Ligand binding to the beta-adrenergic receptor involves its rhodopsin-like core

Recently the genes for several hormone receptors that interact with guanine nucleotide binding proteins (G proteins) have been cloned, including the hamster beta 2-adrenergic receptor (beta 2AR), a human beta AR, the turkey erythrocyte beta AR and the porcine muscarinic acetylcholine receptor (MAR). All these receptors share some amino-acid homology with rhodopsin, particularly in 7 hydrophobic stretches of residues that are believed to represent transmembrane helices. To determine whether differences in ligand specificity result from the divergence in the sequences of the hydrophilic regions of these receptors, we have expressed in mammalian cells genes for the wild-type hamster and human beta AR proteins, and a series of deletion mutant genes of the hamster beta 2AR. The pharmacology of the expressed receptors indicates that most of the hydrophilic residues are not directly involved in the binding of agonists or antagonists to the receptor. In addition, we have identified a mutant receptor that has high agonist affinity but does not couple to adenylate cyclase.     

The genomic clone G-21 which resembles a beta-adrenergic receptor sequence encodes the 5-HT1A receptor

The recent cloning of the complementary DNAs and/or genes for several receptors linked to guanine nucleotide regulatory proteins including the adrenergic receptors (alpha 1, alpha 2A, alpha 2B, beta 1, beta 2), several subtypes of the muscarinic cholinergic receptors, and the visual 'receptor' rhodopsin has revealed considerable similarity in the primary structure of these proteins. In addition, all of these proteins contain seven putative transmembrane alpha-helices. We have previously described a genomic clone, G-21, isolated by cross-hybridization at reduced stringency with a full length beta 2-adrenergic receptor probe. This clone contains an intronless gene which, because of its striking sequence resemblance to the adrenergic receptors, is presumed to encode a G-protein-coupled receptor. Previous attempts to identify this putative receptor by expression studies have failed. We now report that the protein product of the genomic clone, G21, transiently expressed in monkey kidney cells has all the typical ligand-binding characteristics of the 5-hydroxytryptamine (5-HT1A) receptor.     

Cloning of the gene and cDNA for mammalian beta-adrenergic receptor and homology with rhodopsin

The adenylate cyclase system, which consists of a catalytic moiety and regulatory guanine nucleotide-binding proteins, provides the effector mechanism for the intracellular actions of many hormones and drugs. The tissue specificity of the system is determined by the particular receptors that a cell expresses. Of the many receptors known to modulate adenylate cyclase activity, the best characterized and one of the most pharmacologically important is the beta-adrenergic receptor (beta AR). The pharmacologically distinguishable subtypes of the beta-adrenergic receptor, beta 1 and beta 2 receptors, stimulate adenylate cyclase on binding specific catecholamines. Recently, the avian erythrocyte beta 1, the amphibian erythrocyte beta 2 and the mammalian lung beta 2 receptors have been purified to homogeneity and demonstrated to retain binding activity in detergent-solubilized form. Moreover, the beta-adrenergic receptor has been reconstituted with the other components of the adenylate cyclase system in vitro, thus making this hormone receptor particularly attractive for studies of the mechanism of receptor action. This situation is in contrast to that for the receptors for growth factors and insulin, where the primary biochemical effectors of receptor action are unknown. Here, we report the cloning of the gene and cDNA for the mammalian beta 2AR. Analysis of the amino-acid sequence predicted for the beta AR indicates significant amino-acid homology with bovine rhodopsin and suggests that, like rhodopsin, beta AR possesses multiple membrane-spanning regions.     

Removal of phosphorylation sites from the beta 2-adrenergic receptor delays onset of agonist-promoted desensitization

Eukaryotic cells have evolved a variety of mechanisms for dampening their responsiveness to hormonal stimulation in the face of sustained activation. The mechanisms for such processes, collectively referred to as desensitization, often involve alterations in the properties and number of cell-surface hormone receptors. It has been speculated that phosphorylation-dephosphorylation reactions, which are known to regulate the catalytic activities of enzymes, also regulate the function of receptors. Highly specific receptor kinases, such as rhodopsin kinase and beta-adrenergic receptor kinase, which show stimulus-dependent phosphorylation of receptors have been described. Direct evidence for a causal relationship between receptor phosphorylation and desensitization has been lacking however. Here we report that prevention of agonist-stimulated beta 2-adrenergic receptor (beta 2AR) phosphorylation by truncation of its serine and threonine-rich phosphate acceptor segment delays the onset of desensitization. We also show that selective replacement of these serine and threonine residues by alanine and glycine delays desensitization even further. These data provide the first direct evidence that one molecular mechanism of desensitization of G-protein-coupled receptors involves their agonist-induced phosphorylation.     

The mas oncogene encodes an angiotensin receptor

The class of receptors coupled to GTP-binding proteins share a conserved structural motif which is described as a 'seven-transmembrane segment' following the prediction that these hydrophobic segments form membrane-spanning alpha-helices. Identified examples include the mammalian opsins, alpha 1-, alpha 2-, beta 1- and beta 2-adrenergic receptors, the muscarinic receptor family, the 5-HT1C-receptor, and the substance-K receptor. In addition, two mammalian genes have been identified that code for predicted gene products with sequence similarity to these receptors, but whose ligand specificity is unknown namely, G21 and the mas oncogene. The mas oncogene shows the greatest sequence similarity to the substance-K receptor, and on this basis it was predicted that it would encode a peptide receptor with mitogenic activity which would act through the inositol lipid signalling pathways. The mas oncogene product was transiently expressed in Xenopus oocytes, and stably expressed in a transfected mammalian cell line. The results demonstrate that the mas gene product is a functional angiotensin receptor.     

Isoprenylation in regulation of signal transduction by G-protein-coupled receptor kinases.

Rhodopsin kinase and beta-adrenergic receptor kinase (beta ARK) are related members of a serine/threonine kinase family that specifically initiate deactivation of G-protein-coupled receptors. After stimulus-mediated receptor activation, these cytoplasmic kinases translocate to the plasma membrane. Here we show that the molecular basis for this event involves a class of unsaturated lipids called isoprenoids. Covalent modification in vivo of rhodopsin kinase by a 15-C (farnesyl) isoprenoid enables the kinase to anchor to photon-activated rhodopsin. Mutations that alter or eliminate the isoprenoid, fully disable light-specific Rhodopsin kinase translocation. Other receptor kinases (such as beta ARK), which lack an intrinsic lipid, are activated on exposure to brain beta gamma subunits of the signal-transducing G proteins, the gamma subunit of which bears a 20-C (geranylgeranyl) isoprenoid. Using chimaeric beta ARKs that undergo isoprenylation in vitro, we demonstrate that membrane association and activation of these kinases can occur in the absence of beta gamma. These results indicate that rhodopsin kinase (by means of an integral isoprenoid) and beta ARK (through its association with beta gamma) both rely on the function of isoprenyl moieties for their translocation and activity, illustrating distinct, though related, modes of biological regulation of receptor function.     

Light-dependent phosphorylation of rhodopsin by beta-adrenergic receptor kinase

The structural components involved in transduction of extracellular signals as diverse as a photon of light impinging on the retina or a hormone molecule impinging on a cell have been highly conserved. These components include a recognition unit or receptor (for example, the beta-adrenergic receptor (beta AR) for catecholamines or the 'light receptor' rhodopsin), a guanine nucleotide regulatory or transducing protein, and an effector enzyme (for example, adenylate cyclase or cyclic GMP phosphodiesterase). Molecular cloning has revealed that the beta AR shares significant sequence and three-dimensional homology with rhodopsin. The function of the beta AR is diminished by exposure to stimulatory agonists, leading to desensitization. Similarly, 'light adaptation' involves decreased coupling of photoactivated rhodopsin to cGMP phosphodiesterase activation. Both forms of desensitization involve receptor phosphorylation. The latter is mediated by a unique protein kinase, rhodopsin kinase, which phosphorylates only the light-bleached form of rhodopsin. An analogous enzyme (termed beta AR kinase or beta ARK) phosphorylates only the agonist-occupied beta AR. We report here that beta ARK is also capable of phosphorylating rhodopsin in a totally light-dependent fashion. Moreover, rhodopsin kinase can phosphorylate the agonist-occupied beta AR. Thus the mechanisms which regulate the function of these disparate signalling systems also appear to be similar.     

Cloning and expression of a rat D2 dopamine receptor cDNA

Dopamine receptors are classified into D1 and D2 subtypes on the basis of their pharmacological and biochemical characteristics. The D2 dopamine receptor has been implicated in the pathophysiology and treatment of movement disorders, schizophrenia and drug addiction. The D2 dopamine receptor interacts with guanine nucleotide-binding proteins to induce second messenger systems. Other members of the family of receptors that are coupled to G proteins share a significant similarity in primary amino-acid sequence and exhibit an archetypical topology predicted to consist of seven putative transmembrane domains. We have taken advantage of the expected nucleotide sequence similarities among members of this gene family to isolate genes coding for new receptors. Using the hamster beta 2-adrenergic receptor gene as a hybridization probe we have isolated related genes including a cDNA encoding the rat D2 dopamine receptor. This receptor has been characterized on the basis of three criteria: the deduced amino-acid sequence which reveals that it is a member of the family of G-protein-coupled receptors; the tissue distribution of the mRNA which parallels that of the D2 dopamine receptor; and the pharmacological profile of mouse fibroblast cells transfected with the cDNA.     

Structure of a beta1-adrenergic G-protein-coupled receptor

G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.     

A kinase-regulated PDZ-domain interaction controls endocytic sorting of the beta2-adrenergic receptor.

A fundamental question in cell biology is how membrane proteins are sorted in the endocytic pathway. The sorting of internalized beta2-adrenergic receptors between recycling endosomes and lysosomes is responsible for opposite effects on signal transduction and is regulated by physiological stimuli. Here we describe a mechanism that controls this sorting operation, which is mediated by a family of conserved protein-interaction modules called PDZ domains. The phosphoprotein EBP50 (for ezrinradixin-moesin(ERM)-binding phosphoprotein-50) binds to the cytoplasmic tail of the beta2-adrenergic receptor through a PDZ domain and to the cortical actin cytoskeleton through an ERM-binding domain. Disrupting the interaction of EBP50 with either domain or depolymerization of the actin cytoskeleton itself causes missorting of endocytosed beta2-adrenergic receptors but does not affect the recycling of transferrin receptors. A serine residue at position 411 in the tail of the beta2-adrenergic receptor is a substrate for phosphorylation by GRK-5 (for G-protein-coupled-receptor kinase-5) and is required for interaction with EBP50 and for proper recycling of the receptor. Our results identify a new role for PDZ-domain-mediated protein interactions and for the actin cytoskeleton in endocytic sorting, and suggest a mechanism by which GRK-mediated phosphorylation could regulate membrane trafficking of G-protein-coupled receptors after endocytosis.     

Leukocytes express a novel gene encoding a putative transmembrane protein-kinase devoid of an extracellular domain

Tyrosine-specific phosphorylation of proteins is a key to the control of diverse pathways leading to cell growth and differentiation. The protein-tyrosine kinases described to date are either transmembrane proteins having an extracellular ligand binding domain or cytoplasmic proteins related to the v-src oncogene. Most of these proteins are expressed in a wide variety of cells and tissues; few are tissue-specific. Previous studies have suggested that lymphokines could mediate haematopoietic cell survival through their action on glucose transport, regulated in some cells through the protein-tyrosine kinase activity of the insulin receptor. We have investigated the possibility that insulin receptor-like genes are expressed specifically in haematopoietic cells. Using the insulin receptor-related avian sarcoma oncogene v-ros as a probe, we have isolated and characterized the complementary DNA of a novel gene, ltk (leukocyte tyrosine kinase). The ltk gene is expressed mainly in leukocytes, is related to several tyrosine kinase receptor genes of the insulin receptor family and has unique structural properties: it apparently encodes a transmembrane protein devoid of an extracellular domain. Two candidate ltk proteins have been identified with antibodies in the mouse thymus, and have properties indicating that they are integral membrane proteins. These features suggest that ltk could be a signal transduction subunit for one or several of the haematopoietic receptors.     

Complexity of human T-cell antigen receptor beta-chain constant- and variable-region genes

Immune systems of vertebrates function via two types of effector cells, B and T cells, which are capable of antigen-specific recognition. The immunoglobulins, which serve as antigen receptors on B cells, have been well characterized with respect to gene structure, unlike the T-cell receptors. Recently, cDNA clones thought to correspond to the beta-chain locus of the human and mouse T-cell receptor have been described. The presumptive beta-chain clones detect gene rearrangement specifically in T-cell DNA and show homology with immunoglobulin light chains. The similarity of the T-cell beta-chain gene system to the immunoglobulin genes has been further demonstrated by the recent observation of variable- and constant-region gene segments as well as joining segments and putative diversity segments. We report here the characterization of cDNA and genomic clones encoding human T-cell receptor beta-chain genes. There are two constant-region genes (C beta 1 and C beta 2), each capable of rearrangement and expression as RNA. The gene arrangement, analogous to that of mouse beta-chain genes, shows strong evolutionary conservation of the dual C beta gene system in these two species.     

Rearrangement and transcription of the beta-chain genes of the T-cell antigen receptor in different types of murine lymphocytes

Rearrangements of T-cell receptor beta-chain genes are usually found on both chromosomal homologues, occurring by both deletional and non-deletional mechanisms. Two constant-region (C beta) genes have been identified previously and at least one is transcribed in every helper or cytotoxic T cell tested, but the choice of C beta gene expression is not correlated with the specialized functions of these T lymphocytes. By contrast, four of five suppressor T-cell hybridomas examined have deleted all known joining (J beta) gene segments and C beta genes and therefore may have antigen receptors encoded by different T-cell receptor gene families.     

Germ-line transmission of a disrupted beta 2-microglobulin gene produced by homologous recombination in embryonic stem cells

Major histocompatibility complex (MHC) class I molecules are integral membrane proteins present on virtually all vertebrate cells and consist of a heterodimer between the highly polymorphic alpha-chain and the beta 2-microglobulin (beta 2-m) protein of relative molecular mass 12,000 (ref. 1). These cell-surface molecules play a pivotal part in the recognition of antigens, the cytotoxic response of T cells, and the induction of self tolerance. It is possible, however, that the function of MHC class I molecules is not restricted to the immune system, but extends to a wide variety of biological reactions including cell-cell interactions. For example, MHC class I molecules seem to be associated with various cell-surface proteins, including the receptors for insulin, epidermal growth factor, luteinizing hormone and the beta-adrenergic receptor. In mice, class I molecules are secreted in the urine and act as highly specific olfactory cues which influence mating preference. The beta 2-m protein has also been identified as the smaller component of the Fc receptor in neonatal intestinal cells, and it has been suggested that the protein induces collagenase in fibroblasts. Cells lacking beta 2-m are deficient in the expression of MHC class I molecules, indicating that the association with beta 2-m is crucial for the transport of MHC class I molecules to the cell surface. The most direct means of unravelling the many biological functions of beta 2-m is to create a mutant mouse with a defective beta 2-m gene. We have now used the technique of homologous recombination to disrupt the beta 2-m gene. We report here that introduction of a targeting vector into embryonic stem cells resulted in beta 2-m gene disruption with high frequency. Chimaeric mice derived from blastocysts injected with mutant embryonic stem cell clones transmit the mutant allele to their offspring.     

Expression of the T-cell-specific gamma gene is unnecessary in T cells recognizing class II MHC determinants

Subtractive complementary DNA cloning combined with partial protein sequencing has allowed identification of the genes encoding the alpha and beta subunits of T-cell receptors. The subtractive cDNA library prepared from the cytotoxic T lymphocyte (Tc) clone 2C has been found to contain a third type of clone encoding the gamma chain. The gamma gene shares several features with the alpha and beta genes: (1) assembly from gene segments resembling immunoglobulin V, J and C (respectively variable, joining and constant region) DNA segments; (2) rearrangement and expression in T cells and not in B cells; (3) sequences reminiscent of transmembrane and intracytoplasmic regions of integral membrane proteins; (4) a cysteine residue at the position expected for an interchain disulphide bond. The alpha and beta genes are expressed at equivalent levels in both Tc cells and helper T cells (TH). The gamma gene, obtained from 2C, has been found to be expressed in all Tc cells studied. Here we present evidence that strongly suggests that TH cells do not require gamma gene expression.     

High constitutive activity of native H3 receptors regulates histamine neurons in brain

Some G-protein-coupled receptors display 'constitutive activity', that is, spontaneous activity in the absence of agonist. This means that a proportion of the receptor population spontaneously undergoes an allosteric transition, leading to a conformation that can bind G proteins. The process has been shown to occur with recombinant receptors expressed at high density, and/or mutated, but also non-mutated recombinant receptors expressed at physiological concentrations. Transgenic mice that express a constitutively active mutant of the beta2-adrenergic receptor display cardiac anomalies; and spontaneous receptor mutations leading to constitutive activity are at the origin of some human diseases. Nevertheless, this process has not previously been found to occur in animals expressing normal levels of receptor. Here we show that two isoforms of the recombinant rat H3 receptor display high constitutive activity. Using drugs that abrogate this activity ('inverse agonists') and a drug that opposes both agonists and inverse agonists ('neutral antagonist'), we show that constitutive activity of native H3 receptors is present in rodent brain and that it controls histaminergic neuron activity in vivo. Inverse agonists may therefore find therapeutic applications, even in the case of diseases involving non-mutated receptors expressed at normal levels.     

T-cell antigen receptor genes and T-cell recognition

The four distinct T-cell antigen receptor polypeptides (alpha, beta, gamma, delta) form two different heterodimers (alpha:beta and gamma:delta) that are very similar to immunoglobulins in primary sequence, gene organization and modes of rearrangement. Whereas antibodies have both soluble and membrane forms that can bind to antigens alone, T-cell receptors exist only on cell surfaces and recognize antigen fragments only when they are embedded in major histocompatibility complex (MHC) molecules. Patterns of diversity in T-cell receptor genes together with structural features of immunoglobulin and MHC molecules suggest a model for how this recognition might occur. This view of T-cell recognition has implications for how the receptors might be selected in the thymus and how they (and immunoglobulins) may have arisen during evolution.     

Conserved organization of the human and murine T-cell receptor beta-gene families

Generation of an immune response depends on the interaction of haematopoietic cell types, among which T cells and their receptors are of central importance. The T-cell receptor is a heterodimer consisting of disulphide-linked alpha and beta-chains, each chain divided into variable (V) and constant (C) regions. The beta-chain is encoded by the rearrangement of separate variable (V beta), diversity (D beta) and joining (J beta) gene segments during T-cell differentiation. To examine the mechanisms of somatic DNA rearrangement and evolution of the beta-gene segments, we have constructed a physical map of the human T-cell receptor beta-chain family containing 40 V beta gene segments as well as both C beta gene clusters. A comparison of the published nucleotide sequences of human and murine V beta gene segments reveals 12 examples of gene segments sharing 65% or more interspecies homology. The relative order of these human and murine V beta gene segment homologues is also conserved along the chromosome, apart from more extensive human gene duplication, presumably as a consequence of constraints imposed on evolutionary mechanisms operating to diversify these gene families or of selective pressures operating to maintain order.     

Identical V beta T-cell receptor genes used in alloreactive cytotoxic and antigen plus I-A specific helper T cells

T lymphocytes involved in the cellular immune response carry cell-surface receptors responsible for antigen and self recognition. This T-cell receptor molecule is a heterodimeric protein consisting of disulphide-linked alpha- and beta-chains with variable (V) and constant (C) regions. Several complementary DNA and genomic DNA clones have been isolated and characterized. These analyses showed that the genomic arrangement and rearrangement of T-cell receptor genes using VT, diversity (DT), joining (JT) and CT gene segments is very similar to the structure of the known immunoglobulin genes. We have isolated two cDNA clones from an allospecific cytotoxic T cell, one of which shows a productive V beta-J beta-C beta 1 rearrangement without an intervening D beta segment. This V beta gene segment is identical to the V beta gene expressed in a helper T-cell clone specific for chicken red blood cells and H-21. The other clone carries the C beta 2 gene of the T-cell receptor, but the C beta 2 sequence is preceded by a DNA sequence that does not show any similarity to V beta or J beta sequences.