β-arrestin-2 is an essential regulator of pancreatic β-cell function under physiological and pathophysiological conditions

β-arrestins are critical signalling molecules that regulate many fundamental physiological functions including the maintenance of euglycemia and peripheral insulin sensitivity. Here we show that inactivation of the β-arrestin-2 gene, barr2, in β-cells of adult mice greatly impairs insulin release and glucose tolerance in mice fed with a calorie-rich diet. Both glucose and KCl-induced insulin secretion and calcium responses were profoundly reduced in β-arrestin-2 (barr2) deficient β-cells. In human β-cells, barr2 knockdown abolished glucose-induced insulin secretion. We also show that the presence of barr2 is essential for proper CAMKII function in β-cells. Importantly, overexpression of barr2 in β-cells greatly ameliorates the metabolic deficits displayed by mice consuming a high-fat diet. Thus, our data identify barr2 as an important regulator of β-cell function, which may serve as a new target to improve β-cell function.

Preferential Interaction between the dopamine D2 receptor and Arrestin2 in neostriatal neurons

Dopamine D2 receptor interactions with arrestins and arrestin-dependent internalization have been characterized using heterologously expressed D2 receptor and arrestins. The purpose of this study was to investigate D2 receptor interaction with endogenous arrestins. Arrestin2 and arrestin3 in striatal homogenates bound to the third cytoplasmic loop of the D2 receptor, and purified arrestin2 and arrestin3 bound to the second and third loops and C terminus of the D2 receptor, in a glutathione S-transferase pull-down assay. In NS20Y neuroblastoma cells expressing an enhanced green-fluorescent protein-tagged D2 receptor (D2-EGFP), 2-h D2 agonist stimulation enhanced the colocalization of D2-EGFP with endogenous arrestin2 and arrestin3. These results suggest that the D2 receptor has the intrinsic ability to bind both nonvisual arrestins. Agonist treatment of D2-EGFP NS20Y cells induced D2 receptor internalization (36-46%) that was maximal within 20 min, but that was prevented by small interfering RNA-induced depletion of arrestin2 and arrestin3. In neostriatal neurons, 2-h agonist treatment selectively increased the colocalization of the endogenous D2 receptor with arrestin2, whereas receptor colocalization with arrestin3 was reduced. Agonist stimulation caused translocation of arrestin2, but not arrestin3, to the membrane in neurons and selectively enhanced the coimmunoprecipitation of the D2 receptor and arrestin2. All three measures of receptor/arrestin interaction (colocalization, translocation, and coprecipitation) demonstrated selective agonist-induced interaction between the D2 receptor and arrestin2 in neurons.

The role of arrestin alpha-helix I in receptor binding

Arrestins rapidly bind phosphorylated activated forms of their cognate G protein-coupled receptors, thereby preventing G protein coupling and often switching signaling to other pathways. Amphipathic alpha-helix I (residues 100-111) has been implicated in receptor binding, but the mechanism of its action has not been determined yet. Here we show that several mutations in the helix itself and in adjacent hydrophobic residues in the body of the N-domain reduce arrestin1 binding to light-activated phosphorylated rhodopsin (P-Rh*). On the background of phosphorylation-independent mutants that bind with high affinity to both P-Rh* and light-activated unphosphorylated rhodopsin, these mutations reduce the stability of the arrestin complex with P-Rh*, but not with light-activated unphosphorylated rhodopsin. Using site-directed spin labeling, we found that the local structure around alpha-helix I changes upon binding to rhodopsin. However, the intramolecular distances between alpha-helix I and adjacent beta-strand I (or the rest of the N-domain), measured using double electron-electron resonance, do not change, ruling out relocation of the helix due to receptor binding. Collectively, these data demonstrate that alpha-helix I plays an indirect role in receptor binding, likely keeping beta-strand I, which carries several phosphate-binding residues, in a position favorable for its interaction with receptor-attached phosphates.

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.

GPCR-mediated β-arrestin activation deconvoluted with single-molecule precision

β-arrestins bind G protein-coupled receptors to terminate G protein signaling and to facilitate other downstream signaling pathways. Using single-molecule fluorescence resonance energy transfer imaging, we show that β-arrestin is strongly autoinhibited in its basal state. Its engagement with a phosphopeptide mimicking phosphorylated receptor tail efficiently releases the β-arrestin tail from its N domain to assume distinct conformations. Unexpectedly, we find that β-arrestin binding to phosphorylated receptor, with a phosphorylation barcode identical to the isolated phosphopeptide, is highly inefficient and that agonist-promoted receptor activation is required for β-arrestin activation, consistent with the release of a sequestered receptor C tail. These findings, together with focused cellular investigations, reveal that agonism and receptor C-tail release are specific determinants of the rate and efficiency of β-arrestin activation by phosphorylated receptor. We infer that receptor phosphorylation patterns, in combination with receptor agonism, synergistically establish the strength and specificity with which diverse, downstream β-arrestin-mediated events are directed.

The origin and evolution of G protein-coupled receptor kinases

G protein-coupled receptor (GPCR) kinases (GRKs) play key role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors, promoting high affinity binding of arrestins, which precludes G protein coupling. Direct binding to active GPCRs activates GRKs, so that they selectively phosphorylate only the activated form of the receptor regardless of the accessibility of the substrate peptides within it and their Ser/Thr-containing sequence. Mammalian GRKs were classified into three main lineages, but earlier GRK evolution has not been studied. Here we show that GRKs emerged at the early stages of eukaryotic evolution via an insertion of a kinase similar to ribosomal protein S6 kinase into a loop in RGS domain. GRKs in Metazoa fall into two clades, one including GRK2 and GRK3, and the other consisting of all remaining GRKs, split into GRK1-GRK7 lineage and GRK4-GRK5-GRK6 lineage in vertebrates. One representative of each of the two ancient clades is found as early as placozoan Trichoplax adhaerens. Several protists, two oomycetes and unicellular brown algae have one GRK-like protein, suggesting that the insertion of a kinase domain into the RGS domain preceded the origin of Metazoa. The two GRK families acquired distinct structural units in the N- and C-termini responsible for membrane recruitment and receptor association. Thus, GRKs apparently emerged before animals and rapidly expanded in true Metazoa, most likely due to the need for rapid signalling adjustments in fast-moving animals.

Arrestins and two receptor kinases are upregulated in Parkinson's disease with dementia

Arrestins and G proteins-coupled receptor kinases (GRKs) regulate signaling and trafficking of G protein-coupled receptors. We investigated changes in the expression of arrestins and GRKs in the striatum of patients with Parkinson's disease without (PD) or with dementia (PDD) at postmortem using Western blotting and ribonuclease protection assay. Both PD and PDD groups had similar degree of dopamine depletion in all striatal regions. Arrestin proteins and mRNAs were increased in the PDD group throughout striatum. Protein and mRNA of GRK5, the major subtype in the human striatum, and GRK3 were also upregulated, whereas GRK2 and 6 were mostly unchanged. The PD group had lower concentration of arrestins and GRKs than the PDD group. There was no statistical link between the load of Alzheimer's pathology and the expression of these signaling proteins. Upregulation of arrestins and GRK in PDD may confer resistance to the therapeutic effects of levodopa often observed in these patients. In addition, increased arrestin and GRK concentrations may lead to dementia via perturbation of multiple signaling mechanisms.

Direct binding of visual arrestin to microtubules determines the differential subcellular localization of its splice variants in rod photoreceptors

Proper function of visual arrestin is indispensable for rapid signal shut-off in rod photoreceptors. Dramatic light-dependent changes in its subcellular localization are believed to play an important role in light adaptation of photoreceptor cells. Here we show that visual arrestin binds microtubules. The truncated splice variant of visual arrestin, p44, demonstrates dramatically higher affinity for microtubules than the full-length protein (p48). Enhanced microtubule binding of p44 underlies its earlier reported preferential localization to detergent-resistant membranes, where it is anchored via membrane-associated microtubules in a rhodopsin-independent fashion. Experiments with purified proteins demonstrate that arrestin interaction with microtubules is direct and does not require any additional protein partners. Most importantly, arrestin interactions with microtubules and light-activated phosphorylated rhodopsin are mutually exclusive, suggesting that microtubule interaction may play a role in keeping p44 arrestin away from rhodopsin in dark-adapted photoreceptors.

Arrestin-3 binds c-Jun N-terminal kinase 1 (JNK1) and JNK2 and facilitates the activation of these ubiquitous JNK isoforms in cells via scaffolding

Non-visual arrestins scaffold mitogen-activated protein kinase (MAPK) cascades. The c-Jun N-terminal kinases (JNKs) are members of MAPK family. Arrestin-3 has been shown to enhance the activation of JNK3, which is expressed mainly in neurons, heart, and testes, in contrast to ubiquitous JNK1 and JNK2. Although all JNKs are activated by MKK4 and MKK7, both of which bind arrestin-3, the ability of arrestin-3 to facilitate the activation of JNK1 and JNK2 has never been reported. Using purified proteins we found that arrestin-3 directly binds JNK1α1 and JNK2α2, interacting with the latter comparably to JNK3α2. Phosphorylation of purified JNK1α1 and JNK2α2 by MKK4 or MKK7 is increased by arrestin-3. Endogenous arrestin-3 interacted with endogenous JNK1/2 in different cell types. Arrestin-3 also enhanced phosphorylation of endogenous JNK1/2 in intact cells upon expression of upstream kinases ASK1, MKK4, or MKK7. We observed a biphasic effect of arrestin-3 concentrations on phosphorylation of JNK1α1 and JNK2α2 both in vitro and in vivo. Thus, arrestin-3 acts as a scaffold, facilitating JNK1α1 and JNK2α2 phosphorylation by MKK4 and MKK7 via bringing JNKs and their activators together. The data suggest that arrestin-3 modulates the activity of ubiquitous JNK1 and JNK2 in non-neuronal cells, impacting the signaling pathway that regulates their proliferation and survival.

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.

Identification of receptor binding-induced conformational changes in non-visual arrestins

The non-visual arrestins, arrestin-2 and arrestin-3, belong to a small family of multifunctional cytosolic proteins. Non-visual arrestins interact with hundreds of G protein-coupled receptors (GPCRs) and regulate GPCR desensitization by binding active phosphorylated GPCRs and uncoupling them from heterotrimeric G proteins. Recently, non-visual arrestins have been shown to mediate G protein-independent signaling by serving as adaptors and scaffolds that assemble multiprotein complexes. By recruiting various partners, including trafficking and signaling proteins, directly to GPCRs, non-visual arrestins connect activated receptors to diverse signaling pathways. To investigate arrestin-mediated signaling, a structural understanding of arrestin activation and interaction with GPCRs is essential. Here we identified global and local conformational changes in the non-visual arrestins upon binding to the model GPCR rhodopsin. To detect conformational changes, pairs of spin labels were introduced into arrestin-2 and arrestin-3, and the interspin distances in the absence and presence of the receptor were measured by double electron electron resonance spectroscopy. Our data indicate that both non-visual arrestins undergo several conformational changes similar to arrestin-1, including the finger loop moving toward the predicted location of the receptor in the complex as well as the C-tail release upon receptor binding. The arrestin-2 results also suggest that there is no clam shell-like closure of the N- and C-domains and that the loop containing residue 136 (homolog of 139 in arrestin-1) has high flexibility in both free and receptor-bound states.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

GPCRs and Signal Transducers: Interaction Stoichiometry

Until the late 1990s, class A G protein-coupled receptors (GPCRs) were believed to function as monomers. Indirect evidence that they might internalize or even signal as dimers has emerged, along with proof that class C GPCRs are obligatory dimers. Crystal structures of GPCRs and their much larger binding partners were consistent with the idea that two receptors might engage a single G protein, GRK, or arrestin. However, recent biophysical, biochemical, and structural evidence invariably suggests that a single GPCR binds G proteins, GRKs, and arrestins. Here we review existing evidence of the stoichiometry of GPCR interactions with signal transducers and discuss potential biological roles of class A GPCR oligomers, including proposed homo- and heterodimers.

Haloperidol and clozapine differentially affect the expression of arrestins, receptor kinases, and extracellular signal-regulated kinase activation

Dopamine and other G protein-coupled receptors (GPCRs) represent the major target of antipsychotic drugs. GPCRs undergo desensitization via activation-dependent phosphorylation by G protein-coupled receptor kinases (GRKs) followed by arrestin binding. Arrestins and GRKs are major regulators of GPCR signaling. We elucidated changes in expression of two arrestins and four GRKs following chronic (21 days) treatment with haloperidol (1 mg/kg i.p.) or clozapine (20 mg/kg i.p.) 2 or 24 h after the last injection in 11 brain regions. Haloperidol decreased GRK3 in ventrolateral caudate-putamen and transiently down-regulated GRK5 in globus pallidus and caudal caudate-putamen. Clozapine also caused a short-term suppression of the GRK5 expression in the caudal caudate-putamen and globus pallidus, but, unlike haloperidol, elevated GRK5 in the caudal caudate-putamen after 24 h. Unlike haloperidol, clozapine decreased arrestin2 and GRK3 in hippocampus and GRK3 in globus pallidus but increased arrestin2 in the core of nucleus accumbens and ventrolateral caudate-putamen and GRK2 in prefrontal cortex. Clozapine, but not haloperidol, induced long-term activation of extracellular signal-regulated kinase (ERK) 2 in ventrolateral caudate-putamen and transient in prefrontal cortex. The data demonstrate that haloperidol and clozapine differentially affect the expression of arrestins and GRKs and ERK activity, which may play a role in determining their clinical profile.

Critical role of the central 139-loop in stability and binding selectivity of arrestin-1

Arrestin-1 selectively binds active phosphorylated rhodopsin (P-Rh*), demonstrating much lower affinity for inactive phosphorylated (P-Rh) and unphosphorylated active (Rh*) forms. Receptor interaction induces significant conformational changes in arrestin-1, which include large movement of the previously neglected 139-loop in the center of the receptor binding surface, away from the incoming receptor. To elucidate the functional role of this loop, in mouse arrestin-1 we introduced deletions of variable lengths and made several substitutions of Lys-142 in it and Asp-72 in the adjacent loop. Several mutants with perturbations in the 139-loop demonstrate increased binding to P-Rh*, dark P-Rh, Rh*, and phospho-opsin. Enhanced binding of arrestin-1 mutants to non-preferred forms of rhodopsin correlates with decreased thermal stability. The 139-loop perturbations increase P-Rh* binding of arrestin-1 at low temperatures and further change its binding profile on the background of 3A mutant, where the C-tail is detached from the body of the molecule by triple alanine substitution. Thus, the 139-loop stabilizes basal conformation of arrestin-1 and acts as a brake, preventing its binding to non-preferred forms of rhodopsin. Conservation of this loop in other subtypes suggests that it has the same function in all members of the arrestin family.

Aspartic acid 564 in the third cytoplasmic loop of the luteinizing hormone/choriogonadotropin receptor is crucial for phosphorylation-independent interaction with arrestin2

Arrestin2 binding to the active but unphosphorylated luteinizing hormone/choriogonadotropin receptor (LH/CG R) in ovarian follicles is triggered by activation of ADP-ribosylation factor 6 (ARF6) and leads to uncoupling of this receptor from cAMP signaling. We sought to determine how arrestin2 binds to LH/CG R, if binding is of high affinity, and if the receptor also binds arrestin3. Desensitization of intact LH/CG R was equally sensitive to ectopic constructs of arrestin2 that bind other G protein-coupled receptors (GPCRs) either in a phosphorylation-independent or -dependent manner. Intact LH/CG R was not desensitized by ectopic arrestin3 constructs. Surface plasmon resonance studies showed that arrestin2 bound a synthetic third intracellular (3i) LH/CG R loop peptide with picomolar affinity; arrestin3 bound with millimolar affinity. To determine whether Asp-564 in the 3i loop mimicked the phosphorylated residue of other GPCRs, human embryonic kidney (HEK) cells were transfected with wild-type (WT) and D564G LH/CG R. An agonist-stimulated ARF6-dependent arrestin2 undocking pathway to drive desensitization of WT receptor was recapitulated in HEK cell membranes, and ectopic arrestin2 promoted desensitization of WT LH/CG R. However, D564G LH/CG R in HEK cells was not desensitized, and synthetic 3i D564G peptide did not bind arrestin2. Synthetic 3i loop peptides containing D564E, D564V, or D564N also did not bind arrestin2. We conclude that the ARF6-mediated mechanism to release a pool of membrane-delimited arrestin to bind GPCRs may be a widespread mechanism to deliver arrestin to GPCRs for receptor desensitization. Unlike other GPCRs that additionally require receptor phosphorylation, LH/CG R activation is sufficient to expose a conformation in which Asp-564 in the 3i loop confers high affinity binding selectively to arrestin2.

Manipulation of very few receptor discriminator residues greatly enhances receptor specificity of non-visual arrestins

Based on the identification of residues that determine receptor selectivity of arrestins and the analysis of the evolution in the arrestin family, we introduced 10 mutations of "receptor discriminator" residues in arrestin-3. The recruitment of these mutants to M2 muscarinic (M2R), D1 (D1R) and D2 (D2R) dopamine, and β(2)-adrenergic receptors (β(2)AR) was assessed using bioluminescence resonance energy transfer-based assays in cells. Seven of 10 mutations differentially affected arrestin-3 binding to individual receptors. D260K and Q262P reduced the binding to β(2)AR, much more than to other receptors. The combination D260K/Q262P virtually eliminated β(2)AR binding while preserving the interactions with M2R, D1R, and D2R. Conversely, Y239T enhanced arrestin-3 binding to β(2)AR and reduced the binding to M2R, D1R, and D2R, whereas Q256Y selectively reduced recruitment to D2R. The Y239T/Q256Y combination virtually eliminated the binding to D2R and reduced the binding to β(2)AR and M2R, yielding a mutant with high selectivity for D1R. Eleven of 12 mutations significantly changed the binding to light-activated phosphorhodopsin. Thus, manipulation of key residues on the receptor-binding surface modifies receptor preference, enabling the construction of non-visual arrestins specific for particular receptor subtypes. These findings pave the way to the construction of signaling-biased arrestins targeting the receptor of choice for research or therapeutic purposes.

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.

Altered expression and subcellular distribution of GRK subtypes in the dopamine-depleted rat basal ganglia is not normalized by l-DOPA treatment

Dysregulation of dopamine (DA) receptors is believed to underlie Parkinson's disease pathology and l-DOPA-induced motor complications. DA receptors are subject to regulation by G protein-coupled receptor kinases (GRKs) and arrestins. DA lesion with 6-hydroxydopamine caused multiple protein- and brain region-specific changes in the expression of GRKs. In the globus pallidus, all four GRK isoforms (GRK2, 3, 5, 6) were reduced in the lesioned hemisphere. In the caudal caudate-putamen (cCPu) three GRK isoforms (GRK2, 3, 6) were decreased by DA depletion. The decrease in GRK proteins in globus pallidus, but not cCPu, was mirrored by reduction in mRNA. GRK3 protein was reduced in the rostral caudate-putamen (rCPu), whereas other isoforms were either unchanged or up-regulated. GRK6 protein and mRNA were up-regulated in rCPu and nucleus accumbens. l-DOPA (25 mg/kg, twice daily for 10 days) failed to reverse changes caused by DA depletion, whereas D(2)/D(3) agonist pergolide (0.25 mg/kg daily for 10 days) restored normal levels of expression of GRK5 and 6. In rCPu, GRK2 protein was increased in most subcellular fractions by l-DOPA but not by DA depletion alone. Similarly, l-DOPA up-regulated arrestin3 in membrane fractions in both regions. GRK5 was down-regulated by l-DOPA in cCPu in the light membrane fraction, where this isoform is the most abundant. The data suggest that alterations in the expression and subcellular distribution of arrestins and GRKs contribute to pathophysiology of Parkinson's disease. Thus, these proteins may be targets for antiparkinsonian therapy.

Arrestin binding to the M(2) muscarinic acetylcholine receptor is precluded by an inhibitory element in the third intracellular loop of the receptor

Desensitization of G protein-coupled receptors (GPCRs) involves the binding of members of the family of arrestins to the receptors. In the model system involving the visual GPCR rhodopsin, activation and phosphorylation of rhodopsin is thought to convert arrestin from a low to high affinity binding state. Phosphorylation of the M(2) muscarinic acetylcholine receptor (mAChR) has been shown to be required for binding of arrestins 2 and 3 in vitro and for arrestin-enhanced internalization in intact cells (Pals-Rylaarsdam, R., and Hosey, M. M. (1997) J. Biol. Chem. 272, 14152-14158). For the M(2) mAChR, arrestin binding requires phosphorylation at multiple serine and threonine residues at amino acids 307-311 in the third intracellular (i3) loop. Here, we have investigated the molecular basis for the requirement of receptor phosphorylation for arrestin binding. Constructs of arrestin 2 that can bind to other GPCRs in a phosphorylation-independent manner were unable to interact with a mutant M(2) mAChR in which the Ser/Thr residues at 307-311 were mutated to alanines. However, although phosphorylation-deficient mutants of the M(2) mAChR that lacked 50-157 amino acids from the i3 loop were unable to undergo agonist-dependent internalization when expressed alone in tsA201 cells, co-expression of arrestin 2 or 3 restored agonist-dependent internalization. Furthermore, a deletion of only 15 amino acids (amino acids 304-319) was sufficient to allow for phosphorylation-independent arrestin-receptor interaction. These results indicate that phosphorylation at residues 307-311 does not appear to be required to activate arrestin into a high affinity binding state. Instead, phosphorylation at residues 307-311 appears to facilitate the removal of an inhibitory constraint that precludes receptor-arrestin association in the absence of receptor phosphorylation.

Receptor-Arrestin Interactions: The GPCR Perspective

Arrestins are a small family of four proteins in most vertebrates that bind hundreds of different G protein-coupled receptors (GPCRs). Arrestin binding to a GPCR has at least three functions: precluding further receptor coupling to G proteins, facilitating receptor internalization, and initiating distinct arrestin-mediated signaling. The molecular mechanism of arrestin-GPCR interactions has been extensively studied and discussed from the "arrestin perspective", focusing on the roles of arrestin elements in receptor binding. Here, we discuss this phenomenon from the "receptor perspective", focusing on the receptor elements involved in arrestin binding and emphasizing existing gaps in our knowledge that need to be filled. It is vitally important to understand the role of receptor elements in arrestin activation and how the interaction of each of these elements with arrestin contributes to the latter's transition to the high-affinity binding state. A more precise knowledge of the molecular mechanisms of arrestin activation is needed to enable the construction of arrestin mutants with desired functional characteristics.

mu-Opioid receptors desensitize less rapidly than delta-opioid receptors due to less efficient activation of arrestin

Receptor desensitization by G-protein receptor kinases (GRK) and arrestins is likely to be an important component underlying the development of tolerance to opioid drugs. Reconstitution of this process in Xenopus oocytes revealed distinct differences in the kinetics of GRK and arrestin regulation of the closely related opioid receptors mu (MOR), delta (DOR), and kappa (KOR). We demonstrated that under identical conditions, GRK and arrestin-dependent desensitization of MOR proceeds dramatically slower than that of DOR. Furthermore, GRK3 phosphorylation sites required for opioid receptor desensitization also greatly differ. The determinants for DOR and KOR desensitization reside in the carboxyl-terminal tail, whereas MOR depends on Thr-180 in the second intracellular loop. Although this later finding might indicate an inefficient phosphorylation of MOR Thr-180, increasing the amount of arrestin expressed greatly increased the rate of MOR desensitization to a rate comparable with that of DOR. Similarly, coexpression of a constitutively active arrestin 2(R169E) with MOR and DOR desensitized both receptors in an agonist-dependent, GRK-independent manner at rates that were indistinguishable. Together, these data suggest that it is the activation of arrestin, rather than its binding, that is the rate-limiting step in MOR desensitization. In addition, mutation of Thr-161 in DOR, homologous to MOR Thr-180, significantly inhibited the faster desensitization of DOR. These results suggest that DOR desensitization involves phosphorylation of both the carboxyl-terminal tail and the second intracellular loop that together leads to a more efficient activation of arrestin and thus faster desensitization.

Nonvisual arrestins function as simple scaffolds assembling the MKK4-JNK3α2 signaling complex

Arrestins make up a small family of proteins with four mammalian members that play key roles in the regulation of multiple G protein-coupled receptor-dependent and -independent signaling pathways. Although arrestins were reported to serve as scaffolds for MAP kinase cascades, promoting the activation of JNK3, ERK1/2, and p38, the molecular mechanisms involved were not elucidated, and even the direct binding of arrestins with MAP kinases was never demonstrated. Here, using purified proteins, we show that both nonvisual arrestins directly bind JNK3α2 and its upstream activator MKK4, and that the affinity of arrestin-3 for these kinases is higher than that of arrestin-2. Reconstitution of the MKK4-JNK3α2 signaling module from pure proteins in the presence of different arrestin-3 concentrations showed that arrestin-3 acts as a "true" scaffold, facilitating JNK3α2 phosphorylation by bringing the two kinases together. Both the level of JNK3α2 phosphorylation by MKK4 and JNK3α2 activity toward its substrate ATF2 increase at low and then decrease at high arrestin-3 levels, yielding a bell-shaped concentration dependence expected with true scaffolds that do not activate the upstream kinase or its substrate. Thus, direct binding of both kinases and true scaffolding is the molecular mechanism of action of arrestin-3 on the MKK4-JNK3α2 signaling module.

Cone arrestin binding to JNK3 and Mdm2: conformational preference and localization of interaction sites

Arrestins are multi-functional regulators of G protein-coupled receptors. Receptor-bound arrestins interact with >30 remarkably diverse proteins and redirect the signaling to G protein-independent pathways. The functions of free arrestins are poorly understood, and the interaction sites of the non-receptor arrestin partners are largely unknown. In this study, we show that cone arrestin, the least studied member of the family, binds c-Jun N-terminal kinase (JNK3) and Mdm2 and regulates their subcellular distribution. Using arrestin mutants with increased or reduced structural flexibility, we demonstrate that arrestin in all conformations binds JNK3 comparably, whereas Mdm2 preferentially binds cone arrestin 'frozen' in the basal state. To localize the interaction sites, we expressed separate N- and C-domains of cone and rod arrestins and found that individual domains bind JNK3 and remove it from the nucleus as efficiently as full-length proteins. Thus, the arrestin binding site for JNK3 includes elements in both domains with the affinity of partial sites on individual domains sufficient for JNK3 relocalization. N-domain of rod arrestin binds Mdm2, which localizes its main interaction site to this region. Comparable binding of JNK3 and Mdm2 to four arrestin subtypes allowed us to identify conserved residues likely involved in these interactions.

Phosphorylation barcode-dependent signal bias of the dopamine D1 receptor

Agonist-activated G protein-coupled receptors (GPCRs) must correctly select from hundreds of potential downstream signaling cascades and effectors. To accomplish this, GPCRs first bind to an intermediary signaling protein, such as G protein or arrestin. These intermediaries initiate signaling cascades that promote the activity of different effectors, including several protein kinases. The relative roles of G proteins versus arrestins in initiating and directing signaling is hotly debated, and it remains unclear how the correct final signaling pathway is chosen given the ready availability of protein partners. Here, we begin to deconvolute the process of signal bias from the dopamine D1 receptor (D1R) by exploring factors that promote the activation of ERK1/2 or Src, the kinases that lead to cell growth and proliferation. We found that ERK1/2 activation involves both arrestin and Gαs, while Src activation depends solely on arrestin. Interestingly, we found that the phosphorylation pattern influences both arrestin and Gαs coupling, suggesting an additional way the cells regulate G protein signaling. The phosphorylation sites in the D1R intracellular loop 3 are particularly important for directing the binding of G protein versus arrestin and for selecting between the activation of ERK1/2 and Src. Collectively, these studies correlate functional outcomes with a physical basis for signaling bias and provide fundamental information on how GPCR signaling is directed.