An Eight Amino Acid Segment Controls Oligomerization and Preferred Conformation of the two Non-visual Arrestins

Insights into the Activation and Self-Association of Arrestin-1

Arrestins halt signal transduction by binding to the phosphorylated C-termini of activated G protein-coupled receptors. Arrestin-1, the first subtype discovered, binds to rhodopsin in rod cells. Mutations in SAG, the gene encoding Arrestin-1, are linked to Oguchi disease, characterized by delayed dark adaptation. Since the discovery of Arrestin-1, substantial progress has been made in understanding the role of these regulatory proteins in phototransduction, including the characterization of visual phenotypes of animals and humans lacking this protein, discovery of splice variants, and documentation of its binding to inositol-polyphosphates. Arrestin-1 was one of the first structurally characterized proteins in the phototransduction cascade. However, there are knowledge gaps regarding the conformational intermediates leading to its binding to phosphorylated rhodopsin. Among various mammalian Arrestin-1 conformations captured via crystallography, the preactivated state is represented by the mutant R175E-Arrestin-1 and by a C-terminally truncated splice variant (p44). This report describes a novel purification method of Arrestin-1 from bovine retinas followed by limited proteolysis to obtain a protein resembling p44. We solved the crystal structure of this preactivated, shortened 3-367Arrestin-1 at a resolution of 1.40 Å. The structure reveals a more complete picture of the finger loop structure and of the role of the polar core in the activation of Arrestin-1. The structure of 3-367Arrestin-1 captures an intermediate form halfway between the inactive and fully activated conformations of Arrestin-1. Finally, we addressed the question of Arrestin-1 oligomerization by comparing the packing interfaces in different Arrestin-1 crystals and dimer models predicted by AlphaFold 3.

Flavors of GPCR signaling bias

GPCRs are inherently flexible molecules existing in an equilibrium of multiple conformations. Binding of GPCR agonists shifts this equilibrium. Certain agonists can increase the fraction of active-like conformations that predispose the receptor to coupling to a particular signal transducer or a select group of transducers. Such agonists are called biased, in contrast to balanced agonists that facilitate signaling via all transducers the receptor couples to. These biased agonists preferentially channel the signaling of a GPCR to particular G proteins, GRKs, or arrestins. Preferential activation of particular G protein or arrestin subtypes can be beneficial, as it would reduce unwanted on-target side effects, widening the therapeutic window. However, biasing GPCRs has two important limitations: a) complete bias is impossible due to inherent flexibility of GPCRs; b) receptor-independent functions of signal transducer proteins cannot be directly affected by GPCR ligands or differential receptor barcoding by GRK phosphorylation. This article is part of the Special Issue on "Ligand Bias".

Mechanisms of amphibian arrestin 1 self-association and dynamic distribution in retinal photoreceptors

Visual arrestin 1 (Arr1) is an essential protein for termination of the light response in photoreceptors. While mammalian Arr1s form dimers and tetramers at physiological concentrations in vitro, oligomerization in other vertebrates has not been studied. Here we examine self-association of Arr1 from two amphibian species, Xenopus laevis (xArr1) and Ambystoma tigrinum (salArr1). Sedimentation velocity analytical ultracentrifugation showed that xArr1 and salArr1 oligomerization is limited to dimers. The KD for dimer formation was 53 μM for xArr1 and 44 μM for salArr1, similar to the 69 μM KD for bovine Arr1 (bArr1) dimers. Mutations of orthologous amino acids important for mammalian Arr1 oligomerization had no impact on xArr1 dimerization. Crystallography showed that the fold of xArr1 closely resembles that of bArr1 and crystal structures in different space groups revealed two potential xArr1 dimer forms: a symmetric dimer with a C-domain interface (CC dimer), resembling the bArr1 solution dimer, and an asymmetric dimer with an N-domain/C-domain interface. Mutagenesis of residues predicted to interact in either of these two dimer forms yielded modest reduction in dimer affinity, suggesting that the dimer interfaces compete or are not unique. Indeed, small-angle X-ray scattering and protein painting data were consistent with a symmetric anti-parallel solution dimer (AP dimer) distinct from the assemblies observed by crystallography. Finally, a computational model evaluating xArr1 binding to compartment-specific partners and partitioning based on heterogeneity of available cytoplasmic spaces shows that Arr1 distribution in dark-adapted photoreceptors is largely explained by the excluded volume effect together with tuning by oligomerization.

Insight into the regulatory mechanism of β-arrestin2 and its emerging role in diseases

β-arrestin2, a member of the arrestin family, mediates the desensitization and internalization of most G protein-coupled receptors (GPCRs) and functions as a scaffold protein in signalling pathways. Previous studies have demonstrated that β-arrestin2 expression is dysregulated in malignant tumours, fibrotic diseases, cardiovascular diseases and metabolic diseases, suggesting its pathological roles. Transcription and post-transcriptional modifications can affect the expression of β-arrestin2. Furthermore, post-translational modifications, such as phosphorylation, ubiquitination, SUMOylation and S-nitrosylation affect the cellular localization of β-arrestin2 and its interaction with downstream signalling molecules, which further regulate the activity of β-arrestin2. This review summarizes the structure and function of β-arrestin2 and reveals the mechanisms involved in the regulation of β-arrestin2 at multiple levels. Additionally, recent studies on the role of β-arrestin2 in some major diseases and its therapeutic prospects have been discussed to provide a reference for the development of drugs targeting β-arrestin2.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Molecular insights into atypical modes of β-arrestin interaction with seven transmembrane receptors

β-arrestins (βarrs) are multifunctional proteins involved in signaling and regulation of seven transmembrane receptors (7TMRs), and their interaction is driven primarily by agonist-induced receptor activation and phosphorylation. Here, we present seven cryo-electron microscopy structures of βarrs either in the basal state, activated by the muscarinic receptor subtype 2 (M2R) through its third intracellular loop, or activated by the βarr-biased decoy D6 receptor (D6R). Combined with biochemical, cellular, and biophysical experiments, these structural snapshots allow the visualization of atypical engagement of βarrs with 7TMRs and also reveal a structural transition in the carboxyl terminus of βarr2 from a β strand to an α helix upon activation by D6R. Our study provides previously unanticipated molecular insights into the structural and functional diversity encoded in 7TMR-βarr complexes with direct implications for exploring novel therapeutic avenues.

Expression of Untagged Arrestins in E. coli and Their Purification

Purified arrestin proteins are necessary for biochemical, biophysical, and structural studies of these versatile regulators of cell signaling. Described herein is a basic protocol for arrestin expression in Escherichia coli and purification of tag-free wild-type and mutant arrestins. The method includes ammonium sulfate precipitation of arrestins from cell lysates, followed by Heparin-Sepharose chromatography. Depending on the arrestin type and/or mutations, the next step is Q-Sepharose or SP-Sepharose chromatography. In many cases, the nonbinding column is used as a filter to bind contaminants without retaining arrestin. In some cases, both chromatographic steps must be performed sequentially to achieve high purity. Purified arrestins can be concentrated up to 10 mg/ml, remain fully functional, and withstand several cycles of freezing and thawing, provided that the overall salt concentration is maintained at or above physiological levels. © 2023 Wiley Periodicals LLC. Basic Protocol: Large-scale expression and purification of arrestins Alternate Protocol: Purification of arrestin-3 and truncated form of arrestin-1-(1-378) Support Protocol: Small-scale test expression of wild-type and mutant arrestins in E. coli.

GPCR targeting of E3 ubiquitin ligase MDM2 by inactive β-arrestin

E3 ubiquitin ligase Mdm2 facilitates β-arrestin ubiquitination, leading to the internalization of G protein-coupled receptors (GPCRs). In this process, β-arrestins bind to Mdm2 and recruit it to the receptor; however, the molecular architecture of the β-arrestin-Mdm2 complex has not been elucidated yet. Here, we identified the β-arrestin-binding region (ABR) on Mdm2 and solved the crystal structure of β-arrestin1 in complex with Mdm2ABR peptide. The acidic residues of Mdm2ABR bind to the positively charged concave side of the β-arrestin1 N-domain. The C-tail of β-arrestin1 is still bound to the N-domain, indicating that Mdm2 binds to the inactive state of β-arrestin1, whereas the phosphorylated C-terminal tail of GPCRs binds to activate β-arrestins. The overlapped binding site of Mdm2 and GPCR C-tails on β-arrestin1 suggests that the binding of GPCR C-tails might trigger the release of Mdm2. Moreover, hydrogen/deuterium exchange experiments further show that Mdm2ABR binding to β-arrestin1 induces the interdomain interface to be more dynamic and uncouples the IP6-induced oligomer of β-arrestin1. These results show how the E3 ligase, Mdm2, interacts with β-arrestins to promote the internalization of GPCRs.

Structural snapshots uncover a key phosphorylation motif in GPCRs driving β-arrestin activation

Agonist-induced GPCR phosphorylation is a key determinant for the binding and activation of β-arrestins (βarrs). However, it is not entirely clear how different GPCRs harboring divergent phosphorylation patterns impart converging active conformation on βarrs leading to broadly conserved functional responses such as desensitization, endocytosis, and signaling. Here, we present multiple cryo-EM structures of activated βarrs in complex with distinct phosphorylation patterns derived from the carboxyl terminus of different GPCRs. These structures help identify a P-X-P-P type phosphorylation motif in GPCRs that interacts with a spatially organized K-K-R-R-K-K sequence in the N-domain of βarrs. Sequence analysis of the human GPCRome reveals the presence of this phosphorylation pattern in a large number of receptors, and its contribution in βarr activation is demonstrated by targeted mutagenesis experiments combined with an intrabody-based conformational sensor. Taken together, our findings provide important structural insights into the ability of distinct GPCRs to activate βarrs through a significantly conserved mechanism.

Mechanisms of Arrestin-Mediated Signaling

Arrestins were first discovered as proteins that selectively bind active phosphorylated GPCRs and suppress (arrest) their G protein-mediated signaling. Nonvisual arrestins are also recognized as signaling proteins regulating a variety of cellular pathways. Arrestins are highly flexible; they can assume many different conformations. In their receptor-bound conformation, arrestins have higher affinity for a subset of binding partners. This explains how receptor activation regulates certain branches of arrestin-dependent signaling via arrestin recruitment to GPCRs. However, free arrestins are also active molecular entities that regulate other signaling pathways and localize signaling proteins to particular subcellular compartments. Recent findings suggest that the two visuals, arrestin-1 and arrestin-4, which are expressed in photoreceptor cells, not only regulate signaling via binding to photopigments but also interact with several nonreceptor partners, critically affecting the health and survival of photoreceptor cells. Detailed in this overview are GPCR-dependent and independent modes of arrestin-mediated regulation of cellular signaling. © 2023 Wiley Periodicals LLC.

Do arrestin oligomers have specific functions?

Arrestins are a small family of versatile regulators of cell signaling. Arrestins regulate signaling and trafficking of G protein-coupled receptors, regulate and direct to particular subcellular compartments numerous protein kinases, ubiquitin ligases, etc. Three out of four arrestin subtypes expressed in vertebrates self-associate, each forming oligomers of a distinct size and shape. While the structures of the solution oligomers of arrestin-1, -2, and -3 have been elucidated, no function specific for the oligomeric form of either of these three subtypes has been identified thus far. Considering how multi-functional average-sized (~45 kDa) arrestin proteins were found to be, it appears likely that certain functions are predominantly or exclusively fulfilled by monomeric and oligomeric forms of each subtype.

Membrane phosphoinositides regulate GPCR-β-arrestin complex assembly and dynamics

Binding of arrestin to phosphorylated G protein-coupled receptors (GPCRs) is crucial for modulating signaling. Once internalized, some GPCRs remain complexed with β-arrestins, while others interact only transiently; this difference affects GPCR signaling and recycling. Cell-based and in vitro biophysical assays reveal the role of membrane phosphoinositides (PIPs) in β-arrestin recruitment and GPCR-β-arrestin complex dynamics. We find that GPCRs broadly stratify into two groups, one that requires PIP binding for β-arrestin recruitment and one that does not. Plasma membrane PIPs potentiate an active conformation of β-arrestin and stabilize GPCR-β-arrestin complexes by promoting a fully engaged state of the complex. As allosteric modulators of GPCR-β-arrestin complex dynamics, membrane PIPs allow for additional conformational diversity beyond that imposed by GPCR phosphorylation alone. For GPCRs that require membrane PIP binding for β-arrestin recruitment, this provides a mechanism for β-arrestin release upon translocation of the GPCR to endosomes, allowing for its rapid recycling.

Solo vs. Chorus: Monomers and Oligomers of Arrestin Proteins

Three out of four subtypes of arrestin proteins expressed in mammals self-associate, each forming oligomers of a distinct kind. Monomers and oligomers have different subcellular localization and distinct biological functions. Here we summarize existing evidence regarding arrestin oligomerization and discuss specific functions of monomeric and oligomeric forms, although too few of the latter are known. The data on arrestins highlight biological importance of oligomerization of signaling proteins. Distinct modes of oligomerization might be an important contributing factor to the functional differences among highly homologous members of the arrestin protein family.

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.

Structural basis of GPCR coupling to distinct signal transducers: implications for biased signaling

Three classes of G-protein-coupled receptor (GPCR) partners - G proteins, GPCR kinases, and arrestins - preferentially bind active GPCRs. Our analysis suggests that the structures of GPCRs bound to these interaction partners available today do not reveal a clear conformational basis for signaling bias, which would have enabled the rational design of biased GRCR ligands. In view of this, three possibilities are conceivable: (i) there are no generalizable GPCR conformations conducive to binding a particular type of partner; (ii) subtle differences in the orientation of individual residues and/or their interactions not easily detectable in the receptor-transducer structures determine partner preference; or (iii) the dynamics of GPCR binding to different types of partners rather than the structures of the final complexes might underlie transducer bias.

Arrestin-dependent internalization of rhodopsin-like G protein-coupled receptors

The internalization of G protein-coupled receptors (GPCRs) is an important mechanism regulating the signal strength and limiting the opportunity of receptor activation. Based on the importance of GPCRs, the detailed knowledge about the regulation of signal transduction is crucial. Here, current knowledge about the agonist-induced, arrestin-dependent internalization process of rhodopsin-like GPCRs is reviewed. Arrestins are conserved molecules that act as key players within the internalization process of many GPCRs. Based on highly conserved structural characteristics within the rhodopsin-like GPCRs, the identification of arrestin interaction sites in model systems can be compared and used for the investigation of internalization processes of other receptors. The increasing understanding of this essential regulation mechanism of receptors can be used for drug development targeting rhodopsin-like GPCRs. Here, we focus on the neuropeptide Y receptor family, as these receptors transmit various physiological processes such as food intake, energy homeostasis, and regulation of emotional behavior, and are further involved in pathophysiological processes like cancer, obesity and mood disorders. Hence, this receptor family represents an interesting target for the development of novel therapeutics requiring the understanding of the regulatory mechanisms influencing receptor mediated signaling.