The molecular acrobatics of arrestin activation

Below the Surface: IGF-1R Therapeutic Targeting and Its Endocytic Journey

Ligand-activated plasma membrane receptors follow pathways of endocytosis through the endosomal sorting apparatus. Receptors cluster in clathrin-coated pits that bud inwards and enter the cell as clathrin-coated vesicles. These vesicles travel through the acidic endosome whereby receptors and ligands are sorted to be either recycled or degraded. The traditional paradigm postulated that the endocytosis role lay in signal termination through the removal of the receptor from the cell surface. It is now becoming clear that the internalization process governs more than receptor signal cessation and instead reigns over the entire spatial and temporal wiring of receptor signaling. Governing the localization, the post-translational modifications, and the scaffolding of receptors and downstream signal components established the endosomal platform as the master regulator of receptor function. Confinement of components within or between distinct organelles means that the endosome instructs the cell on how to interpret and translate the signal emanating from any given receptor complex into biological effects. This review explores this emerging paradigm with respect to the cancer-relevant insulin-like growth factor type 1 receptor (IGF-1R) and discusses how this perspective could inform future targeting strategies.

Genetic code expansion and photocross-linking identify different β-arrestin binding modes to the angiotensin II type 1 receptor

The angiotensin II (AngII) type 1 receptor (AT1R) is a member of the G protein-coupled receptor (GPCR) family and binds β-arrestins (β-arrs), which regulate AT1R signaling and trafficking. These processes can be biased by different ligands or mutations in the AGTR1 gene. As for many GPCRs, the exact details for AT1R-β-arr interactions driven by AngII or β-arr-biased ligands remain largely unknown. Here, we used the amber-suppression technology to site-specifically introduce the unnatural amino acid (UAA) p-azido-l-phenylalanine (azF) into the intracellular loops (ICLs) and the C-tail of AT1R. Our goal was to generate competent photoreactive receptors that can be cross-linked to β-arrs in cells. We performed UV-mediated photolysis of 25 different azF-labeled AT1Rs to cross-link β-arr1 to AngII-bound receptors, enabling us to map important contact sites in the C-tail and in the ICL2 and ICL3 of the receptor. The extent of AT1R-β-arr1 cross-linking among azF-labeled receptors differed, revealing variability in β-arr's contact mode with the different AT1R domains. Moreover, the signature of ligated AT1R-β-arr complexes from a subset of azF-labeled receptors also differed between AngII and β-arr-biased ligand stimulation of receptors and between azF-labeled AT1R bearing and that lacking a bias signaling mutation. These observations further implied distinct interaction modalities of the AT1R-β-arr1 complex in biased signaling conditions. Our findings demonstrate that this photocross-linking approach is useful for understanding GPCR-β-arr complexes in different activation states and could be extended to study other protein-protein interactions in cells.

Encoding the β-Arrestin Trafficking Fate of Ghrelin Receptor GHSR1a: C-Tail-Independent Molecular Determinants in GPCRs

G-protein-coupled receptors (GPCRs) can bias signaling through distinct biochemical pathways that originate from G-protein/receptor and β-arrestin/receptor complexes. Receptor conformations supporting β-arrestin engagement depend on multiple receptor determinants. Using ghrelin receptor GHR1a, we demonstrate by bioluminescence resonance energy transfer and fluorescence microscopy a critical role for its second intracellular loop 2 (ICL2) domain in stabilizing β-arrestin/GHSR1a core interactions and determining receptor trafficking fate. We validate our findings in ICL2 gain- and loss-of-function experiments assessing β-arrestin and ubiquitin-dependent internalization of the CC chemokine receptor, CCR1. Like all CC and CXC subfamily chemokine receptors, CCR1 lacks a critical proline residue found in the ICL2 consensus domain of rhodopsin-family GPCRs. Our study indicates that ICL2, C-tail determinants, and the orthosteric binding pocket that regulates β-arrestin/receptor complex stability are sufficient to encode a broad repertoire of the trafficking fates observed for rhodopsin-family GPCRs, suggesting they provide the essential elements for regulating a large fraction of β-arrestin signaling bias.

Distinct G protein-coupled receptor phosphorylation motifs modulate arrestin affinity and activation and global conformation

Cellular functions of arrestins are determined in part by the pattern of phosphorylation on the G protein-coupled receptors (GPCRs) to which arrestins bind. Despite high-resolution structural data of arrestins bound to phosphorylated receptor C-termini, the functional role of each phosphorylation site remains obscure. Here, we employ a library of synthetic phosphopeptide analogues of the GPCR rhodopsin C-terminus and determine the ability of these peptides to bind and activate arrestins using a variety of biochemical and biophysical methods. We further characterize how these peptides modulate the conformation of arrestin-1 by nuclear magnetic resonance (NMR). Our results indicate different functional classes of phosphorylation sites: 'key sites' required for arrestin binding and activation, an 'inhibitory site' that abrogates arrestin binding, and 'modulator sites' that influence the global conformation of arrestin. These functional motifs allow a better understanding of how different GPCR phosphorylation patterns might control how arrestin functions in the cell.

The structural basis of the arrestin binding to GPCRs

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins targeted by more clinically used drugs than any other protein family. GPCR signaling via G proteins is quenched (desensitized) by the phosphorylation of the active receptor by specific GPCR kinases (GRKs) followed by tight binding of arrestins to active phosphorylated receptors. Thus, arrestins engage two types of receptor elements: those that contain GRK-added phosphates and those that change conformation upon activation. GRKs attach phosphates to serines and threonines in the GPCR C-terminus or any one of the cytoplasmic loops. In addition to these phosphates, arrestins engage the cavity that appears between trans-membrane helices upon receptor activation and several other non-phosphorylated elements. The residues that bind GPCRs are localized on the concave side of both arrestin domains. Arrestins undergo a global conformational change upon receptor binding (become activated). Arrestins serve as important hubs of cellular signaling, emanating from activated GPCRs and receptor-independent.

Critical role of the finger loop in arrestin binding to the receptors

We tested the interactions with four different G protein-coupled receptors (GPCRs) of arrestin-3 mutants with substitutions in the four loops, three of which contact the receptor in the structure of the arrestin-1-rhodopsin complex. Point mutations in the loop at the distal tip of the N-domain (Glu157Ala), in the C-loop (Phe255Ala), back loop (Lys313Ala), and one of the mutations in the finger loop (Gly65Pro) had mild variable effects on receptor binding. In contrast, the deletion of Gly65 at the beginning of the finger loop reduced the binding to all GPCRs tested, with the binding to dopamine D2 receptor being affected most dramatically. Thus, the presence of a glycine at the beginning of the finger loop appears to be critical for the arrestin-receptor interaction.

GPCR Signaling Regulation: The Role of GRKs and Arrestins

Every animal species expresses hundreds of different G protein-coupled receptors (GPCRs) that respond to a wide variety of external stimuli. GPCRs-driven signaling pathways are involved in pretty much every physiological function and in many pathologies. Therefore, GPCRs are targeted by about a third of clinically used drugs. The signaling of most GPCRs via G proteins is terminated by the phosphorylation of active receptor by specific kinases (GPCR kinases, or GRKs) and subsequent binding of arrestin proteins, that selectively recognize active phosphorylated receptors. In addition, GRKs and arrestins play a role in multiple signaling pathways in the cell, both GPCR-initiated and receptor-independent. Here we focus on the mechanisms of GRK- and arrestin-mediated regulation of GPCR signaling, which includes homologous desensitization and redirection of signaling to additional pathways by bound arrestins.

Arrestin-3 scaffolding of the JNK3 cascade suggests a mechanism for signal amplification

Scaffold proteins tether and orient components of a signaling cascade to facilitate signaling. Although much is known about how scaffolds colocalize signaling proteins, it is unclear whether scaffolds promote signal amplification. Here, we used arrestin-3, a scaffold of the ASK1-MKK4/7-JNK3 cascade, as a model to understand signal amplification by a scaffold protein. We found that arrestin-3 exhibited >15-fold higher affinity for inactive JNK3 than for active JNK3, and this change involved a shift in the binding site following JNK3 activation. We used systems biochemistry modeling and Bayesian inference to evaluate how the activation of upstream kinases contributed to JNK3 phosphorylation. Our combined experimental and computational approach suggested that the catalytic phosphorylation rate of JNK3 at Thr-221 by MKK7 is two orders of magnitude faster than the corresponding phosphorylation of Tyr-223 by MKK4 with or without arrestin-3. Finally, we showed that the release of activated JNK3 was critical for signal amplification. Collectively, our data suggest a "conveyor belt" mechanism for signal amplification by scaffold proteins. This mechanism informs on a long-standing mystery for how few upstream kinase molecules activate numerous downstream kinases to amplify signaling.

Arrestin-mediated signaling: Is there a controversy?

The activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK)1/2 was traditionally used as a readout of signaling of G protein-coupled receptors (GPCRs) via arrestins, as opposed to conventional GPCR signaling via G proteins. Several recent studies using HEK293 cells where all G proteins were genetically ablated or inactivated, or both non-visual arrestins were knocked out, demonstrated that ERK1/2 phosphorylation requires G protein activity, but does not necessarily require the presence of non-visual arrestins. This appears to contradict the prevailing paradigm. Here we discuss these results along with the recent data on gene edited cells and arrestin-mediated signaling. We suggest that there is no real controversy. G proteins might be involved in the activation of the upstream-most MAP3Ks, although in vivo most MAP3K activation is independent of heterotrimeric G proteins, being initiated by receptor tyrosine kinases and/or integrins. As far as MAP kinases are concerned, the best-established role of arrestins is scaffolding of the three-tiered cascades (MAP3K-MAP2K-MAPK). Thus, it seems likely that arrestins, GPCR-bound and free, facilitate the propagation of signals in these cascades, whereas signal initiation via MAP3K activation may be independent of arrestins. Different MAP3Ks are activated by various inputs, some of which are mediated by G proteins, particularly in cell culture, where we artificially prevent signaling by receptor tyrosine kinases and integrins, thereby favoring GPCR-induced signaling. Thus, there is no reason to change the paradigm: Arrestins and G proteins play distinct non-overlapping roles in cell signaling.

V1b vasopressin receptor trafficking and signaling: Role of arrestins, G proteins and Src kinase

The signaling pathway of G protein-coupled receptors is strongly linked to their trafficking profile. Little is known about the molecular mechanisms involved in the vasopressin receptor V_1b subtype (V_1b R) trafficking and its impact on receptor signaling and regulation. For this purpose, we investigated the role of β-arrestins in receptor desensitization, internalization and recycling and attempted to dissect the V_1b R-mediated MAP kinase pathway. Using MEF cells Knocked-out for β-arrestins 1 and 2, we demonstrated that both β-arrestins 1 and 2 play a fundamental role in internalization and recycling of V_1b R with a rapid and transient V_1b R-β-arrestin interaction in contrast to a slow and long-lasting β-arrestin recruitment of the V₂ vasopressin receptor subtype (V₂ R). Using V_1b R-V₂ R chimeras and V_1b R C-terminus truncations, we demonstrated the critical role of the V_1b R C-terminus in its interaction with β-arrestins thereby regulating the receptor internalization and recycling kinetics in a phosphorylation-independent manner. In parallel, V_1b R MAP kinase activation was dependent on arrestins and Src-kinase but independent on G proteins. Interestingly, Src interacted with hV_1b R at basal state and dissociated when receptor internalization occurred. Altogether, our data describe for the first time the trafficking profile and MAP kinase pathway of V_1b R involving both arrestins and Src kinase family.

Arrestins: structural disorder creates rich functionality

Arrestins are soluble relatively small 44-46 kDa proteins that specifically bind hundreds of active phosphorylated GPCRs and dozens of non-receptor partners. There are binding partners that demonstrate preference for each of the known arrestin conformations: free, receptor-bound, and microtubule-bound. Recent evidence suggests that conformational flexibility in every functional state is the defining characteristic of arrestins. Flexibility, or plasticity, of proteins is often described as structural disorder, in contrast to the fixed conformational order observed in high-resolution crystal structures. However, protein-protein interactions often involve highly flexible elements that can assume many distinct conformations upon binding to different partners. Existing evidence suggests that arrestins are no exception to this rule: their flexibility is necessary for functional versatility. The data on arrestins and many other multi-functional proteins indicate that in many cases, "order" might be artificially imposed by highly non-physiological crystallization conditions and/or crystal packing forces. In contrast, conformational flexibility (and its extreme case, intrinsic disorder) is a more natural state of proteins, representing true biological order that underlies their physiologically relevant functions.

Therapeutic Targets for Treatment of Heart Failure: Focus on GRKs and β-Arrestins Affecting βAR Signaling

Heart failure (HF) is a heart disease that is classified into two main types: HF with reduced ejection fraction (HFrEF) and HF with preserved ejection fraction (HFpEF). Both types of HF lead to significant risk of mortality and morbidity. Pharmacological treatment with β-adrenergic receptor (βAR) antagonists (also called β-blockers) has been shown to reduce the overall hospitalization and mortality rates and improve the clinical outcomes in HF patients with HFrEF but not HFpEF. Although, the survival rate of patients suffering from HF continues to drop, the management of HF still faces several limitations and discrepancies highlighting the need to develop new treatment strategies. Overstimulation of the sympathetic nervous system is an adaptive neurohormonal response to acute myocardial injury and heart damage, whereas prolonged exposure to catecholamines causes defects in βAR regulation, including a reduction in the amount of βARs and an increase in βAR desensitization due to the upregulation of G protein-coupled receptor kinases (GRKs) in the heart, contributing in turn to the progression of HF. Several studies show that myocardial GRK2 activity and expression are raised in the failing heart. Furthermore, β-arrestins play a pivotal role in βAR desensitization and, interestingly, can mediate their own signal transduction without any G protein-dependent pathway involved. In this review, we provide new insight into the role of GRKs and β-arrestins on how they affect βAR signaling regarding the molecular and cellular pathophysiology of HF. Additionally, we discuss the therapeutic potential of targeting GRKs and β-arrestins for the treatment of HF.

Arrestin mutations: Some cause diseases, others promise cure

Arrestins play a key role in homologous desensitization of G protein-coupled receptors (GPCRs) and regulate several other vital signaling pathways in cells. Considering the critical roles of these proteins in cellular signaling, surprisingly few disease-causing mutations in human arrestins were described. Most of these are loss-of-function mutations of visual arrestin-1 that cause excessive rhodopsin signaling and hence night blindness. Only one dominant arrestin-1 mutation was discovered so far. It reduces the thermal stability of the protein, which likely results in photoreceptor death via unfolded protein response. In case of the two nonvisual arrestins, only polymorphisms were described, some of which appear to be associated with neurological disorders and altered response to certain treatments. Structure-function studies revealed several ways of enhancing arrestins' ability to quench GPCR signaling. These enhanced arrestins have potential as tools for gene therapy of disorders associated with excessive signaling of mutant GPCRs.

Calcium influx mediates the chemoattractant-induced translocation of the arrestin-related protein AdcC in Dictyostelium

Arrestins are key adaptor proteins that control the fate of cell-surface membrane proteins and modulate downstream signaling cascades. The Dictyostelium discoideum genome encodes six arrestin-related proteins, harboring additional modules besides the arrestin domain. Here, we studied AdcB and AdcC, two homologs that contain C2 and SAM domains. We showed that AdcC - in contrast to AdcB - responds to various stimuli (such as the chemoattractants cAMP and folate) known to induce an increase in cytosolic calcium by transiently translocating to the plasma membrane, and that calcium is a direct regulator of AdcC localization. This response requires the calcium-dependent membrane-targeting C2 domain and the double SAM domain involved in AdcC oligomerization, revealing a mode of membrane targeting and regulation unique among members of the arrestin clan. AdcB shares several biochemical properties with AdcC, including in vitro binding to anionic lipids in a calcium-dependent manner and auto-assembly as large homo-oligomers. AdcB can interact with AdcC; however, its intracellular localization is insensitive to calcium. Therefore, despite their high degree of homology and common characteristics, AdcB and AdcC are likely to fulfill distinct functions in amoebae.

Arrestins: Introducing Signaling Bias Into Multifunctional Proteins

Arrestins were discovered as proteins that bind active phosphorylated G protein-coupled receptors (GPCRs) and block their interactions with G proteins, i.e., for their role in homologous desensitization of GPCRs. Mammals express only four arrestin subtypes, two of which are largely restricted to the retina. Two nonvisual arrestins are ubiquitous and interact with hundreds of different GPCRs and dozens of other binding partners. Changes of just a few residues on the receptor-binding surface were shown to dramatically affect GPCR preference of inherently promiscuous nonvisual arrestins. Mutations on the cytosol-facing side of arrestins modulate their interactions with individual downstream signaling molecules. Thus, it appears feasible to construct arrestin mutants specifically linking particular GPCRs with signaling pathways of choice or mutants that sever the links between selected GPCRs and unwanted pathways. Signaling-biased "designer arrestins" have the potential to become valuable molecular tools for research and therapy.

Allosteric mechanisms underlie GPCR signaling to SH3-domain proteins through arrestin

Signals from 800 G-protein-coupled receptors (GPCRs) to many SH3 domain-containing proteins (SH3-CPs) regulate important physiological functions. These GPCRs may share a common pathway by signaling to SH3-CPs via agonist-dependent arrestin recruitment rather than through direct interactions. In the present study, ¹⁹F-NMR and cellular studies revealed that downstream of GPCR activation engagement of the receptor-phospho-tail with arrestin allosterically regulates the specific conformational states and functional outcomes of remote β-arrestin 1 proline regions (PRs). The observed NMR chemical shifts of arrestin PRs were consistent with the intrinsic efficacy and specificity of SH3 domain recruitment, which was controlled by defined propagation pathways. Moreover, in vitro reconstitution experiments and biophysical results showed that the receptor-arrestin complex promoted SRC kinase activity through an allosteric mechanism. Thus, allosteric regulation of the conformational states of β-arrestin 1 PRs by GPCRs and the allosteric activation of downstream effectors by arrestin are two important mechanisms underlying GPCR-to-SH3-CP signaling.

Molecular mechanism of modulating arrestin conformation by GPCR phosphorylation

Arrestins regulate the signaling of ligand-activated, phosphorylated G-protein-coupled receptors (GPCRs). Different patterns of receptor phosphorylation (phosphorylation barcode) can modulate arrestin conformations, resulting in distinct functional outcomes (for example, desensitization, internalization, and downstream signaling). However, the mechanism of arrestin activation and how distinct receptor phosphorylation patterns could induce different conformational changes on arrestin are not fully understood. We analyzed how each arrestin amino acid contributes to its different conformational states. We identified a conserved structural motif that restricts the mobility of the arrestin finger loop in the inactive state and appears to be regulated by receptor phosphorylation. Distal and proximal receptor phosphorylation sites appear to selectively engage with distinct arrestin structural motifs (that is, micro-locks) to induce different arrestin conformations. These observations suggest a model in which different phosphorylation patterns of the GPCR C terminus can combinatorially modulate the conformation of the finger loop and other phosphorylation-sensitive structural elements to drive distinct arrestin conformation and functional outcomes.

Structural Basis of Arrestin-Dependent Signal Transduction

Arrestins are a small family of proteins with four isoforms in humans. Remarkably, two arrestins regulate signaling from >800 G protein-coupled receptors (GPCRs) or nonreceptor activators by simultaneously binding an activator and one out of hundreds of other signaling proteins. When arrestins are bound to GPCRs or other activators, the affinity for these signaling partners changes. Thus, it is proposed that an activator alters arrestin's ability to transduce a signal. The comparison of all available arrestin structures identifies several common conformational rearrangements associated with activation. In particular, it identifies elements that are directly involved in binding to GPCRs or other activators, elements that likely engage distinct downstream effectors, and elements that likely link the activator-binding sites with the effector-binding sites.

S-Nitrosylation of β-Arrestins Biases Receptor Signaling and Confers Ligand Independence

Most G protein-coupled receptors (GPCRs) signal through both heterotrimeric G proteins and β-arrestins (βarr1 and βarr2). Although synthetic ligands can elicit biased signaling by G protein- vis-à-vis βarr-mediated transduction, endogenous mechanisms for biasing signaling remain elusive. Here we report that S-nitrosylation of a novel site within βarr1/2 provides a general mechanism to bias ligand-induced signaling through GPCRs by selectively inhibiting βarr-mediated transduction. Concomitantly, S-nitrosylation endows cytosolic βarrs with receptor-independent function. Enhanced βarr S-nitrosylation characterizes inflammation and aging as well as human and murine heart failure. In genetically engineered mice lacking βarr2-Cys253 S-nitrosylation, heart failure is exacerbated in association with greatly compromised β-adrenergic chronotropy and inotropy, reflecting βarr-biased transduction and β-adrenergic receptor downregulation. Thus, S-nitrosylation regulates βarr function and, thereby, biases transduction through GPCRs, demonstrating a novel role for nitric oxide in cellular signaling with potentially broad implications for patho/physiological GPCR function, including a previously unrecognized role in heart failure.

Structural Basis for G Protein-Coupled Receptor Signaling

G protein-coupled receptors (GPCRs), which mediate processes as diverse as olfaction and maintenance of metabolic homeostasis, have become the single most effective class of therapeutic drug targets. As a result, understanding the molecular basis for their activity is of paramount importance. Recent technological advances have made GPCR structural biology increasingly tractable, offering views of these receptors in unprecedented atomic detail. Structural and biophysical data have shown that GPCRs function as complex allosteric machines, communicating ligand-binding events through conformational change. Changes in receptor conformation lead to activation of effector proteins, such as G proteins and arrestins, which are themselves conformational switches. Here, we review how structural biology has illuminated the agonist-induced cascade of conformational changes that culminate in a cellular response to GPCR activation.

Molecular dissection of the human A3 adenosine receptor coupling with β-arrestin2

Besides classical G protein coupling, G protein-coupled receptors (GPCRs) are nowadays well known to show significant signalling via other adaptor proteins, such as β-arrestin2 (βarr2). The elucidation of the molecular mechanism of the GPCR-βarr2 interaction is a prerequisite for the structure-activity based design of biased ligands, which introduces a new chapter in drug discovery. The general mechanism of the interaction is believed to rely on phosphorylation sites, exposed upon agonist binding. However, it is not known whether this mechanism is universal throughout the GPCR family or if GPCR-specific patterns are involved. In recent years, promising orally active agonists for the human A₃ adenosine receptor (A₃AR), a GPCR highly expressed in inflammatory and cancer cells, have been evaluated in clinical trials for the treatment of rheumatoid arthritis, psoriasis, and hepatocellular carcinoma. In this study, the effect of cytoplasmic modifications of the A₃AR on βarr2 recruitment was evaluated in transiently transfected HEK293T cells, using a live-cell split-reporter system (NanoBit®, Promega), based on the structural complementation of NanoLuc luciferase, allowing real-time βarr2 monitoring. The A₃AR-selective reference agonist 2-Cl-IB-MECA yielded a robust, concentration dependent (5 nM-1 µM) recruitment of βarr2 (logEC50: -7.798 ± 0.076). The role of putative phosphorylation sites, located in the C-terminal part and cytoplasmic loops, and the role of the 'DRY' motif was evaluated. It was shown that the A₃AR C-terminus was dispensable for βarr2 recruitment. This contrasts with studies in the past for the rat A₃AR, which pointed at crucial C-terminal phosphorylation sites. When combining truncation of the A₃AR with modification of the 'DRY' motif to 'AAY', the βarr2 recruitment was drastically reduced. Recruitment could be partly rescued by back-mutation to 'NQY', or by extending the C-terminus again. In conclusion, other parts of the human A₃AR, either cytosolic or exposed upon receptor activation, rather than the C-terminus alone, are responsible for βarr2 recruitment in a complementary or synergistic way.

Computational insights into the G-protein-biased activation and inactivation mechanisms of the μ opioid receptor

The μ opioid receptor (OR), a member of the class A subfamily of G-protein coupled receptors (GPCRs), is a major target for the treatment of pain. G-protein biased μ-OR agonists promise to be developed as analgesics. Thus, TRV130, the first representative μ-OR ligand with G-protein bias, has entered into phase III clinical trials. To identify the detailed G-protein-biased activation and inactivation mechanisms of the μ-OR, we constructed five μ-OR systems that were in complexes with the G-protein-biased agonists TRV130 and BU72, the antagonists β-FNA and naltrexone, as well as the free receptor. We performed a series of conventional molecular dynamics simulations and analyses of G-protein-biased activation and inactivation mechanisms of μ-OR. Our results, together with previously reported mutation results, revealed the operating mode of the activation switch composed of residues W^6.48 and Y^7.43 (Ballesteros/Weinstein numbering), the activity of which was responsible for down- and up-regulation, respectively, of the β-arrestin signaling, which in turn affected G-protein-biased activation of μ-OR. TRV130 was found to stabilize W^6.48 by interacting with Y^7.43. In addition, we obtained useful information regarding μ-OR-biased activation, such as strong stabilization of W^7.35 through a hydrophobic ring interaction in the TRV130 system. These findings may facilitate understanding of μ-OR biased activation and the design of new biased ligands for GPCRs.

Molecular Defects of the Disease-Causing Human Arrestin-1 C147F Mutant

PURPOSE

The purpose of this study was to identify the molecular defect in the disease-causing human arrestin-1 C147F mutant.

METHODS

The binding of wild-type (WT) human arrestin-1 and several mutants with substitutions in position 147 (including C147F, which causes dominant retinitis pigmentosa in humans) to phosphorylated and unphosphorylated light-activated rhodopsin was determined. Thermal stability of WT and mutant human arrestin-1, as well as unfolded protein response in 661W cells, were also evaluated.

RESULTS

WT human arrestin-1 was selective for phosphorylated light-activated rhodopsin. Substitutions of Cys-147 with smaller side chain residues, Ala or Val, did not substantially affect binding selectivity, whereas residues with bulky side chains in the position 147 (Ile, Leu, and disease-causing Phe) greatly increased the binding to unphosphorylated rhodopsin. Functional survival of mutant proteins with bulky substitutions at physiological and elevated temperature was also compromised. C147F mutant induced unfolded protein response in cultured cells.

CONCLUSIONS

Bulky Phe substitution of Cys-147 in human arrestin-1 likely causes rod degeneration due to reduced stability of the protein, which induces unfolded protein response in expressing cells.

Molecular Mechanisms of GPCR Signaling: A Structural Perspective

G protein-coupled receptors (GPCRs) are cell surface receptors that respond to a wide variety of stimuli, from light, odorants, hormones, and neurotransmitters to proteins and extracellular calcium. GPCRs represent the largest family of signaling proteins targeted by many clinically used drugs. Recent studies shed light on the conformational changes that accompany GPCR activation and the structural state of the receptor necessary for the interactions with the three classes of proteins that preferentially bind active GPCRs, G proteins, G protein-coupled receptor kinases (GRKs), and arrestins. Importantly, structural and biophysical studies also revealed activation-related conformational changes in these three types of signal transducers. Here, we summarize what is already known and point out questions that still need to be answered. Clear understanding of the structural basis of signaling by GPCRs and their interaction partners would pave the way to designing signaling-biased proteins with scientific and therapeutic potential.

Retromer stops beta-arrestin 1-mediated signaling from internalized cannabinoid 2 receptors

The retromer acts as the gatekeeper blocking signaling mediated by beta-arrestin 1 from internalized cannabinoid 2 receptors. This work provides further confirmation of the relevance and prevalence of signaling from internalized receptors at endosomal compartments after ligand-induced endocytosis.

G protein–coupled receptors mediate their complex functions through activation of signaling cascades from receptors localized at the cell surface and endosomal compartments. These signaling pathways are modulated by heterotrimeric G proteins and the scaffold proteins beta-arrestin 1 and 2. However, in contrast to the events occurring at the cell surface, our knowledge of the mechanisms controlling signaling from receptors localized at intracellular compartments is still very limited. Here we sought to investigate the intracellular signaling from cannabinoid 2 receptor (CB₂R). First, we show that receptor internalization is required for agonist-induced phosphorylation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). Then we demonstrate that ERK1/2 activation is mediated by beta-arrestin 1 from receptors localized exclusively at Rab4/5 compartments. Finally, we identify the retromer complex as a gatekeeper, terminating beta-arrestin 1–mediated ERK phosphorylation. These findings extend our understanding of the events controlling signaling from endocytosed receptors and identify the retromer as a modulator of beta-arrestin–mediated signaling from CB₂R.

Differential manipulation of arrestin-3 binding to basal and agonist-activated G protein-coupled receptors

Non-visual arrestins interact with hundreds of different G protein-coupled receptors (GPCRs). Here we show that by introducing mutations into elements that directly bind receptors, the specificity of arrestin-3 can be altered. Several mutations in the two parts of the central "crest" of the arrestin molecule, middle-loop and C-loop, enhanced or reduced arrestin-3 interactions with several GPCRs in receptor subtype and functional state-specific manner. For example, the Lys139Ile substitution in the middle-loop dramatically enhanced the binding to inactive M2 muscarinic receptor, so that agonist activation of the M2 did not further increase arrestin-3 binding. Thus, the Lys139Ile mutation made arrestin-3 essentially an activation-independent binding partner of M2, whereas its interactions with other receptors, including the β2-adrenergic receptor and the D1 and D2 dopamine receptors, retained normal activation dependence. In contrast, the Ala248Val mutation enhanced agonist-induced arrestin-3 binding to the β2-adrenergic and D2 dopamine receptors, while reducing its interaction with the D1 dopamine receptor. These mutations represent the first example of altering arrestin specificity via enhancement of the arrestin-receptor interactions rather than selective reduction of the binding to certain subtypes.

Functional role of the three conserved cysteines in the N domain of visual arrestin-1

Arrestins specifically bind active and phosphorylated forms of their cognate G protein-coupled receptors, blocking G protein coupling and often redirecting the signaling to alternative pathways. High-affinity receptor binding is accompanied by two major structural changes in arrestin: release of the C-tail and rotation of the two domains relative to each other. The first requires detachment of the arrestin C-tail from the body of the molecule, whereas the second requires disruption of the network of charge-charge interactions at the interdomain interface, termed the polar core. These events can be facilitated by mutations destabilizing the polar core or the anchoring of the C-tail that yield "preactivated" arrestins that bind phosphorylated and unphosphorylated receptors with high affinity. Here we explored the functional role in arrestin activation of the three native cysteines in the N domain, which are conserved in all arrestin subtypes. Using visual arrestin-1 and rhodopsin as a model, we found that substitution of these cysteines with serine, alanine, or valine virtually eliminates the effects of the activating polar core mutations on the binding to unphosphorylated rhodopsin while only slightly reducing the effects of the C-tail mutations. Thus, these three conserved cysteines play a role in the domain rotation but not in the C-tail release.

Functional consequences of chemically-induced β-arrestin binding to chemokine receptors CXCR4 and CCR5 in the absence of ligand stimulation

Chemokine receptor signaling is a tightly regulated process which was for a long time exclusively attributed to heterotrimeric G proteins. β-Arrestins constitute a separable signaling arm from classical heterotrimeric G proteins, in addition to their well-established roles in receptor desensitization and endocytosis. In order to clearly dissect β-arrestin- from G protein-dependent effects we forced the recruitment of β-arrestin to CXCR4 and CCR5 independently of agonist-promoted receptor activation through chemically-induced dimerization. Targeting β-arrestins to receptors at the plasma membrane prior to chemokine stimulation attenuated G protein-mediated calcium release. Association of β-arrestins to the receptors was sufficient to induce their internalization in the absence of ligand and this effect could be further enhanced by translocation of a constitutively active β-arrestin 1 variant. CXCR4 and CCR5 were targeted to different intracellular compartments upon chemical-induced dimerization with β-arrestins and reproduced the intracellular distribution of receptors after activation with their respective ligands. Our data further provide evidence for direct β-arrestin-mediated signaling via MAP kinases ERK 1/2. These results provide clear evidence that CXCR4- or CCR5-β-arrestin complexes induce receptor endocytosis and signaling in the absence of G protein coupling and ligand-induced conformational changes of the receptor.

Novel Structural Insights into GPCR-β-Arrestin Interaction and Signaling

G protein-coupled receptors (GPCRs) are major signal recognition and transmission units in the plasma membrane. The interaction of activated and phosphorylated GPCRs with the multifunctional adaptor proteins β-arrestins (βarrs) is crucial for regulation of their signaling and functional outcomes. Over the past few years, a range of structural, biochemical, and cellular studies have revealed novel insights into GPCR-βarr interaction and signaling. Some of these findings have come as a surprise and therefore have the potential to significantly refine the conceptual framework of the GPCR-βarr system. Here we discuss these recent advances with particular emphasis on biphasic GPCR-βarr interaction, the formation of GPCR-G-protein-βarr supercomplexes, and receptor-specific conformational signatures in βarrs. We also underline the emerging research areas that are likely to be at the center stage of investigations in the coming years.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

Arresting the Development of Addiction: The Role of β-Arrestin 2 in Drug Abuse

The protein β-arrestin (βarr) 2 directly interacts with receptors and signaling pathways that mediate the behavioral effects of drugs of abuse, making it a prime candidate for therapeutic interventions. βarr2 drives desensitization and internalization of G protein-coupled receptors, including dopamine, opioid, and cannabinoid receptors, and it can also trigger G protein-independent intracellular signaling. βarr2 mediates several drug-induced behaviors, but the relationship is complex and dependent on the type of behavior (e.g., psychomotor versus reward), the class of drug (e.g., psychostimulant versus opioid), and the circuit being interrogated (e.g., brain region, cell type, and specific receptor ligand). Here we discuss the current state of research concerning the contribution of βarr2 to the psychomotor and rewarding effects of addictive drugs. Next we identify key knowledge gaps and suggest new tools and approaches needed to further elucidate the neuroanatomical substrates and neurobiological mechanisms to explain how βarr2 modulates behavioral responses to drugs of abuse, as well as its potential as a therapeutic target.

Two serines in the distal C-terminus of the human ß1-adrenoceptor determine ß-arrestin2 recruitment

G protein-coupled receptors (GPCRs) undergo phosphorylation at several intracellular residues by G protein-coupled receptor kinases. The resulting phosphorylation pattern triggers arrestin recruitment and receptor desensitization. The exact sites of phosphorylation and their function remained largely unknown for the human β1-adrenoceptor (ADRB1), a key GPCR in adrenergic signal transduction and the target of widely used drugs such as β-blockers. The present study aimed to identify the intracellular phosphorylation sites in the ADRB1 and to delineate their function. The human ADRB1 was expressed in HEK293 cells and its phosphorylation pattern was determined by mass spectrometric analysis before and after stimulation with a receptor agonist. We identified a total of eight phosphorylation sites in the receptor's third intracellular loop and C-terminus. Analyzing the functional relevance of individual sites using phosphosite-deficient receptor mutants we found phosphorylation of the ADRB1 at Ser461/Ser462 in the distal part of the C-terminus to determine β-arrestin2 recruitment and receptor internalization. Our data reveal the phosphorylation pattern of the human ADRB1 and the site that mediates recruitment of β-arrestin2.

A new inhibitor of the β-arrestin/AP2 endocytic complex reveals interplay between GPCR internalization and signalling

In addition to G protein-coupled receptor (GPCR) desensitization and endocytosis, β-arrestin recruitment to ligand-stimulated GPCRs promotes non-canonical signalling cascades. Distinguishing the respective contributions of β-arrestin recruitment to the receptor and β-arrestin-promoted endocytosis in propagating receptor signalling has been limited by the lack of selective analytical tools. Here, using a combination of virtual screening and cell-based assays, we have identified a small molecule that selectively inhibits the interaction between β-arrestin and the β2-adaptin subunit of the clathrin adaptor protein AP2 without interfering with the formation of receptor/β-arrestin complexes. This selective β-arrestin/β2-adaptin inhibitor (Barbadin) blocks agonist-promoted endocytosis of the prototypical β2-adrenergic (β2AR), V2-vasopressin (V2R) and angiotensin-II type-1 (AT1R) receptors, but does not affect β-arrestin-independent (transferrin) or AP2-independent (endothelin-A) receptor internalization. Interestingly, Barbadin fully blocks V2R-stimulated ERK1/2 activation and blunts cAMP accumulation promoted by both V2R and β2AR, supporting the concept of β-arrestin/AP2-dependent signalling for both G protein-dependent and -independent pathways.

Beta-arrestins play central roles in the mechanisms regulating GPCR signalling and trafficking. Here the authors identify a selective inhibitor of the interaction between β-arrestin and the β2-adaptin subunit of the clathrin adaptor protein AP-2, which they use to dissect the role of the β-arrestin/β2-adaptin interaction in GPCR signalling.

Core engagement with β-arrestin is dispensable for agonist-induced vasopressin receptor endocytosis and ERK activation

G protein-coupled receptors (GPCRs) exhibit highly conserved activation and signaling mechanisms by which agonist stimulation leads to coupling of heterotrimeric G proteins and generation of second messenger response. This is followed by receptor phosphorylation, primarily in the carboxyl terminus but also in the cytoplasmic loops, and subsequent binding of arrestins. GPCRs typically recruit arrestins through two different sets of interactions, one involving phosphorylated receptor tail and the other mediated by the receptor core. The engagement of both set of interactions (tail and core) is generally believed to be necessary for arrestin-dependent functional outcomes such as receptor desensitization, endocytosis, and G protein-independent signaling. Here we demonstrate that a vasopressin receptor (V₂R) mutant with truncated third intracellular loop (V₂R^ΔICL3) can interact with β-arrestin 1 (βarr1) only through the phosphorylated tail without engaging the core interaction. Of interest, such a partially engaged V₂R^ΔICL3-βarr1 complex can efficiently interact with clathrin terminal domain and ERK2 MAPK in vitro. Furthermore, this core interaction-deficient V₂R mutant exhibits efficient endocytosis and ERK activation upon agonist stimulation. Our data suggest that core interaction with βarr is dispensable for V₂R endocytosis and ERK activation and therefore provide novel insights into refining the current understanding of functional requirements in biphasic GPCR-βarr interaction.

Functional competence of a partially engaged GPCR-β-arrestin complex

G Protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors and drug targets. GPCR signalling and desensitization is critically regulated by β-arrestins (βarr). GPCR-βarr interaction is biphasic where the phosphorylated carboxyl terminus of GPCRs docks to the N-domain of βarr first and then seven transmembrane core of the receptor engages with βarr. It is currently unknown whether fully engaged GPCR-βarr complex is essential for functional outcomes or partially engaged complex can also be functionally competent. Here we assemble partially and fully engaged complexes of a chimeric β2V2R with βarr1, and discover that the core interaction is dispensable for receptor endocytosis, ERK MAP kinase binding and activation. Furthermore, we observe that carvedilol, a βarr biased ligand, does not promote detectable engagement between βarr1 and the receptor core. These findings uncover a previously unknown aspect of GPCR-βarr interaction and provide novel insights into GPCR signalling and regulatory paradigms.

The Multiple Waves of Cannabinoid 1 Receptor Signaling

The cannabinoid 1 receptor (CB1R) is one of the most abundant G protein-coupled receptors (GPCRs) in the central nervous system, with key roles during neurotransmitter release and synaptic plasticity. Upon ligand activation, CB1Rs may signal in three different spatiotemporal waves. The first wave, which is transient (<10 minutes) and initiated by heterotrimeric G proteins, is followed by a second wave (>5 minutes) that is mediated by β-arrestins. The third and final wave occurs at intracellular compartments and could be elicited by G proteins or β-arrestins. This complexity presents multiple challenges, including the correct classification of receptor ligands, the identification of the signaling pathways regulated by each wave, and the underlying molecular mechanisms and physiologic impacts of these waves. Simultaneously, it provides new opportunities to harness the therapeutic potential of the cannabinoid system and other GPCRs. Over the last several years, we have significantly expanded our understanding of the mechanisms and pathways downstream from the CB1R. The identification of receptor mutations that can bias signaling to specific pathways and the use of siRNA technology have been key tools to identifying which signaling cascades are controlled by G proteins or β-arrestins. Here, we review our current knowledge on CB1R signaling, with particular emphasis on the mechanisms and cascades mediated by β-arrestins downstream from the CB1R.

Time Course Analysis of Skeletal Muscle Pathology of GDE5 Transgenic Mouse

Glycerophosphodiesterase 5 (GDE5) selectively hydrolyses glycerophosphocholine to choline and is highly expressed in type II fiber-rich skeletal muscles. We have previously generated that a truncated mutant of GDE5 (GDE5dC471) that lacks phosphodiesterase activity and shown that transgenic mice overexpressing GDE5dC471 in skeletal muscles show less skeletal muscle mass than control mice. However, the molecular mechanism and pathophysiological features underlying decreased skeletal muscle mass in GDE5dC471 mice remain unclear. In this study, we characterized the skeletal muscle disorder throughout development and investigated the primary cause of muscle atrophy. While type I fiber-rich soleus muscle mass was not altered in GDE5dC471 mice, type II fiber-rich muscle mass was reduced in 8-week-old GDE5dC471 mice. Type II fiber-rich muscle mass continued to decrease irreversibly in 1-year-old transgenic mice with an increase in apoptotic cell. Adipose tissue weight and blood triglyceride levels in 8-week-old and 1-year-old transgenic mice were higher than those in control mice. This study also demonstrated compensatory mRNA expression of neuromuscular junction (NMJ) components, including nicotinic acetylcholine receptors (α1, γ, and ε subunits) and acetylcholinesterase in type II fiber-rich quadriceps muscles in GDE5dC471 mice. However, we did not observe morphological changes in NMJs associated with skeletal muscle atrophy in GDE5dC471 mice. We also found that HSP70 protein levels are significantly increased in the skeletal muscles of 2-week-old GDE5dC471 mice and in mouse myoblastic C2C12 cells overexpressing GDE5dC471. These findings suggest that GDE5dC471 mouse is a novel model of early-onset irreversible type II fiber-rich myopathy associated with cellular stress.

Understanding the Differential Selectivity of Arrestins toward the Phosphorylation State of the Receptor

Proteins in the arrestin family exhibit a conserved structural fold that nevertheless allows for significant differences in their selectivity for G-protein coupled receptors (GPCRs) and their phosphorylation states. To reveal the mechanism of activation that prepares arrestin for selective interaction with GPCRs, and to understand the basis for these differences, we used unbiased molecular dynamics simulations to compare the structural and dynamic properties of wild type Arr1 (Arr1-WT), Arr3 (Arr3-WT), and a constitutively active Arr1 mutant, Arr1-R175E, characterized by a perturbation of the phosphate recognition region called "polar core". We find that in our simulations the mutant evolves toward a conformation that resembles the known preactivated structures of an Arr1 splice-variant, and the structurally similar phosphopeptide-bound Arr2-WT, while this does not happen for Arr1-WT. Hence, we propose an activation allosteric mechanism connecting the perturbation of the polar core to a global conformational change, including the relative reorientation of N- and C-domains, and the emergence of electrostatic properties of putative binding surfaces. The underlying local structural changes are interpreted as markers of the evolution of an arrestin structure toward an active-like conformation. Similar activation related changes occur in Arr3-WT in the absence of any perturbation of the polar core, suggesting that this system could spontaneously visit preactivated states in solution. This hypothesis is proposed to explain the lower selectivity of Arr3 toward nonphosphorylated receptors. Moreover, by elucidating the allosteric mechanism underlying activation, we identify functionally critical regions on arrestin structure that can be targeted with drugs or chemical tools for functional modulation.

G protein-coupled receptor kinases as regulators of dopamine receptor functions

Actions of the neurotransmitter dopamine in the brain are mediated by dopamine receptors that belong to the superfamily of G protein-coupled receptors (GPCRs). Mammals have five dopamine receptor subtypes, D1 through D5. D1 and D5 couple to Gs/olf and activate adenylyl cyclase, whereas D2, D3, and D4 couple to Gi/o and inhibit it. Most GPCRs upon activation by an agonist are phosphorylated by GPCR kinases (GRKs). The GRK phosphorylation makes receptors high-affinity binding partners for arrestin proteins. Arrestin binding to active phosphorylated receptors stops further G protein activation and promotes receptor internalization, recycling or degradation, thereby regulating their signaling and trafficking. Four non- visual GRKs are expressed in striatal neurons. Here we describe known effects of individual GRKs on dopamine receptors in cell culture and in the two in vivo models of dopamine-mediated signaling: behavioral response to psychostimulants and L-DOPA- induced dyskinesia. Dyskinesia, associated with dopamine super-sensitivity of striatal neurons, is a debilitating side effect of L-DOPA therapy in Parkinson's disease. In vivo, GRK subtypes show greater receptor specificity than in vitro or in cultured cells. Overexpression, knockdown, and knockout of individual GRKs, particularly GRK2 and GRK6, have differential effects on signaling of dopamine receptor subtypes in the brain. Furthermore, deletion of GRK isoforms in select striatal neuronal types differentially affects psychostimulant-induced behaviors. In addition, anti-dyskinetic effect of GRK3 does not require its kinase activity: it is mediated by the binding of its RGS-like domain to Gαq/11, which suppresses Gq/11 signaling. The data demonstrate that the dopamine signaling in defined neuronal types in vivo is regulated by specific and finely orchestrated actions of GRK isoforms.

Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.

Resonance Energy Transfer-Based Approaches to Study GPCRs

Since their discovery, G protein-coupled receptors (GPCRs) constitute one of the most studied proteins leading to important discoveries and perspectives in terms of their biology and implication in physiology and pathophysiology. This is mostly linked to the remarkable advances in the development and application of the biophysical resonance energy transfer (RET)-based approaches, including bioluminescence and fluorescence resonance energy transfer (BRET and FRET, respectively). Indeed, BRET and FRET have been extensively applied to study different aspects of GPCR functioning such as their activation and regulation either statically or dynamically, in real-time and intact cells. Consequently, our view on GPCRs has considerably changed opening new challenges for the study of GPCRs in their native tissues in the aim to get more knowledge on how these receptors control the biological responses. Moreover, the technological aspect of this field of research promises further developments for robust and reliable new RET-based assays that may be compatible with high-throughput screening as well as drug discovery programs.

Structural evidence for the role of polar core residue Arg175 in arrestin activation

Binding mechanism of arrestin requires photoactivation and phosphorylation of the receptor protein rhodopsin, where the receptor bound phosphate groups cause displacement of the long C-tail ‘activating’ arrestin. Mutation of arginine 175 to glutamic acid (R175E), a central residue in the polar core and previously predicted as the ‘phosphosensor’ leads to a pre-active arrestin that is able to terminate phototransduction by binding to non-phosphorylated, light-activated rhodopsin. Here, we report the first crystal structure of a R175E mutant arrestin at 2.7 Å resolution that reveals significant differences compared to the basal state reported in full-length arrestin structures. These differences comprise disruption of hydrogen bond network in the polar core, and three-element interaction including disordering of several residues in the receptor-binding finger loop and the C-terminus (residues 361–404). Additionally, R175E structure shows a 7.5° rotation of the amino and carboxy-terminal domains relative to each other. Consistent to the biochemical data, our structure suggests an important role of R29 in the initial activation step of C-tail release. Comparison of the crystal structures of basal arrestin and R175E mutant provide insights into the mechanism of arrestin activation, where binding of the receptor likely induces structural changes mimicked as in R175E.

Phospho-selective mechanisms of arrestin conformations and functions revealed by unnatural amino acid incorporation and (19)F-NMR

Specific arrestin conformations are coupled to distinct downstream effectors, which underlie the functions of many G-protein-coupled receptors (GPCRs). Here, using unnatural amino acid incorporation and fluorine-19 nuclear magnetic resonance (¹⁹F-NMR) spectroscopy, we demonstrate that distinct receptor phospho-barcodes are translated to specific β-arrestin-1 conformations and direct selective signalling. With its phosphate-binding concave surface, β-arrestin-1 ‘reads' the message in the receptor phospho-C-tails and distinct phospho-interaction patterns are revealed by ¹⁹F-NMR. Whereas all functional phosphopeptides interact with a common phosphate binding site and induce the movements of finger and middle loops, different phospho-interaction patterns induce distinct structural states of β-arrestin-1 that are coupled to distinct arrestin functions. Only clathrin recognizes and stabilizes GRK2-specific β-arrestin-1 conformations. The identified receptor-phospho-selective mechanism for arrestin conformation and the spacing of the multiple phosphate-binding sites in the arrestin enable arrestin to recognize plethora phosphorylation states of numerous GPCRs, contributing to the functional diversity of receptors.

G-protein-coupled receptors (GPCRs) signal via G proteins or arrestin-mediated pathways; the plasticity of arrestin proteins is thought to underlie their function. Here, the authors use NMR to examine how β-arrestin-1 recognizes different GPCR phospho-barcodes, and how this triggers structural rearrangements to fulfill selective functions.

Beyond traditional pharmacology: new tools and approaches

Traditional pharmacology is defined as the science that deals with drugs and their actions. While small molecule drugs have clear advantages, there are many cases where they have proved to be ineffective, prone to unacceptable side effects, or where due to a particular disease aetiology they cannot possibly be effective. A dominant feature of the small molecule drugs is their single mindedness: they provide either continuous inhibition or continuous activation of the target. Because of that, these drugs tend to engage compensatory mechanisms leading to drug tolerance, drug resistance or, in some cases, sensitization and consequent loss of therapeutic efficacy over time and/or unwanted side effects. Here we discuss new and emerging therapeutic tools and approaches that have potential for treating the majority of disorders for which small molecules are either failing or cannot be developed. These new tools include biologics, such as recombinant hormones and antibodies, as well as approaches involving gene transfer (gene therapy and genome editing) and the introduction of specially designed self-replicating cells. It is clear that no single method is going to be a 'silver bullet', but collectively, these novel approaches hold promise for curing practically every disorder.

How genetic errors in GPCRs affect their function: Possible therapeutic strategies

Activating and inactivating mutations in numerous human G protein-coupled receptors (GPCRs) are associated with a wide range of disease phenotypes. Here we use several class A GPCRs with a particularly large set of identified disease-associated mutations, many of which were biochemically characterized, along with known GPCR structures and current models of GPCR activation, to understand the molecular mechanisms yielding pathological phenotypes. Based on this mechanistic understanding we also propose different therapeutic approaches, both conventional, using small molecule ligands, and novel, involving gene therapy.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.