Identification and sequence of a binding site peptide of the beta 2-adrenergic receptor

Three-dimensional structure for the beta 2 adrenergic receptor protein based on computer modeling studies

Computer-aided model building techniques have been used to construct three-dimensional model structures for hamster beta 2 adrenergic receptor. Experimental data were used as constraints to guide the model building procedure, and a number of rather strict criteria were applied to assess the physical plausibility of model structures. We present details of our best model structure to date, which is consistent with a large body of experimental data. We also discuss in detail our model building procedures and evaluation criteria, which we believe may be of general utility in modeling projects.

In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors

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Immunoblots with rhodopsin antisera suggest that a purified mu opioid binding protein has structural characteristics of a G-protein-coupled receptor

A mu opioid binding protein (OBP), previously purified to homogeneity from bovine striatal membranes, was examined by immunoblotting with six antisera against bovine rhodopsin. An antibody against the carboxyl-terminal tail of rhodopsin and one against membrane-associated rhodopsin gave strong signals at the appropriate molecular mass (65 kDa). An antibody directed against the first cytoplasmic loop of rhodopsin was weakly reactive. Three other antibodies did not recognize OBP. This pattern of crossreactivity was identical to that previously seen with beta-adrenergic receptors. The existence of domains in the OBP, which are antigenically similar to those in two other guanine nucleotide regulatory protein-coupled receptors, supports the hypothesis that mu opioid receptors have the structure characteristic of this receptor family.

Sequence alignment of the G-protein coupled receptor superfamily

The multitude of G-protein coupled receptor (GPR) superfamily cDNAs recently isolated has exceeded the number of receptor subtypes anticipated by pharmacological studies. Analysis of the sequence similarities and unique features of the members of this family is valuable for designing strategies to isolate related cDNAs, for developing hypotheses concerning substrate-ligand and receptor-effector interactions, and for understanding the evolution of these genes. We have compiled and aligned the 74 unique amino acid sequences published to date and review the present understanding of the structural motifs contributing to ligand binding and G-protein coupling.

Approaches to molecular modeling studies and specific application to serotonin ligands and receptors

Molecular modeling studies are useful in as much as they may allow us to understand the activity and selectivity of currently existing agents, and, furthermore, may aid in the design of completely novel therapeutic agents. There are two basic modeling strategies: the ligand-ligand approach and the ligand-receptor approach. Both approaches possess certain inherent advantages and disadvantages and, in addition, make certain assumptions about the agents and/or receptors being investigated. Keeping with the spirit of this minisymposium, we describe these two approaches, their general usefulness, and their limitations. Using serotonin (5-HT) receptors as a focal point, we review and provide novel examples of molecular modeling studies involving both strategies. Presented for the first time are examples of ligand-receptor models to account for the binding of serotonergic agents at 5-HT2 and 5-HT1C receptors.

Interactions between the argininyl moieties of neurotensin and the catechol protons of dopamine

The interaction between dopamine and neurotensin as well as other Arg-containing peptides was studied to provide more chemical details of how dopamine binds to the neuropeptide neurotensin. The stoichiometry of 1:1, dopamine to neurotensin, was confirmed by additional electroanalytical and ultraviolet-visible spectroscopic studies. By analyses of the 205- to 340-nm difference spectra of fixed concentrations of dopamine in the presence of increasing amounts of neurotensin, the dissociation constant of the interaction was found to be 5.9 x 10(-8) mol/L. This finding confirmed (by a second physical method) the previously reported KD value obtained by electroanalytical techniques. The associations between dopamine and neurotensin as well as the neurotensin fragment Pro7-Arg8-Arg9-Pro10 were found to be pH dependent when the dissociation constant was measured as a function of pH (in 150 mmol/L NaCl). The results of studies of the formal potential of dopamine in the presence of Arg and Arg-containing peptides confirmed that catechol protons are directly involved in the association and that the chemical species of dopamine associated with neurotensin is a catecholate form. The (pseudo)-first-order rate constant of dissociation of the complex at pH 7.6, measured by the chronoamperometric and rotating disk electroanalytical techniques, was found to be approximately 10(5) s-1, indicating that the rate of formation of the complex is under diffusion control. A hypothetical chemical structure of the neurotensin-dopamine complex is suggested.

Structure/function relationship of proteins belonging to the family of receptors coupled to GTP-binding proteins

The structural properties of a number of proteins belonging to the family of receptors coupled to GTP binding proteins are discussed in relation to their function. The structure of the ligand binding site and of the regions involved in coupling to the G proteins are analyzed mainly for the adrenergic and muscarinic cholinergic receptors, for which site-directed mutagenesis and chimaeric constructions have been studied. The structure of the genes are compared and the presence of various regulatory elements is discussed in relation to control of expression. Mechanism of desensitization and internalization, while mostly studied for the beta 2-adrenergic receptor, are proposed to be generally applicable to all G-protein-coupled receptors.

Receptors regulating acid secretion

Histamine stimulation of gastric acid secretion has for a long time been known to be mediated by an H2-type receptor located on the parietal cell surface, but the biochemical nature of this receptor has only very recently been elucidated. It is a 70-kDa glycoprotein showing structural analogies with the beta 2-adrenergic receptor and the other seven membrane-spanning domains/G protein-coupled receptors. It activates adenylated cyclase through a cholera toxin-sensitive, pertussis toxin-insensitive, guanosine 5'-triphosphatase-binding regulatory Gs protein. The cAMP thereby produced is believed to play a crucial role in the opening of the Cl- channel associated with the (H+,K+)-ATPase in the secretory membrane. However, other sites of action are likely to be involved, since several histamine- or cAMP-dependent phosphoproteins have been detected in the parietal cell. In addition to its action on cAMP production, histamine was found to produce a transient increase in the intracellular Ca2+ concentration, but this effect remains unexplained. On the other hand, the intervention of an H3-type histamine receptor in the regulation of gastric acid secretion has recently been documented, but the cellular location of this new receptor has not been yet investigated.

A human homologue of the yeast HDEL receptor

Retention of resident proteins in the lumen of the endoplasmic reticulum is achieved in both yeast and animal cells by their continual retrieval from the cis-Golgi, or a pre-Golgi compartment. Sorting of these proteins is dependent on a C-terminal tetrapeptide signal, usually Lys-Asp-Glu-Leu (KDEL in the single letter code) in animal cells, His-Asp-Glu-Leu (HDEL) in Saccharomyces cerevisiae. There is evidence that the ERD2 gene encodes the sorting receptor that recognizes HDEL in yeast; its product is an integral membrane protein of relative molecular mass 26,000 (26K) that is not glycosylated. In contrast, Vaux et al. suggest that the mammalian KDEL receptor is a 72K glycoprotein that they detected using an anti-idiotypic antibody approach. If this were so, it would indicate a surprising divergence of the retrieval machinery between yeast and animal cells. We report here that human cells express a protein similar in sequence, size and properties to the ERD2 product, and propose that this protein is the human KDEL receptor.

Biotechnology of beta-adrenergic receptors

The emergence of Biotechnology has provided pharmacologists with a variety of methods for investigating the structure, the function, and the regulation of membrane-bound receptors with a precision that was not imagined even five years ago. These new tools have been developed and used to analyze the known catecholamine beta 1- and beta 2-adrenergic receptors and to discover and study a new subtype, the beta 3 receptor. We review here the salient features of each of these three receptors, compare their structural and functional properties, and propose models to explain their differential regulation in time and space. A whole family of proteins has now been found to share with the beta-adrenergic receptors their most prominent features, including seven transmembrane domains and coupling with GTP-binding "G" proteins. We therefore propose that the biotechnology-based procedures developed for the beta-adrenergic receptors will be well applicable to the other members of this "R7G" family of receptors.

Protein engineering and the study of structure--function relationships in receptors

Protein engineering is a powerful tool for studying relationships between receptor structure and function--providing that it is used and interpreted appropriately. Site-directed mutagenesis, deletion mutagenesis and construction of chimaeric proteins have all been used to characterize receptors. In this review, Walter Ward, David Timms and Alan Fersht describe the application of protein engineering, illustrating important concepts with experimental data. They explain that detailed study of function requires careful dissection of mechanistic steps. Care must also be taken when selecting replacement residues; mutation should not cause delocalized structural reorganization or else the true significance of functional change will remain unclear.

Location in muscarinic acetylcholine receptors of sites for [3H]propylbenzilylcholine mustard binding and for phosphorylation with protein kinase C

Muscarinic acetylcholine receptors purified from porcine cerebra or atria were covalently labeled with [3H]propylbenzilylcholine mustard ([3H]PrBCM), and then the labeled receptors were subjected to limited hydrolysis with trypsin, V8 protease, and lysyl endopeptidase, followed by analysis involving sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fluorography, autoradiography, or immunostaining. The labeled peptides were located on the basis of their reactivity with antibodies raised against three synthetic peptides with partial sequences of the m1 or m2 receptor, and of their sensitivity to endoglycosidase F, which was taken as evidence that they contain glycosylation sites near the N terminus. The [3H]PrBCM-binding site in both cerebral and atrial receptors was found to be located between the N terminus and the second intracellular loop, because the size of the smallest deglycosylated peptide that contained both the [3H]PrBCM-binding and glycosylation sites was approximately 16 kDa. Cerebral receptors were 32P-phosphorylated with protein kinase C, and the major phosphorylation sites in cerebral muscarinic receptors were found to be located in a C-terminal segment including a part of the third intracellular loop, because a 32P-labeled peptide of 12-14 kDa reacted with anti-(m1 C-terminal peptide) antiserum. The presence of an intramolecular disulfide bond, probably between Cys 98 and Cys 178 in the first and second extracellular loops, respectively, was suggested by the finding that a peptide of approximately 17 kDa containing the [3H]PrBCM-binding site, but not the glycosylation sites, was partly converted to a peptide of approximately 12 kDa on treatment with beta-mercaptoethanol.

Human cytomegalovirus encodes three G protein-coupled receptor homologues

Human cytomegalovirus (HCMV) is a herpesvirus with a genome of 230 kilobases (Kb) encoding about 200 genes. Although infection is generally innocuous, HCMV causes serious congenital and neonatal disease, and is a dangerous opportunistic pathogen in immune-deficient individuals. We have identified a family of three HCMV genes which encode polypeptides containing seven putative membrane-spanning domains, and a series of well-defined motifs characteristic of the rhodopsin-like G protein-coupled receptors (GCRs). By these criteria all three of the HCMV sequences are homologous to cellular GCRs. Members of this receptor family function in visual signal transduction, regulation of homeostasis, and development, and include known and potential oncogenes. These receptors are activated by photons or small molecules such as neurotransmitters, and glycoprotein hormones. The finding of viral-encoded GCR homologues implies a further level of complexity in the interactions between HCMV and its host, and may provide a potential pathway for virally transformed cell proliferation. Their identification could permit the development of a novel class of antiviral drugs analogous to beta-adrenergic receptor antagonists.

Limited proteolysis of the vasoactive intestinal peptide receptor: comparison of its folded structure in the membrane-bound and detergent-solubilized states

Limited proteolysis was used to probe and compare the conformation of the rat lung vasoactive intestinal peptide (VIP) receptor in membrane-bound and detergent-solubilized states. It had been shown previously that the activity of the detergent-solubilized VIP receptor is sensitive to the nature of the detergent used for extraction (Patthi, S., Simerson S. and Velicelebi, G. (1988) J. Biol. Chem., 263, 19363-19369). Receptors that were extracted from the membrane using digitonin retained the ability to bind 125I-VIP, while those solubilized in Triton X-100 displayed little or no detectable activity. In order to correlate the differences observed in the activity of the receptor with its folded state, membrane-bound and detergent-solubilized receptors were covalently labeled with 125I-VIP and subjected to limited proteolysis using trypsin, chymotrypsin or carboxypeptidase Y. Digitonin-solubilized receptors most closely resembled the membrane-bound protein in terms of protease sensitivity and proteolytic cleavage products. By contrast, receptors solubilized in Triton X-100 displayed increased sensitivity to proteases and produced distinctly different proteolytic patterns. Thus, the differences observed in the activities of receptors solubilized in digitonin and those solubilized in Triton X-100 could be correlated with detectable differences in the conformation of the protein in each respective detergent solution. These results suggest that digitonin provides an environment that is more compatible with the native folded state of the receptor, similar to its conformation in the membrane.

Role of extracellular disulfide-bonded cysteines in the ligand binding function of the beta 2-adrenergic receptor

Evidence is presented for a role of disulfide bridging in forming the ligand binding site of the beta 2-adrenergic receptor (beta AR). The presence of disulfide bonds at the ligand binding site is indicated by "competitive" inhibition by dithiothreitol (DTT) in radioligand binding assays, by specific protection by beta-adrenergic ligands of these effects, and by the requirement of disulfide reduction for limit proteolysis of affinity ligand labeled receptor. The kinetics of binding inhibition by DTT suggest at least two pairs of disulfide-bonded cysteines essential for normal binding. Through site-directed mutagenesis, we indeed were able to identify four cysteines which are critical for normal ligand binding affinities and for the proper expression of functional beta AR at the cell surface. Unexpectedly, the four cysteines required for normal ligand binding are not those located within the hydrophobic transmembrane domains of the receptor (where ligand binding is presumed to occur) but lie in the extracellular hydrophilic loops connecting these transmembrane segments. These findings indicate that, in addition to the well-documented involvement of the membrane-spanning domains of the receptor in ligand binding, there is an important and previously unsuspected role of the hydrophilic extracellular domains in forming the ligand binding site.

Molecular characterization of a functional cDNA encoding the rat substance P receptor

Substance P is a member of the tachykinin peptide family and participates in the regulation of diverse biological processes. The polymerase chain reaction and conventional library screening were used to isolate a complementary DNA (cDNA) encoding the rat substance P receptor from brain and submandibular gland. By homology analysis, this receptor belongs to the G protein-coupled receptor superfamily. The receptor cDNA was expressed in a mammalian cell line and the ligand binding properties of the encoded receptor were pharmacologically defined by Scatchard analysis and tachykinin peptide displacement as those of a substance P receptor. The distribution of the messenger RNA for this receptor is highest in urinary bladder, submandibular gland, striatum, and spinal cord, which is consistent with the known distribution of substance P receptor binding sites. Thus, this receptor appears to mediate the primary actions of substance P in various brain regions and peripheral tissues.

Adrenergic receptors. Models for regulation of signal transduction processes

Adrenergic receptors are prototypic models for the study of the relations between structure and function of G protein-coupled receptors. Each receptor is encoded by a distinct gene. These receptors are integral membrane proteins with several striking structural features. They consist of a single subunit containing seven stretches of 20-28 hydrophobic amino acids that represent potential membrane-spanning alpha-helixes. Many of these receptors share considerable amino acid sequence homology, particularly in the transmembrane domains. All of these macromolecules share other similarities that include one or more potential sites of extracellular N-linked glycosylation near the amino terminus and several potential sites of regulatory phosphorylation that are located intracellularly. By using a variety of techniques, it has been demonstrated that various regions of the receptor molecules are critical for different receptor functions. The seven transmembrane regions of the receptors appear to form a ligand-binding pocket. Cysteine residues in the extracellular domains may stabilize the ligand-binding pocket by participating in disulfide bonds. The cytoplasmic domains contain regions capable of interacting with G proteins and various kinases and are therefore important in such processes as signal transduction, receptor-G protein coupling, receptor sequestration, and down-regulation. Finally, regions of these macromolecules may undergo posttranslational modifications important in the regulation of receptor function. Our understanding of these complex relations is constantly evolving and much work remains to be done. Greater understanding of the basic mechanisms involved in G protein-coupled, receptor-mediated signal transduction may provide leads into the nature of certain pathophysiological states.

Structure of the lutropin/choriogonadotropin receptor

In summary, the LH/CG receptor is a single polypeptide which contains a large hydrophilic domain that is situated extracellularly, attached to a region that spans the plasma membrane seven times, the carboxy-terminal region being intracellular. This topology was predicted by the amino acid sequence and has been confirmed by our immunofluorescence studies. The extracellular domain, which is related to a family of leucine-rich glycoproteins, is presumably involved in binding the large glycoprotein hormones hCG and LH. The carboxy-terminal half of the receptor, which is related to the family of rhodopsinlike receptors, is (by analogy with these receptors) presumably involved in the coupling of the receptor to the G protein. Our transfection studies confirm that this single polypeptide is capable of binding hormone and activating adenylyl cyclase. Therefore, not only is the structure of the LH/CG receptor unique compared to other cell surface receptors characterized to date, but also its structure suggests that the mechanism of the translation of hormone binding to G protein coupling in this receptor is different from other G protein-coupled receptors whose ligands are much smaller and intercalcate among the transmembrane helices. We predict that, due to the homology among the glycoprotein hormones, the structures of the FSH and TSH receptors share extensive amino acid and structural homology with the LH/CG receptor. Last, our newly acquired knowledge about the structure of the LH/CG receptor, and the development of a cDNA and antibodies for this receptor, should enable more detailed studies on the function and regulation of the LH/CG receptor, not previously possible.

Muscarinic cholinergic receptor of rat cerebral cortex. Location and characterization of ligand binding site-carrying peptides in synaptosomal membranes and isolated neuronal perikarya

A careful examination of the location and biochemical properties of the tryptic peptides identified by site-specific labelling of the muscarinic cholinergic receptor (mAChR) of rat cerebral cortex has been carried out. In brain synaptosomal membranes and isolated neuronal perikarya, mAChR labelled with [3H]propylbenzilylcholine mustard (PrBCM) was tryptically cleaved to peptides of Mr 50,000, 30,000. 18,000 and a limiting fragment of Mr 8000. All of these binding site-carrying fragments, characterized in terms of their content of carbohydrates and thiol groups, were quantitatively recovered as membrane-bound peptides. The delipidated [3H]PrBCM-labelled tryptic limiting fragment was found to be highly hydrophobic and insoluble in aqueous media. Experiments performed with proteinase on the tryptic limiting fragment suggest the existence of an ester linkage between the ligand and the peptide. The results strongly support the hydropathicity profile which predicts the location of the muscarinic receptor protein with respect to the membrane bilayer.

Non classical, multiple-site interaction of [3H]-prazosin with the alpha 1-adrenoceptor of intact BC3H1 cells

1. In intact BC3H1 cells the EC50 of noradrenaline (NA) for the inositol phosphate response measured at 37 degrees C (EC50 = 193 nM) was much lower than its apparent dissociation constant (Ki37 degrees C = 83.211 microM) determined at this temperature by [3H]-prazosin binding. 2. After pretreatment of the cells with NA at 37 degrees C for 45 min, the time used in binding assays at this temperature, this difference between EC50 and Ki37 degrees C did not decrease significantly. An agonist-induced reduction in alpha 1-adrenoceptor affinity can therefore not explain the very high Ki37 degrees C value. 3. NA pretreatment at 37 degrees C decreased the number of [3H]-prazosin binding sites (assessed by whole cell binding at 2 degrees C) by only 49%; not by 100%, the value expected if agonist-induced receptor internalization were the origin of the very low Ki37 degrees C. 4. The EC50 of NA for the inositol phosphate response in the presence of 156 pM [3H]-prazosin was 1.841 microM but the IC50 of NA for the inhibition of [3H]-prazosin binding (126 pM) was 316 microM. As there is no alpha 1-adrenoceptor reserve in these cells we propose that at 37 degrees C [3H]-prazosin interacts, not only with the catecholamine recognition site (site 1) of the receptor, but also reacts weakly with another site from which it cannot be directly displaced by catecholamine-like substances (site 2).

Evidence that the voltage-dependent component in the fertilization process is contributed by the sperm

To investigate the mechanisms that account for the voltage dependence of fertilization and provide an electrical block to polyspermy, we studied cross-fertilizations between three species of amphibians having different degrees of voltage dependence. Anurans, such as the toad Bufo japonicus, as well as the primitive urodele Hynobius nebulosus, have voltage-dependent fertilization; other urodeles, such as Cynops pyrrhogaster, have voltage-independent fertilization (Y. Iwao, 1989, Dev. Biol. 134, 438-445). Entry of Hynobius sperm into Cynops eggs was blocked by clamping the egg's membrane potential at +40 mV, as is the case for fertilization of Hynobius eggs with Hynobius sperm, but not for fertilization of Cynops eggs with Cynops sperm. Therefore, fertilization was voltage dependent in an experimental condition where only the sperm could be contributing this characteristic. The voltage-dependent properties of fertilization between Bufo eggs and Hynobius sperm were also characteristic of the sperm species; fertilization was blocked at +50 mV as in Hynobius fertilization, but not at +20 mV as in Bufo fertilization. These results support the conclusion that the voltage dependence of fertilization results from a component contributed by the sperm.

Molecular modeling of a putative antagonist binding site on helix III of the beta-adrenoceptor

In recent biochemical studies it was demonstrated that residue Asp113 of the beta-adrenoceptor (beta-AR) is an indispensable amino acid for the binding of beta-AR antagonists. Earlier fluorescence studies showed that a tryptophan-rich region of the beta-AR is involved in the binding of propranolol, the prototype beta-AR antagonist. Bearing these two biochemical findings in mind, we explored the beta-AR part containing Asp113, for an energetically favorable antagonist binding site. This was done by performing molecular docking studies with the antagonist propranolol and a specific beta-AR peptide which included, besides Asp113, two possibly relevant tryptophan residues. In the docking calculations, the propranolol molecule was allowed to vary all its internal torsional angles. The receptor peptide was kept in an alpha-helix conformation, while side chains relevant to ligand binding were flexible to enable optimal adaptations to the ligand's binding conformation. By means of force-field calculations the total energy was minimized, consisting of the intramolecular energies of both ligand and receptor peptide, and the intermolecular energy. We found an antagonist binding site, consisting of amino acids Asp113 and Trp109, which enabled energetically favorable interactions with the receptor-binding groups of propranolol. According to these results, binding involves three main interaction points: (i) a reinforced ionic bond; (ii) a hydrogen bond; and (iii) a hydrophobic/charge transfer interaction. The deduced binding site shows a difference in affinity between the levo- and dextrorotatory isomers of propranolol caused by a difference in ability to form a hydrogen bond, which is in conformity with the experimentally observed stereoselectivity. Moreover, it also provides an explanation for the beta 1-selectivity of p-phenyl substituted phenoxypropanolamines like betaxolol. The p-phenyl substituent of betaxolol was shown to be sterically hindered upon binding to the beta 2-AR peptide, whereas this hindrance is very likely to be much less with the beta 1-AR peptide. Finally, the proposed antagonist binding site is discussed in the light of some recent biochemical findings and theories.

Consensus residues at the acetylcholine binding site of cholinergic proteins

The nicotinic (nAcChR) and muscarinic (mAcCh) acetylcholine receptors and acetylcholinesterase (AcChEase) are structurally unrelated but share a common functional property: interaction with acetylcholine (AcCh). Alignment of the probable AcCh binding site regions of the nAcChR and mAcChR protein sequences revealed the presence of ten nearly identically spaced consensus residues, six of which contain potentially ligand-interactive side chains. Important elements of the consensus residues also were found in one unique sequence region of the AcChEases. Alignments among the two receptors and AcChEase outside the apparent binding region were rare, and the consensus AcCh binding residues were largely substituted in the homologous proteins, which do not bind AcCh. The consensus residues include two possible anionic subsite Asp residues and a Ser that may hydrogen bond to the AcCh carbonyl in the receptors. These residues correspond to positions Asp-166, Ser-173, and Asp-200 in the neuromuscular nAcChR; Asp-71, Ser-78, and Asp-105 in the M1 mAcChR; and Asp-93 and Asp-128 in Torpedo AcChEase. No corresponding consensus Ser is found in the AcChEase sequence; this is expected because of a downstream esterase active-site Ser-200 (Torpedo). A receptor-conserved and disulfide-linked Cys corresponding to neuromuscular nAcChR residue 193 and M1 mAcChR residue 97 may be important in energy transduction associated with agonist-mediated events. The presence of additional binding-site aromatic residues that may form a hydrophobic environment near the anionic subsite are aligned within, but not between, the three cholinergic protein groups. These observations target specific regions and residues within these proteins for structure-function studies of the cholinergic binding domain.

Location of a region of the muscarinic acetylcholine receptor involved in selective effector coupling

Chimaeric muscarinic acetylcholine receptors (mAChR) in which corresponding portions of mAChR I and mAChR II are replaced with each other have been produced in Xenopus oocytes by expression of cDNA constructs encoding them. Functional analysis of the chimaeric mAChRs indicates that a region mostly comprising the putative cytoplasmic portion between the proposed transmembrane segments V and VI is involved in selective coupling of mAChR I and mAChR II with different effector systems. In contrast, the exchange of this region between mAChR I and mAChR II does not significantly affect the antagonist binding properties of the two mAChR subtypes.

Structural basis of beta-adrenergic receptor subtype specificity studied with chimeric beta 1/beta 2-adrenergic receptors

The beta 1- and beta 2-adrenergic receptors are two structurally related, but pharmacologically distinguishable, receptor subtypes, both of which activate adenylyl cyclase in a catecholamine-dependent manner through the guanine nucleotide-binding regulatory protein Gs. The receptors are approximately 50% identical in amino acid sequence and each is characterized by the presence of seven putative transmembrane domains. To elucidate the structural basis for the pharmacological distinctions between these two receptor subtypes, we constructed a series of chimeric beta 1/beta 2-adrenergic receptor genes and expressed them by injection of RNA into Xenopus laevis oocytes. The pharmacological properties of the expressed chimeric receptor proteins were assessed by radioligand binding and adenylyl cyclase assays utilizing subtype-selective agonists and antagonists. Our data indicate that transmembrane region IV is largely responsible for determining beta 1 vs. beta 2 properties with respect to agonist binding (relative affinities for epinephrine and norepinephrine). Transmembrane regions VI and VII play an important role in determining binding of beta 1 vs. beta 2 selective antagonists. However, a number of the other transmembrane regions also contribute, to a lesser extent, to the determination of beta-adrenergic receptor subtype specificity for agonists and antagonists. Thus, several of the membrane-spanning regions appear to be involved in the determination of receptor subtype specificity, presumably by formation of a ligand-binding pocket, with determinants for agonist and antagonist binding being distinguishable.

The carboxy-terminal segment of the yeast alpha-factor receptor is a regulatory domain

The alpha-factor receptor is rapidly hyperphosphorylated on Thr and Ser residues in its hydrophilic C-terminal domain after cells are exposed to pheromone. Mutant receptors in which this domain is altered or removed are biologically active and bind alpha-factor with nearly normal affinity. However, cells expressing the mutant receptors are hypersensitive to pheromone action and appear to be defective in recovery from alpha-factor-induced growth arrest. Mutant receptors with partial C-terminal truncations undergo ligand-induced endocytosis, suggesting that down-regulation of receptor number is not the sole process for adaptation at the receptor level. A mutant receptor lacking the entire C-terminal domain (134 residues) does not display ligand-induced endocytosis. Genetic experiments indicate that the contribution of SST2 function to adaptation does not require the C-terminal domain of the receptor.

From epinephrine to cyclic AMP

Binding of catecholamines to the beta-adrenergic receptor results in the activation of adenylate cyclase and the intracellular formation of adenosine 3',5'-monophosphate (cAMP). In the past 20 years the events that lead from hormone binding at the cell surface receptor site to the synthesis of cAMP at the inner layer of the membrane have been intensively studied. Signal transduction in this system involves the sequential interaction of the beta-adrenergic receptor with the guanine nucleotide-binding protein (Gs) and the adenylate cyclase catalyst (C). The mechanism of signal transduction from the receptor through Gs to C, as well as the role of the adenylate cyclase inhibitory G protein Gi, is discussed.

Molecular characterization of a functional cDNA encoding the serotonin 1c receptor

Neurons that release serotonin as a neurotransmitter project to most regions of the central and peripheral nervous system and mediate diverse neural functions. The physiological effects of serotonin are initiated by the activation of multiple, distinct receptor subtypes. Cloning in RNA expression vectors was combined with a sensitive electrophysiological assay in Xenopus oocytes in order to isolate a functional cDNA clone encoding the 5HTlc serotonin receptor. Injection of RNA transcribed in vitro from this clone into Xenopus oocytes elicits serotonin sensitivity. Mouse fibroblasts transformed with this clone bind serotonin agonists and antagonists and exhibit an increase in intracellular Ca2+ concentrations in response to serotonin. The sequence of the 5HTlc receptor reveals that it belongs to the family of G protein-coupled receptors, which are thought to traverse the cytoplasmic membrane seven times. Moreover, in situ hybridization and RNA blot analysis indicate that the 5HTlc receptor is expressed in neurons in many regions of the central nervous system and suggest that this subclass of receptor may mediate many of the central actions of serotonin.

Chimeric alpha 2-,beta 2-adrenergic receptors: delineation of domains involved in effector coupling and ligand binding specificity

The alpha 2 and beta 2 adrenergic receptors, both of which are activated by epinephrine, but which can be differentiated by selective drugs, have opposite effects (inhibitory and stimulatory) on the adenylyl cyclase system. The two receptors are homologous with each other, rhodopsin, and other receptors coupled to guanine nucleotide regulatory proteins and they contain seven hydrophobic domains, which may represent transmembrane spanning segments. The function of specific structural domains of these receptors was determined after construction and expression of a series of chimeric alpha 2-,beta 2-adrenergic receptor genes. The specificity for coupling to the stimulatory guanine nucleotide regulatory protein lies within a region extending from the amino terminus of the fifth hydrophobic domain to the carboxyl terminus of the sixth. Major determinants of alpha 2- and beta 2-adrenergic receptor agonist and antagonist ligand binding specificity are contained within the seventh membrane spanning domain. Chimeric receptors should prove useful for elucidating the structural basis of receptor function.