Biochemical characterization of phosphorylated beta-adrenergic receptors from catecholamine-desensitized turkey erythrocytes

Isoproterenol-induced desensitization of turkey erythrocyte adenylate cyclase is accompanied (1) by a decrease in the mobility of beta-adrenergic receptor proteins, specifically photoaffinity labeled with 125I-(p-azidobenzyl)carazolol (125I-PABC), on sodium dodecyl sulfate (SDS)-polyacrylamide gels and (2) by a 2-3-fold increase in phosphate incorporation into the beta receptor [Stadel, J.M., Nambi, P., Shorr, R. G. L., Sawyer, D. F., Caron, M. G., & Lefkowitz, R. J. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 3173]. Analysis of 32P-labeled beta receptors partially purified by affinity chromatography and subsequently hydrolyzed in 6 N HCl revealed that the beta receptor from control erythrocytes contained only phosphoserine and that agonist-promoted phosphorylation of the receptor in desensitized cells occurred on serine residues. Comparison of limited-digest peptide maps of 125I-PABC-labeled beta receptors from control and desensitized erythrocytes reveals distinctly different sensitivities of the two beta receptors to cleavage by chymotrypsin and Staphylococcus aureus protease. The altered mobility of the 125I-PABC-labeled beta receptor from desensitized erythrocytes was eliminated when 5 M urea was included in the SDS-polyacrylamide gels. Limited-digest peptide mapping of 32P-labeled beta receptors from control and desensitized cells with the protease papain identified a unique phosphorylated peptide in desensitized preparations. Our results are consistent with the hypothesis that the altered mobility of beta-receptor proteins on SDS gels following desensitization is due to changes in conformation promoted by prolonged exposure to agonists.

Studies on the use of a novel affinity matrix, sepharose amine-succinyl-amine haloperidol hemisuccinate, ASA-HHS, for purification of canine dopamine (D2) receptor

Haloperidol Hemisuccinate (HHS) was synthesized specifically as a ligand for an affinity chromatography matrix. The affinity chromatography matrix, ASA-HHS was developed which had high affinity and capacity for dopamine D2 receptors in solubilized canine striatal preparations. ASA-HHS also demonstrated nonspecific interaction with the D2 receptor. Two fractions, which bound 3H-spiroperidol specifically, with similar one dimensional SDS-PAGE patterns could be eluted successfully with 20 microM haloperidol in only 30% of the runs. Both fractions represented 300-400 fold purification. Two dimensional IEF-PAGE analysis of one of the fractions demonstrated coelution of beta and gamma actin, alpha and beta tubulin with the 3H-spiroperidol binding sites. The pattern of the proteins eluted from ASA-HHS and the inconsistent recovery of active D2 receptors are discussed.

A novel catecholamine-activated adenosine cyclic 3',5'-phosphate independent pathway for beta-adrenergic receptor phosphorylation in wild-type and mutant S49 lymphoma cells: mechanism of homologous desensitization of adenylate cyclase

Virtually all known biological actions stimulated by beta-adrenergic and other adenylate cyclase coupled receptors are mediated by cAMP-dependent protein kinase. Nonetheless, "homologous" or beta-adrenergic agonist-specific desensitization does not require cAMP. Since beta-adrenergic receptor phosphorylation may be involved in desensitization, we studied agonist-promoted receptor phosphorylation during homologous desensitization in wild-type S49 lymphoma cells (WT) and two mutants defective in the cAMP-dependent pathway of beta-agonist-stimulated protein phosphorylation (cyc- cannot generate cAMP in response to beta-adrenergic agonists; kin- lacks cAMP-dependent kinase). All three cell types demonstrate rapid, beta-adrenergic agonist-promoted, stoichiometric phosphorylation of the receptor which is clearly not cAMP mediated. The amino acid residue phosphorylated is solely serine. These data demonstrate, for the first time, that catecholamines can promote phosphorylation of a cellular protein (the beta-adrenergic receptor) via a cAMP-independent pathway. Moreover, the ability of cells with mutations in the adenylate cyclase-cAMP-dependent protein kinase pathway to both homologously desensitize and phosphorylate the beta-adrenergic receptors provides very strong support for the notion that receptor phosphorylation may indeed be central to the molecular mechanism of desensitization.

Large-scale purification of beta-adrenergic receptors from mammalian cells in culture

S49 Mouse lymphoma wild-type cells were grown in spinner cultures of 40 liters to a density of approximately 3 million cells/ml. Growth of cells to high density (2-3 million cells/ml) required that the cell suspensions be bubbled with oxygen. Cells from 40 liter cultures were collected by centrifugation and disrupted by nitrogen cavitation. Highly purified membranes (0.35 g membrane protein) that were rich in beta-adrenergic receptor (0.4-0.7 pmol receptor/mg membrane protein) were prepared by differential centrifugation and then solubilized with the plant glycoside, digitonin (1.5% digitonin at 3 mg of membrane protein/ml). Beta-adrenergic receptors were isolated and purified by sequential affinity chromatography, ion-exchange chromatography, and steric exclusion high-pressure liquid chromatography. The extract was subjected to affinity chromatography on a derivatized Sepharose-4B CL column to which the high-affinity, beta-adrenergic antagonist (-)alprenolol had been immobilized. Following extensive washing, the receptor bound to this matrix was eluted using a 0-100 micromolar linear gradient of (-)alprenolol. The receptor eluted as a sharp peak at 30 micromolar ligand and displayed a specific activity of 280 pmol receptor/mg of protein. Ion-exchange chromatography on DEAE-Sephacel increased the specific activity to 950 pmol/mg of protein. The final step in the purification, steric-exclusion high-pressure liquid chromatography on two TSK-3000 and one TSK-2000 columns, tandem linked, resulted in a beta-adrenergic receptor preparation with a specific activity of 6700 pmol/mg of protein (15,900-fold purification). Autoradiography of the radioiodinated pure receptor, the receptor photolabeled with [125I]iodoazidobenzylpindolol or silver-staining of chemical amounts of protein revealed that the Mr of the pure receptor is 66,000 upon polyacrylamide gel electrophoresis in sodium dodecyl sulfate under reducing conditions. The receptor is a beta2-subtype adrenergic receptor.

Molecular structure of the beta-adrenergic receptor

The beta-adrenergic receptor from several tissues has been purified to homogeneity or photoaffinity radiolabeled and its subunit molecular weight determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. In this study we have examined the oligomeric structure of nondenatured beta 1- and beta 2-adrenergic receptor proteins, as solubilized with the detergent digitonin. Model systems used were frog and turkey red blood cell as well as rat, rabbit, and bovine lung plasma membrane preparations. To correct for the effects of detergent binding, sedimentation equilibrium analysis in various solvents, as adapted for the air-driven ultracentrifuge, was used. With this approach an estimate of 6 g of digitonin/g of protein binding was determined, corresponding to a ratio of 180 mol of digitonin/mol of protein. Protein molecular weights estimated by this method were 43 500 for the turkey red blood cell beta 1 receptor and 54 000 for the frog red blood cell beta 2 receptor. Molecular weights of 60 000-65 000 were estimated for beta 1 and beta 2 receptors present in mammalian lungs. These values agree with estimates of subunit molecular weight obtained by SDS gel electrophoresis of purified or photoradiolabeled preparations and suggest beta-adrenergic receptors to be digitonin solubilized from the membrane as single polypeptide chains.

Fat cell beta 1-adrenergic receptor: structural evidence for existence of disulfide bridges essential for ligand binding

Agents that react chemically with sulfhydryl groups of proteins modify the response of adenylate cyclase to stimulation by beta-adrenergic agonists. N-Ethylmaleimide, an agent that alkylates sulfhydryl groups, inactivates both the catalytic moiety of adenylate cyclase and the stimulatory, regulatory guanine nucleotide binding protein Ns of rat fat cells but fails to affect binding of antagonists to the beta-adrenergic receptor [Malbon, C. C., Graziano, M. P., & Johnson, G. L. (1984) J. Biol. Chem. 259, 3254-3260]. Treating membranes of rat fat cells with dithiothreitol or beta-mercaptoethanol, agents that reduce disulfide bridges of proteins, results in a loss of binding of beta-adrenergic radioligands to the receptor. The specific binding of radioligands to beta-adrenergic receptors that are solubilized in digitonin is affected similarly by treatment with disulfide bridge reducing agents. beta-Adrenergic receptor purified from rat fat cells and treated with beta-mercaptoethanol (10%) and then subjected to gel electrophoresis in the presence of sodium dodecyl sulfate migrates as a Mr 67 000 peptide [Cubero, A., & Malbon, C. C. (1984) J. Biol. Chem. 259, 1344-1350]. In the absence of disulfide bridge reducing agents, however, the purified receptor exhibits greater electrophoretic mobility, migrating as a peptide with Mr 54 000. Treating the native form of the purified receptor with beta-mercaptoethanol (0.1-10%) or dithiothreitol (0.1-10 mM) decreases the ability of the receptor to bind beta-adrenergic ligands, decreases the electrophoretic mobility of the receptor, and results in receptor peptides migrating with molecular weight ranging from 54 000 to 67 000 when subjected to gel electrophoresis in the presence of sodium dodecyl sulfate.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptor structure, synthesis, antibodies and human disease

Lung beta 2-adrenergic receptors have been isolated using a covalent affinity label and monoclonal and autoantibodies with specificity toward the receptor. The beta 2-receptor monomer has a molecular mass of 58-64,000 daltons. Target size analysis indicates that the beta 2-receptor exists as a dimer in lung membranes. The half life of the beta 2-receptor on cultured lung cells is on the order of 20-30 h. Glucocorticoids induce the synthesis of beta 2-receptors, resulting in a doubling of the membrane concentration of receptors in 24 h. Autoantibodies to beta 2-adrenergic receptors may play a role in beta 2-receptor associated human diseases including asthma. Autoantibodies to beta 2-receptors in humans are associated with decreased sensitivity of beta 2-receptor functions and increased responsiveness of alpha-adrenergic and muscarinic cholinergic receptors.

The oligosaccharide moiety of the beta 1-adrenergic receptor from turkey erythrocytes has a biantennary, N-acetyllactosamine-containing structure

The turkey erythrocyte beta 1-adrenergic receptor can be purified by affinity chromatography on alprenolol-Sepharose and characterized by photoaffinity labeling with N-(p-azido-m-[125I]iodobenzyl)-carazolol. Through the use of the specific glycosidases neuraminidase and endo-beta-N-acetylglucosaminidase H and affinity chromatography on lectin-Sepharose gels, we show here that the receptor is an N-glycosyl protein that contains complex carbohydrate chains. No high-mannose carbohydrate chains appear to be present. The binding of the radiolabeled antagonist dihydroalprenolol to the receptor is affected neither by the enzymic treatments nor by the presence of lectins, suggesting that the carbohydrate moiety is not involved in the catecholamine binding site.

High-yield purification of plasma membranes from transformed human keratinocytes in culture

The density pertubation technique with cationic silica microbeads was applied to prepare highly purified plasma membranes from cultured human keratinocytes. Trypsinized cells were coated successively with the beads (diameter approximately 50 nm, gravity greater than 2 g/cm3) and polyacrylic acid before they were lysed by osmotic shock and mechanical shear. The plasma membranes remained in the form of large open sheets which could easily be separated from other cell organelles and the cytosol by low-speed centrifugation. The membrane preparation was characterized by scanning and transmission electron microscopy, marker enzyme activities, one-dimensional sodium dodecyl sulfate polyacrylamide electrophoresis, and the specific beta-adrenergic receptor count. A yield of 79 +/- 9% was calculated by comparing the amount of beta-adrenoceptors in the purified membrane preparation with that of a crude cellular particulate fraction. The specific beta-adrenoceptor count of these two preparations was 1.2 +/- 0.02 and 0.2 +/- 0.05 pmol/mg protein, respectively, indicating a 6-fold improved purification with this microbead technique. The purified membranes were essentially free from contamination of other cell organelles.

Reconstitution of membrane receptor systems

This review makes an attempt to summarize the present status of the field of receptor reconstitution. First a general discussion on the problem of receptor to effector coupling is discussed with an emphasis on the approaches used to solubilize, purify and reconstitute receptors with their respective biochemical effectors. Two categories of receptors have thus far been studied in great detail: (1) receptors linked to ion channels best represented by the nicotinic acetylcholine receptor and (2) receptors linked to adenylate cyclase. Through a detailed discussion of these two receptor systems the reader should get an idea of where the field of receptor reconstitution is headed. Only in the beta-adrenergic-receptor-dependent adenylate cyclase have the receptor and the effector systems been completely separated, purified and reconstituted. Therefore, a detailed discussion on that system occupies a very significant portion of this article. A summary of the state-of-the-art on a number of other receptor systems is also given in the last part of the review.

Beta-adrenoreceptors in rat brown adipose tissue: proportions of beta 1- and beta 2-subtypes

Binding of (-)-[3H]dihydroalprenolol [( 3H]DHA) to isolated brown adipose tissue (BAT) microsomal membranes was used to estimate beta-adrenoreceptor density, and this was found to be saturable, reversible, and stereospecific. Scatchard analysis indicated a single class of binding sites of equilibrium dissociation constant 2.2 nM, with a maximum number of binding sites of 160-170 fmol/mg protein. These values were unaffected by the age or sex of the animals, and similar Kd values were obtained for binding to heart and lung membranes. The kinetically derived Kd from forward and reverse reactions was 1.4 nM for BAT. Hofstee analysis of displacement of [3H]DHA produced linear plots for propranolol but curvilinear plots for atenolol and ICI 118,551. Atenolol showed 50-fold selectivity for beta 1-receptors and ICI 118,551 50-fold selectivity for beta 2-receptors in heart, lung, and BAT membranes. These plots indicated beta 1:beta 2-receptor ratios of 59:41 for BAT and heart and 25:75 for lung. It is possible that both beta-adrenoreceptor subtypes in BAT may be involved in the activation of thermogenesis in the tissue.

Cellular redistribution of beta-adrenergic receptors in a human astrocytoma cell line: a comparison with the epidermal growth factor receptor in murine fibroblasts

The redistribution of beta-adrenergic receptors (beta-AR) during agonist-induced desensitization has been compared to the process of receptor-mediated endocytosis of epidermal growth factor (EGF) in human astrocytoma cells (1321N1). [125I]EGF exhibited saturable binding to high affinity (KD = 1-2 nM) receptor sites on intact 1321N1 cells. [125I]EGF was found to internalize rapidly using an acid wash technique to remove surface bound hormone. Sucrose density gradient fractionation following exposure to EGF revealed a redistribution of EGF binding sites from high density (heavy peak) to low density (light peak) regions of the gradient. The light peak binding probably represents EGF in internalized vesicles formed during endocytosis. Low temperature (4 degrees C) or the presence of the lectin concanavalin A (con A) inhibited this ligand-induced movement of EGF receptors. When cells were incubated simultaneously with EGF and the beta-AR agonist isoproterenol, both receptors were found to co-migrate in the low density regions of sucrose gradients. No evidence of heterologous ligand-induced receptor endocytosis was found. These results suggest that the EGF receptors and beta-AR are processed in parallel by 1321N1 cells.

Reconstitution of beta-adrenergic receptor with components of adenylate cyclase

Beta 1-Adrenergic receptor proteins were extracted from turkey erythrocyte membranes with lauroyl sucrose and digitonin and purified by affinity chromatography on a column of alprenolol agarose Affi-gel 10 or 15. The 5000-fold purified receptor is able to couple functionally with the stimulatory GTP-binding protein (GS) from either turkey or duck erythrocytes. Functional coupling was achieved by three different approaches. (i) Purified beta-receptor polypeptides were coupled in phospholipid (asolectin) vesicles with GS from a crude cholate or lauroyl sucrose extract of turkey erythrocyte membranes. The detergent was removed and vesicles were formed with SM-2 beads. (ii) Purified beta-receptor was reconstituted with pure, homogeneous GS in asolectin vesicles. (iii) Purified beta-receptors were either coupled in asolectin vesicles with a mixture of pure, homogeneous Gpp(NH)p-activated GS and a lauroyl sucrose extract of turkey erythrocyte membranes, or with pure, homogeneous Gpp(NH)p-activated GS alone. The decay of activity was measured on addition of GTP and hormone. In (ii) and (iii), the detergent was removed and vesicles were formed by gel filtration on Sephadex G-50 columns. In each of the three different experimental conditions, the beta-receptor was activated with l-isoproterenol and activation was blocked with d,l-propranolol. Activated GS were measured separately by means of their capacity to activate a crude Lubrol PX-solubilized adenylate cyclase preparation from rabbit myocardial membrane. The kinetics of GS activation by purified beta-receptors occupied by l-isoproterenol was first order and activation was linearly dependent on receptor concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoaffinity labeling of beta-adrenergic receptors in mammalian tissues

Photoaffinity labeling of beta 1- and beta 2-adrenergic receptors in plasma membranes from various mammalian tissues has been been performed utilizing the recently developed beta-adrenergic antagonist probe [125I]para-azidobenzylcarazolol. Tissues studied and their proportions of beta 1 and beta 2 receptors were: rat lung (18% beta 1, 82% beta 2), rabbit lung (72% beta 1, 28% beta 2), guinea pig lung (15% beta 1, 85% beta 2), dog lung (20% beta 1, 80% beta 2) and rabbit skeletal muscle (10% beta 1, 90% beta 2). As assessed by autoradiograms of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two to three specifically protected bands of Mr 62,000-65,000, 50,000-55,000 and 38,000-42,000 were observed in each tissue system. In each case, beta-adrenergic agonists and antagonists protected against photolabeling with appropriate beta 1 and beta 2 selectivity. Thus, in rat lung the beta 2 selective antagonist ICI-118,551 was more potent in blocking incorporation than the beta 1 selective antagonist betaxolol, whereas in rat, dog and guinea pig lung and rabbit skeletal muscle epinephrine was more potent than norepinephrine in blocking labeling, indicating a beta 2 specificity in these tissues. Conversely, in rabbit lung membranes, norepinephrine was approximately equipotent with epinephrine in blocking photoincorporation, indicating a beta 1 selectivity. In some systems protease inhibitors, especially those specific for metalloproteases (EDTA, EGTA), markedly diminished the amount of the smaller Mr peptides. For example, in rat lung the ratio of Mr 62,000:47,000:36,000 peptides changed from 30:40:30 to 60:35:5 in the presence of inhibitors. These results demonstrate the applicability of using [125I]para-azidobenzylcarazolol to covalently label mammalian beta-adrenergic receptors and suggest that mammalian beta 1 and beta 2 receptor binding sites primarily reside on peptides of Mr 62,000-65,000 and that smaller ligand binding fragments may arise by proteolysis.

Use of a density shift method to assess beta-adrenergic receptor synthesis during recovery from catecholamine-induced down-regulation in human astrocytoma cells

Exposure of postconfluent 1321N1 human astrocytoma cells to 1.0 microM isoproterenol for 12-24 hr results in a 90% loss of beta-adrenergic receptors. Upon removal of agonist, recovery of beta-receptors to control levels occurs within 72 hr. The recovery of receptors is completely blocked by cycloheximide [R. C. Doss, J. P. Perkins, and T. K. Harden, J. Biol. Chem. 256:12281-12286 (1981)]. In contrast cycloheximide does not block recovery of beta-receptors after down-regulation in preconfluent cultures. To determine unambiguously if beta-receptor synthesis accounts for the recovery of receptors after down-regulation, post confluent cultures were incubated with isoproterenol and then transferred to agonist-free medium containing either normal or "heavy" (2H, 13C, 15N) amino acids. The rate and extent of beta-receptor recovery were similar in both normal and heavy amino acid-containing medium. When beta-receptors that had recovered in the heavy amino acid-containing medium were labeled with 125I-cyanopindolol, solubilized in Lubrol PX, and subjected to centrifugation on a 5-15% sucrose density gradient, they exhibited an increased mass compared to beta-receptors that recovered in the presence of normal amino acids. These results confirm that the density shift method is a useful approach for the study of beta-receptor synthesis and that new receptor synthesis occurs during recovery of beta-receptors from catecholamine-induced down-regulation in postconfluent cultures.

Localization of cAMP in dog and cat trachea: effects of beta-adrenergic agonists

Adenosine 3',5'-cyclic monophosphate (cAMP) is believed to mediate the transport of ions, water, and mucous glycoproteins in the respiratory tract. Because chemical measurements of total tissue levels of cAMP may not always reflect changes in specific cell types, we adapted standard immunocytochemical methods to examine the cellular localization of cAMP in dog and cat tracheae. The beta-adrenergic agonists terbutaline and isoproterenol increased immunoreactive cAMP in ciliated epithelial cells of dog and cat tracheae and in both serous and mucous gland cells of cat tracheae. Epithelial goblet cells did not respond to beta-adrenergic agonists in either species. This study provides information about the location of beta-receptors on individual cells in the trachea that is not available from chemical assays of either cAMP or beta-receptors in these tissues. Our results support the hypothesis that secretory functions in both serous and mucous submucosal gland cells and ciliated epithelial cells, but not goblet cells, may involve cyclic AMP-dependent mechanisms.

Isolation and characterization of beta 1-adrenergic receptors from adult, rat cardiac myocytes

The beta-receptors were isolated from rat cardiac myocytes and characterized. Isolated myocytes were prepared from adult rat hearts and characterized for viability. Membrane proteins were solubilized from myocytes with 1% Triton X-102. The solubilized membrane proteins were fractionated by DEAE-Sephacel ion exchange column chromatography. Two major protein peaks were obtained. The second protein peak sample was found to contain beta-receptors to which 125I-15-(4'-azido-3'-iodobenzyl)-carazolol (125I-ABC) was specifically bound. This sample was labeled covalently with 125I-ABC by UV irradiation. The radiolabeled sample was applied to a Sepharose CL-6B gel column. Two radiolabeled protein peaks, one with a molecular weight of approximately 570,000 and the other with a molecular weight of approximately 95,000 were found. When the 570,000-dalton complex was subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions, it was dissociated into a component with a molecular weight of 66,000. The 95,000-dalton complex was dissociated into a 58,000-dalton component upon SDS-PAGE under reducing conditions. An excess amount of isoproterenol and propranolol decreased photolabeling of the beta-receptors with 125I-ABC by 60% and 40%, respectively.

The mammalian beta 2-adrenergic receptor: reconstitution of functional interactions between pure receptor and pure stimulatory nucleotide binding protein of the adenylate cyclase system

Pure beta-adrenergic receptors (beta-AR) isolated from guinea pig lung and pure guanine nucleotide binding regulatory protein (NS) of adenylate cyclase isolated from human erythrocytes have been inserted into phospholipid vesicles, resulting in the functional coupling of these two components. The reconstitution of receptor and NS interactions results in the establishment of a guanine nucleotide sensitive state of the receptor that binds agonists with high affinity. Competition curves of isoproterenol for labeled antagonist binding to vesicles containing both beta-AR and NS are biphasic and reveal two affinity states, one of high (approximately 2 nM) and the other of low affinity (approximately 300 nM). In the presence of guanine nucleotides, the competition curves become monophasic and are shifted to a single low-affinity state for the agonist similar to the situation observed in membrane preparations. In addition, the interactions of the receptor and NS lead to the induction of a GTPase activity in NS. The GTPase activity can be stimulated by beta-adrenergic agonists such as isoproterenol (2-5-fold) and is completely blocked by antagonists such as alprenolol in a stereoselective manner. The established hormone responsive activity retains the beta 2-adrenergic specificity conferred by the pure receptor, and similar extents of stimulation (up to 4-fold) are observed with pure receptor from frog erythrocytes, indicating a similar efficiency of coupling between receptors from different species and NS.(ABSTRACT TRUNCATED AT 250 WORDS)

The mammalian beta 2-adrenergic receptor: purification and characterization

The beta 2-adrenergic receptors from hamster, guinea pig, and rat lungs have been solubilized with digitonin and purified by sequential Sepharose-alprenolol affinity and high-performance steric-exclusion liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography of iodinated purified receptor preparations reveal a peptide with an apparent Mr of 64 000 in all three systems that coincides with the peptide labeled by the specific beta-adrenergic photoaffinity probe (p-azido-m-[125I]iodobenzyl)carazolol. A single polypeptide was observed in all three systems, suggesting that lower molecular weight peptides identified previously by affinity labeling or purification in mammalian systems may represent proteolyzed forms of the receptor. Purification of the beta-adrenergic receptor has also been assessed by silver staining, iodinated lectin binding, and measurement of the specific activity (approximately 15 000 pmol of [3H]dihydroalprenolol bound/mg of protein). Overall yields approximate 10% of the initial crude particulate binding, with 1-3 pmol of purified receptor obtained/g of tissue. The purified receptor preparations bind agonist and antagonist ligands with the expected beta 2-adrenergic specificity and stereoselectivity. Peptide mapping and lectin binding studies of the hamster, guinea pig, and rat lung beta 2-adrenergic receptors reveal significant similarities suggestive of evolutionary homology.

Reconstitution of a hormone-sensitive adenylate cyclase system. The pure beta-adrenergic receptor and guanine nucleotide regulatory protein confer hormone responsiveness on the resolved catalytic unit

A hormone responsive adenylate cyclase has been reconstituted in phosphatidylcholine vesicles from its isolated protein components. The proteins used were the affinity chromatography purified (500-2000-fold) or pure Mr = 64,000 beta-adrenergic receptors (beta AR) isolated from hamster and guinea pig lung membranes, the pure heterotrimeric (Mr: alpha = 42,000; beta = 35,000; gamma approximately equal to 5,000) guanine nucleotide regulatory protein (Ns) isolated from human erythrocyte membranes; and the catalytic unit of the adenylate cyclase (C) solubilized from bovine brain caudate nucleus and resolved from beta AR and Ns by gel filtration. Adenylate cyclase activity in vesicles containing C alone was stimulated by forskolin but not by guanine nucleotides or by the beta-adrenergic agonist isoproterenol. Reconstitution of Ns and C interactions in the lipid vesicles resulted in guanine nucleotide but not beta-adrenergic agonist sensitivity. When beta AR was inserted together with Ns and C into lipid vesicles, the catalytic unit became responsive to beta-adrenergic agonists as well and this stimulation was blocked in a stereoselective manner by the beta-adrenergic antagonist alprenolol. Regulation of adenylate cyclase activity in the reconstituted system by beta-adrenergic agonists, guanine nucleotides, and Mg2+ showed properties similar to those observed in native membranes. The interactions of the various protein components in the reconstituted system were also monitored by GTPase activity. Such activity was observed to occur primarily as a result of receptor-Ns interactions. The results described in this report document the feasibility of studying hormone-responsive adenylate cyclase in a totally reconstituted system which retains the major regulatory properties of the enzyme in its native membrane-bound environment.

Direct demonstration of impaired functionality of a purified desensitized beta-adrenergic receptor in a reconstituted system

Long-term exposure of various cell types to beta-adrenergic agonists such as isoproterenol leads to an attenuated responsiveness ("desensitization") of the adenylate cyclase system to further challenge with these agonists. The turkey erythrocyte model system was used earlier to show that a covalent modification of the receptor (phosphorylation) is associated with this process. The functionality of the "desensitized" beta-adrenergic receptor was assessed by implanting purified beta-adrenergic receptor preparations from control and desensitized turkey erythrocytes into phospholipid mixtures and then fusing them with receptor-deficient cells (Xenopus laevis erythrocytes). Desensitized beta-adrenergic receptors showed a 40 to 50 percent reduction in their ability to couple to the heterologous adenylate cyclase system, comparable to the reduction in their functionality observed in their original membrane environment. These results demonstrate the utility of recently developed receptor reconstitution techniques for assessing the functionality of purified receptors and show a direct link between a covalent modification of a membrane-bound receptor and its impaired functionality in a reconstituted system.

Mammalian beta-adrenergic receptors. Distinct glycoprotein populations containing high mannose or complex type carbohydrate chains

Mammalian beta-adrenergic receptor binding peptides can be visualized by covalently labeling them with the photoaffinity reagent p-azido-m-[125I]iodobenzylcarazolol followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The receptor peptides migrate as broad bands of Mr approximately equal to 62,000. In the present study, we examined the carbohydrate composition of the mammalian beta receptor through the use of specific exo- and endoglycosidases and lectin affinity chromatography. Treatment of p-azido-m-[125I]iodobenzylcarazolol-labeled beta2-adrenergic receptors from hamster lung or rat erythrocyte with the exoglycosidases neuraminidase and alpha-mannosidase provided evidence for the existence of both high mannose and complex type carbohydrate chains on beta 2-adrenergic receptors. The nonadditivity of the effect of sequential treatments with these enzymes suggested discrete populations of beta-adrenergic receptors containing either complex or high mannose type chains. Deglycosylation of receptor with endoglycosidase F results in a single labeled polypeptide at Mr = 49,000 for both systems. The same two populations of the beta receptors (high mannose or complex type chain) could also be fractionated by lectin affinity chromatography of solubilized p-azido-m-[125I]iodobenzylcarazolol-labeled receptors. The high mannose-containing receptors could be absorbed to and specifically eluted from concanavalin A-agarose. Those containing complex type carbohydrates could be adsorbed to and eluted from wheat germ agglutinin-agarose. Taken together, these data suggest that mammalian beta-adrenergic receptors contain both complex and high mannose type carbohydrate chains and that microheterogeneity of these chains likely explains the broad band pattern typically obtained on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

The beta-adrenergic receptor survives solubilization in deoxycholate while forming a stable association with the agonist

Agonist, but not antagonist, protects the beta-adrenergic receptor from inactivation during solubilization in deoxycholate. Protection is apparently due to locking of the agonist in the receptor, a high affinity interaction induced or stabilized by this detergent. The guanyl nucleotide-binding protein which normally interacts with the receptor to induce high affinity binding of the hormone is apparently not required in this agonist-specific locking process. It is therefore possible that deoxycholate mimics the effect of the guanyl nucleotide-binding protein on the conformation of the hormone-occupied receptor. The experiments further show that the receptor in deoxycholate is quite stable for days, provided that it is occupied by the agonist. Removal of deoxycholate brings about the release of the locked agonist and the return of the receptor to the dynamic functional state.