fMet-Leu-Phe binding and guanine nucleotide sensitivity were reconstituted upon the addition of purified brain G-proteins. Similar results were obtained when the receptor was reconstituted with brain G-proteins into phospholipid vesicles by gel filtration chromatography. In addition, we also demonstrated fMet-Leu-Phe-dependent GTP hydrolysis in the reconstituted vesicles. The results of this work indicate that coupling of the fMet-Leu-Phe receptor to G-proteins converts the receptor to a high-affinity binding state and that agonist produces activation of G-proteins. The resolution and functional reconstitution of this receptor should provide an important step toward the elucidation of the molecular mechanism of the fMet-Leu-Phe transduction system in neutrophils.