Identification of residues required for ligand binding to the beta-adrenergic receptor

Role of tyrosine kinase and membrane-spanning domains in signal transduction by the platelet-derived growth factor receptor

Three types of mutations were introduced into the platelet-derived growth factor (PDGF) receptor to cause a loss of PDGF-stimulated tyrosine kinase activity: (i) a point mutation of the ATP-binding site, (ii) a deletion of the carboxyl-terminal region, and (iii) replacement of the membrane-spanning sequences by analogous transmembrane sequences of other receptors. Transfectants expressing mutated receptors bind, 125I-labeled PDGF with a high affinity but had no PDGF-sensitive tyrosine kinase activity, phosphatidylinositol turnover, increase in the intracellular calcium concentration, change in cellular pH, or stimulation of DNA synthesis. However, PDGF-induced receptor down regulation was normal in the mutant cells. These results indicate that the transmembrane sequence has a specific signal-transducing function other than merely serving as a membrane anchor and that the receptor kinase activity is necessary for most responses to PDGF but is not required for receptor down regulation.

Cysteine residues 110 and 187 are essential for the formation of correct structure in bovine rhodopsin

To investigate the role of different cysteine residues in bovine rhodopsin, a series of mutants were prepared in which the cysteine residues were systematically replaced by serines. The mutant genes were expressed in monkey kidney cells (COS-1) and the mutant opsins were evaluated for their levels of expression, glycosylation patterns, and ability to form the chromophore characteristic of rhodopsin and to activate transducin. Substitution of the three cytoplasmic cysteines (Cys-316, Cys-322, and Cys-323) and the four membrane-embedded cysteines (Cys-140, Cys-167, Cys-222, and Cys-264) produced proteins with wild-type phenotype. Also, single substitutions of Cys-185 gave rise to a wild-type phenotype. In contrast, substitution of the three intradiscal cysteines (Cys-110, Cys-185, and Cys-187) or single substitution of Cys-110 or Cys-187 gave proteins that were expressed at reduced levels, glycosylated abnormally, and unable to bind 11-cis-retinal. Thus, of the 10 cysteines in bovine rhodopsin, only intradiscal Cys-110 and Cys-187 are essential for the correct tertiary structure of the protein.

A chemoattractant receptor controls development in Dictyostelium discoideum

During the early stages of its developmental program, Dictyostelium discoideum expresses cell surface cyclic adenosine monophosphate (cyclic AMP) receptors. It has been suggested that these receptors coordinate the aggregation of individual cells into a multicellular organism and regulate the expression of a large number of developmentally regulated genes. The complementary DNA (cDNA) for the cyclic AMP receptor has now been cloned from lambda gt-11 libraries by screening with specific antiserum. The 2-kilobase messenger RNA (mRNA) that encodes the receptor is undetectable in growing cells, rises to a maximum at 3 to 4 hours of development, and then declines. In vitro transcribed complementary RNA, when hybridized to cellular mRNA, specifically arrests in vitro translation of the receptor polypeptide. When the cDNA is expressed in Dictyostelium cells, the undifferentiated cells specifically bind cyclic AMP. Cell lines transformed with a vector that expresses complementary mRNA (antisense) do not express the cyclic AMP receptor protein. These cells fail to enter the aggregation stage of development during starvation, whereas control and wild-type cells aggregate and complete the developmental program within 24 hours. The phenotype of the antisense transformants suggests that the cyclic AMP receptor is essential for development. The deduced amino acid sequence of the receptor reveals a high percentage of hydrophobic residues grouped in seven domains, similar to the rhodopsins and other receptors believed to interact with G proteins. It shares amino acid sequence identity and is immunologically cross-reactive with bovine rhodopsin. A model is proposed in which the cyclic AMP receptor crosses the bilayer seven times with a serine-rich cytoplasmic carboxyl terminus, the proposed site of ligand-induced receptor phosphorylation.

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.

Temperature-dependence of the kinetics of the binding of [3H]-(+)-N-methyl-4-methyldiphenhydramine to the histamine H1-receptor: comparison with the kinetics of [3H]-mepyramine

1. The dissociation of [3H]-(+)-N-methyl-4-methyldiphenhydramine ([3H]-QMDP) from the histamine H1-receptor was markedly temperature-dependent. The t1/2 was 4 min at 37 degrees C and 16 h at 6 degrees C. The association rate constant, k1, was also temperature-dependent, but not to the same extent as k-1. 2. Plots of the observed rate constant for [3H]-QMDP-receptor complex formation, kon, versus [3H-QMDP] were linear at both 30 degrees C and 10 degrees C, consistent with the interaction of [3H]-QMDP with the H1-receptor being a simple, one-step equilibrium. 3. The ratio of the kinetic constants, k1/k-1, indicated that the affinity constant of [3H]-QMDP for the H1-receptor should increase with decreasing temperature. Measurement of (+)-QMDP antagonism of the contraction of longitudinal muscle strips from guinea-pig small intestine induced by histamine at 37 degrees C, 30 degrees C and 25 degrees C provided some evidence that the affinity of (+)-QMDP is greater at 25 degrees C than 37 degrees C. However, the flattening of the concentration-response curves for histamine at low concentrations of (+)-QMDP at 30 degrees C and 25 degrees C is consistent with a slow dissociation of the (+)-QMDP-receptor complex and hence an incomplete equilibration with the agonist. 4. Arrhenius plots for k1 and k-1 for [3H]-QMDP were linear between 37 degrees C and 6 degrees C. The activation energies, Ea, for complex formation and dissociation were 77 +/- 4 and 129 +/- 3 kJ mol-1, respectively. 5. An Arrhenius plot for k-1 for the dissociation of [3H]-mepyramine from the H1-receptor was also linear between 37 degrees C and 6 degrees C. The activation energy was 140 +/- 2 kJ mol-1. 6. Activation energies for complex formation with the H1-receptor, Eaf, and complex dissociation, Ead, were similar for [3H]-QMDP and [3H]-mepyramine. The energy difference, Eaf--Ead, equivalent to the enthalpy change, did not differ significantly for the two ligands (-52 and -48 kJ mol-1, respectively). The larger values of k1 and k-1 for [3H]-mepyramine compared to [3H]-QMDP imply the presence of an entropic component in the interaction. 7. The simplest explanation for these observations is that transfer from the aqueous phase into a hydrophobic region is a significant factor in antagonist-H1-receptor interaction. This would be entropically more favourable for [3H]-mepyramine, a tertiary amine, than for [3H]-QMDP, a quaternary amine.

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.

A molecular graphics study exploring a putative ligand binding site of the beta-adrenoceptor

The recent elucidation of the primary structure of the cell membrane-bound beta-adrenoceptor has prompted us to explore putative ligand binding sites on this physiologically important receptor. By minimizing the energies of the 'prototype' ligand propranolol, (part of) the receptor and the proposed ligand-receptor complex with the aid of force field and quantum chemical calculations, we identified amino acid residue Trp313 as a highly probable candidate for interaction with the aromatic moiety of propranolol. The charge distribution on the indole nucleus of another beta-blocker, pindolol, with higher affinity for the beta-adrenoceptor, enables an even stronger interaction with the tryptophan residue. The carboxylic amino acid residue Glu306, located near the extracellular space of the cell membrane, interacts favorably with the positively charged nitrogen atom in the aliphatic side chain of the ligands. Finally, this putative model is discussed in the light of recent findings in mutagenesis studies, and compared to other ideas with respect to ligand-receptor interactions.

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

p-(Bromoacetamido)benzyl-1-[125I]iodocarazolol (125I-pBABC) is a potent derivative of the beta-adrenergic receptor antagonist p-aminobenzylcarazolol. Treatment of the receptor with 125I-pBABC results in efficient covalent incorporation of the ligand into the receptor binding site. Extensive degradation of 125I-pBABC-labeled beta 2-adrenergic receptor with either cyanogen bromide or Staphylococcus aureus V8 protease results in specifically labeled fragments having Mr's of about 1600 and 3500, respectively. Because the primary structure of the beta 2-adrenergic receptor is known, and these proteolytic reagents are highly sequence specific, the site of 125I-pBABC incorporation may be deduced from the sizes of the specifically labeled fragments. Thus the fragment generated by cyanogen bromide cleavage corresponds to residues 83-96, a region of 14 amino acids included in the second membrane spanning domain (helix II) of the beta 2-adrenergic receptor. This assignment was confirmed by direct amino acid sequencing of this labeled fragment, though the actual amino acid modified could not be determined. These data permit the assignment of a part of the hormone binding region of the beta 2-adrenergic receptor.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.