The multiple membrane spanning topography of the beta 2-adrenergic receptor. Localization of the sites of binding, glycosylation, and regulatory phosphorylation by limited proteolysis

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

The Open Question of How GPCRs Interact with GPCR Kinases (GRKs)

G protein-coupled receptors (GPCRs), which regulate a vast number of eukaryotic processes, are desensitized by various mechanisms but, most importantly, by the GPCR kinases (GRKs). Ever since GRKs were first identified, investigators have sought to determine which structural features of GRKs are used to select for the agonist-bound states of GPCRs and how this binding event in turn enhances GRK catalytic activity. Despite a wealth of molecular information from high-resolution crystal structures of GRKs, the mechanisms driving activation have remained elusive, in part because the GRK N-terminus and active site tether region, previously proposed to serve as a receptor docking site and to be key to kinase domain closure, are often disordered or adopt inconsistent conformations. However, two recent studies have implicated other regions of GRKs as being involved in direct interactions with active GPCRs. Atomic resolution structures of GPCR–GRK complexes would help refine these models but are, so far, lacking. Here, we assess three distinct models for how GRKs recognize activated GPCRs, discuss limitations in the approaches used to generate them, and then experimentally test a hypothetical GPCR interaction site in GRK2 suggested by the two newest models.

A simple and rapid pipeline for identification of receptor-binding sites on the surface proteins of pathogens

Ligand-receptor interactions play a crucial role in the plethora of biological processes. Several methods have been established to reveal ligand-receptor interface, however, the majority of methods are time-consuming, laborious and expensive. Here we present a straightforward and simple pipeline to identify putative receptor-binding sites on the pathogen ligands. Two model ligands (bait proteins), domain III of protein E of West Nile virus and NadA of Neisseria meningitidis, were incubated with the proteins of human brain microvascular endothelial cells immobilized on nitrocellulose or PVDF membrane, the complex was trypsinized on-membrane, bound peptides of the bait proteins were recovered and detected on MALDI-TOF. Two peptides of DIII (~916 Da and ~2003 Da) and four peptides of NadA (~1453 Da, ~1810 Da, ~2051 Da and ~2433 Da) were identified as plausible receptor-binders. Further, binding of the identified peptides to the proteins of endothelial cells was corroborated using biotinylated synthetic analogues in ELISA and immunocytochemistry. Experimental pipeline presented here can be upscaled easily to map receptor-binding sites on several ligands simultaneously. The approach is rapid, cost-effective and less laborious. The proposed experimental pipeline could be a simpler alternative or complementary method to the existing techniques used to reveal amino-acids involved in the ligand-receptor interface.

Myocyte Response to Stimulation of Receptors and Ion Channels

Ventricular myocytes dissociated from adult rat heart and cultured chick embryo ventricular cells were utilized to examine mechanisms by which neurotransmitters, hormones, and ontogeny modulate expression and function of β-adrenergic receptors and L-type calcium channels. Either freshly dissociated cells or cultured cells were studied by an optical-video system to characterize contractility and, in some instances, by a microspectrofluorimeter to determine [Ca2+]i as reported by fura 2. Ligand binding studies in intact cells and membranes were conducted with receptor and ion channel antagonists and agonists. Exposure of intact cells to isoproterenol produced contractile de-sensitization, loss of high affinity receptors from the sarcolemma and closely coupled decline in hormone-sensitive adenylate cyclase activity. Desensitization was by a microfilament-dependent process. Down-regulation depended upon microtubular function. During development of the chick heart, there was an increase in number of dihydropyridine binding sites, taken as a measure of number of L-type calcium channels, at a time when sensitivity to [Ca2+]o and to Bay k 8644 declined. Thyroid hormone was capable of up-regulating L-type calcium channels. Prolonged exposure to a β-adrenergic agonist produced coordinate down-regulation of β-receptors and calcium channels. Down-regulation was a cAMP-dependent process. Thus, the β-adrenergic receptor and a distal component of the effector-response coupling system, the L-type calcium channel, can be regulated independently and in concert by physiologically and pathophysiologically important mechanisms.

Platelet Signaling and Disease: Targeted Therapy for Thrombosis and Other Related Diseases

Platelets are essential for clotting in the blood and maintenance of normal hemostasis. Under pathologic conditions such as atherosclerosis, vascular injury often results in hyperactive platelet activation, resulting in occlusive thrombus formation, myocardial infarction, and stroke. Recent work in the field has elucidated a number of platelet functions unique from that of maintaining hemostasis, including regulation of tumor growth and metastasis, inflammation, infection, and immune response. Traditional therapeutic targets for inhibiting platelet activation have primarily been limited to cyclooxygenase-1, integrin αIIbβ3, and the P2Y12 receptor. Recently identified signaling pathways regulating platelet function have made it possible to develop novel approaches for pharmacological intervention in the blood to limit platelet reactivity. In this review, we cover the newly discovered roles for platelets as well as their role in hemostasis and thrombosis. These new roles for platelets lend importance to the development of new therapies targeted to the platelet. Additionally, we highlight the promising receptor and enzymatic targets that may further decrease platelet activation and help to address the myriad of pathologic conditions now known to involve platelets without significant effects on hemostasis.

The GPCR crystallography boom: providing an invaluable source of structural information and expanding the scope of homology modeling

G protein-coupled receptors (GPCRs) are integral membrane proteins of high pharmaceutical interest. Until relatively recently, their structures have been particularly elusive, and rhodopsin has been for many years the only member of the superfamily with experimentally elucidated structures. However, a number of recent technical and scientific advancements made the determination of GPCR structures more feasible, thus leading to the solution of the structures of several receptors. Besides providing direct structural information, these experimental GPCR structures also provide templates for the construction of GPCR models. In depth studies have been performed to probe the accuracy of these models, in particular with respect to the interactions with their ligands, and to assess their applicability the rational discovery of GPCR modulators. Given the current state of the art and the pace of the field, the future of GPCR structural studies is likely to be characterized by a landscape populated by an increasingly higher number of experimental and theoretical structures.

Arrestin interactions with G protein-coupled receptors

G-protein-coupled receptors (GPCRs) are the primary interaction partners for arrestins. The visual arrestins, arrestin1 and arrestin4, physiologically bind to only very few receptors, i.e., rhodopsin and the color opsins, respectively. In contrast, the ubiquitously expressed nonvisual variants β-arrestin1 and 2 bind to a large number of receptors in a fairly nonspecific manner. This binding requires two triggers, agonist activation and receptor phosphorylation by a G-protein-coupled receptor kinase (GRK). These two triggers are mediated by two different regions of the arrestins, the "phosphorylation sensor" in the core of the protein and a less well-defined "activation sensor." Binding appears to occur mostly in a 1:1 stoichiometry, involving the N-terminal domain of GPCRs, but in addition a second GPCR may loosely bind to the C-terminal domain when active receptors are abundant.Arrestin binding initially uncouples GPCRs from their G-proteins. It stabilizes receptors in an active conformation and also induces a conformational change in the arrestins that involves a rotation of the two domains relative to each other plus changes in the polar core. This conformational change appears to permit the interaction with further downstream proteins. The latter interaction, demonstrated mostly for β-arrestins, triggers receptor internalization as well as a number of nonclassical signaling pathways.Open questions concern the exact stoichiometry of the interaction, possible specificity with regard to the type of agonist and of GRK involved, selective regulation of downstream signaling (=biased signaling), and the options to use these mechanisms as therapeutic targets.

Abnormal fat distribution in PMM2-CDG

We hypothesize that abnormal fat distribution, a common feature of PMM2-CDG, is associated with abnormal perinatal hormone regulation. We assessed 32 cases with PMM2-CDG, for the comorbidity of hypoglycemia/hyperinsulinism and fat pads. Ninety percent of patients with hypoketotic hypoglycemia and/or hyperinsulinism had abnormal fat distribution, while normoglycemic patients showed this feature in 50% of the cases. This statistically significant difference suggests an etiological role of the insulin receptor in developing abnormal fat distribution in PMM2-CDG.

Extracellular loop II modulates GTP sensitivity of the prostaglandin EP3 receptor

Unlike the majority of G protein-coupled receptors, the prostaglandin E(2) (PGE(2)) E-prostanoid 3 (EP3) receptor binds agonist with high affinity that is insensitive to the presence of guanosine 5[prime]-O-(3-thio)triphosphate (GTPγS). We report the identification of mutations that confer GTPγS sensitivity to agonist binding. Seven point mutations were introduced into the conserved motif in the second extracellular loop (ECII) of EP3, resulting in acquisition of GTP-sensitive agonist binding. One receptor mutation W203A was studied in detail. Loss of agonist binding was observed on intact human embryonic kidney 293 cells expressing the W203A receptor, conditions where high GTP levels are present; however, high affinity binding [(3)H]PGE(2) was observed in broken cell preparations washed free of GTP. The [(3)H]PGE(2) binding of W203A in broken cell membrane fractions was inhibited by addition of GTPγS (IC(50) 21 ± 1.8 nM). Taken together, these results suggest that the wild-type EP3 receptor displays unusual characteristics of the complex coupled equilibria between agonist-receptor and receptor-G protein interaction. Moreover, mutation of ECII can alter this coupled equilibrium from GTP-insensitive agonist binding to more conventional GTP-sensitive binding. This suggests that for the mutant receptors, ECII plays a critical role in linking the agonist bound receptor conformation to the G protein nucleotide bound state.

In silico screening for agonists and blockers of the β(2) adrenergic receptor: implications of inactive and activated state structures

Ten crystal structures of the β(2) adrenergic receptor have been published, reflecting different signaling states. Here, through controlled-docking experiments, we examined the implications of using inactive or activated structures on the in silico screening for agonists and blockers of the receptor. Specifically, we targeted the crystal structures solved in complex with carazolol (2RH1), the neutral antagonist alprenalol, the irreversible agonist FAUC50 (3PDS), and the full agonist BI-167017 (3P0G). Our results indicate that activated structures favor agonists over blockers, whereas inactive structures favor blockers over agonists. This tendency is more marked for activated than for inactive structures. Additionally, agonists tend to receive more favorable docking scores when docked at activated rather than inactive structures, while blockers do the opposite. Hence, the difference between the docking scores attained with an activated and an inactive structure is an excellent means for the classification of ligands into agonists and blockers as we determined through receiver operating characteristic curves and linear discriminant analysis. With respect to virtual screening, all structures prioritized well agonists and blockers over nonbinders. However, inactive structures worked better for blockers and activated structures worked better for agonists, respectively. Notably, the combination of individual docking experiments through receptor ensemble docking resulted in an excellent performance in the retrieval of both agonists and blockers. Finally, we demonstrated that the induced-fit docking of agonists is a viable way of modifying an inactive crystal structure and bias it toward the in silico recognition of agonists rather than blockers.

On the applicability of GPCR homology models to computer-aided drug discovery: a comparison between in silico and crystal structures of the beta2-adrenergic receptor

The publication of the crystal structure of the beta2-adrenergic receptor (beta2-AR) proved that G protein-coupled receptors (GPCRs) share a structurally conserved rhodopsin-like 7TM core. Here, to probe to which extent realistic GPCR structures can be recreated through modeling, carazolol was docked at two rhodopsin-based homology models of the human beta 2-AR. The first featured a rhodopsin-like second extracellular loop, which interfered with ligand docking and with the orientation of several residues in the binding pocket. The second featured a second extracellular loop built completely de novo, which afforded a more accurate model of the binding pocket and a better docking of the ligand. Furthermore, incorporating available biochemical and computational data to the model by correcting the conformation of a single residue lining the binding pocket --Phe290(6.52)--, resulted in significantly improved docking poses. These results support the applicability of GPCR modeling to the design of site-directed mutagenesis experiments and to drug discovery.

Regulation of anterograde transport of adrenergic and angiotensin II receptors by Rab2 and Rab6 GTPases

Three Rab GTPases, Rab1, Rab2 and Rab6, are involved in protein transport between the endoplasmic reticulum (ER) and the Golgi. Whereas Rab1 regulates the anterograde ER-to-Golgi transport, Rab2 and Rab6 coordinate the retrograde Golgi-to-ER transport. We have previously demonstrated that Rab1 differentially modulates the export trafficking of distinct G protein-coupled receptors (GPCRs). In this report, we determined the role of Rab2 and Rab6 in the cell-surface expression and signaling of alpha(2B)-adrenergic (alpha(2B)-AR), beta(2)-AR and angiotensin II type 1 receptors (AT1R). Expression of the GTP-bound mutant Rab2Q65L significantly attenuated the cell-surface expression of both alpha(2B)-AR and beta(2)-AR, whereas the GTP-bound mutant Rab6Q72L selectively inhibited the transport of beta(2)-AR, but not alpha(2B)-AR. Similar results were obtained by siRNA-mediated selective knockdown of endogenous Rab2 and Rab6. Consistently, Rab2Q65L and Rab2 siRNA inhibited alpha(2B)-AR and beta(2)-AR signaling measured as ERK1/2 activation and cAMP production, respectively, whereas Rab6Q72L and Rab6 siRNA reduced signaling of beta(2)-AR, but not alpha(2B)-AR. Similar to the beta(2)-AR, AT1R expression at the cell surface and AT1R-promoted inositol phosphate accumulation were inhibited by Rab6Q72L. Furthermore, the nucleotide-free mutant Rab6N126I selectively attenuated the cell-surface expression of beta(2)-AR and AT1R, but not alpha(2B)-AR. These data demonstrate that Rab2 and Rab6 differentially influence anterograde transport and signaling of GPCRs. These data also provide the first evidence indicating that Rab6-coordinated retrograde transport selectively modulates intracellular trafficking and signaling of GPCRs.

The structural basis of arrestin-mediated regulation of G-protein-coupled receptors

The 4 mammalian arrestins serve as almost universal regulators of the largest known family of signaling proteins, G-protein-coupled receptors (GPCRs). Arrestins terminate receptor interactions with G proteins, redirect the signaling to a variety of alternative pathways, and orchestrate receptor internalization and subsequent intracellular trafficking. The elucidation of the structural basis and fine molecular mechanisms of the arrestin-receptor interaction paved the way to the targeted manipulation of this interaction from both sides to produce very stable or extremely transient complexes that helped to understand the regulation of many biologically important processes initiated by active GPCRs. The elucidation of the structural basis of arrestin interactions with numerous non-receptor-binding partners is long overdue. It will allow the construction of fully functional arrestins in which the ability to interact with individual partners is specifically disrupted or enhanced by targeted mutagenesis. These "custom-designed" arrestin mutants will be valuable tools in defining the role of various interactions in the intricate interplay of multiple signaling pathways in the living cell. The identification of arrestin-binding sites for various signaling molecules will also set the stage for designing molecular tools for therapeutic intervention that may prove useful in numerous disorders associated with congenital or acquired disregulation of GPCR signaling.

Identification of a novel aminergic-like G protein-coupled receptor in the cnidarian Renilla koellikeri

Biogenic amines exert various physiological effects in cnidarians, but the receptors involved in these responses are not known. We have cloned a novel G protein-coupled receptor cDNA from an anthozoan, the sea pansy Renilla koellikeri, that shows homology to mammalian catecholamine receptors and, to a lesser extent, to peptidergic receptors. This putative receptor, named Ren2, has a DRC pattern that replaces the well-conserved DRY motif on the cytoplasmic side of the transmembrane III and lacks the cysteine residues usually found in the second extracellular loop and C-terminus tail. Both the second extracellular loop and the N-terminal tail were seen to be short (six and three amino acids, respectively). Northern blot analysis suggests that the receptor gene codes for two transcripts. Localization of these transcripts by in situ hybridization demonstrated abundant expression in the epithelium of the pharyngeal wall, the oral disk and tentacles as well as in the endodermal epithelium lining the gastrovascular cavities.

Distinct pathways for the trafficking of angiotensin II and adrenergic receptors from the endoplasmic reticulum to the cell surface: Rab1-independent transport of a G protein-coupled receptor

The molecular mechanism underlying the transport of G protein-coupled receptors from the endoplasmic reticulum (ER) to the cell surface is poorly understood. This issue was addressed by determining the role of Rab1, a Ras-related small GTPase that coordinates vesicular protein transport in the early secretory pathway, in the subcellular distribution and function of the angiotensin II type 1A receptor (AT1R), beta2-adrenergic receptor (AR), and alpha2B-AR in HEK293T cells. Inhibition of endogenous Rab1 function by transient expression of dominant-negative Rab1 mutants or Rab1 small interfering RNA (siRNA) induced a marked perinuclear accumulation and a significant reduction in cell-surface expression of AT1R and beta2-AR. The accumulated receptors were colocalized with calregulin (an ER marker) and GM130 (a Golgi marker), consistent with Rab1 function in regulating protein transport from the ER to the Golgi. In contrast, dominant-negative Rab1 mutants and siRNA had no effect on the subcellular distribution of alpha2B-AR. Similarly, expression of dominant-negative Rab1 mutants and siRNA depletion of Rab1 significantly attenuated AT1R-mediated inositol phosphate accumulation and ERK1/2 activation and beta2-AR-mediated ERK1/2 activation, but not alpha2B-AR-stimulated ERK1/2 activation. These data indicate that Rab1 GTPase selectively regulates intracellular trafficking and signaling of G protein-coupled receptors and suggest a novel, as yet undefined pathway for movement of G protein-coupled receptors from the ER to the cell surface.

Membrane topology of a metabotropic glutamate receptor

The metabotropic glutamate receptors (mGluRs) have been predicted to have a classical seven transmembrane domain structure similar to that seen for members of the G-protein-coupled receptor (GPCR) superfamily. However, the mGluRs (and other members of the family C GPCRs) show no sequence homology to the rhodopsin-like GPCRs, for which this seven transmembrane domain structure has been experimentally confirmed. Furthermore, several transmembrane domain prediction algorithms suggest that the mGluRs have a topology that is distinct from these receptors. In the present study, we set out to test whether mGluR5 has seven true transmembrane domains. Using a variety of approaches in both prokaryotic and eukaryotic systems, our data provide strong support for the proposed seven transmembrane domain model of mGluR5. We propose that this membrane topology can be extended to all members of the family C GPCRs.

Mutation of Asn293 to Asp in transmembrane helix VI abolishes agonist-induced but not constitutive activity of the beta(2)-adrenergic receptor

The beta(2)-adrenergic receptor has been shown to display significant constitutive activity (i.e., in the absence of agonist) in addition to agonist-induced activation. Various studies have suggested that a movement in transmembrane helix VI plays a role in activation of various G-protein-coupled receptors. Here we show that a mutation in this domain of the beta(2)-adrenergic receptor abolishes agonist activation but not constitutive activity. An Asn293Asp mutant of the human beta(2)-adrenergic receptor was expressed either transiently in COS-7 cells or stably in Chinese hamster ovary cells. The mutant receptors were unable to couple to G(s), as seen by the lack of high-affinity agonist binding as well as a reduction of the affinities of several agonists correlating with their intrinsic activities. The mutant receptors caused only minimal activation of adenylyl cyclase (2.5% of wild-type activity) and also failed to show agonist-induced phosphorylation by G-protein-coupled receptor kinase 2. In contrast, the mutant receptors were much less affected in their constitutive activity: transient transfection of wild-type and mutant receptors into COS-7 cells caused an increase in intracellular cAMP-levels that was dependent on the level of receptor expression and was maximally 5.4-fold for the mutant and 6.8-fold for the wild-type receptors (67% of wild-type activity). Introduction of the Asn293Asp mutation into a constitutively active mutant receptor did not affect the constitutive activity of this mutant. These results underscore the importance of transmembrane helix VI in controlling agonist-induced activation of the receptor and suggest that constitutive activity is different from agonist-induced activity. Furthermore, they indicate that Asn293 is a key residue in transferring conformational information from the agonist-binding site to the intracellular surface.

Modeling and docking the endothelin G-protein-coupled receptor

A model of the endothelin G-protein-coupled receptor (ET(A)) has been constructed using a segmented approach. The model was produced using a bovine rhodopsin model as a template for the seven transmembrane alpha-helices. The three cytoplasmic loop regions and the C-terminal region were modeled on NMR structures of corresponding segments from bovine rhodopsin. The three extracellular loops were modeled on homologous loop regions in other proteins of known structure. The N-terminal region was modeled as a three-helix domain based on its homology with a hydrolase protein. To test the model, the FTDOCK algorithm was used to predict the ligand-binding site for the crystal structure of human endothelin. The site of docking is consistent with mutational and biochemical data. The principal sites of interaction in the endothelin ligand all lie on one face of a helix that has been implicated by structure-activity relationship studies as being essential for binding. As further support for the model, attempts to dock bigET, an inactive precursor to endothelin that does not bind to the receptor, found no sites for tight binding. The model of the receptor-ligand complex produced forms a basis for rational drug design of agonists and antagonists for this G-protein-coupled receptor.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

Lack of a C-terminal tail in the mammalian gonadotropin-releasing hormone receptor confers resistance to agonist-dependent phosphorylation and rapid desensitization

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is, at present, the only G-protein-coupled receptor that activates phospholipase C and lacks a C-terminal tail. We have previously demonstrated that this unique structural feature is associated with resistance to rapid desensitization of phosphoinositide signaling in COS-7 and HEK-293 cells (Heding, A., Vrecl, M., Bogerd, J., McGregor, A., Sellar, R., Taylor, P. L., and Eidne, K. A. (1998) J. Biol. Chem. 273, 11472-11477). Using receptors tagged with a nonapeptide of the influenza hemagglutinin protein to enable immunoprecipitation, we now demonstrate that the mammalian GnRH-R is not phosphorylated in an agonist-dependent manner. In contrast, the mammalian thyrotropin-releasing hormone receptor and the African catfish GnRH-R, both of which have a C-terminal tail, are phosphorylated in response to agonist challenge. Furthermore, chimeras of the mammalian GnRH-R with the C-terminal tail of either the mammalian thyrotropin-releasing hormone receptor or the catfish GnRH-R are also phosphorylated in an agonist-dependent manner. Only those receptors having C-terminal tails showed desensitization of phosphoinositide responses within 5-10 min of agonist challenge. We also show that the internalization of all these receptors when expressed transiently in COS-7 cells is similar. This dissociates receptor internalization from rapid desensitization and demonstrates that the lack of a C-terminal tail in the mammalian GnRH-R results in an inability of the receptor to undergo agonist-dependent phosphorylation and that this results directly in a resistance to rapid desensitization.

Structural features of heterotrimeric G-protein-coupled receptors and their modulatory proteins

Over the past 20 years, the general mechanism for signaling through 7-transmembrane helix receptors coupled to GTP hydrolysis has been worked out. Although similar in overall organization, subtype variability and subcellular localization of components have built in considerable signaling specificity. Atomic resolution structures for many of the components have delineated the domain organization of these complex proteins and have given physical form to the idea of subtype specificity. This review describes what is known about the physical structures of the 7-transmembrane helix receptors, the heterotrimeric GTP binding coupling proteins, the adenylate cyclase and phospholipase C effector proteins, and signaling modulatory proteins, such as arrestin, phosducin, recoverin-type myristoyl switch proteins, and the pleckstrin homology domain of G-protein receptor kinase-2. These images allow experimenters to contemplate the details of the supramolecular organization of the multiprotein complexes involved in the transmission of signals across the cellular lipid bilayer.

Structure-function studies of the eighth hydrophobic domain of a serotonin receptor

The most prominent structural feature of the G protein-coupled receptor superfamily is their seven hydrophobic domains, which are postulated to form membrane-spanning alpha helices. Some members of the G protein-coupled receptor family, specifically several serotonin (5-HT) receptors, possess eight hydrophobic domains. The importance of this extra hydrophobic domain, located at the N terminus of the receptor, is unknown. This question was addressed by deleting the extra hydrophobic region from the 5-HT2C receptor and comparing its function and topology with those of the wild-type receptor. Immunofluorescence microscopy was used to determine the location of the N terminus of the epitope-tagged wild-type and mutant receptors. The N terminus of both receptors was extracellular, suggesting that the extra hydrophobic domain does not change the topology of this receptor and is unlikely to be a membrane-spanning alpha helix. Radioligand-binding studies in transfected cells and expression studies in Xenopus oocytes demonstrated that seven hydrophobic domains were sufficient for normal function in these assays. Interestingly, the mutant receptor, now containing seven hydrophobic domains, is expressed at higher levels in transfected cells than the wild-type receptor containing eight hydrophobic domains, suggesting that the extra hydrophobic domain does impact the activity of this receptor by regulating its expression.

Regulation of second messengers associated with airway smooth muscle contraction and relaxation

Agonist-receptor interactions regulate airway smooth muscle tone through activation of guanine nucleotide binding proteins (G proteins), which are coupled to second messenger pathways that mediate changes in the tissue's contractile state. With respect to airway smooth muscle (ASM) contraction, receptor activation elicits phosphatidylinositol turnover that results in the formation of the second messengers, 1,2,-diacylglyserol, which activates protein kinase C (PKC), and inositol 1,4,5,-trisphosphate (Ins[1,4,5]P3), which binds to its intracellular receptor to mobilize intracellular calcium (Ca2+). Both the mobilization of Ca2+ and activation PKC play critical roles in initiating and acutely modulating the intensity and duration of the ASM contraction response. In contrast, bronchodilator agonist-mediated receptor activation is typically coupled to an enhanced accumulation of the second messenger, adenosine 3',5'-cyclic monophosphate (cAMP) which, through activation of cAMP-dependent protein kinase, induces the phosphorylation of specific proteins, leading to ASM relaxation. For activation of both of these functionally distinct signal transduction pathways, the agonist-receptor complexes interact with specific G proteins, which in turn modulate the enzymes regulating the production of their respective second messengers. Perturbations in Ins(1,4,5)P3 accumulation, its metabolism and intracellular binding may underlie changes in ASM contractility. Comparably, changes in ASM relaxation responsiveness, secondary to perturbations in cAMP accumulation, may be due to altered receptor/G protein modulation of adenylate cyclase activity, as well as to altered binding of Ins(1,4,5)P3 to its Ca2+-mobilizing intracellular receptor. This review begins with an overview of the structural and functional characteristics of G protein-linked receptors, followed by descriptions of the role of G proteins, their transmembrane signaling processes, and mechanisms regulating second messenger-coupled ASM contraction and relaxation, and concludes with new information underscoring the important roles of altered receptor/G protein-coupled expression and regulatory interactions between signaling pathways in modulating second-messenger accumulation and action in the "pro-asthmatic" sensitized airway smooth muscle.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

An agonist-like monoclonal antibody against the human beta2-adrenoceptor

Monoclonal antibodies were produced against a peptide corresponding to the second extracellular loop of the human beta2-adrenoceptor. One of these monoclonals, inducing an agonist-like effect in neonatal rat cardiomyocytes, was used to define the structural and physiological basis of this activity. The epitope recognized by the antibody corresponds to the sequence Trp-Tyr-Arg-Ala-Thr-His-Gln-Glu as determined by peptide scanning. Analysis by alanine modification of the peptide epitope showed the importance of the Trp, and Glu residues in antibody recognition The apparent affinity of the antibody assessed either by surface plasmon resonance or by functional titration on its agonist-like activity showed a similar value (10(8) M(-1)). The antibody recognized the receptor in its native form as shown by immunofluorescence experiments on A431 cells but not in its denatured form as shown by its absence of staining in immunoblots. The positive chronotropic effect in vitro was specifically blocked by both the antigenic peptide and the specific beta2-antagonist (+/-)-1-[2,3-(Dihydro-7-methyl1H-inden-4-yl)oxy]-3-[(1-methy lethyl)amino]-2-butanol hydrochloride (ICI1118,551). This activity was mediated through activation of Ca2+ L-type channels as assessed in guinea pig cardiomyocytes. These results suggest that the epitope is located in an extracellular alpha-helix, whose recognition by the antibody could stabilize the receptor in its 'active' conformation.

Site-specific fluorescence labeling of the beta2 adrenergic receptor amino terminus

A modified human beta2 receptor, designated 0K-beta2, was developed for site-specific labeling at the amino terminus with amine reactive fluorescent probes. 0K-beta2 has the following modifications: (1) all 16 lysines in the wild-type beta2 receptor were mutated to arginines, (2) a FLAG epitope preceded by a cleaved hemagglutinin signal sequence was fused to the amino terminus, and (3) a hexahistidine tail was added to the carboxyl terminus. The FLAG epitope and hexahistidine tail were added to facilitate purification while lysine to arginine mutations eliminate potential labeling sites for amine-reactive fluorescent probes. The remaining primary amines in the 0K-beta2 receptor, the amino terminal amine and the epsilon-amine of Lys3, both reside in the amino-terminal FLAG epitope. The 0K-beta2 receptor expressed in Sf9 insect cells exhibited ligand binding and G-protein coupling characteristics similar to the wild-type beta2 receptor. The modified receptor was labeled with fluorescamine, an amine-reactive fluorescent probe. Proteolysis with factor Xa showed that labeling was confined to the amino terminus of the 0K-beta2 receptor. Our results demonstrate site-specific fluorescamine labeling at the amino terminus of the 0K-beta2 receptor, a lysine-depleted beta2 receptor that retains functional characteristics of the wild-type receptor.

The C-terminal domain of the human EP4 receptor confers agonist-induced receptor desensitization in a receptor hybrid with the rat EP3beta receptor

Prostaglandin E2 receptors (EPR), which belong to the family of heterotrimeric G protein-coupled ectoreceptors with seven transmembrane domains, can be classified into four subtypes according to their ligand binding and G protein coupling specificity. Of these, EP3betaR is coupled to Gi, whereas EP4R is coupled to Gs. EP4R, in contrast to EP3betaR, shows agonist-induced desensitization. The C-terminal domain and the third intracellular loop of these receptors have been implicated in G protein coupling specificity and desensitization. Here, receptor hybrids consisting of the main portion of rat EP3betaR and either the C-terminal domain or the third intracellular loop of human EP4R were used to study the contribution of the respective receptor domains to G protein coupling and desensitization. Neither the EP4R C-terminal domain nor the EP4R third intracellular loop alone was sufficient to change the coupling specificity of the rEP3hEP4 receptor hybrids from Gi to Gs or to confer additional Gs coupling. However, the EP4R C-terminal domain but not the third intracellular loop was necessary and sufficient to mediate rapid agonist-induced, second messenger-independent desensitization in the Gi-coupled hybrid receptors.

Phosphorylation of phospholipase C-coupled receptors

Early work on G-protein-coupled receptor (GPCR) phosphorylation focused on the adenylyl cyclase-linked beta-adrenoceptor, where phosphorylation at sites on the C-terminal tail and within the third intracellular loop results in receptor desensitisation. In recent years, intense research activity has revealed that a large number of GPCR subtypes exist as phosphoproteins, where the level of phosphorylation is dramatically increased subsequent to receptor stimulation. Among these receptor subtypes are those receptors coupled to phospholipase C (PLC). It appears, therefore, that regulation via receptor phosphorylation is a mechanism employed by all but a few GPCRs, including those coupled to PLC. Because the majority of GPCRs are coupled to the phosphoinositide signalling pathway, receptor phosphorylation of PLC-coupled receptors is a regulatory process with profound physiological significance for a huge array of biological responses. This review discusses the properties of homologous and heterologous phosphorylation of PLC-coupled receptors, together with the receptor kinases involved and the functional significance of receptor phosphorylation.

Proteolytic mapping and substrate protection of the Escherichia coli melibiose permease

The topology and substrate-induced conformational change(s) of the Na+ (Li+ or H+)-melibiose cotransporter (MelB) of Escherichia coli were investigated by limited protease digestion. To facilitate these analyses, MelB was epitope-tagged both at its carboxyl-terminus and at its amino-terminus. Limited digestion with different proteases indicates that the cytoplasmic loops connecting transmembrane domains 4-5, 6-7, and 10-11 together with the carboxyl-terminus of MelB are exposed in the cytoplasm. In contrast, periplasmic loops are highly resistant to all the proteases examined, including nonspecific proteases such as proteinase K and thermolysin. The effect of Na+ or Li+ and/or melibiose on the rate of protease digestion of the cytoplasmic loops was also analyzed. The rate of protease digestion of loop 4-5 is specifically reduced, by approximately 3-fold, by the presence of Na+ or Li+. These results suggest that loop 4-5 is near or part of the cation binding site. Moreover, the presence of both melibiose and either Na+ or Li+ further reduced the rate of protease digestion of this loop 4-5 by up to 9-fold, although no protection from protease digestion was observed when melibiose was added alone. The increase in resistance to proteases observed in the presence of the cation alone or the cation plus melibiose suggests that the interaction of the two cosubstrate with MelB results in change(s) of MelB conformation.

The genomic structure of the rat corticotropin releasing factor receptor. A member of the class II G protein-coupled receptors

Isolation and structural characterization of the rat corticotropin releasing factor receptor (CRFR) gene was performed to determine the exon/intron organization of the coding region and the potential for splice variants. The CRFR gene contains 13 exons and 12 introns, and the positions of the exon/intron junctions are similar to those of other Class II G protein-coupled receptor genes including the parathyroid hormone and glucagon receptors. The promoter resides within 593 base pairs of the initiation codon and the major transcriptional start site at nucleotide -238. This domain does not possess a TATA box but contains multiple Sp1 and AP-2 sites upstream and downstream of the major transcriptional start site. Intron junctions were identified in the extracellular, transmembrane (TM), and cytoplasmic (C) domains of the CRFR, giving the potential for differential signal transduction by splice variants. CRFR cDNAs derived from rat Leydig cell mRNA included the pituitary Form A, which spans exons 1-13, and two splice variants with deletion of exon 3 or exons 7, 11, and 12. An evolutionary link between the intronless TM/C module of the glycoprotein hormone receptors and the intron-containing TM/C module of the CRFR is suggested by the common position of the luteinizing hormone receptor Form D alternate acceptor splice site and the CRFR intron 12.

Identification of the G protein-coupled receptor kinase phosphorylation sites in the human beta2-adrenergic receptor

Rapid desensitization of G protein-coupled receptors is mediated, at least in part, by their phosphorylation by the G protein-coupled receptor kinases (GRKs). However, only in the case of rhodopsin have the actual sites of receptor phosphorylation been unambiguously determined. Although previous studies have implicated the cytoplasmic tail of the beta2-adrenergic receptor (beta2AR) as the site of GRK-mediated phosphorylation, the identities of the phosphorylated residues were unknown. Here we report the identification of the sites of GRK2- and GRK5-mediated beta2AR phosphorylation. The phosphorylation sites of both serine/threonine kinases reside exclusively in a 40-amino acid peptide located at the extreme carboxyl terminus of the beta2AR. Of the seven phosphorylatable residues within this peptide, six are phosphorylated by GRK5 (Thr-384, Thr-393, Ser-396, Ser-401, Ser-407, and Ser-411) and four are phosphorylated by GRK2 (Thr-384, Ser-396, Ser-401, and Ser-407) at equivalent phosphorylation stoichiometries (approximately 1.0 mol Pi/mol receptor). In addition to the GRK5-specific phosphorylation of Thr-393 and Ser-411, differences in the distribution of phosphate between sites are observed for GRK2 and GRK5. Increasing the stoichiometry of GRK2-mediated beta2AR phosphorylation from approximately 1.0 to 5.0 mol Pi/mol receptor increases the stoichiometry of phosphorylation of Thr-384, Ser-396, Ser-401, and Ser-407 rather than increasing the number of phosphoacceptor sites. The location of multiple GRK2 and GRK5 phosphoacceptor sites at the extreme carboxyl terminus of the beta2AR is highly reminiscent of GRK1-mediated phosphorylation of rhodopsin.

The role of prostaglandin receptors in regulating cerebral blood flow in the perinatal period

Prostaglandins exert significant effects on the range of cerebral blood flow autoregulation. However, the newborn exhibits a narrow cerebral blood flow autoregulatory range compared to the adult, and this apparently contributes to the susceptibility of the newborn to major perinatal complications such as intraventricular cerebral haemorrhage. Reduced vasoconstriction in response to prostaglandins due to the fewer prostaglandin receptors, especially for PGE2 (EP) and PGF2 alpha (FP), seems to contribute in part to the narrower range of cerebral blood flow autoregulation in the newborn. Evidence suggests that high levels of prostaglandins in the perinatal period are responsible for the down-regulation of neurovascular EP and FP receptors. We review the pharmacology of prostaglandin receptors, in particular PGE2 and PGF2 alpha receptors, their ontogeny on the neural vasculature, the perinatal regulation of their expression, and how these changes relate to the control of neural blood flow autoregulation.

Conformational changes upon binding of a receptor loop to lipid structures: possible role in signal transduction

The mas oncogene codes for a seven transmembrane helix protein. The amino acid sequence 253-266, from the third extracellular loop and beginning of helix 7, was synthesized either blocked or carrying an amino acid spin label at the N-terminus. Peptide binding to bilayers and micelles was monitored by ESR, fluorescence and circular dichroism. Binding induced tighter lipid packing, and caused an increase of peptide secondary structure. While binding to bilayers occurred only when peptide and phospholipid bore opposite charges, in micelles the interaction took place irrespective of charge. The results suggest that changes in lipid packing could modulate conformational changes in receptor loops related to the triggering of signal transduction.

Role of phosphorylation in agonist-promoted beta 2-adrenergic receptor sequestration. Rescue of a sequestration-defective mutant receptor by beta ARK1

The beta 2-adrenergic receptor (beta 2AR) belongs to the large family of G protein-coupled receptors. Mutation of tyrosine residue 326 to an alanine resulted in a beta 2AR mutant (beta 2AR-Y326A) that was defective in its ability to sequester and was less well coupled to adenylyl cyclase than the wild-type beta 2AR. However, this mutant receptor not only desensitized in response to agonist stimulation but down-regulated normally. In an attempt to understand the basis for the properties of this mutant, we have examined the ability of this regulation-defective mutant to undergo agonist-mediated phosphorylation. When expressed in 293 cells, the maximal response for phosphorylation of the beta 2AR-Y326A mutant was impaired by 75%. Further characterization of this phosphorylation, using either forskolin stimulation or phosphorylation site-deficient beta 2AR-Y326A mutants, demonstrated that the beta 2AR-Y326A mutant can be phosphorylated by cAMP-dependent protein kinase (PKA) but does not serve as a substrate for the beta-adrenergic receptor kinase 1 (beta ARK1). However, overexpression of beta ARK1 led to the agonist-dependent phosphorylation of the beta 2AR-Y326A mutant and rescue of its sequestration. beta ARK1-mediated rescue of beta 2AR-Y326A sequestration could be prevented by mutating putative beta ARK phosphorylation sites, but not PKA phosphorylation sites. In addition, both sequestration and phosphorylation of the wild-type beta 2AR could be attenuated by overexpressing a dominant-negative mutant of beta ARK1 (C20 beta ARK1-K220M). These findings implicate a role for beta ARK1-mediated phosphorylation in facilitating wild-type beta 2AR sequestration.

Phosphorylation of the N-formyl peptide receptor carboxyl terminus by the G protein-coupled receptor kinase, GRK2

Attenuation of receptor-mediated signal amplification in response to external stimuli, an essential step in the balance of cellular activation, may be mediated by receptor phosphorylation. We have recently shown that the carboxyl-terminal cytoplasmic domain of the N-formyl peptide receptor (FPR) interacts with G proteins and demonstrate here that this same region of the FPR is specifically phosphorylated by a neutrophil cytosolic kinase with properties similar to the G protein-coupled receptor kinase, GRK2. Both kinase activities show a lack of sensitivity toward protein kinase A, protein kinase C, and tyrosine kinase inhibitors but demonstrate almost identical sensitivity toward the kinase inhibitor heparin. Kinetic studies demonstrated that GRK2 has a Km for the carboxyl-terminal domain of the FPR of approximately 1.5 microM and that denaturation of the substrate results in an almost complete loss of phosphorylation. Comparative studies reveal that GRK3 has approximately 50% of the activity of GRK2 toward the FPR carboxyl terminus, whereas GRK5 and GRK6 have no detectable activity. Site-directed mutagenesis of numerous regions of the FPR carboxyl terminus demonstrated that, whereas Glu326/Asp327 and Asp333 are critical for phosphorylation, the carboxyl-terminal 10 amino acids are not required. Simultaneous substitution of Thr334, Thr336, Ser338, and Thr339 resulted in an approximately 50% reduction in phosphorylation, whereas simultaneous substitution of the upstream Ser328, Thr329, Thr331, and Ser332 or merely the Ser328 and Thr329 residues resulted in an approximately 80% reduction in phosphorylation. The introduction of negatively charged glutamate residues for Ser328 and Thr329 or Thr331 and Ser332 resulted in marked stimulation of phosphorylation. These results suggest a hierarchical mechanism in which phosphorylation of amino-terminal serine and threonine residues is required for the subsequent phosphorylation of carboxyl-terminal residues. These results provide the first direct evidence that an intracellular domain of a chemoattractant receptor is a high affinity substrate for GRK2 and further suggest a role for GRK2 or a closely related kinase in the attenuation of receptor-mediated activation of inflammatory cells.