Molecular and cellular biology of adrenergic receptors

experimental system in molecular genetics. lygenes and homogeneous subtypes. J Car- Science 240:1483-1488. diovasc Phatmacol 12:S7-S20.

Williams RR: 1989. Will gene markers predict Yamori Y: 1983. Physiopathology of the vari- hypertension? Hypertension 14:6 1 O-6 13. ous strains of spontaneously hypertensive

Williams RR, Hunt SC, Hasstedt SJ, et al.: rats. In Genest J, Kuchel 0, Hamet P, eds. 1988. Definition of genetic factors in hyper- Hypertension. New York, McGraw-Hill, p tension: a search for major genes, po- 556. TCM

Molecular and Cellular Biology of Adrenergic Receptors Brian Kobilka

Advenergic receptors form the intevface between the sympathetic nervous system and the cardiovascular system. Genomic or cDNA clones fey 8 types of mammalian advenevgic receptors have been obtained. Much has been learned about the structure and functional properties of the P,-adrenergic receptor. Less is known about the functional properties and the physiologic vole of the other advenevgic receptors. Further progress in this field may lead to the development of more selective drugs to modify the physiologic processes controlled by these receptors. (Trends Cardiovasc Med 199 1; 1: 189- 194)

Cardiovascular function is tightly regulated by the autonomic nervous system. Cardiac muscle, cardiac conducting tissue, and coronary and systemic blood vessels are richly innervated by autonomic nerves. Sensory nerves monitor the volume and pressure status of the heart and blood vessels, as well as the metabolic state of cardiac and systemic tissues. This information is processed by the central nervous system, and im- pulses sent via the autonomic motor nerves modulate cardiac rate and con- tractility, as well as coronary and systemic vascular resistance. In addition to these direct controls over the cardiovascular system, the sympathetic nervous

Brian Kobilka is at the Howard Hughes Medical Institute and the Departments of Cardiology and Molecular and Cellular Physi- ology, Stanford University Medical Center, Stanford, CA 94305, USA.

system innervation of the kidney influ- ences fluid and electrolyte balance.

Adrenergic receptors form the inter- face between the sympathetic nervous system and the cardiovascular system. Sympathetic nerves release the neuro- transmitter norepinephrine from nerve terminals, and predominantly epinephrine from the adrenal medulla. Catechol- amines are also important neurotransmitters within the central nervous system. The effect produced upon releasing norepinephrine from a nerve terminal depends on the type of cell at the synapse, the type of receptors on the cell surface, and the type of effector molecules found within the cell.

The importance of adrenergic receptors in the control of cardiovascular function can be appreciated by observ- ing the physiologic effects of the admini- stration of pharmacologic doses of agonists or antagonists for these receptors. Pharmacologic control over the activity

of these adrenergic receptors has proved useful for a number of acute and chronic cardiovascular disorders such as hypertension, coronary artery disease, acute hypotension, congestive heart failure, and bradyarrhythmias. Currently available drugs, however, lack the selectivity to enable specific control over all aspects of the cardiovascular system that are regulated by the sympathetic nervous system. Undesirable side effects may be due to the lack of selectivity of a drug for a particular subtype of receptor, or may be due to the different effects a drug may have on different tissues. Finally, the effect of many agonist compounds such as clonidine, isoproterenol, and dopam- ine may be limited by the tendency of cells to adapt to the drug by decreasing the number and/or functional capacity of the receptors for the drug. A better understanding of the complexity of the sympathetic nervous system at the cellular and molecular levels may provide new approaches to modulate its activity pharmacologically.

Over the past several years, as a result of advances in molecular biology, a great deal has been learned about the structure, pharmacology, and cell biology of the adrenergic receptors. This review sum- marizes our current understanding of how these receptors work at the molecular level, and how this research may lead to improved approaches to pharmacologic control over receptor function.

l Purifhation and Cloning

of Receptors

The adrenergic receptors were first identified through the physiologic response produced by agonist activation; however, the development of radioligand binding assays and selective synthetic agonists and antagonists provided the critical tools for pharmacologic, physiologic, and biochemical characterization of these receptors. Based on these studies, four types of mammalian adrenergic receptors were clearly distinguished: at, a2, fit, and &,. More recent pharmacologic studies have suggested that several subtypes of both a, and a2 receptors may exist (Bylund 1988; Han et al. 1987).

The first DNA clones for adrenergic receptors were obtained as a result of having amino-acid sequence from puri- fied receptor protein for hamster pz (Dixon et al. 1986) and a, (Cotecchia et

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HUMAN hADRENERGIC RECEPTOR

EXTRACELLULAR SURFACE

CYTOPLASMIC SURFACE

PARK _

’ PKA

Figure 1. Diagram of the proposed membrane topology for the human P,-adrenergic receptor. Circks with letters represent amino acids, identified by the single-letter amino-acid code.

Proposed membrane-spanning domains are numbered with ~omun numerds. Extracellular domains are designated by a prefix e-. Intracellular domains are designated by a prefix i-. Black circles with white letters indicate potential sites for phosphorylation by protein kinase A (PKA)

or P-adrenergic receptor kinase @ARK).

al. 1988), and human a, (Kobilka et al. libraries under conditions that would I987a) receptors, as well as the turkey permit identification of closely related erythrocyte fi receptor (Yarden et al. DNA sequences. With this low-strin-

1986). These DNA clones were then used gency screening technique, a cDNA clone to design oligonucleotide and cDNA for the human P,-adrenergic receptor probes to isolate clones coding for other has been obtained (Frielle et al. 1987) as adrenergic receptors by screening DNA well as clones for adrenergic receptors

that had previously been only partially characterized by pharmacologic or physiologic studies. As a result of this cloning work, the following receptors have been identified: two subtypes of the a,- adrenergic receptor (Cotecchia et al. 1988; Schwinn et al. 1990), three subtypes of a,-adrenergic receptor (Kobilka et al. 1987a; Regan et al. 1988; Iomasney et al. 1990), and three subtypes of fl receptors [&, &, and p3 (Emorine et al. 1989)]. In addition, using these tech- niques, the clone for the serotonin receptor subtype 1A was obtained with the use of P,-adrenergic receptor as a probe

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(Kobilka et al. 1987b; Fargin et al. 1988). This is of interest, since this serotonin receptor and the & receptor both bind to the antagonist cyanopindolol.

The discovery of genes encoding addi- tional subtypes of a- and P-adrenergic receptors raises questions about the physiologic role of specific receptor subtypes. Agonists and antagonists currently available for therapeutic use do not enable selective stimulation or blockade of specific a1 or a2 receptor subtypes. The development of subtype selective drugs and the use of these drugs to characterize the physiologic role of specific subtypes may advance our understanding of the pathophysiology behind chronic diseases such as hypertension and may provide new therapeutic tools to modulate the activity of the sympathetic nervous system with fewer undesirable effects.

l Receptor Structure

Analysis of the primary amino acid sequence deduced from the cloned DNA sequences provided the first clues to the structure of these membrane proteins. All of the adrenergic receptors cloned thus far share the general features illustrated in Figure 1, using the Pz-adrenergic receptor as an example. Seven clus- ters of 20 or more hydrophobic amino acids have been proposed to be membrane- spanning domains. The amino terminus lies on the outside of the cell and the carboxyl terminus is in the cytoplasm. The amino terminus has one or more glycosylation sites, and the cytoplasmic domains have amino-acid sequences that are potential substrates for a variety of kinases, enzymes capable of phosphorylat- ing the receptor. In comparing the sequences of these receptors with each other, the greatest degree of similarity is found in the hydrophobic domains. For example, when comparing two closely related receptors such as two subtypes of the a, receptor, 65%85% of the amino acids in the hydrophobic domains are identical. For less closely related receptors, such as the 8 and the a receptors, only 40%50% of the amino acids in the hydrophobic domains are identical. The most noticeable differences between any two receptors are found in the amino terminus, the third cytoplasmic loop (i-3) and the carboxyl terminus. These regions not only differ in the specific sequence of amino acids, but also in the

Figure 2. Outline of a cycle of signal transduction for the P,-adrenergic receptor. (1) The components include the hormone (H) epinephrine, the pz receptor (R), the subunits of G, (a, p, and r), and adenylyl cyclase (AC). (2) Receptors bound to G, have a higher affinity for the hormone. (3) The binding of the hormone to the pz receptor leads to a structural change in the receptor facilitating the release of GDP and the binding of GTP to the a subunit of G,. (4) The binding of GTP to the a subunit results in a release of the a subunit from both the receptor and the /3-r subunit. (5) The GTP-G,, subunit binds to and activates adenylyl cyclase, which catalyzes the conversion of ATP to cyclic AMP (CAMP). CAMP activates protein kinase A (PKA), which phosphorylates a variety of cytosolic and nuclear proteins (not shown). (6) The activation of adenylyl cyclase continues until GTP is hydrolyzed to GDP and the G,, subunit is released from the enzyme and complexes with the l3-r subunit.

size of these domains. The function performed by some of these structural domains is discussed next.

l Signal Transduction

In discussing membrane receptors, signal transduction refers to the process of transmitting information across the plasma membrane without the direct transport of molecules across the plasma membrane. Thus, receptors detect extracellular hormones or neurotransmitters and respond by changes in structure. The changes in the receptor structure can then be detected by intracellular molecules. In the case of the adrenergic receptors, the intracellular molecules that detect changes in receptor structure are membrane-associated guanosine triphosphate (GTP)-binding proteins, which

are referred to as G proteins [for review, see Iyengar and Birnbaumer (1990)]. After a G protein detects a hormone- occupied receptor, it proceeds to modify the activity of an intracellular enzyme or an ion channel. The G proteins are heterotrimeric proteins consisting of a, p, and ysubunits. The a subunit interacts most closely with the receptor and detects changes in the receptor when it binds agonists. The a subunit contains the GTP-binding site and becomes de- tached from the receptor and the 8 and y subunits during the process of signal transduction. The l3 and y subunits are tightly, but noncovalently, bound to each other and anchor the G-protein complex to the inner surface of the plasma membrane. The role of the /3-y-subunit complex in interactions between the receptor and the G protein is not well understood;

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however, efficient coupling of the recep- nase C (PKC), which modulates the activ- tor to the G protein requires an intact ity of cytosolic and nuclear proteins o-f&r trimer. through phosphorylation.

The mechanism of receptor-G-protein coupling, illustrated for the pZ receptor in Figure 2, is likely to be similar for the other adrenergic receptors. Each of these receptors has a preference for a specific G protein or a class of G proteins that can regulate one or more cellular processes. The pi and & receptors couple to a G protein (G,) that stimulates adenylyl cyclase. The a2 receptors couple to a G protein (Gi) that inhibits adenylyl cyclase. Three types of Gi proteins have been identified: Gi,, Giz, and Gi,. The interaction between specific Gi’s and specific receptor subtypes has not yet been fully characterized. The a,-adrenergic receptor couples to a G protein (not yet identified) that activates phospholipase C. There is evidence that receptors may interact with functionally different G proteins. For example, stimulation of the az-adrenergic receptor not only leads to inhibition of adenylyl cyc!ase, but can also lead to activation of phospholipase C (Cotecchia et al. 1990). There is no evidence at present that any of the adrenergic receptors bypass G proteins in the process of transmitting a signal.

Structural Domains Involved in Signal Transduction

Mutagenesis studies indicate that the membrane-spanning domains of adrenergic receptors (Figure 1) are likely to form the ligand binding site. This differs from other types of plasma membrane receptors such as the LDL receptor, the insulin receptor, and the platelet-derived growth factor receptor, where ligand binding takes place on an extracellular domain. Results from several different experimental approaches suggest that membrane-spanning domains 3, 5, and 7 may participate in the formation of the ligand- binding site (Strader et al. 1989; Kobilka et al. 1988; Wong et al. 1988).

Second Messengers

The activation of adenylyl cyclase by G, leads to the generation of CAMP that positively regulates the activity of protein kinase A (PKA). A common mechanism of conveying messages within the cell is the transfer of phosphates from adenosine triphosphate (ATP) to proteins by kinases such as PKA. Enzymes, channels, and proteins that modify gene expression can be turned on or off by the addition of phosphates by kinases or the removal of phosphates by phosphatases. PKA modulates a variety of processes involved in cellular metabolism as well as the activity of ion channels. In cardiac tissue, the activity of Ca2+ channels is regulated by phosphorylation of the channel protein by PKA.

Determining the location of the domain on adrenergic receptors that interacts with G proteins has been an active area of investigation. The G-protein coupling specificity for a receptor can be changed by changing the third cytoplasmic loop (Kobilka et al. 1988; Wong et al. 1990). For example, the o2 receptor, which normally couples to Gi, can be made to couple with G, if its i-3 domain is replaced with the i-3 domain of the p2 receptor. Within this domain appear to be sequences in the amino and carboxyl terminal regions that are particularly important in mediating interactions between the receptor and the G protein. Portions of the second cytoplasmic loop (i-2) and the carboxyl terminus may also be important in receptor-G-protein cou-

Figure 4. Comparison of desensitization mediated by protein kinase A (PKA) and P-adrenergic receptor kinase (PARK). Activation of the P,-adrenergic receptor by epinephrine (E) can result in phosphorylation of the receptor by PKA and PARK. Phosphorylation of the receptor by PKA interferes with the interaction of the receptor and G,. Phosphorylation of the receptor by PARK is not sufficient to interfere with the activation of G, by the receptor. l3-Arrestin recognizes PARK phosphorylated receptor and blocks activation of G,.

Activation of phospholipase C following a,-adrenergic receptor stimulation leads to the cleavage of membrane phospholip- ids to produce inositol phosphates and diacylglycerol. Inositol phosphates regulate intracellular calcium, which in turn regulates a variety of enzymes and channels. Diacylglycerol activates protein ki-

Figure 3. Conceptual model of the three- dimensional structure of an adrenergic receptor. The membrane-spanning domains are numbered. G, asparagine-linked giycosyla- tion; N, the amino terminus of the receptor; and C, the carboxyl terminus of the receptor.

pling (O’Dowd et al. 1988).

The model shown in Figure 1 is useful in comparing the primary and secon- dary structures of these receptors; however, it does not represent the three- dimensional structure. While the three- dimensional structure of adrenergic receptors has not been determined, a possible model based on bacteriorho- dopsin, a protein with a similar primary structure, is shown in Figure 3. In this model, the hydrophobic domains form a

p- ARRESTlN

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channel-like ligand-binding pocket in the plasma membrane.

l Receptor Regulation

Signal transduction by adrenergic receptors is constantly regulated by ad- justments in the functional capacity of receptors and in receptor density at the cell surface. For the purposes of this discussion, the term desensitization will be used to refer to a variety of processes by which the functional interaction of a receptor and its G protein are impaired, leading to a reduction in the cellular response to a hormone or neurotransmit- ter. This process usually follows pro- longed stimulation of the receptor or related receptors that activate the same G protein.

Desensitization is a clinically important phenomenon that is likely to be part of the normal physiologic regulation of adrenergic receptor function. In pathol- ogic conditions such as chronic lung disease or congestive heart failure, however, desensitization may limit the effi- cacy of hormones, neurotransmitters, or drugs that may otherwise be clinically beneficial. The process of desensitization to a drug is h-equently called tachyphy- laxis. Better understanding of the mechanism of desensitization may lead to new ways of modifying this process for therapeutic purposes.

The process of desensitization of the l3 receptor has been most extensively characterized [for reviews, see Hausdorff et al. (1990) and Lefkowitz et al. (1990)]. In mechanistic terms, the process of desensitization can be brought about by changes in receptor structure by phosphorylation and by changes in receptor density.

Receptor Phosphorylation

The functional properties of cellular proteins and enzymes are often regulated by phosphorylation and dephosphorylation. Stimulation of the P-adrenergic receptors leads to change in the activity of PKA, which is activated by CAMP. PKA recognizes specific amino- acid sequences (consensus sequences) on the pz receptor and transfers phosphate from ATP to two serine residues on the protein. A more recently characterized kinase has been identified that phosphorylates receptors only when they are occupied by an agonist. This kinase has been named (3-adrenergic receptor

kinase (BARK) (Benovic et al. 1989). Mutagenesis studies have characterized the location of functionally important sites for phosphorylation of the pZ receptor (see Figure 1) by PKA and BARK (Bouvier et al. 1988). Phosphorylation at these sites impairs the ability of the receipt to activate G,.

Desensitization of the l!$ receptor mediated by BARK and PKA differs in two important ways. First, BARK phosphorylation of the fl receptor depends entirely on the presence of agonist, Unoccupied receptor or antagonist- occupied receptor cannot be phosphorylated by BARK. This differs from PKA phosphorylation of the receptor, which does not require agonist occupancy. In fact, PKA phosphorylation of the f$ receptor may occur following activation of this kinase by other receptors. The second difference in desensitization mediated by these kinases is that phosphorylation of the pZ receptor by PKA impairs pZ receptor function directly, whereas the disrup- tion of receptor-G-protein interaction following BARK phosphorylation of & receptor requires another protein (Ben- ovic et al. 1987). This protein, which has recently been cloned (Lohse et al. 1990), has been called p-arrestin because of its structural and functional similarity to arrestin, a protein that regulates visual signal transduction. A model comparing PKA and BARK mediated desensitization of the p receptor is illustrated in Figure 4.

Regulation of Receptor Density

Signal transduction by G proteins can be regulated by altering the density of functional receptors at the cell surface. Re- ceptor density can be modified by changes in the rate of synthesis of new receptors. The rate of &-receptor mR.NA synthesis and destruction are both modulated by intracellular CAMP (Collins et al. 1989; Hadcock et al. 1989). The density of pz receptors at the cell surface can also be modified by at least two processes that actively remove receptors from the plasma membrane: sequestration, which is a reversible process; and downregulation, which probably leads to destruction of receptor protein. The molecular mechanisms involved in these processes have not yet been characterized; however, interactions between these receptors and cytoskeletal proteins are probably involved.

l Conclusion

The application of the tools of molecular biology to the study of adrenergic receptors has revealed that this family of receptors is much more complex than previously appreciated. Of particular importance has been the identification of genes encoding subtypes of a, and ~1~ receptors. Much emphasis will be placed on characterizing the cellular responses following stimulation of each receptor subtype and the role each subtype plays in normal physiologic processes. As more is learned about the three-dimensional structure of the ligand-binding site and the sites of interaction between these receptors and regulatory proteins, it may be possible to develop more selective drugs to modify the biologic processes controlled by these receptors through the autonomic nervous system.

References

Benovic JL, Kuhn H, Weyland I, Codina J, Caron MG, Lefkowitz RJ: 1987. Functional desensitization of the isolated P-adrenergic receptor by the 8-adrenergic receptor kinase: potential role of an analog of the retinal protein arrestin (48kDa protein). Proc Nat1 Acad Sci USA 84:8879-8882.

Benovic JL, De Blasi A, Stone WC, Caron MG, Leflcowitz RJ: 1989. 8-adrenergic receptor kinase: primary structure delineates a mul- tigene family. Science 246:235-240.

Bouvier M, Hausdorff WP, De Blasi A, et al.: 1988. Removal of phosphorylation sites from the 8,-adrenergic receptor delays onset of agonist-promoted desensitization. Na- ture 333:370-373.

Bouvier M, Collins S, O’Dowd BF, et al.: 1989. Two distinct pathways for CAMP-mediated down-regulation of the /3,-adrenergic receptor. J Biol Chem 264:16,786-16,792.

Bylund DB: 1988: Subtypes of cxz-adreno- ceptors: pharmacological and molecular biological evidence converge. Trends Phar- macol Sci 91356-361.

Collins S, Bouvier M, Bolanowski MA, Caron MG, Lefkowitz RJ: 1989. CAMP stimulates transcription of the P,-adrenergic receptor gene in response to short-term agonist exposure. Proc Nat1 Acad Sci USA 86:4853- 4857.

Cotecchia S, Schwinn D, Randall RR, Lefkow- itz RJ, Caron MG, Kobilka BK: 1988. Molecular cloning and expression of the cDNA for the hamster qadrenergic receptor. Proc Nat1 Acad Sci USA 85:7159-7163.

Cotecchia S, Kobilka BK, Daniel KW, et al.: 1990. Multiple second messenger pathways

TCM Vol. 1, No. 5, 1991 01991. Elsevier Science Publishing Co.. 1050-1738/91/$2.00 193

of a-adrenergic receptor subtypes expressed in eukaryotic cells. J Med Biol Chem 265:63- 69.

Dixon RAF, Kobilka BK, Strader DJ, et al.: 1986. Cloning of the gene and cDNA for mammalian 8,-adrenergic receptor and ho- mology with rhodopsin. Nature 321:75-79.

Emorine LJ, Marullo S, Briend-Sutren MM, et al.: 1989. Molecular characterization of the human 8,-adrenergic receptor. Science 245:1118-1121.

Fargin A, Raymond JR, Lohse MJ, Kobilka BK, Caron MG, Leflcowitz RJ: 1988. The genomic clone G-21 encodes the serotonin (S-HT-1A) receptor. Nature 335:358-360.

Frielle T Collins S., Daniel KW, Caron MG, Lefkowitz RJ, Kobilka BK: 1987. Cloning of the cDNA for the human P,-adrenergic receptor. Proc Nat1 Acad Sci USA 1987; 84~7920-7924.

Hadcock JR, Wang HY, Malbon CC: 1989. Agonist-induced destabilization of p-adrenergic receptor mRNA: attenuation of gluco- corticoid-induced up-regulation of 8- adrenergic receptors. J Biol Chem 264:19,928-19,933.

Han C, Abael PW, Minneman Kp: 1987. ai subtypes linked to different mechanisms for increasing intracellular CaZ+ in smooth muscle. Nature 329:333-335.

Hausdorff WP, Caron MG, Lefkowitz RJ: 1990. Turning off the signal: desensitiza-

tion of 8-adrenergic receptor function. FASEB J 412881-2889.

Iyengar R, Bimbaumer L, eds: 1990. G Pro- teins. San Diego, Academic Press.

Kobilka BK, Matsui H, Kobilka TS, et al.: 1987a. Cloning, sequencing, and expression of the gene coding for the human platelet a,-adrenergic receptor. Science 238:650-656.

Kobilka BK, Frielle T, Yang-Feng T, et al.: 1987b. An intronless gene encoding a potential member of the family of receptors coupled to guanine nucleotide regulatory proteins. Nature 329:75-79.

Kobilka BK, Kobilka TS, Daniel K, et al.: 1988. Chimericaz-8,-adrenergic receptors: delineation of domains involved in effector coupling and ligand binding specificity. Science 240:1310-1316.

Lefkowitz RJ, Hausdorff WP, Caron MG: 1990. Role of phosphorylation in desensitization of the 8-adrenoceptor. Trends Phar- macol Sci 11:190-194.

Lohse MJ, Benovic JL, Codina J, Caron MG, Lefkowitz RJ: 1990. j.%Arrestin: a protein that regulates 8-adrenergic receptor function. Science 248:1547-50.

Lomasney JW, Lorenz W, Allen LF, et al.: 1990. Expansion of a,-adrenergic receptor family: cloning and characterization of a human aZ-adrenergic receptor subtype, the gene for which is located on chromosome 2. Proc Nat1 Acad Sci USA 87:5094-5098.

O’Dowd BF, Hnatowich M, Regan Jw, Leder WM, Caron MG, Lefkowitz RJ: 1988. Site- directed mutagenesis of the cytoplasmic domains of the human P,-adrenergic receptor: location of regions involved in a G protein-receptor coupling. J Biol Chem 263:15,985-15,992.

Regan Jw, Kobilka TS, Yang-Feng TL, Caron MG, Lefkowitz RJ, Kobilka BK: 1988. Clon- ing and expression of a human kidney cDNA for a novel oz-adrenergic receptor. Proc Nat1 Acad Sci USA 85:6301-6305.

Schwinn, DA, Lomasney JW, Lorenz W, et al.: 1990. Molecular cloning and expression of the cDNA for a novel a,-adrenergic receptor subtype. J Biol Chem 265:8183-8189.

Strader CD, Irving SS, Dixon RAF: 1989. Structural basis of J3-adrenergic receptor function. FASEB J 3:1825-1832.

Wong SK, Slaughter C, Ruoho AE, Ross EM: 1988. The catecholamine binding site of the 8-adrenergic receptor is formed by juxta- posed membrane-spanning domains. J Biol Chem 263:7925-7928.

Wong SK, Parker, EM, Ross EM: 1990. Chim- eric muscarinic cholinergic: 8-adrenergic receptors that activate G, in response to muscarinic agonists. JBiol Chem 265:6219- 6224.

Yarden Y, Rodriguez H, Wong SK, et al.: 1986. The avian J3-adrenergic receptor: primary structure and membrane topology. Proc Natl Acad Sci USA 83:6795-6799. TCM

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Molecular and cellular biology of adrenergic receptors