The differential engagement of arrestin surface charges by the various functional forms of the receptor

Ubiquitin ligase parkin promotes Mdm2-arrestin interaction but inhibits arrestin ubiquitination

Numerous mutations in E3 ubiquitin ligase parkin were shown to associate with familial Parkinson's disease. Here we show that parkin binds arrestins, versatile regulators of cell signaling. Arrestin-parkin interaction was demonstrated by coimmunoprecipitation of endogenous proteins from brain tissue and shown to be direct using purified proteins. Parkin binding enhances arrestin interactions with another E3 ubiquitin ligase, Mdm2, apparently by shifting arrestin conformational equilibrium to the basal state preferred by Mdm2. Although Mdm2 was reported to ubiquitinate arrestins, parkin-dependent increase in Mdm2 binding dramatically reduces the ubiquitination of both nonvisual arrestins, basal and stimulated by receptor activation, without affecting receptor internalization. Several disease-associated parkin mutations differentially affect the stimulation of Mdm2 binding. All parkin mutants tested effectively suppress arrestin ubiquitination, suggesting that bound parkin shields arrestin lysines targeted by Mdm2. Parkin binding to arrestins along with its effects on arrestin interaction with Mdm2 and ubiquitination is a novel function of this protein with implications for Parkinson's disease pathology.

Few residues within an extensive binding interface drive receptor interaction and determine the specificity of arrestin proteins

Arrestins bind active phosphorylated forms of G protein-coupled receptors, terminating G protein activation, orchestrating receptor trafficking, and redirecting signaling to alternative pathways. Visual arrestin-1 preferentially binds rhodopsin, whereas the two non-visual arrestins interact with hundreds of G protein-coupled receptor subtypes. Here we show that an extensive surface on the concave side of both arrestin-2 domains is involved in receptor binding. We also identified a small number of residues on the receptor binding surface of the N- and C-domains that largely determine the receptor specificity of arrestins. We show that alanine substitution of these residues blocks the binding of arrestin-1 to rhodopsin in vitro and of arrestin-2 and -3 to β2-adrenergic, M2 muscarinic cholinergic, and D2 dopamine receptors in intact cells, suggesting that these elements critically contribute to the energy of the interaction. Thus, in contrast to arrestin-1, where direct phosphate binding is crucial, the interaction of non-visual arrestins with their cognate receptors depends to a lesser extent on phosphate binding and more on the binding to non-phosphorylated receptor elements.

Binding between a distal C-terminus fragment of cannabinoid receptor 1 and arrestin-2

Internalization of G-protein-coupled receptors is mediated by phosphorylation of the C-terminus, followed by binding with the cytosolic protein arrestin. To explore structural factors that may play a role in internalization of cannabinoid receptor 1 (CB1), we utilize a phosphorylated peptide derived from the distal C-terminus of CB1 (CB1(5P)(454-473)). Complexes formed between the peptide and human arrestin-2 (wt-arr2(1-418)) were compared to those formed with a truncated arrestin-2 mutant (tr-arr2(1-382)) using isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. The pentaphosphopeptide CB1(5P)(454-473) adopts a helix-loop conformation, whether binding to full-length arrestin-2 or its truncated mutant. This structure is similar to that of a heptaphosphopeptide, mimicking the distal segment of the rhodopsin C-tail (Rh(7P)(330-348)), binding to visual arrestin, suggesting that this adopted structure bears functional significance. Isothermal titration calorimetry (ITC) experiments show that the CB1(5P)(454-473) peptide binds to tr-arr2(1-382) with higher affinity than to the full-length wt-arr2(1-418). As the observed structure of the bound peptides is similar in either case, we attribute the increased affinity to a more exposed binding site on the N-domain of the truncated arrestin construct. The transferred NOE data from the bound phosphopeptides are used to predict a model describing the interaction with arrestin, using the data driven HADDOCK docking program. The truncation of arrestin-2 provides scope for positively charged residues in the polar core of the protein to interact with phosphates present in the loop of the CB1(5P)(454-473) peptide.

Robust self-association is a common feature of mammalian visual arrestin-1

Arrestin-1 binds light-activated phosphorhodopsin and ensures rapid signal termination. Its deficiency in humans and mice results in prolonged signaling and rod degeneration. However, most of the biochemical studies were performed on bovine arrestin-1, which was shown to self-associate forming dimers and tetramers, although only the monomer binds rhodopsin. It is unclear whether self-association is a property of arrestin-1 in all mammals or a specific feature of bovine protein. To address this issue, we compared self-association parameters of purified human and mouse arrestin-1 with those of its bovine counterpart using multiangle light scattering. We found that mouse and human arrestin-1 also robustly self-associate, existing in a monomer-dimer-tetramer equilibrium. Interestingly, the combination of dimerization and tetramerization constants in these three species is strikingly different. While tetramerization of bovine arrestin-1 is highly cooperative (K(D,dim)(4) > K(D,tet)), K(D,dim) ∼ K(D,tet) in the mouse form and K(D,dim) ≪ K(D,tet) in the human form. Importantly, in all three species at very high physiological concentrations of arrestin-1 in rod photoreceptors, most of it is predicted to exist in oligomeric form, with a relatively low concentration of the free monomer. Thus, it appears that maintenance of low levels of the active monomer is the biological role of arrestin-1 self-association.

Crystal structure of arrestin-3 reveals the basis of the difference in receptor binding between two non-visual subtypes

Arrestins are multi-functional proteins that regulate signaling and trafficking of the majority of G protein-coupled receptors (GPCRs), as well as sub-cellular localization and activity of many other signaling proteins. We report the first crystal structure of arrestin-3, solved at 3.0 Å resolution. Arrestin-3 is an elongated two-domain molecule with overall fold and key inter-domain interactions that hold the free protein in the basal conformation similar to the other subtypes. Arrestin-3 is the least selective member of the family, binding a wide variety of GPCRs with high affinity and demonstrating lower preference for active phosphorylated forms of the receptors. In contrast to the other three arrestins, part of the receptor-binding surface in the arrestin-3 C-domain does not form a contiguous β-sheet, which is consistent with increased flexibility. By swapping the corresponding elements between arrestin-2 and arrestin-3 we show that the presence of this loose structure is correlated with reduced arrestin selectivity for activated receptors, consistent with a conformational change in this β-sheet upon receptor binding.

Elucidation of inositol hexaphosphate and heparin interaction sites and conformational changes in arrestin-1 by solution nuclear magnetic resonance

Arrestins specifically bind activated and phosphorylated G protein-coupled receptors and orchestrate both receptor trafficking and channel signaling through G protein-independent pathways via direct interactions with numerous nonreceptor partners. Here we report the first successful use of solution NMR in mapping the binding sites in arrestin-1 (visual arrestin) for two polyanionic compounds that mimic phosphorylated light-activated rhodopsin: inositol hexaphosphate (IP6) and heparin. This yielded an identification of residues involved in the binding with these ligands that was more complete than what has previously been feasible. IP6 and heparin appear to bind to the same site on arrestin-1, centered on a positively charged region in the N-domain. We present the first direct evidence that both IP6 and heparin induced a complete release of the arrestin C-tail. These observations provide novel insight into the nature of the transition of arrestin from the basal to active state and demonstrate the potential of NMR-based methods in the study of protein-protein interactions involving members of the arrestin family.

Monomeric rhodopsin is sufficient for normal rhodopsin kinase (GRK1) phosphorylation and arrestin-1 binding

G-protein-coupled receptor (GPCR) oligomerization has been observed in a wide variety of experimental contexts, but the functional significance of this phenomenon at different stages of the life cycle of class A GPCRs remains to be elucidated. Rhodopsin (Rh), a prototypical class A GPCR of visual transduction, is also capable of forming dimers and higher order oligomers. The recent demonstration that Rh monomer is sufficient to activate its cognate G protein, transducin, prompted us to test whether the same monomeric state is sufficient for rhodopsin phosphorylation and arrestin-1 binding. Here we show that monomeric active rhodopsin is phosphorylated by rhodopsin kinase (GRK1) as efficiently as rhodopsin in the native disc membrane. Monomeric phosphorylated light-activated Rh (P-Rh*) in nanodiscs binds arrestin-1 essentially as well as P-Rh* in native disc membranes. We also measured the affinity of arrestin-1 for P-Rh* in nanodiscs using a fluorescence-based assay and found that arrestin-1 interacts with monomeric P-Rh* with low nanomolar affinity and 1:1 stoichiometry, as previously determined in native disc membranes. Thus, similar to transducin activation, rhodopsin phosphorylation by GRK1 and high affinity arrestin-1 binding only requires a rhodopsin monomer.

Beta-arrestins as regulators of signal termination and transduction: how do they determine what to scaffold?

Over the last decade β-arrestins have emerged as pleiotropic scaffold proteins, capable of mediating numerous diverse responses to multiple agonists. Most well characterized are the G-protein-coupled receptor (GPCR) stimulated β-arrestin signals, which are sometimes synergistic with, and sometimes independent of, heterotrimeric G-protein signals. β-arrestin signaling involves the recruitment of downstream signaling moieties to β-arrestins; in many cases specific sites of interaction between β-arrestins and the downstream target have been identified. As more information unfolds about the nature of β-arrestin scaffolding interactions, it is evident that these proteins are capable of adopting multiple conformations which in turn reveal a specific set of interacting domains. Recruitment of β-arrestin to a specific GPCR can promote formation of a specific subset of available β-arrestin scaffolds, allowing for a higher level of specificity to given agonists. This review discusses recent advances in β-arrestin signaling, discussing the molecular details of a subset of known β-arrestin scaffolds and the significance of specific binding interactions on the ultimate cellular response.

Custom-designed proteins as novel therapeutic tools? The case of arrestins

Multiple genetic disorders can be associated with excessive signalling by mutant G-protein-coupled receptors (GPCRs) that are either constitutively active or have lost sites where phosphorylation by GPCR kinases is necessary for desensitisation by cognate arrestins. Phosphorylation-independent arrestin1 can compensate for defects in phosphorylation of the GPCR rhodopsin in retinal rod cells, facilitating recovery, improving light responsiveness, and promoting photoreceptor survival. These proof-of-principle experiments show that, based on mechanistic understanding of the inner workings of a protein, one can modify its functional characteristics to generate custom-designed mutants that improve the balance of signalling in congenital and acquired disorders. Manipulations of arrestin elements responsible for scaffolding mitogen-activated protein kinase cascades and binding other signalling proteins involved in life-or-death decisions in the cell are likely to yield mutants that affect cell survival and proliferation in the desired direction. Although this approach is still in its infancy, targeted redesign of individual functions of many proteins offers a promise of a completely new therapeutic toolbox with huge potential.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.

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.

Enhanced arrestin facilitates recovery and protects rods lacking rhodopsin phosphorylation

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins expressed in every cell in the body and are targeted by the majority of clinically used drugs [1]. GPCR signaling, including rhodopsin-driven phototransduction, is terminated by receptor phosphorylation followed by arrestin binding [2]. Genetic defects in receptor phosphorylation and excessive signaling by overactive GPCR mutants result in a wide variety of diseases, from retinal degeneration to cancer [3-6]. Here, we tested whether arrestin1 mutants with enhanced ability to bind active unphosphorylated rhodopsin [7-10] can suppress uncontrolled signaling, bypassing receptor phosphorylation by rhodopsin kinase (RK) and replacing this two-step mechanism with a single-step deactivation in rod photoreceptors. We show that in this precisely timed signaling system with single-photon sensitivity [11], an enhanced arrestin1 mutant partially compensates for defects in rhodopsin phosphorylation, promoting photoreceptor survival, improving functional performance, and facilitating photoresponse recovery. These proof-of-principle experiments demonstrate the feasibility of functional compensation in vivo for the first time, which is a promising approach for correcting genetic defects associated with gain-of-function mutations. Successful modification of protein-protein interactions by appropriate mutations paves the way to targeted redesign of signaling pathways to achieve desired functional outcomes.

A model for the solution structure of the rod arrestin tetramer

Visual rod arrestin has the ability to self-associate at physiological concentrations. We previously demonstrated that only monomeric arrestin can bind the receptor and that the arrestin tetramer in solution differs from that in the crystal. We employed the Rosetta docking software to generate molecular models of the physiologically relevant solution tetramer based on the monomeric arrestin crystal structure. The resulting models were filtered using the Rosetta energy function, experimental intersubunit distances measured with DEER spectroscopy, and intersubunit contact sites identified by mutagenesis and site-directed spin labeling. This resulted in a unique model for subsequent evaluation. The validity of the model is strongly supported by model-directed crosslinking and targeted mutagenesis that yields arrestin variants deficient in self-association. The structure of the solution tetramer explains its inability to bind rhodopsin and paves the way for experimental studies of the physiological role of rod arrestin self-association.

Arrestin binds to different phosphorylated regions of the thyrotropin-releasing hormone receptor with distinct functional consequences

Arrestin binding to agonist-occupied phosphorylated G protein-coupled receptors typically increases the affinity of agonist binding, increases resistance of receptor-bound agonist to removal with high acid/salt buffer, and leads to receptor desensitization and internalization. We tested whether thyrotropin-releasing hormone (TRH) receptors lacking phosphosites in the C-terminal tail could form stable and functional complexes with arrestin. Fibroblasts from mice lacking arrestins 2 and 3 were used to distinguish between arrestin-dependent and -independent effects. Arrestin did not promote internalization or desensitization of a receptor that had key Ser/Thr phosphosites mutated to Ala (4Ala receptor). Nevertheless, arrestin greatly increased acid/salt resistance and the affinity of 4Ala receptor for TRH. Truncation of 4Ala receptor just distal to the key phosphosites (4AlaStop receptor) abolished arrestin-dependent acid/salt resistance but not the effect of arrestin on agonist affinity. Arrestin formed stable complexes with activated wild-type and 4Ala receptors but not with 4AlaStop receptor, as measured by translocation of arrestin-green fluorescent protein to the plasma membrane or chemical cross-linking. An arrestin mutant that does not interact with clathrin and AP2 did not internalize receptor but still promoted high affinity TRH binding, acid/salt resistance, and desensitization. A sterically restricted arrestin mutant did not cause receptor internalization or desensitization but did promote acid/salt resistance and high agonist affinity. The results demonstrate that arrestin binds to proximal or distal phosphosites in the receptor tail. Arrestin binding at either site causes increased agonist affinity and acid/salt resistance, but only the proximal phosphosites evoke the necessary conformational changes in arrestin for receptor desensitization and internalization.

How and why do GPCRs dimerize?

Dimerization is fairly common in the G-protein-coupled receptor (GPCR) superfamily. First attempts to rationalize this phenomenon gave rise to an idea that two receptors in a dimer could be necessary to bind a single molecule of G protein or arrestin. Although GPCRs, G proteins and arrestins were crystallized only in their inactive conformations (in which they do not interact), the structures appeared temptingly compatible with this beautiful model. However, it did not survive the rigors of experimental testing: several recent studies unambiguously demonstrated that one receptor molecule is sufficient to activate a G protein and bind arrestin. Thus, to figure out the biological role of receptor self-association we must focus on other functions of GPCRs at different stages of their functional cycle.

Arrestins as multi-functional signaling adaptors

Arrestins are versatile regulators of cellular signaling expressed in every cell in the body. Arrestins bind active phosphorylated forms of their cognate G-protein-coupled receptors, shutting down G-protein activation and linking receptors to alternative signaling pathways. Arrestins directly interact with more than 20 surprisingly diverse proteins, such as several Src family kinases, ubiquitin ligases, protein phosphatases, microtubules, etc., and serve as scaffolds facilitating signaling in two MAP kinase cascades, leading to the activation of ERK1/2 and JNK3. A number of arrestin-binding partners are key players in signaling pathways that regulate cell proliferation, survival, and apoptotic death, which make arrestin interactions with these proteins inviting targets for therapeutic intervention. For example, enhancement of pro-survival or pro-apoptotic arrestin-dependent signaling is a promising strategy in treating disorders such as neurodegenerative diseases or cancer, respectively. Recent studies show that in the cell arrestin exists in at least three distinct conformations, free, receptor-bound, and microtubule-bound, with very different signaling capabilities. Precise identification of arrestin elements mediating its interactions with each partner and elucidation of conformational dependence of these interactions will pave the way to the development of molecular tools for targeted enhancement or attenuation of arrestin interactions with individual partners. This structural information is necessary to devise conventional drug-based approaches and to engineer specialized "designer" arrestins that can compensate for defects in receptor regulation associated with congenital disorders and/or redirect arrestin-mediated signaling to desired pathways. Arrestins are at the crossroads of crucial pathways that determine cell fate and behavior. Therefore, targeted manipulation of arrestin-dependent signaling has an enormous therapeutic potential.

Opposing effects of inositol hexakisphosphate on rod arrestin and arrestin2 self-association

The robust cooperative formation of rod arrestin tetramers has been well-established, whereas the ability of other members of the arrestin family to self-associate remains controversial. Here, we used purified arrestins and multi-angle light scattering to quantitatively compare the propensity of the four mammalian arrestin subtypes to self-associate. Both non-visual and cone arrestins only form oligomers at very high non-physiological concentrations. However, inositol hexakisphosphate (IP6), a fairly abundant form of inositol in the cytoplasm, greatly facilitates self-association of arrestin2. Arrestin2 self-association equilibrium constants in the presence of 100 microM IP6 suggest that an appreciable proportion could exist in an oligomeric state but only in intracellular compartments where its concentration is 5-10-fold higher than average. In contrast to arrestin2, IP6 inhibits self-association of rod arrestin, indicating that the structure of these two tetramers in solution is likely different.

Regulation of arrestin binding by rhodopsin phosphorylation level

Arrestins ensure the timely termination of receptor signaling. The role of rhodopsin phosphorylation in visual arrestin binding was established more than 20 years ago, but the effects of the number of receptor-attached phosphates on this interaction remain controversial. Here we use purified rhodopsin fractions with carefully quantified content of individual phosphorylated rhodopsin species to elucidate the impact of phosphorylation level on arrestin interaction with three biologically relevant functional forms of rhodopsin: light-activated and dark phosphorhodopsin and phospho-opsin. We found that a single receptor-attached phosphate does not facilitate arrestin binding, two are necessary to induce high affinity interaction, and three phosphates fully activate arrestin. Higher phosphorylation levels do not increase the stability of arrestin complex with light-activated rhodopsin but enhance its binding to the dark phosphorhodopsin and phospho-opsin. The complex of arrestin with hyperphosphorylated light-activated rhodopsin is less sensitive to high salt and appears to release retinal faster. These data suggest that arrestin likely quenches rhodopsin signaling after the third phosphate is added by rhodopsin kinase. The complex of arrestin with heavily phosphorylated rhodopsin, which appears to form in certain disease states, has distinct characteristics that may contribute to the phenotype of these visual disorders.

N-terminal and C-terminal domains of arrestin both contribute in binding to rhodopsin

Visual arrestin terminates the signal amplification cascade in photoreceptor cells by blocking the interaction of light activated phosphorylated rhodopsin with the G-protein transducin. Although crystal structures of arrestin and rhodopsin are available, it is still unknown how the complex of the two proteins is formed. To investigate the interaction sites of arrestin with rhodopsin various surface regions of recombinant arrestin were sterically blocked by different numbers of fluorophores (Alexa 633). The binding was recorded by time-resolved light scattering. To accomplish site-specific shielding of protein regions, in a first step all three wild-type cysteines were replaced by alanines. Nevertheless, regarding the magnitude and specificity of rhodopsin binding, the protein is still fully active. In a second step, new cysteines were introduced at selected sites to allow covalent binding of fluorophores. Upon attachment of Alexa 633 to the recombinant cysteines we observed that these bulky labels residing in the concave area of either the N- or the C-terminal domain do not perturb the activity of arrestin. By simultaneously modifying both domains with one Alexa 633 the binding capacity was reduced. The presence of two Alexa 633 molecules in each domain prevented binding of rhodopsin to arrestin. This observation indicates that both concave sites participate in binding.

Functional selectivity through protean and biased agonism: who steers the ship?

This article describes functional selectivity of agonists and antagonists and distinguishes conventional cell-based functional selectivity, where the strength of signal produces selective signaling in various organs, from true receptor active-state based selectivity, also alternatively referred to in the literature as "stimulus trafficking," "biased agonism," and "collateral efficacy." This latter mechanism of selectivity depends on the ligand-related conformation of the receptor and is not compatible with the parsimonious view that agonists produce a single receptor active state. In addition, protean agonism is described, whereby a ligand produces positive agonism in quiescent systems and inverse agonism in constitutively active systems. This is a special case of active state-based selectivity in which the ligand produces an active state that is of lower efficacy than the natural constitutively active state. It is postulated that receptor active-state based selectivity, unlike cell-based functional selectivity, is controllable through the chemical structure of the ligand and is therefore more likely to be a viable avenue for therapeutic selectivity in the clinic. Reasons are given for differentiating receptor active-state based selectivity from conventional functional organ selectivity.

Dynamics of arrestin-rhodopsin interactions: loop movement is involved in arrestin activation and receptor binding

In this study we investigate conformational changes in Loop V-VI of visual arrestin during binding to light-activated, phosphorylated rhodopsin (Rho*-P) using a combination of site-specific cysteine mutagenesis and intramolecular fluorescence quenching. Introduction of cysteines at positions in the N-domain at residues predicted to be in close proximity to Ile-72 in Loop V-VI of arrestin (i.e. Glu-148 and Lys-298) appear to form an intramolecular disulfide bond with I72C, significantly diminishing the binding of arrestin to Rho*-P. Using a fluorescence approach, we show that the steady-state emission from a monobromobimane fluorophore in Loop V-VI is quenched by tryptophan residues placed at 148 or 298. This quenching is relieved upon binding of arrestin to Rho*-P. These results suggest that arrestin Loop V-VI moves during binding to Rho*-P and that conformational flexibility of this loop is essential for arrestin to adopt a high affinity binding state.

Phosphorylation of the delta-opioid receptor regulates its beta-arrestins selectivity and subsequent receptor internalization and adenylyl cyclase desensitization

In the current study, we investigated the role of receptor phosphorylation and beta-arrestins in delta-opioid receptor (DOR) signaling and trafficking by using a DOR mutant in which all Ser/Thr residues in the C terminus were mutated to Ala (DTS). We demonstrated that the DOR agonist D-[Pen(2),Pen(5)]enkephalin could induce receptor internalization and adenylyl cyclase (AC) desensitization of DTS, but with comparatively slower kinetics than those observed with wild type DOR. Blockade of the internalization of DTS by the dominant-negative mutant dynamin, dynamin K44E, did not affect AC desensitization. However, depletion of beta-arrestins almost totally blocked both internalization and AC desensitization of DTS. A BRET assay suggested that DOR phosphorylation promotes receptor selectivity for beta-arrestin 2 over beta-arrestin 1. Furthermore, in mouse embryonic fibroblast (MEF) cells lacking either beta-arrestin 1 (beta arr1(-/-)) or beta-arrestin 2 (beta arr2(-/-)), agonist-induced DTS desensitization and internalization were similar to that observed in wild type MEFs. In contrast, although DOR internalization decreased in both beta arr1(-/-) MEFs and beta arr2(-/-) MEFs, DPDPE-induced DOR desensitization was significantly reduced in beta arr2(-/-) MEFs, but not in beta arr1(-/-) MEFs. Additionally, the BRET assay suggested that depletion of phosphorylation did not influence the stability of the receptor-beta-arrestin complex. Consistent with this observation, DTS did not recycle after internalization, which is like wild type DOR. Taken together, these results indicate that receptor phosphorylation confers DOR selectivity for beta-arrestin 2 without affecting the stability of the receptor-beta-arrestin complex and the fate of the internalized receptor.

Regulation of receptor trafficking by GRKs and arrestins

To ensure that extracellular stimuli are translated into intracellular signals of appropriate magnitude and specificity, most signaling cascades are tightly regulated. One of the major mechanisms involved in the regulation of G protein-coupled receptors (GPCRs) involves their endocytic trafficking. GPCR endocytic trafficking entails the targeting of receptors to discrete endocytic sites at the plasma membrane, followed by receptor internalization and intracellular sorting. This regulates the level of cell surface receptors, the sorting of receptors to degradative or recycling pathways, and in some cases the specific signaling pathways. In this chapter we discuss the mechanisms that regulate receptor endocytic trafficking, emphasizing the role of GPCR kinases (GRKs) and arrestins in this process.

Arrestin mobilizes signaling proteins to the cytoskeleton and redirects their activity

Arrestins regulate the activity and subcellular localization of G protein-coupled receptors and other signaling molecules. Here, we demonstrate that arrestins bind microtubules (MTs) in vitro and in vivo. The MT-binding site on arrestins overlaps significantly with the receptor-binding site, but the conformations of MT-bound and receptor-bound arrestin are different. Arrestins recruit ERK1/2 and the E3 ubiquitin ligase Mdm2 to MTs in cells, similar to the arrestin-dependent mobilization of these proteins to the receptor. Arrestin-mediated sequestration of ERK to MTs reduces the level of ERK activation. In contrast, recruitment of Mdm2 to MTs by arrestin channels Mdm2 activity toward cytoskeleton-associated proteins, increasing their ubiquitination dramatically. The mobilization of signaling molecules to MTs is a novel biological function of arrestin proteins.

Arrestins: ubiquitous regulators of cellular signaling pathways

The arrestins are a small family of proteins that regulate the signaling and trafficking of G-protein-coupled receptors and also serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

In vertebrates, the arrestins are a family of four proteins that regulate the signaling and trafficking of hundreds of different G-protein-coupled receptors (GPCRs). Arrestin homologs are also found in insects, protochordates and nematodes. Fungi and protists have related proteins but do not have true arrestins. Structural information is available only for free (unbound) vertebrate arrestins, and shows that the conserved overall fold is elongated and composed of two domains, with the core of each domain consisting of a seven-stranded β-sandwich. Two main intramolecular interactions keep the two domains in the correct relative orientation, but both of these interactions are destabilized in the process of receptor binding, suggesting that the conformation of bound arrestin is quite different. As well as binding to hundreds of GPCR subtypes, arrestins interact with other classes of membrane receptors and more than 20 surprisingly diverse types of soluble signaling protein. Arrestins thus serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

Arrestin binding to calmodulin: a direct interaction between two ubiquitous signaling proteins

Arrestins serve as multi-functional regulators of G-protein coupled receptors, interacting with hundreds of different receptor subtypes and a variety of other signaling proteins. Here we identify calmodulin as a novel arrestin interaction partner using three independent methods in vitro and in cells. Arrestin preferentially binds calcium-loaded calmodulin with a Kd value of approximately 7 microM, which is within range of endogenous calmodulin concentrations. The calmodulin binding site is localized on the concave side of the C-domain and a loop in the center of the arrestin molecule, significantly overlapping with receptor and microtubule-binding sites. Using purified proteins, we found that arrestins sequester calmodulin, preventing its binding to microtubules. Nanomolar affinity of arrestins for their cognate receptors makes calmodulin an ineffective competitor for arrestin binding at relatively high receptor concentrations. The arrestin-calmodulin interaction likely regulates the localization of both proteins and their availability for other interaction partners.

Visual arrestin binding to microtubules involves a distinct conformational change

Recently we found that visual arrestin binds microtubules and that this interaction plays an important role in arrestin localization in photoreceptor cells. Here we use site-directed mutagenesis and spin labeling to explore the molecular mechanism of this novel regulatory interaction. The microtubule binding site maps to the concave sides of the two arrestin domains, overlapping with the rhodopsin binding site, which makes arrestin interactions with rhodopsin and microtubules mutually exclusive. Arrestin interaction with microtubules is enhanced by several "activating mutations" and involves multiple positive charges and hydrophobic elements. The comparable affinity of visual arrestin for microtubules and unpolymerized tubulin (K(D) > 40 mum and >65 mum, respectively) suggests that the arrestin binding site is largely localized on the individual alphabeta-dimer. The changes in the spin-spin interaction of a double-labeled arrestin indicate that the conformation of microtubule-bound arrestin differs from that of free arrestin in solution. In sharp contrast to rhodopsin, where tight binding requires an extended interdomain hinge, arrestin binding to microtubules is enhanced by deletions in this region, suggesting that in the process of microtubule binding the domains may move in the opposite direction. Thus, microtubule and rhodopsin binding induce different conformational changes in arrestin, suggesting that arrestin assumes three distinct conformations in the cell, likely with different functional properties.

Soluble mimics of the cytoplasmic face of the human V1-vascular vasopressin receptor bind arrestin2 and calmodulin

Signal transduction by G protein-coupled receptors (GPCRs) is mediated by interactions between intracellular proteins and exposed motifs on the cytoplasmic face of these receptors. Arrestins bind to GPCRs and modulate receptor function either by interfering with heterotrimeric G protein signaling or by serving as signaling adaptors themselves. Calmodulin interacts with GPCRs triggering a calcium response. We have studied the interaction of arrestin2 and calmodulin with intracellular elements of the human V1-vascular vasopressin receptor (hV1R). For this purpose, we designed, expressed, and purified soluble fusion proteins with the maltose-binding protein (MBP) from Escherichia coli that mimic the intracellular surface of the hV1R. These MBP fusion proteins bind arrestin2 and calmodulin with affinities in the micromolar range. A different series of soluble receptor analogs, named vasopressin receptor 1 elements on a soluble scaffold (V1ROSS) proteins, consist of the third intracellular loop and/or the C-terminal segment of the hV1R receptor juxtaposed on the surface of the MBP. V1ROSS proteins bind calmodulin and a truncated, phosphorylation-independent form of arrestin2 more tightly than the corresponding linear fusion proteins. Thus, embedding receptor loops within the three-dimensional structure of the MBP yields a better representation of the active conformation of these receptor loops than linear receptor peptides fused onto the C terminus of the MBP. V1ROSS proteins provide a valuable tool to study receptor interactions because they are more amenable to structural analysis than the native membrane receptor. These findings set the stage for the detailed structural analysis of these protein-protein interactions that are important for understanding the mechanism of signaling.

Differential interaction of spin-labeled arrestin with inactive and active phosphorhodopsin

Arrestins regulate signaling and trafficking of G protein-coupled receptors by virtue of their preferential binding to the phosphorylated active form of the receptor. To identify sites in arrestin involved in receptor interaction, a nitroxide-containing side chain was introduced at each of 28 different positions in visual arrestin, and the dynamics of the side chain was used to monitor arrestin interaction with phosphorylated forms of its cognate receptor, rhodopsin. At physiological concentrations, visual arrestin associates with both inactive dark phosphorylated rhodopsin (P-Rh) and light-activated phosphorylated rhodopsin (P-Rh*). Residues distributed over the concave surfaces of the two arrestin domains are involved in weak interactions with both states of phosphorhodopsin, and the flexible C-terminal sequence (C-tail) of arrestin becomes dynamically disordered in both complexes. A large-scale movement of the C-tail is demonstrated by direct distance measurements using a doubly labeled arrestin with one nitroxide in the C-tail and the other in the N-domain. Despite some overlap, the molecular "footprint" of arrestin bound to P-Rh and P-Rh* is different, showing the structure of the complexes to be unique. Strong immobilizing interactions with residues in a highly flexible loop between beta-strands V and VI are only observed in complex with the activated state. This result identifies this loop as a key recognition site in the arrestin-P-Rh* complex and supports the view that flexible sequences are key elements in protein-protein interactions.