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

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.

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.