The catecholamine binding site of the beta-adrenergic receptor is formed by juxtaposed membrane-spanning domains

The catecholamine binding domain of the turkey erythrocyte beta-adrenergic receptor was mapped by determining the sites of covalent labeling of the purified receptor by two beta-adrenergic photoaffinity reagents, [125I]iodocyanopindolol-diazirine (ICYP-da) and [125I] iodoazidobenzylpindolol (IABP). Both labels were incorporated at two separate sites. By sequencing a labeled peptide, one site of labeling was found to lie at Trp330 in the extracellular half of the seventh membrane span. This position is homologous to the retinal attachment site in rhodopsin. The second labeled site was isolated on an 8000-Da peptide and immunoprecipitated using sequence-directed antibodies. This site lies in membrane spans 3-5. Labeling of the two sites was equal using ICYP-da and 3-10-fold greater in the span 7 site using IABP. These data indicate that the catecholamine binding site is formed from the juxtaposition of span 7 and spans 3-5 in a tertiary structure probably similar to that of rhodopsin.

Mastoparan, a peptide toxin from wasp venom, mimics receptors by activating GTP-binding regulatory proteins (G proteins)

Mastoparan, a peptide toxin from wasp venom, is a nonspecific secretagogue. We show here that mastoparan increases the GTPase activity and the rate of nucleotide binding of several purified GTP-binding regulatory proteins (G proteins) whose function is to couple cell-surface receptors to intracellular mediators. Mastoparan accelerated guanosine-5'-(3-O-thiotriphosphate binding and consequent G protein activation in part by promoting the dissociation of bound GDP, the mechanism by which receptors regulate G proteins. ADP-ribosylation by pertussis toxin, which uncouples receptors from G proteins, selectively inhibited mastoparan-stimulated activation. Like receptors, mastoparan was more potent if the G protein was reconstituted in phospholipid vesicles and was active at micromolar concentrations of Mg2+. The structure of mastoparan in a lipid bilayer is similar to that predicted for a cationic intracellular loop of G protein-coupled receptors. Mastoparan thus displays a novel mode of toxicity by acting directly on G proteins to mimic the role normally played by agonist-liganded receptors.

Beta-adrenergic receptors display intramolecular disulfide bridges in situ: analysis by immunoblotting and functional reconstitution

The molecular nature of mammalian beta-adrenergic receptors in situ was probed using immunoblotting and functional reconstitution techniques. Membrane proteins of cells replete with beta-adrenergic receptors were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the resolved proteins were transferred to nitrocellulose and then probed with anti-receptor antibodies. When cell membranes were first treated with agents that cleave disulfides of proteins, immunoblots of these membranes revealed intense immunoreactive bands with electrophoretic mobility similar to that of protein standards of Mr 65,000-67,000, comigrating with purified, reduced, and alkylated beta-adrenergic receptors. However, when cell membranes were prepared under anaerobic conditions, solubilized in the presence of agents that alkylate thiols, and denatured in the absence of added thiols, immunoblotting revealed receptor with Mr 55,000, rather than 65,000. This faster electrophoretic mobility is associated with the presence of intramolecular disulfides in the purified receptor and demonstrates that beta-adrenergic receptors possess intramolecular disulfide bridges in situ. Purified receptors that demonstrate this faster mobility (Mr 55,000 under nonreducing conditions) were co-reconstituted into phospholipid vesicles with the stimulatory GTP-binding protein GS and their ability to catalyze the binding of [35S]guanosine-5'-O-(3-thio)triphosphate to GS was measured. Agonist (isoproterenol) as well as thiol increased the receptor-promoted activation of GS. Taken together, these data demonstrate that native beta-adrenergic receptors possess one or more intramolecular disulfide bridges in situ, reduction of which causes functional activation of the receptor.

The hydrophobic tryptic core of the beta-adrenergic receptor retains Gs regulatory activity in response to agonists and thiols

The function of structural domains of the beta-adrenergic receptor were probed by studying the ability of tryptic fragments of the receptor to catalyze the binding of guanosine-5'-O-(3-thiotriphosphate (GTP gamma S) to the GTP-binding regulatory protein, Gs. beta-Adrenergic receptor purified from turkey erythrocytes was treated with trypsin under nondenaturing conditions. Such treatment decreased beta-adrenergic ligand binding activity by only 15-25%. Active components of the limit digest were repurified by affinity chromatography on alprenolol-agarose and then reconstituted with purified Gs into unilamellar phospholipid vesicles. After reconstitution, the proteolyzed receptor was able to catalyze agonist-stimulated binding of GTP gamma S to Gs at a rate and extent equivalent to that of the nonproteolyzed receptor. The proteolyzed receptor was also partially activated upon reduction by dithiothreitol, as previously reported for the intact receptor (Pedersen, S.E., and Ross, E.M. (1985) J. Biol. Chem. 260, 14150-14157). The repurified, active tryptic digest contained two detectable peptides. One, of approximately 2 X 10(4) Da, contained either four or five of the amino-terminal membrane-spanning domains plus the intervening hydrophilic loops but not the amino-terminal extracellular, glycosylated peptide. The second, of 9,000-10,000 Da, was composed essentially of the two carboxyl-terminal membrane-spanning domains and the intervening extracellular, hydrophilic loop. These data indicate that most of the large intracellular hydrophilic loop and the hydrophilic, carboxyl-terminal region of the receptor are not necessary for the agonist-stimulated regulation of Gs.

The effect of activating ligands on the intrinsic fluorescence of guanine nucleotide-binding regulatory proteins

The intensity of the tryptophan fluorescence of the alpha subunits of guanine nucleotide-binding regulatory proteins increases when they bind guanosine 5'-O-(3-thio)triphosphate (GTY gamma S). The kinetics of the fluorescence enhancement and of the measured binding of [35S]GTP gamma S are well correlated. The addition of Mg2+ to the nucleotide-bound proteins causes a further, rapid increase in the fluorescence intensity. Similar effects result from exposure of the proteins to F- and Mg2+, and the required concentration of F- is reduced by the inclusion of Al3+. It is presumed that the more highly fluorescent state of the G protein alpha subunits represents their active conformation.

Effect of Al3+ plus F- on the catecholamine-stimulated GTPase activity of purified and reconstituted Gs

The effects of Al3+ and F- on the catecholamine-stimulated GTPase cycle were studied by using reconstituted phospholipid vesicles that contained purified beta-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, Gs. Al3+/F- activated reconstituted Gs to levels previously reported for detergent-solubilized, purified Gs, although both activation and deactivation were faster in the reconstituted preparation. Under these conditions, Al3+/F- did not inhibit by more than 15% the beta-adrenergic agonist-stimulated GTPase activity of the vesicles nor did it significantly inhibit the rates of GTP binding, GTP hydrolysis, or GDP release. When Mg2+ (50 mM) was used instead of agonist to promote GTP hydrolysis in the receptor-Gs vesicles, Al3+/F- was found to inhibit GTP gamma S binding, GDP release, and steady-state GTPase activity to unstimulated levels. These data can be interpreted as indicating that the receptor catalyzes nucleotide exchange by Gs faster or more efficiently than does Mg2+.

Catecholamine-stimulated GTPase cycle. Multiple sites of regulation by beta-adrenergic receptor and Mg2+ studied in reconstituted receptor-Gs vesicles

The regulation of the intermediary steps of the catecholamine-stimulated GTPase cycle by beta-adrenergic agonists and Mg2+ was investigated using unilamellar phosphatidylethanolamine-phosphatidylserine vesicles that contained purified beta-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, Gs. The steady-state turnover number of the agonist-stimulated GTPase, normalized according to the receptor-responsive pool of Gs, was 0.8 min-1 for untreated vesicles and 1.7 min-1 for vesicles that had been treated with dithiothreitol to activate the receptors. The binding and release of [alpha-32P]GTP, [3H] GTP, and [gamma-32P]GTP were used to measure the binding and hydrolysis of GTP and the release of GDP. Agonist-liganded receptor stimulated both the binding of GTP and the release of the GDP product, and GDP release per se did not appear to be the mechanism by which receptor stimulated the binding of GTP. Both processes displayed apparent first order rate constants of about 0.5 min-1 for untreated vesicles and both rates increased about 5-fold after dithiothreitol treatment. Both processes were formally catalytic with respect to receptor, in that several (up to 8) molecules of Gs were stimulated per molecule of receptor. The hydrolysis of Gs X GTP to Gs X GDP was unaltered by agonist and occurred with a rate constant of about 4 min-1. The rates of these partial reactions were consistent with the overall rate of steady-state hydrolysis and with the ability of the agonist-liganded receptor to promote the formation of sufficient Gs X GTP to fully stimulate adenylate cyclase in a native membrane. The Mg2+ dependence of agonist-stimulated, steady-state GTPase activity appeared to consist of at least two, distinct Mg2+-requiring processes. Very low concentrations of Mg2+ (approximately 20 nM) were required for hydrolysis of Gs X GTP, and 10 microM Mg2+ was required to maximize the initial rate of agonist-stimulated [alpha-32P] GTP binding.

Severe atherosclerotic coronary artery disease, healed myocardial infarction and chronic congestive heart failure: analysis of 81 patients studied at necropsy

Observations are described in 81 necropsy patients (aged 29 to 91 years [mean 62]; 77 [95%] men) with severe congestive heart failure (CHF) more than 3 months in duration, left ventricular (LV) transmural scar and greater than 75% cross-sectional area narrowing by atherosclerotic plaque of 1 or more of the 4 major epicardial coronary arteries. The duration of symptoms from initial onset of acute myocardial infarction (59 patients) or CHF (18 patients) or angina pectoris (2 patients) to death ranged from 0.5 to 18 years (mean 7.1) (2 unknown). Angina pectoris occurred at some time, however, in 31 patients (38%). Cause of death was CHF in 48 patients (59%), sudden (arrhythmia) in 16 (20%), acute myocardial infarction in 11 (14%), and emboli in 6 (7%). The heart weight ranged from 410 to 800 g (mean 585). Left or right ventricular thrombi or both occurred in 37 patients (46%), only 4 (10%) of whom had systemic emboli; of the 44 patients without intracardiac thrombi, none had any form of emboli. The severity of coronary narrowing was variable. In 24 patients (30%) only 1 artery was narrowed greater than 75% in cross-sectional area; in 22 patients (27%), 2 arteries were so narrowed; in 32 patients (39%), 3 arteries; and in 3 patients (4%), 4 arteries. The size of the LV scar also varied. Of the 81 patients, 58 (72%) had large scars (involving greater than 40% of the LV wall); 10 (12%) had moderate-sized scars (6 to 40% of the LV wall); and 13 (16%) had small scars (less than or equal to 5% of the LV wall).(ABSTRACT TRUNCATED AT 250 WORDS)

Severe atherosclerotic coronary arterial narrowing and chronic congestive heart failure without myocardial infarction: analysis of 18 patients studied at necropsy

Observations are reported on 18 patients (aged 38 to 73 years ([mean 58]; 16 [89%] men) studied at necropsy who had had chronic congestive heart failure (CHF) more than 3 months in duration, greater than 75% cross-sectional area (XSA) narrowing of 1 or more of the 4 major epicardial coronary arteries, and no left ventricular fibrosis or necrosis. Duration of symptoms from onset of CHF to death ranged from 0.3 to 13 years (mean 5.7). Angina pectoris occurred in 2 patients (11%). The mode of death was CHF in 12 (67%), sudden (arrhythmia) in 5 (28%), and emboli in 1 (5%). Heart weight ranged from 410 to 890 g (mean 632). Of 72 major epicardial coronary arteries (right, left main, left anterior descending, left circumflex) in the 18 patients, 30 (42%) were narrowed 76 to 100% in XSA by atherosclerotic plaque (right = 10, left main = 0, left anterior descending = 9 and left circumflex = 11). A mean of 1.7 of 4 major epicardial coronary arteries per patient were narrowed 76 to 100% in XSA by atherosclerotic plaque. In 10 patients, each 5-mm segment of the 4 major coronary arteries was examined histologically (mean 53 per patient): 23 segments (3%) were narrowed 96 to 100% in XSA by atherosclerotic plaque; 58 (11%), 76 to 95; 93 (18%), 51 to 75%; 209 (40%), 26 to 50%, and 146 (28%), 0 to 25%. Left and right ventricular thrombi were found in 9 patients (50%); of the 9 patients, 1 had a systemic embolus; of the 9 patients without intraventricular thrombi, none had systemic emboli.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of catecholamine-stimulated adenylate cyclase activity using three purified proteins

beta-Adrenergic receptors, the GTP-binding regulatory protein that stimulates adenylate cyclase (Gs), and adenylate cyclase were each purified and reconstituted into unilamellar vesicles composed of phosphatidylethanolamine and phosphatidylserine (3:2, w/w). The molar ratio of receptor:Gs:adenylate cyclase was estimated to be about 1:10:1. Adenylate cyclase activity in the vesicles was stimulated up to 2.6-fold by beta-adrenergic agonists. Stimulation was dependent on the presence of guanine nucleotide, displayed appropriate beta-adrenergic selectivity and stereoselectivity for agonists, and was blocked appropriately by beta-adrenergic antagonists. Therefore, while additional proteins may modulate adenylate cyclase activity in native membranes, these results show that these three proteins are sufficient for the expression of hormone-stimulated adenylate cyclase.

Functional activation of beta-adrenergic receptors by thiols in the presence or absence of agonists

Treatment of beta-adrenergic receptor with dithiothreitol (DTT) or other thiol compounds caused its functional activation in the presence or absence of agonist ligands. Such activation was observed in reconstituted unilamellar phospholipid vesicles that contained beta-adrenergic receptors, purified to greater than or equal to 95% homogeneity from turkey erythrocyte plasma membranes, and the stimulatory GTP-binding protein of the adenylate cyclase system (Gs) purified from rabbit liver. Incubation of the vesicles with 2-10 mM DTT at 0 degrees C for 1 h increased the rate (4-5-fold) and the extent (3-4-fold) of activation of Gs by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding, an effect about equivalent to the addition of beta-adrenergic agonists. Treatment with DTT also markedly potentiated the ability of agonists to stimulate GTP gamma S binding, increasing the initial rate about 10-fold. DTT treatment was as effective as agonist in stimulating GTPase activity, and maximal stimulation was obtained when DTT-treated vesicles were assayed in the presence of agonist. Other thiol compounds produced effects similar to those of DTT but were at least 10-fold less potent. Stimulation of GTP gamma S binding or GTPase activity required active receptor, and treatment of the receptor with DTT prior to reconstitution also increased its efficacy. There was no effect of DTT on Gs alone. Thus, the site of action of DTT appears to be on the beta-adrenergic receptor itself, and the reduction of disulfides and the binding of agonist act synergistically to activate the receptor. DTT treatment made the receptor more labile to thermal denaturation. Inclusion of cholesterol or cholesteryl-hemisuccinate (5-25%) in the vesicles protected the reduced receptor against such denaturation and enhanced its recovery during reconstitution. No effect of cholesterol or cholesteryl-hemisuccinate was observed on the stability of the nonreduced receptor, which was comparable to that observed in native membranes.

A developmental test based on the STYCAR sequences used in the National Childhood Encephalopathy Study

During the National Childhood Encephalopathy Study, standard neurodevelopmental assessments had to be performed on a large number of children under 3 years of age scattered throughout Great Britain. Currently available tests were reviewed but were found to be impractical for this purpose. We describe a simple test procedure, based on the STYCAR sequences, suitable for use in a clinic or at home with the minimum of special equipment. Results were reliable and provided an informative record of a child's developmental progress. It may be possible to modify the scheme for general use.

The carcinoid syndrome: comparison of 21 necropsy subjects with carcinoid heart disease to 15 necropsy subjects without carcinoid heart disease

Carcinoid heart disease is a morphologically specific type of cardiac disorder that involves the mural and valvular endocardium on the right side of the heart. Twenty-one subjects (57 percent) (Group I) with carcinoid heart disease and 15 subjects (43 percent) (Group II) without carcinoid heart disease were studied at necropsy. The two groups were similar in mean age (54 years versus 55 years), duration of clinical illness (4.7 years versus 6.3 years), body weight (50 kg versus 52 kg), systemic blood pressure (117/77 mm Hg versus 128/77 mm Hg), blood hematocrit levels (37 percent versus 36 percent), total serum protein levels (6.0 g/dl), and serum albumin levels (2.2 g/dl versus 2.6 g/dl). The two groups were different in the frequency of the presence of precordial murmurs consistent with tricuspid regurgitation and/or pulmonic stenosis (95 percent versus 13 percent), cardiomegaly by chest radiography (38 percent versus 0), low voltage on electrocardiography (47 percent versus 0), and location of the primary site of the carcinoid tumor. Total electrocardiographic 12-lead QRS voltage was similar in each group (105 mm versus 132 mm) (10 mm = 1 mV). Of Group I subjects, 43 percent died of cardiac causes; none of the Group II subjects died of cardiac causes. Of the 21 subjects with carcinoid heart disease, seven had left-sided cardiac involvement, but in none was it of functional significance. Thus, although carcinoid heart disease frequently is the cause of death in patients with the carcinoid syndrome, the development of carcinoid heart disease is not related to the duration of symptoms of the carcinoid syndrome.