Neurotensin Receptor Allosterism Revealed in Complex with a Biased Allosteric Modulator.

The NTSR1 neurotensin receptor (NTSR1) is a G protein-coupled receptor (GPCR) found in the brain and peripheral tissues with neurotensin (NTS) being its endogenous peptide ligand. In the brain, NTS modulates dopamine neuronal activity, induces opioid-independent analgesia, and regulates food intake. Recent studies indicate that biasing NTSR1 toward ß-arrestin signaling can attenuate the actions of psychostimulants and other drugs of abuse. Here, we provide the cryoEM structures of NTSR1 ternary complexes with heterotrimeric Gq and GoA with and without the brain-penetrant small-molecule SBI-553. In functional studies, we discovered that SBI-553 displays complex allosteric actions exemplified by negative allosteric modulation for G proteins that are Gα subunit selective and positive allosteric modulation and agonism for ß-arrestin translocation at NTSR1. Detailed structural analysis of the allosteric binding site illuminated the structural determinants for biased allosteric modulation of SBI-553 on NTSR1.

Structure-Based Design of a Chemical Probe Set for the 5-HT5A Serotonin Receptor.

The 5-HT5A receptor (5-HT5AR), for which no selective agonists and a few antagonists exist, remains the least understood serotonin receptor. A single commercial antagonist, SB-699551, has been widely used to investigate the 5-HT5AR function in neurological disorders, including pain, but this molecule has substantial liabilities as a chemical probe. Accordingly, we sought to develop an internally controlled probe set. Docking over 6 million molecules against a 5-HT5AR homology model identified 5 mid-µM ligands, one of which was optimized to UCSF678, a 42 nM arrestin-biased partial agonist at the 5-HT5AR with a more restricted off-target profile and decreased assay liabilities versus SB-699551. Site-directed mutagenesis supported the docked pose of UCSF678. Surprisingly, analogs of UCSF678 that lost the 5-HT5AR activity revealed that 5-HT5AR engagement is nonessential for alleviating pain, contrary to studies with less-selective ligands. UCSF678 and analogs constitute a selective probe set with which to study the function of the 5-HT5AR.

TRUPATH, an open-source biosensor platform for interrogating the GPCR transducerome.

G-protein-coupled receptors (GPCRs) remain major drug targets, despite our incomplete understanding of how they signal through 16 non-visual G-protein signal transducers (collectively named the transducerome) to exert their actions. To address this gap, we have developed an open-source suite of 14 optimized bioluminescence resonance energy transfer (BRET) Gαßγ biosensors (named TRUPATH) to interrogate the transducerome with single pathway resolution in cells. Generated through exhaustive protein engineering and empirical testing, the TRUPATH suite of Gαßγ biosensors includes the first Gα15 and GαGustducin probes. In head-to-head studies, TRUPATH biosensors outperformed first-generation sensors at multiple GPCRs and in different cell lines. Benchmarking studies with TRUPATH biosensors recapitulated previously documented signaling bias and revealed new coupling preferences for prototypic and understudied GPCRs with potential in vivo relevance. To enable a greater understanding of GPCR molecular pharmacology by the scientific community, we have made TRUPATH biosensors easily accessible as a kit through Addgene.

Discovery of Human Signaling Systems: Pairing Peptides to G Protein-Coupled Receptors.

The peptidergic system is the most abundant network of ligand-receptor-mediated signaling in humans. However, the physiological roles remain elusive for numerous peptides and more than 100 G protein-coupled receptors (GPCRs). Here we report the pairing of cognate peptides and receptors. Integrating comparative genomics across 313 species and bioinformatics on all protein sequences and structures of human class A GPCRs, we identify universal characteristics that uncover additional potential peptidergic signaling systems. Using three orthogonal biochemical assays, we pair 17 proposed endogenous ligands with five orphan GPCRs that are associated with diseases, including genetic, neoplastic, nervous and reproductive system disorders. We also identify additional peptides for nine receptors with recognized ligands and pathophysiological roles. This integrated computational and multifaceted experimental approach expands the peptide-GPCR network and opens the way for studies to elucidate the roles of these signaling systems in human physiology and disease. VIDEO ABSTRACT.

VEGAS as a Platform for Facile Directed Evolution in Mammalian Cells.

Directed evolution, artificial selection toward designed objectives, is routinely used to develop new molecular tools and therapeutics. Successful directed molecular evolution campaigns repeatedly test diverse sequences with a designed selective pressure. Unicellular organisms and their viral pathogens are exceptional for this purpose and have been used for decades. However, many desirable targets of directed evolution perform poorly or unnaturally in unicellular backgrounds. Here, we present a system for facile directed evolution in mammalian cells. Using the RNA alphavirus Sindbis as a vector for heredity and diversity, we achieved 24-h selection cycles surpassing 10-3 mutations per base. Selection is achieved through genetically actuated sequences internal to the host cell, thus the system's name: viral evolution of genetically actuating sequences, or "VEGAS." Using VEGAS, we evolve transcription factors, GPCRs, and allosteric nanobodies toward functional signaling endpoints each in less than 1 weeks' time.

PAM-Antagonists: A Better Way to Block Pathological Receptor Signaling?

Seven transmembrane receptor (7TMR) responses are modulated by orthosteric and allosteric ligands to great therapeutic advantage. Here we introduce a unique class of negative allosteric modulator (NAM) - the positive allosteric modulator (PAM)-antagonist - that increases the affinity of the receptor for the agonist but concomitantly decreases agonist efficacy when cobound. Notably, the reciprocation of allosteric energy causes the orthosteric agonist to increase the affinity of the receptor for the PAM-antagonist; thus, this modulator seeks out and destroys agonist-bound receptor complexes. When contrasted with standard orthosteric and allosteric antagonists it is clear that PAM-antagonists are uniquely well suited to reversing ongoing persistent agonism and provide favorable target coverage in vivo. Specifically, the therapeutic application of PAM-antagonists to reverse pathological overactivation (e.g., endothelin vasoconstriction) is emphasized.

Structure of the Nanobody-Stabilized Active State of the Kappa Opioid Receptor.

The κ-opioid receptor (KOP) mediates the actions of opioids with hallucinogenic, dysphoric, and analgesic activities. The design of KOP analgesics devoid of hallucinatory and dysphoric effects has been hindered by an incomplete structural and mechanistic understanding of KOP agonist actions. Here, we provide a crystal structure of human KOP in complex with the potent epoxymorphinan opioid agonist MP1104 and an active-state-stabilizing nanobody. Comparisons between inactive- and active-state opioid receptor structures reveal substantial conformational changes in the binding pocket and intracellular and extracellular regions. Extensive structural analysis and experimental validation illuminate key residues that propagate larger-scale structural rearrangements and transducer binding that, collectively, elucidate the structural determinants of KOP pharmacology, function, and biased signaling. These molecular insights promise to accelerate the structure-guided design of safer and more effective κ-opioid receptor therapeutics.

Mdm2 regulates cardiac contractility by inhibiting GRK2-mediated desensitization of ß-adrenergic receptor signaling.

The oncoprotein Mdm2 is a RING domain-containing E3 ubiquitin ligase that ubiquitinates G protein-coupled receptor kinase 2 (GRK2) and ß-arrestin2, thereby regulating ß-adrenergic receptor (ßAR) signaling and endocytosis. Previous studies showed that cardiac Mdm2 expression is critical for controlling p53-dependent apoptosis during early embryonic development, but the role of Mdm2 in the developed adult heart is unknown. We aimed to identify if Mdm2 affects ßAR signaling and cardiac function in adult mice. Using Mdm2/p53-KO mice, which survive for 9-12 months, we identified a critical and potentially novel role for Mdm2 in the adult mouse heart through its regulation of cardiac ß1AR signaling. While baseline cardiac function was mostly similar in both Mdm2/p53-KO and wild-type (WT) mice, isoproterenol-induced cardiac contractility in Mdm2/p53-KO was significantly blunted compared with WT mice. Isoproterenol increased cAMP in left ventricles of WT but not of Mdm2/p53-KO mice. Additionally, while basal and forskolin-induced calcium handling in isolated Mdm2/p53-KO and WT cardiomyocytes were equivalent, isoproterenol-induced calcium handling in Mdm2/p53-KO was impaired. Mdm2/p53-KO hearts expressed 2-fold more GRK2 than WT. GRK2 polyubiquitination via lysine-48 linkages was significantly reduced in Mdm2/p53-KO hearts. Tamoxifen-inducible cardiomyocyte-specific deletion of Mdm2 in adult mice also led to a significant increase in GRK2, and resulted in severely impaired cardiac function, high mortality, and no detectable ßAR responsiveness. Gene delivery of either Mdm2 or GRK2-CT in vivo using adeno-associated virus 9 (AAV9) effectively rescued ß1AR-induced cardiac contractility in Mdm2/p53-KO. These findings reveal a critical p53-independent physiological role of Mdm2 in adult hearts, namely, regulation of GRK2-mediated desensitization of ßAR signaling.

Allosteric "beta-blocker" isolated from a DNA-encoded small molecule library.

The ß2-adrenergic receptor (ß2AR) has been a model system for understanding regulatory mechanisms of G-protein-coupled receptor (GPCR) actions and plays a significant role in cardiovascular and pulmonary diseases. Because all known ß-adrenergic receptor drugs target the orthosteric binding site of the receptor, we set out to isolate allosteric ligands for this receptor by panning DNA-encoded small-molecule libraries comprising 190 million distinct compounds against purified human ß2AR. Here, we report the discovery of a small-molecule negative allosteric modulator (antagonist), compound 15 [([4-((2S)-3-(((S)-3-(3-bromophenyl)-1-(methylamino)-1-oxopropan-2-yl)amino)-2-(2-cyclohexyl-2-phenylacetamido)-3-oxopropyl)benzamide], exhibiting a unique chemotype and low micromolar affinity for the ß2AR. Binding of 15 to the receptor cooperatively enhances orthosteric inverse agonist binding while negatively modulating binding of orthosteric agonists. Studies with a specific antibody that binds to an intracellular region of the ß2AR suggest that 15 binds in proximity to the G-protein binding site on the cytosolic surface of the ß2AR. In cell-signaling studies, 15 inhibits cAMP production through the ß2AR, but not that mediated by other Gs-coupled receptors. Compound 15 also similarly inhibits ß-arrestin recruitment to the activated ß2AR. This study presents an allosteric small-molecule ligand for the ß2AR and introduces a broadly applicable method for screening DNA-encoded small-molecule libraries against purified GPCR targets. Importantly, such an approach could facilitate the discovery of GPCR drugs with tailored allosteric effects.

GPCR-G Protein-ß-Arrestin Super-Complex Mediates Sustained G Protein Signaling.

Classically, G protein-coupled receptor (GPCR) stimulation promotes G protein signaling at the plasma membrane, followed by rapid ß-arrestin-mediated desensitization and receptor internalization into endosomes. However, it has been demonstrated that some GPCRs activate G proteins from within internalized cellular compartments, resulting in sustained signaling. We have used a variety of biochemical, biophysical, and cell-based methods to demonstrate the existence, functionality, and architecture of internalized receptor complexes composed of a single GPCR, ß-arrestin, and G protein. These super-complexes or "megaplexes" more readily form at receptors that interact strongly with ß-arrestins via a C-terminal tail containing clusters of serine/threonine phosphorylation sites. Single-particle electron microscopy analysis of negative-stained purified megaplexes reveals that a single receptor simultaneously binds through its core region with G protein and through its phosphorylated C-terminal tail with ß-arrestin. The formation of such megaplexes provides a potential physical basis for the newly appreciated sustained G protein signaling from internalized GPCRs.

Conformationally selective RNA aptamers allosterically modulate the ß2-adrenoceptor.

G-protein-coupled receptor (GPCR) ligands function by stabilizing multiple, functionally distinct receptor conformations. This property underlies the ability of 'biased agonists' to activate specific subsets of a given receptor's signaling profile. However, stabilizing distinct active GPCR conformations to enable structural characterization of mechanisms underlying GPCR activation remains difficult. These challenges have accentuated the need for receptor tools that allosterically stabilize and regulate receptor function through unique, previously unappreciated mechanisms. Here, using a highly diverse RNA library combined with advanced selection strategies involving state-of-the-art next-generation sequencing and bioinformatics analyses, we identify RNA aptamers that bind a prototypical GPCR, the ß2-adrenoceptor (ß2AR). Using biochemical, pharmacological, and biophysical approaches, we demonstrate that these aptamers bind with nanomolar affinity at defined surfaces of the receptor, allosterically stabilizing active, inactive, and ligand-specific receptor conformations. The discovery of RNA aptamers as allosteric GPCR modulators significantly expands the diversity of ligands available to study the structural and functional regulation of GPCRs.

Allosteric nanobodies reveal the dynamic range and diverse mechanisms of G-protein-coupled receptor activation.

G-protein-coupled receptors (GPCRs) modulate many physiological processes by transducing a variety of extracellular cues into intracellular responses. Ligand binding to an extracellular orthosteric pocket propagates conformational change to the receptor cytosolic region to promote binding and activation of downstream signalling effectors such as G proteins and ß-arrestins. It is well known that different agonists can share the same binding pocket but evoke unique receptor conformations leading to a wide range of downstream responses ('efficacy'). Furthermore, increasing biophysical evidence, primarily using the ß2-adrenergic receptor (ß2AR) as a model system, supports the existence of multiple active and inactive conformational states. However, how agonists with varying efficacy modulate these receptor states to initiate cellular responses is not well understood. Here we report stabilization of two distinct ß2AR conformations using single domain camelid antibodies (nanobodies)­a previously described positive allosteric nanobody (Nb80) and a newly identified negative allosteric nanobody (Nb60). We show that Nb60 stabilizes a previously unappreciated low-affinity receptor state which corresponds to one of two inactive receptor conformations as delineated by X-ray crystallography and NMR spectroscopy. We find that the agonist isoprenaline has a 15,000-fold higher affinity for ß2AR in the presence of Nb80 compared to the affinity of isoprenaline for ß2AR in the presence of Nb60, highlighting the full allosteric range of a GPCR. Assessing the binding of 17 ligands of varying efficacy to the ß2AR in the absence and presence of Nb60 or Nb80 reveals large ligand-specific effects that can only be explained using an allosteric model which assumes equilibrium amongst at least three receptor states. Agonists generally exert efficacy by stabilizing the active Nb80-stabilized receptor state (R80). In contrast, for a number of partial agonists, both stabilization of R80 and destabilization of the inactive, Nb60-bound state (R60) contribute to their ability to modulate receptor activation. These data demonstrate that ligands can initiate a wide range of cellular responses by differentially stabilizing multiple receptor states.

Circulating Exosomes Induced by Cardiac Pressure Overload Contain Functional Angiotensin II Type 1 Receptors.

BACKGROUND: Whether biomechanical force on the heart can induce exosome secretion to modulate cardiovascular function is not known. We investigated the secretion and activity of exosomes containing a key receptor in cardiovascular function, the angiotensin II type I receptor (AT1R). METHODS AND RESULTS: Exosomes containing AT1Rs were isolated from the media overlying AT1R-overexpressing cells exposed to osmotic stretch and from sera of mice undergoing cardiac pressure overload. The presence of AT1Rs in exosomes was confirmed by both electron microscopy and radioligand receptor binding assays and shown to require ß-arrestin2, a multifunctional adaptor protein essential for receptor trafficking. We show that functional AT1Rs are transferred via exosomes in an in vitro model of cellular stretch. Using mice with global and cardiomyocyte conditional deletion of ß-arrestin2, we show that under conditions of in vivo pressure overload the cellular source of the exocytosis of exosomes containing AT1R is the cardiomyocyte. Exogenously administered AT1R-enriched exosomes target cardiomyocytes, skeletal myocytes, and mesenteric resistance vessels and are sufficient to confer blood pressure responsiveness to angiotensin II infusion in AT1R knockout mice. CONCLUSIONS: AT1R-enriched exosomes are released from the heart under conditions of in vivo cellular stress to likely modulate vascular responses to neurohormonal stimulation. In the context of the whole organism, the concept of G protein-coupled receptor trafficking should consider circulating exosomes as part of the reservoir of functional AT1Rs.

Quantification of beta adrenergic receptor subtypes in beta-arrestin knockout mouse airways.

In allergic asthma Beta 2 adrenergic receptors (ß2ARs) are important mediators of bronchorelaxation and, paradoxically, asthma development. This contradiction is likely due to the activation of dual signaling pathways that are downstream of G proteins or ß-arrestins. Our group has recently shown that ß-arrestin-2 acts in its classical role to desensitize and constrain ß2AR-induced relaxation of both human and murine airway smooth muscle. To assess the role of ß-arrestins in regulating ß2AR function in asthma, we and others have utilized ß-arrestin-1 and -2 knockout mice. However, it is unknown if genetic deletion of ß-arrestins in these mice influences ß2AR expression in the airways. Furthermore, there is lack of data on compensatory expression of ßAR subtypes when either of the ß-arrestins is genetically deleted, thus necessitating a detailed ßAR subtype expression study in these ß-arrestin knockout mice. Here we standardized a radioligand binding methodology to characterize and quantitate ßAR subtype distribution in the airway smooth muscle of wild-type C57BL/6J and ß-arrestin-1 and ß-arrestin-2 knockout mice. Using complementary competition and single-point saturation binding assays we found that ß2ARs predominate over ß1ARs in the whole lung and epithelium-denuded tracheobronchial smooth muscle of C57BL/6J mice. Quantification of ßAR subtypes in ß-arrestin-1 and ß-arrestin-2 knockout mouse lung and epithelium-denuded tracheobronchial tissue showed that, similar to the C57BL/6J mice, both knockouts display a predominance of ß2AR expression. These data provide further evidence that ß2ARs are expressed in greater abundance than ß1ARs in the tracheobronchial smooth muscle and that loss of either ß-arrestin does not significantly affect the expression or relative proportions of ßAR subtypes. As ß-arrestins are known to modulate ß2AR function, our analysis of ßAR subtype expression in ß-arrestin knockout mice airways sets a reference point for future studies exploiting these knockout mice in various disease models including asthma.

Allosteric modulation of ß-arrestin-biased angiotensin II type 1 receptor signaling by membrane stretch.

It has recently been appreciated that the angiotensin II type 1 receptor (AT1R), a prototypic member of the G protein-coupled receptor superfamily, also functions as a mechanosensor. Specifically, mechanical stretch activates the AT1R to promote downstream signaling mediated exclusively by the multifunctional scaffold protein, ß-arrestin, in a manner consistent with previously identified ß-arrestin-biased ligands. However, the ligand-independent mechanism by which mechanical stretch promotes ß-arrestin-biased signaling remains unknown. Implicit in the concept of biased agonism (i.e. the ability of an agonist to activate a subset of receptor-mediated signaling pathways) is the notion that distinct active conformations of the receptor mediate differential activation of signaling pathways. Here we determined whether mechanical stretch stabilizes distinct ß-arrestin-activating conformations of the AT1R by using ß-arrestin2-biased agonists as conformational probes in pharmacological and biophysical assays. When tested at cells expressing the AT1R fused to ß-arrestin (AT1R-ß-arrestin2), we found that osmotic stretch increased the binding affinity and potency of the ß-arrestin-biased agonist TRV120023, with no effect on the balanced agonist AngII. In addition, the effect of osmotic stretch on ERK activation was markedly augmented in cells expressing the AT1R-ß-arrestin2 fusion compared with the wild type AT1R and completely blocked in cells expressing the AT1R-Gq fusion. Biophysical experiments with an intramolecular BRET ß-arrestin2 biosensor revealed that osmotic stretch and TRV120023 activate AT1Rs to stabilize ß-arrestin2 active conformations that differ from those stabilized by the AT1R activated by angiotensin II. Together, these data support a novel ligand-independent mechanism whereby mechanical stretch allosterically stabilizes specific ß-arrestin-biased active conformations of the AT1R and has important implications for understanding pathophysiological AT1R signaling.

Divergent transducer-specific molecular efficacies generate biased agonism at a G protein-coupled receptor (GPCR).

The concept of "biased agonism" arises from the recognition that the ability of an agonist to induce a receptor-mediated response (i.e. "efficacy") can differ across the multiple signal transduction pathways (e.g. G protein and ß-arrestin (ßarr)) emanating from a single GPCR. Despite the therapeutic promise of biased agonism, the molecular mechanism(s) whereby biased agonists selectively engage signaling pathways remain elusive. This is due in large part to the challenges associated with quantifying ligand efficacy in cells. To address this, we developed a cell-free approach to directly quantify the transducer-specific molecular efficacies of balanced and biased ligands for the angiotensin II type 1 receptor (AT1R), a prototypic GPCR. Specifically, we defined efficacy in allosteric terms, equating shifts in ligand affinity (i.e. KLo/KHi) at AT1R-Gq and AT1R-ßarr2 fusion proteins with their respective molecular efficacies for activating Gq and ßarr2. Consistent with ternary complex model predictions, transducer-specific molecular efficacies were strongly correlated with cellular efficacies for activating Gq and ßarr2. Subsequent comparisons across transducers revealed that biased AT1R agonists possess biased molecular efficacies that were in strong agreement with the signaling bias observed in cellular assays. These findings not only represent the first measurements of the thermodynamic driving forces underlying differences in ligand efficacy between transducers but also support a molecular mechanism whereby divergent transducer-specific molecular efficacies generate biased agonism at a GPCR.

Regulation of ß2-adrenergic receptor function by conformationally selective single-domain intrabodies.

The biologic activity induced by ligand binding to orthosteric or allosteric sites on a G protein-coupled receptor (GPCR) is mediated by stabilization of specific receptor conformations. In the case of the ß2 adrenergic receptor, these ligands are generally small-molecule agonists or antagonists. However, a monomeric single-domain antibody (nanobody) from the Camelid family was recently found to allosterically bind and stabilize an active conformation of the ß2-adrenergic receptor (ß2AR). Here, we set out to study the functional interaction of 18 related nanobodies with the ß2AR to investigate their roles as novel tools for studying GPCR biology. Our studies revealed several sequence-related nanobody families with preferences for active (agonist-occupied) or inactive (antagonist-occupied) receptors. Flow cytometry analysis indicates that all nanobodies bind to epitopes displayed on the intracellular receptor surface; therefore, we transiently expressed them intracellularly as "intrabodies" to test their effects on ß2AR-dependent signaling. Conformational specificity was preserved after intrabody conversion as demonstrated by the ability for the intracellularly expressed nanobodies to selectively bind agonist- or antagonist-occupied receptors. When expressed as intrabodies, they inhibited G protein activation (cyclic AMP accumulation), G protein-coupled receptor kinase (GRK)-mediated receptor phosphorylation, ß-arrestin recruitment, and receptor internalization to varying extents. These functional effects were likely due to either steric blockade of downstream effector (Gs, ß-arrestin, GRK) interactions or stabilization of specific receptor conformations which do not support effector coupling. Together, these findings strongly implicate nanobody-derived intrabodies as novel tools to study GPCR biology.

Chronic treatment in vivo with ß-adrenoceptor agonists induces dysfunction of airway ß(2) -adrenoceptors and exacerbates lung inflammation in mice.

BACKGROUND AND PURPOSE: Inhalation of a ß-adrenoceptor agonist (ß-agonist) is first-line asthma therapy, used for both prophylaxis against, and acute relief of, bronchoconstriction. However, repeated clinical use of ß-agonists leads to impaired bronchoprotection and, in some cases, adverse patient outcomes. Mechanisms underlying this ß(2) -adrenoceptor dysfunction are not well understood, due largely to the lack of a comprehensive animal model and the uncertainty as to whether or not bronchorelaxation in mice is mediated by ß(2) -adrenoceptors. Thus, we aimed to develop a mouse model that demonstrated functional ß-agonist-induced ß(2) -adrenoceptor desensitization in the context of allergic inflammatory airway disease. EXPERIMENTAL APPROACH: We combined chronic allergen exposure with repeated ß-agonist inhalation in allergen-treated BALB/C mice and examined the contribution of ß(2) -adrenoceptors to albuterol-induced bronchoprotection using FVB/NJ mice with genetic deletion of ß(2) -adrenoceptors (KO). Associated inflammatory changes - cytokines (ELISA), cells in bronchoalevolar lavage and airway remodelling (histology) and ß(2) -adrenoceptor density (radioligand binding) - were also measured. KEY RESULTS ß(2) -Adrenoceptors mediated albuterol-induced bronchoprotection in mice. Chronic treatment with albuterol induced loss of bronchoprotection, associated with exacerbation of the inflammatory components of the asthma phenotype. CONCLUSIONS AND IMPLICATIONS: This animal model reproduced salient features of human asthma and linked loss of bronchoprotection with airway pathobiology. Accordingly, the model offers an advanced tool for understanding the mechanisms of the effects of chronic ß- agonist treatment on ß-adrenoceptor function in asthma. Such information may guide the clinical use of ß-agonists and provide insight into development of novel ß-adrenoceptor ligands for the treatment of asthma.

Distinct phosphorylation sites on the ß(2)-adrenergic receptor establish a barcode that encodes differential functions of ß-arrestin.

Phosphorylation of G protein-coupled receptors (GPCRs, which are also known as seven-transmembrane spanning receptors) by GPCR kinases (GRKs) plays essential roles in the regulation of receptor function by promoting interactions of the receptors with ß-arrestins. These multifunctional adaptor proteins desensitize GPCRs, by reducing receptor coupling to G proteins and facilitating receptor internalization, and mediate GPCR signaling through ß-arrestin-specific pathways. Detailed mapping of the phosphorylation sites on GPCRs targeted by individual GRKs and an understanding of how these sites regulate the specific functional consequences of ß-arrestin engagement may aid in the discovery of therapeutic agents targeting individual ß-arrestin functions. The ß(2)-adrenergic receptor (ß(2)AR) has many serine and threonine residues in the carboxyl-terminal tail and the intracellular loops, which are potential sites of phosphorylation. We monitored the phosphorylation of the ß(2)AR at specific sites upon stimulation with an agonist that promotes signaling by both G protein-mediated and ß-arrestin-mediated pathways or with a biased ligand that promotes signaling only through ß-arrestin-mediated events in the presence of the full complement of GRKs or when either GRK2 or GRK6 was depleted. We correlated the specific and distinct patterns of receptor phosphorylation by individual GRKs with the functions of ß-arrestins and propose that the distinct phosphorylation patterns established by different GRKs establish a "barcode" that imparts distinct conformations to the recruited ß-arrestin, thus regulating its functional activities.