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

ArreSTick motif controls β-arrestin-binding stability and extends phosphorylation-dependent β-arrestin interactions to non-receptor proteins

The binding and function of β-arrestins are regulated by specific phosphorylation motifs present in G protein-coupled receptors (GPCRs). However, the exact arrangement of phosphorylated amino acids responsible for establishing a stable interaction remains unclear. We employ a 1D sequence convolution model trained on GPCRs with established β-arrestin-binding properties. With this approach, amino acid motifs characteristic of GPCRs that form stable interactions with β-arrestins can be identified, a pattern that we name "arreSTick." Intriguingly, the arreSTick pattern is also present in numerous non-receptor proteins. Using proximity biotinylation assay and mass spectrometry analysis, we demonstrate that the arreSTick motif controls the interaction between many non-receptor proteins and β-arrestin2. The HIV-1 Tat-specific factor 1 (HTSF1 or HTATSF1), a nuclear transcription factor, contains the arreSTick pattern, and its subcellular localization is influenced by β-arrestin2. Our findings unveil a broader role for β-arrestins in phosphorylation-dependent interactions, extending beyond GPCRs to encompass non-receptor proteins as well.

Multiphosphorylation-Dependent Recognition of Anti-pS2 Antibodies against RNA Polymerase II C-Terminal Domain Revealed by Chemical Synthesis

Phosphorylation is a major constituent of the CTD code, which describes the set of post-translational modifications on 52 repeats of a YSPTSPS consensus heptad that orchestrates the binding of regulatory proteins to the C-terminal domain (CTD) of RNA polymerase II. Phospho-specific antibodies are used to detect CTD phosphorylation patterns. However, their recognition repertoire is underexplored due to limitations in the synthesis of long multiphosphorylated peptides. Herein, we describe the development of a synthesis strategy that provides access to multiphosphorylated CTD peptides in high purity without HPLC purification for immobilization onto microtiter plates. Native chemical ligation was used to assemble 12 heptad repeats in various phosphoforms. The synthesis of >60 CTD peptides, 48-90 amino acids in length and containing up to 6 phosphosites, enabled a detailed and rapid analysis of the binding characteristics of different anti-pSer2 antibodies. The three antibodies tested showed positional selectivity with marked differences in the affinity of the antibodies for pSer2-containing peptides. Furthermore, the length of the phosphopeptides allowed a systematic analysis of the multivalent chelate-type interactions. The absence of multivalency-induced binding enhancements is probably due to the high flexibility of the CTD scaffold. The effect of clustered phosphorylation proved to be more complex. Recognition of pSer2 by anti-pSer2-antibodies can be prevented and, perhaps surprisingly, enhanced by the phosphorylation of "bystander" amino acids in the vicinity. The results have relevance for functional analysis of the CTD in cell biological experiments.

The role of G protein-coupled receptor kinases in GLP-1R β-arrestin recruitment and internalisation

The glucagon-like peptide 1 receptor (GLP-1R) is a validated clinical target for the treatment of type 2 diabetes and obesity. Unlike most G protein-coupled receptors (GPCRs), the GLP-1R undergoes an atypical mode of internalisation that does not require β-arrestins. While differences in GLP-1R trafficking and β-arrestin recruitment have been observed between clinically used GLP-1R agonists, the role of G protein-coupled receptor kinases (GRKs) in affecting these pathways has not been comprehensively assessed. In this study, we quantified the contribution of GRKs to agonist-mediated GLP-1R internalisation and β-arrestin recruitment profiles using cells where endogenous β-arrestins, or non-visual GRKs were knocked out using CRISPR/Cas9 genome editing. Our results confirm the previously established atypical β-arrestin-independent mode of GLP-1R internalisation and revealed that GLP-1R internalisation is dependent on the expression of GRKs. Interestingly, agonist-mediated GLP-1R β-arrestin 1 and β-arrestin 2 recruitment were differentially affected by endogenous GRK knockout with β-arrestin 1 recruitment more sensitive to GRK knockout than β-arrestin 2 recruitment. Moreover, individual overexpression of GRK2, GRK3, GRK5 or GRK6 in a newly generated GRK2/3/4/5/6 HEK293 cells, rescued agonist-mediated β-arrestin 1 recruitment and internalisation profiles to similar levels, suggesting that there is no specific GRK isoform that drives these pathways. This study advances mechanistic understanding of agonist-mediated GLP-1R internalisation and provides novel insights into how GRKs may fine-tune GLP-1R signalling.

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.

Ready for the sheet: β-strand folding of phosphorylation clusters guides GPCR binding to arrestin

The molecular dynamics of arrestin binding to G protein-coupled receptors (GPCRs) are still poorly understood. In this issue of Structure, Guillien et al. show that negative charges in GPCR key phosphorylation clusters induce the formation of a transient β-strand that participates in an intermolecular β-sheet in the associated complex.

Phosphorylation motif dictates GPCR C-terminal domain conformation and arrestin interaction

Arrestin-dependent G protein-coupled receptor (GPCR) signaling pathway is regulated by the phosphorylation state of GPCR's C-terminal domain, but the molecular bases of arrestin:receptor interaction are to be further illuminated. Here we investigated the impact of phosphorylation on the conformational features of the C-terminal region from three rhodopsin-like GPCRs, the vasopressin V2 receptor (V2R), the growth hormone secretagogue or ghrelin receptor type 1a (GHSR), and the β2-adernergic receptor (β2AR). Using phosphomimetic variants, we identified pre-formed secondary structure elements, or short linear motifs (SLiMs), that undergo specific conformational transitions upon phosphorylation. Of importance, such conformational transitions appear to favor arrestin-2 binding. Hence, our results suggest a model in which the phosphorylation-dependent structuration of the GPCR C-terminal regions would modulate arrestin binding and therefore signaling outcomes in arrestin-dependent pathways.

Realizing integration in structural biology: The 2022 ISBUC Annual Meeting

This meeting report presents the 2022 Annual Meeting of the cluster for Integrative Structural Biology at the University of Copenhagen (ISBUC) and discusses the cluster approach to interdisciplinary research management. This approach successfully facilitates cross-faculty and inter-departmental collaboration. Innovative integrative research collaborations ignited by ISBUC, as well as research presented at the meeting, are showcased.

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.

A key GPCR phosphorylation motif discovered in arrestin2⋅CCR5 phosphopeptide complexes

The two non-visual arrestins, arrestin2 and arrestin3, bind hundreds of GPCRs with different phosphorylation patterns, leading to distinct functional outcomes. Structural information on these interactions is available only for very few GPCRs. Here, we have characterized the interactions between the phosphorylated human CC chemokine receptor 5 (CCR5) and arrestin2. We identified several new CCR5 phosphorylation sites necessary for stable arrestin2 complex formation. Structures of arrestin2 in the apo form and complexes with CCR5 C-terminal phosphopeptides, together with NMR, biochemical, and functional assays, revealed three phosphoresidues in a pXpp motif that are essential for arrestin2 binding and activation. The identified motif appears responsible for robust arrestin2 recruitment in many other GPCRs. An analysis of receptor sequences and available structural and functional information provides hints on the molecular basis of arrestin2/arrestin3 isoform specificity. Our findings demonstrate how multi-site phosphorylation controls GPCR⋅arrestin interactions and provide a framework to probe the intricate details of arrestin signaling.

Phosphorylation barcodes direct biased chemokine signaling at CXCR3

G protein-coupled receptor (GPCR)-biased agonism, selective activation of certain signaling pathways relative to others, is thought to be directed by differential GPCR phosphorylation "barcodes." At chemokine receptors, endogenous chemokines can act as "biased agonists", which may contribute to the limited success when pharmacologically targeting these receptors. Here, mass spectrometry-based global phosphoproteomics revealed that CXCR3 chemokines generate different phosphorylation barcodes associated with differential transducer activation. Chemokine stimulation resulted in distinct changes throughout the kinome in global phosphoproteomics studies. Mutation of CXCR3 phosphosites altered β-arrestin 2 conformation in cellular assays and was consistent with conformational changes observed in molecular dynamics simulations. T cells expressing phosphorylation-deficient CXCR3 mutants resulted in agonist- and receptor-specific chemotactic profiles. Our results demonstrate that CXCR3 chemokines are non-redundant and act as biased agonists through differential encoding of phosphorylation barcodes, leading to distinct physiological processes.

Phosphorylation barcodes direct biased chemokine signaling at CXCR3

G protein-coupled receptor (GPCR) biased agonism, the activation of some signaling pathways over others, is thought to largely be due to differential receptor phosphorylation, or "phosphorylation barcodes." At chemokine receptors, ligands act as "biased agonists" with complex signaling profiles, which contributes to the limited success in pharmacologically targeting these receptors. Here, mass spectrometry-based global phosphoproteomics revealed that CXCR3 chemokines generate different phosphorylation barcodes associated with differential transducer activation. Chemokine stimulation resulted in distinct changes throughout the kinome in global phosphoproteomic studies. Mutation of CXCR3 phosphosites altered β-arrestin conformation in cellular assays and was confirmed by molecular dynamics simulations. T cells expressing phosphorylation-deficient CXCR3 mutants resulted in agonist- and receptor-specific chemotactic profiles. Our results demonstrate that CXCR3 chemokines are non-redundant and act as biased agonists through differential encoding of phosphorylation barcodes and lead to distinct physiological processes.

Structural details of a Class B GPCR-arrestin complex revealed by genetically encoded crosslinkers in living cells

Understanding the molecular basis of arrestin-mediated regulation of GPCRs is critical for deciphering signaling mechanisms and designing functional selectivity. However, structural studies of GPCR-arrestin complexes are hampered by their highly dynamic nature. Here, we dissect the interaction of arrestin-2 (arr2) with the secretin-like parathyroid hormone 1 receptor PTH1R using genetically encoded crosslinking amino acids in live cells. We identify 136 intermolecular proximity points that guide the construction of energy-optimized molecular models for the PTH1R-arr2 complex. Our data reveal flexible receptor elements missing in existing structures, including intracellular loop 3 and the proximal C-tail, and suggest a functional role of a hitherto overlooked positively charged region at the arrestin N-edge. Unbiased MD simulations highlight the stability and dynamic nature of the complex. Our integrative approach yields structural insights into protein-protein complexes in a biologically relevant live-cell environment and provides information inaccessible to classical structural methods, while also revealing the dynamics of the system.

Cannabinoid 1 (CB1 ) receptor arrestin subtype-selectivity and phosphorylation dependence

BACKGROUND AND PURPOSE

Arrestin or G protein bias may be desirable for novel cannabinoid therapeutics. Arrestin-2 and arrestin-3 translocation to CB₁ receptor have been suggested to mediate different functions that may be exploited with biased ligands. Here, the requirement of a recently described phosphorylation motif 'pxxp' (where 'p' denotes phosphorylatable serine or threonine and 'x' denotes any other amino acid) within the CB₁ receptor C-terminus for interaction with different arrestin subtypes was examined.

EXPERIMENTAL APPROACH

Site-directed mutagenesis was conducted to generate nine different phosphorylation-impaired CB₁ receptor C-terminal mutants. Bioluminescence resonance energy transfer (BRET) was employed to measure arrestin-2/3 translocation and G protein dissociation of a high efficacy agonist for each mutant. Immunocytochemistry was used to quantify receptor expression.

KEY RESULTS

The effects of each mutation were shared for arrestin-2 and arrestin-3 translocation to CB₁ receptor pxxp motifs are partially required for arrestin-2/3 translocation, but translocation was not completely inhibited until all phosphorylation sites were mutated. The rate of arrestin translocation was reduced with simultaneous mutation of S425 and S429. Desensitisation of G protein dissociation was inhibited in different mutants proportional to the extent of their respective loss of arrestin translocation.

CONCLUSIONS AND IMPLICATIONS

These data do not support the existence of an 'essential' pxxp motif for arrestin translocation to CB₁ receptor. These data also identify that arrestin-2 and arrestin-3 have equivalent phosphorylation requirements within the CB₁ receptor C-terminus, suggesting arrestin subtype-selective biased ligands may not be viable and that different regions of the C-terminus contribute differently to arrestin translocation.

The Role of Reversible Phosphorylation of Drosophila Rhodopsin

Vertebrate and fly rhodopsins are prototypical GPCRs that have served for a long time as model systems for understanding GPCR signaling. Although all rhodopsins seem to become phosphorylated at their C-terminal region following activation by light, the role of this phosphorylation is not uniform. Two major functions of rhodopsin phosphorylation have been described: (1) inactivation of the activated rhodopsin either directly or by facilitating binding of arrestins in order to shut down the visual signaling cascade and thus eventually enabling a high-temporal resolution of the visual system. (2) Facilitating endocytosis of activated receptors via arrestin binding that in turn recruits clathrin to the membrane for clathrin-mediated endocytosis. In vertebrate rhodopsins the shutdown of the signaling cascade may be the main function of rhodopsin phosphorylation, as phosphorylation alone already quenches transducin activation and, in addition, strongly enhances arrestin binding. In the Drosophila visual system rhodopsin phosphorylation is not needed for receptor inactivation. Its role here may rather lie in the recruitment of arrestin 1 and subsequent endocytosis of the activated receptor. In this review, we summarize investigations of fly rhodopsin phosphorylation spanning four decades and contextualize them with regard to the most recent insights from vertebrate phosphorylation barcode theory.

How Carvedilol activates β2-adrenoceptors

Carvedilol is among the most effective β-blockers for improving survival after myocardial infarction. Yet the mechanisms by which carvedilol achieves this superior clinical profile are still unclear. Beyond blockade of β1-adrenoceptors, arrestin-biased signalling via β2-adrenoceptors is a molecular mechanism proposed to explain the survival benefits. Here, we offer an alternative mechanism to rationalize carvedilol's cellular signalling. Using primary and immortalized cells genome-edited by CRISPR/Cas9 to lack either G proteins or arrestins; and combining biological, biochemical, and signalling assays with molecular dynamics simulations, we demonstrate that G proteins drive all detectable carvedilol signalling through β2ARs. Because a clear understanding of how drugs act is imperative to data interpretation in basic and clinical research, to the stratification of clinical trials or to the monitoring of drug effects on the target pathway, the mechanistic insight gained here provides a foundation for the rational development of signalling prototypes that target the β-adrenoceptor system.

Molecular Mechanisms of Desensitization Underlying the Differential Effects of Formyl Peptide Receptor 2 Agonists on Cardiac Structure-Function Post Myocardial Infarction

Formyl peptide receptor 2 (FPR2) plays an integral role in the transition of macrophages from a pro-inflammatory program to one that is pro-resolving. FPR2-mediated stimulation of resolution post myocardial infarction has demonstrated efficacy in rodent models and is hypothesized to reduce progression into heart failure. FPR2 agonists that promote long-lasting receptor internalization can lead to persistent desensitization and diminished therapeutic benefits. In vitro signaling profiles and propensities for receptor desensitization of two clinically studied FPR2 agonists, namely, BMS-986235 and ACT-389949, were evaluated. In contrast to BMS-986235, pre-stimulation with ACT-389949 led to a decrease in its potency to inhibit cAMP production. Moreover, ACT-389949 displayed greater efficacy for β-arrestin recruitment, while efficacy of Gi activation was similar for both agonists. Following agonist-promoted FPR2 internalization, effective recycling to the plasma membrane was observed only with BMS-986235. Use of G protein-coupled receptor kinase (GRK) knock-out cells revealed a differential impact of GRK2 versus GRK5/6 on β-arrestin recruitment and Gi activation promoted by the two FPR2 agonists. In vivo, decreases of granulocytes in circulation were greatly diminished in mice treated with ACT-389949 but not for BMS-986235. With short-term dosing, both compounds induced a pro-resolution polarization state in cardiac monocyte/macrophages post myocardial infarction. By contrast, with long-term dosing, only BMS-986235 preserved the infarct wall thickness and increased left ventricular ejection fraction in a rat model of myocardial infarction. Altogether, the study shows that differences in the desensitization profiles induced by ACT-389949 and BMS-986235 at the molecular level may explain their distinct inflammatory/pro-resolving activities in vivo.

Signaling snapshots of a serotonin receptor activated by the prototypical psychedelic LSD

Serotonin (5-hydroxytryptamine [5-HT]) 5-HT2-family receptors represent essential targets for lysergic acid diethylamide (LSD) and all other psychedelic drugs. Although the primary psychedelic drug effects are mediated by the 5-HT2A serotonin receptor (HTR2A), the 5-HT2B serotonin receptor (HTR2B) has been used as a model receptor to study the activation mechanisms of psychedelic drugs due to its high expression and similarity to HTR2A. In this study, we determined the cryo-EM structures of LSD-bound HTR2B in the transducer-free, Gq-protein-coupled, and β-arrestin-1-coupled states. These structures provide distinct signaling snapshots of LSD's action, ranging from the transducer-free, partially active state to the transducer-coupled, fully active states. Insights from this study will both provide comprehensive molecular insights into the signaling mechanisms of the prototypical psychedelic LSD and accelerate the discovery of novel psychedelic drugs.

G protein-biased GPR3 signaling ameliorates amyloid pathology in a preclinical Alzheimer's disease mouse model

Biased G protein-coupled receptor (GPCR) ligands, which preferentially activate G protein or β-arrestin signaling pathways, are leading to the development of drugs with superior efficacy and reduced side effects in heart disease, pain management, and neuropsychiatric disorders. Although GPCRs are implicated in the pathophysiology of Alzheimer's disease (AD), biased GPCR signaling is a largely unexplored area of investigation in AD. Our previous work demonstrated that GPR3-mediated β-arrestin signaling modulates amyloid-β (Aβ) generation in vitro and that Gpr3 deficiency ameliorates Aβ pathology in vivo. However, Gpr3-deficient mice display several adverse phenotypes, including elevated anxiety-like behavior, reduced fertility, and memory impairment, which are potentially associated with impaired G protein signaling. Here, we generated a G protein-biased GPR3 mouse model to investigate the physiological and pathophysiological consequences of selective elimination of GPR3-mediated β-arrestin signaling in vivo. In contrast to Gpr3-deficient mice, G protein-biased GPR3 mice do not display elevated anxiety levels, reduced fertility, or cognitive impairment. We further determined that G protein-biased signaling reduces soluble Aβ levels and leads to a decrease in the area and compaction of amyloid plaques in the preclinical AppNL-G-F AD mouse model. The changes in amyloid pathology are accompanied by robust microglial and astrocytic hypertrophy, which suggest a protective glial response that may limit amyloid plaque development in G protein-biased GPR3 AD mice. Collectively, these studies indicate that GPR3-mediated G protein and β-arrestin signaling produce discrete and separable effects and provide proof of concept for the development of safer GPCR-targeting therapeutics with more directed pharmacological action for AD.

β-arrestin1 and 2 exhibit distinct phosphorylation-dependent conformations when coupling to the same GPCR in living cells

β-arrestins mediate regulatory processes for over 800 different G protein-coupled receptors (GPCRs) by adopting specific conformations that result from the geometry of the GPCR–β-arrestin complex. However, whether β-arrestin1 and 2 respond differently for binding to the same GPCR is still unknown. Employing GRK knockout cells and β-arrestins lacking the finger-loop-region, we show that the two isoforms prefer to associate with the active parathyroid hormone 1 receptor (PTH1R) in different complex configurations (“hanging” and “core”). Furthermore, the utilisation of advanced NanoLuc/FlAsH-based biosensors reveals distinct conformational signatures of β-arrestin1 and 2 when bound to active PTH1R (P-R*). Moreover, we assess β-arrestin conformational changes that are induced specifically by proximal and distal C-terminal phosphorylation and in the absence of GPCR kinases (GRKs) (R*). Here, we show differences between conformational changes that are induced by P-R* or R* receptor states and further disclose the impact of site-specific GPCR phosphorylation on arrestin-coupling and function.

Here the authors present improved intramolecular sensors for β-arrestin2 and 1, which enable assessment of conformational changes of both isoforms in living cells. These reveal that the same GPCR induces differential conformational rearrangements that determine the functional diversity between the two β-arrestins.

Accelerated Multiphosphorylated Peptide Synthesis

Preparing phosphorylated peptides with multiple adjacent phosphorylations is synthetically difficult, leads to β-elimination, results in low yields, and is extremely slow. We combined synthetic chemical methodologies with computational studies and engineering approaches to develop a strategy that takes advantage of fast stirring, high temperature, and a very low concentration of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to produce multiphosphorylated peptides at an extremely rapid time and high purity.

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.

Reciprocal Regulation of Mitofusin 2-Mediated Mitophagy and Mitochondrial Fusion by Different PINK1 Phosphorylation Events

Mitochondrial repair is essential to metabolic homeostasis. Outer mitochondrial membrane mitofusin (MFN) proteins orchestrate mitochondrial fusion that opposes mitochondrial degeneration caused by senescence. Depending upon physiological context, MFN2 can either mediate mitochondrial fusion or recruit cytosolic Parkin to initiate mitophagic elimination. Because it is not clear how these events are counter-regulated we engineered and expressed MFN2 mutants that mimic phosphorylated or non-phosphorylatable MFN2 at its PINK1 phosphorylation sites: T111, S378, and S442. By interrogating mitochondrial fusion, polarization status, and Parkin binding/mitophagy as a function of inferred MFN2 phosphorylation, we discovered that individual MFN2 phosphorylation events act as a biological “bar-code”, directing mitochondrial fate based on phosphorylation site state. Experiments in Pink1 deficient cells supported a central role for PINK1 kinase as the pivotal regulator of MFN2 functionality. Contrary to popular wisdom that Parkin-mediated ubiquitination regulates MFN-mediated mitochondrial fusion, results in Prkn null cells demonstrated the dispensability of Parkin for MFN2 inactivation. These data demonstrate that PINK1-mediated phosphorylation is necessary and sufficient, and that Parkin is expendable, to switch MFN2 from fusion protein to mitophagy effector.

Structural Insights into the Intrinsically Disordered GPCR C-Terminal Region, Major Actor in Arrestin-GPCR Interaction

Arrestin-dependent pathways are a central component of G protein-coupled receptor (GPCRs) signaling. However, the molecular processes regulating arrestin binding are to be further illuminated, in particular with regard to the structural impact of GPCR C-terminal disordered regions. Here, we used an integrated biophysical strategy to describe the basal conformations of the C-terminal domains of three class A GPCRs, the vasopressin V2 receptor (V2R), the growth hormone secretagogue or ghrelin receptor type 1a (GHSR) and the β2-adernergic receptor (β2AR). By doing so, we revealed the presence of transient secondary structures in these regions that are potentially involved in the interaction with arrestin. These secondary structure elements differ from those described in the literature in interaction with arrestin. This suggests a mechanism where the secondary structure conformational preferences in the C-terminal regions of GPCRs could be a central feature for optimizing arrestins recognition.

Residency time of agonists does not affect the stability of GPCR-arrestin complexes

BACKGROUND AND PURPOSE

The interaction of arrestins with G-protein-coupled receptors (GPCRs) desensitizes agonist-dependent receptor responses and often leads to receptor internalization. GPCRs that internalize without arrestin have been classified as "class A" GPCRs whereas "class B" GPCRs co-internalize with arrestin into endosomes. The interaction of arrestins with GPCRs requires both agonist activation and receptor phosphorylation. Here we ask the question whether agonists with very slow off-rate can cause the formation of particularly stable receptor-arrestin complexes.

EXPERIMENTAL APPROACH

The stability of GPCR-arrestin-3 complexes at two class A GPCRs, the β₂ -adrenergic receptor and the μ-opioid receptor, was assessed using two different techniques, fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching (FRAP) employing several ligands with very different off-rates. Arrestin trafficking was determined by confocal microscopy.

KEY RESULTS

Upon agonist washout, GPCR-arrestin-3 complexes showed markedly different dissociation rates in single-cell FRET experiments. In FRAP experiments, however, all full agonists led to the formation of receptor-arrestin complexes of identical stability whereas the complex between the μ-opioid receptor and arrestin-3 induced by the partial agonist morphine was less stable. Agonists with very slow off-rates could not mediate the co-internalization of arrestin-3 with class A GPCRs into endosomes.

CONCLUSIONS AND IMPLICATIONS

Agonist off-rates do not affect the stability of GPCR-arrestin complexes but phosphorylation patterns do. Our results imply that orthosteric agonists will not be able to pharmacologically convert class A into class B GPCRs.

GPCR kinase knockout cells reveal the impact of individual GRKs on arrestin binding and GPCR regulation

G protein-coupled receptors (GPCRs) activate G proteins and undergo a complex regulation by interaction with GPCR kinases (GRKs) and the formation of receptor-arrestin complexes. However, the impact of individual GRKs on arrestin binding is not clear. We report the creation of eleven combinatorial HEK293 knockout cell clones lacking GRK2/3/5/6, including single, double, triple and the quadruple GRK knockout. Analysis of β-arrestin1/2 interactions for twelve GPCRs in our GRK knockout cells enables the differentiation of two main receptor subsets: GRK2/3-regulated and GRK2/3/5/6-regulated receptors. Furthermore, we identify GPCRs that interact with β-arrestins via the overexpression of specific GRKs even in the absence of agonists. Finally, using GRK knockout cells, PKC inhibitors and β-arrestin mutants, we present evidence for differential receptor-β-arrestin1/2 complex configurations mediated by selective engagement of kinases. We anticipate our GRK knockout platform to facilitate the elucidation of previously unappreciated details of GRK-specific GPCR regulation and β-arrestin complex formation.

Scaffolding of Mitogen-Activated Protein Kinase Signaling by β-Arrestins

β-arrestins were initially identified to desensitize and internalize G-protein-coupled receptors (GPCRs). Receptor-bound β-arrestins also initiate a second wave of signaling by scaffolding mitogen-activated protein kinase (MAPK) signaling components, MAPK kinase kinase, MAPK kinase, and MAPK. In particular, β-arrestins facilitate ERK1/2 or JNK3 activation by scaffolding signal cascade components such as ERK1/2-MEK1-cRaf or JNK3-MKK4/7-ASK1. Understanding the precise molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly would deepen our understanding of GPCR-mediated MAPK activation and provide clues for the selective regulation of the MAPK signaling cascade for therapeutic purposes. Over the last decade, numerous research groups have attempted to understand the molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly. Although not providing the complete mechanism, these efforts suggest potential binding interfaces between β-arrestins and MAPK signaling components and the mechanism for MAPK signal amplification by β-arrestin-mediated scaffolding. This review summarizes recent developments of cellular and molecular works on the scaffolding mechanism of β-arrestin for MAPK signaling cascade.

Biphasic activation of β-arrestin 1 upon interaction with a GPCR revealed by methyl-TROSY NMR

β-arrestins (βarrs) play multifaceted roles in the function of G protein-coupled receptors (GPCRs). βarrs typically interact with phosphorylated C-terminal tail (C tail) and transmembrane core (TM core) of GPCRs. However, the effects of the C tail- and TM core-mediated interactions on the conformational activation of βarrs have remained elusive. Here, we show the conformational changes for βarr activation upon the C tail- and TM core-mediated interactions with a prototypical GPCR by nuclear magnetic resonance (NMR) spectroscopy. Our NMR analyses demonstrated that while the C tail-mediated interaction alone induces partial activation, in which βarr exists in equilibrium between basal and activated conformations, the TM core- and the C tail-mediated interactions together completely shift the equilibrium toward the activated conformation. The conformation-selective antibody, Fab30, promotes partially activated βarr into the activated-like conformation. This plasticity of βarr conformation in complex with GPCRs engaged in different binding modes may explain the multifunctionality of βarrs.

β-arrestins commonly bind to two distinct elements in GPCRs: the phosphorylated carboxyl terminal tail (C tail) and the cytoplasmic face of the transmembrane region (TM core). Here, the authors use methyl-TROSY NMR measurements to characterise the interactions between β-arrestin 1 (βarr1) and a GPCR and observe that C tail-mediated interaction with a GPCR alone induces the partial activation of βarr1, whereas the TM core- and C tail-mediated interactions together stabilize the activated conformation of βarr1.

Two-point immobilization of a conformation-specific beta2-adrenoceptor for recognizing the receptor agonists or antagonists inspired by binding-induced DNA assembly

Immobilized protein has advanced in many areas like drug discovery. While this field evolved rapidly over the last three decades, the immobilization platform for the G-protein-coupled receptor (GPCR) remains unpromising due to its instability under the relatively harsh conditions of current methodologies. Taking beta₂-adrenoceptor (β₂-AR) as an example, we presented here a general strategy for immobilization of GPCRs by combining the His₆-tag trap system, conformation-specific aptamer, and target binding induced DNA hybridization. Morphology characterization by diverse assays confirmed a monolayer of β₂-AR on the microsphere surface. The radio-ligand binding assay and immuno-transmission electron microscopy showed desirable ligand- and antibody-binding activities. A case study of chromatography using the immobilized receptor as a stationary phase exhibited a demonstrable conformation specificity that enables the selective recognition of the receptor agonists or antagonists. Owing to the competitive strand displacement during the immobilization, the method proved to be capable of sensitively and directly determining the receptor density on the surface which enormously challenges most of the reported assays. This method is possible to turn into a general strategy for the immobilization of GPCRs with a defined orientation, conformation, function, and density, thus paving the way for precisely realizing the receptor-ligand binding interaction and screening the receptor agonist or antagonist with high efficiency.

Delineating the interactions between the cannabinoid CB2 receptor and its regulatory effectors; β-arrestins and G protein-coupled receptor kinases

BACKGROUND AND PURPOSE

The cannabinoid CB₂ receptor (CB₂ ) is a promising therapeutic target for modulating inflammation. However, little is known surrounding the mechanisms underpinning CB₂ desensitisation and regulation, particularly the role of G protein-coupled receptor kinases (GRKs). Here, we evaluated the role of six GRK isoforms in β-arrestin recruitment to CB₂ . Mutagenesis of several distal C-terminal aspartic acid residues was also performed in an attempt to delineate additional structural elements involved in the regulation of CB₂ .

EXPERIMENTAL APPROACH

In CB₂ -expressing HEK 293 cells, β-arrestin translocation was measured using real-time BRET assays. G protein dissociation BRET assays were performed to assess the activation and desensitisation of CB₂ in the presence of β-arrestin 2.

KEY RESULTS

Overexpression of GRK isoforms 1-6 failed to considerably improve translocation of either β-arrestin 1 or β-arrestin 2 to CB₂ . Consistent with this, inhibition of endogenous GRK2/3 did not substantially reduce β-arrestin 2 translocation. Mutagenesis of C-terminal aspartic acid residues resulted in attenuation of β-arrestin 2 translocation, which translated to a reduction in desensitisation of G protein activation.

CONCLUSION AND IMPLICATIONS

Our findings suggest that CB₂ does not adhere to the classical GPCR regulatory paradigm, entailing GRK- and β-arrestin-mediated desensitisation. Instead, C-terminal aspartic acid residues may act as phospho-mimics to induce β-arrestin activation. This study provides novel insights into the regulatory mechanisms of CB₂ , which may aid in our understanding of drug tolerance and dependence.

Structural Basis of Arrestin Selectivity for Active Phosphorylated G Protein-Coupled Receptors

Arrestins are a small family of proteins that bind G protein-coupled receptors (GPCRs). Arrestin binds to active phosphorylated GPCRs with higher affinity than to all other functional forms of the receptor, including inactive phosphorylated and active unphosphorylated. The selectivity of arrestins suggests that they must have two sensors, which detect receptor-attached phosphates and the active receptor conformation independently. Simultaneous engagement of both sensors enables arrestin transition into a high-affinity receptor-binding state. This transition involves a global conformational rearrangement that brings additional elements of the arrestin molecule, including the middle loop, in contact with a GPCR, thereby stabilizing the complex. Here, we review structural and mutagenesis data that identify these two sensors and additional receptor-binding elements within the arrestin molecule. While most data were obtained with the arrestin-1-rhodopsin pair, the evidence suggests that all arrestins use similar mechanisms to achieve preferential binding to active phosphorylated GPCRs.

Chemical Synthesis of a Full-Length G-Protein-Coupled Receptor β2-Adrenergic Receptor with Defined Modification Patterns at the C-Terminus

The β₂-adrenergic receptor (β₂AR) is a G-protein-coupled receptor (GPCR) that responds to the hormone adrenaline and is an important drug target in the context of respiratory diseases, including asthma. β₂AR function can be regulated by post-translational modifications such as phosphorylation and ubiquitination at the C-terminus, but access to the full-length β₂AR with well-defined and homogeneous modification patterns critical for biochemical and biophysical studies remains challenging. Here, we report a practical synthesis of differentially modified, full-length β₂AR based on a combined native chemical ligation (NCL) and sortase ligation strategy. An array of homogeneous samples of full-length β₂ARs with distinct modification patterns, including a full-length β₂AR bearing both monoubiquitination and octaphosphorylation modifications, were successfully prepared for the first time. Using these homogeneously modified full-length β₂AR receptors, we found that different phosphorylation patterns mediate different interactions with β-arrestin1 as reflected in different agonist binding affinities. Our experiments also indicated that ubiquitination can further modulate interactions between β₂AR and β-arrestin1. Access to full-length β₂AR with well-defined and homogeneous modification patterns at the C-terminus opens a door to further in-depth mechanistic studies into the structure and dynamics of β₂AR complexes with downstream transducer proteins, including G proteins, arrestins, and GPCR kinases.

Spatial bias in cAMP generation determines biological responses to PTH type 1 receptor activation

[Figure: see text].

Assays for detecting arrestin interaction with GPCRs

The four vertebrate arrestins play a key role in the desensitization and internalization of G protein-coupled receptors (GPCRs) and also mediate receptor-dependent signaling. Recent work has shown that bias for arrestin vs G protein signaling could offer certain therapeutic advantages (or disadvantages) in different systems, making assays that measure arrestin binding to receptors important for drug discovery efforts. Herein, we briefly review several commonly used techniques for measuring arrestin binding to receptors, as well as provide an in-depth and methodologically focused review of two methods that do not require receptor modification. The first approach measures direct binding between purified arrestin and rhodopsin, and the second measures the recruitment of arrestin to receptors in living cells.

The LPA3 Receptor: Regulation and Activation of Signaling Pathways

The lysophosphatidic acid 3 receptor (LPA₃) participates in different physiological actions and in the pathogenesis of many diseases through the activation of different signal pathways. Knowledge of the regulation of the function of the LPA₃ receptor is a crucial element for defining its roles in health and disease. This review describes what is known about the signaling pathways activated in terms of its various actions. Next, we review knowledge on the structure of the LPA₃ receptor, the domains found, and the roles that the latter might play in ligand recognition, signaling, and cellular localization. Currently, there is some information on the action of LPA₃ in different cells and whole organisms, but very little is known about the regulation of its function. Areas in which there is a gap in our knowledge are indicated in order to further stimulate experimental work on this receptor and on other members of the LPA receptor family. We are convinced that knowledge on how this receptor is activated, the signaling pathways employed and how the receptor internalization and desensitization are controlled will help design new therapeutic interventions for treating diseases in which the LPA₃ receptor is implicated.

Differential Regulation of GPCRs-Are GRK Expression Levels the Key?

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors and their signal transduction is tightly regulated by GPCR kinases (GRKs) and β-arrestins. In this review, we discuss novel aspects of the regulatory GRK/β-arrestin system. Therefore, we briefly revise the origin of the "barcode" hypothesis for GPCR/β-arrestin interactions, which states that β-arrestins recognize different receptor phosphorylation states to induce specific functions. We emphasize two important parameters which may influence resulting GPCR phosphorylation patterns: (A) direct GPCR-GRK interactions and (B) tissue-specific expression and availability of GRKs and β-arrestins. In most studies that focus on the molecular mechanisms of GPCR regulation, these expression profiles are underappreciated. Hence we analyzed expression data for GRKs and β-arrestins in 61 tissues annotated in the Human Protein Atlas. We present our analysis in the context of pathophysiological dysregulation of the GPCR/GRK/β-arrestin system. This tissue-specific point of view might be the key to unraveling the individual impact of different GRK isoforms on GPCR regulation.

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

FGFR/Heartless and Smog interact synergistically to negatively regulate Fog mediated G-protein coupled receptor signaling in the Drosophila nervous system

Folded gastrulation (Fog) is a secreted ligand that signals through the G-protein-coupled receptors Mist and Smog and the G-protein Concertina to activate downstream effectors to elicit cell-shape change during gastrulation. In the embryonic central nervous system (CNS), Fog has roles in axon guidance and glial morphogenesis. However, the elements of the pathway as well as mechanisms required for transducing the signal in this context have not been determined. We find that while Concertina is essential for Fog signaling, Mist is dispensable and Smog, surprisingly, functions as a negative regulator of the pathway in the CNS. Interestingly Heartless, a fibroblast growth factor receptor, also functions as a negative regulator. Furthermore, both Heartless and Smog interact in a synergistic manner to regulate Fog signaling. Our results thus identify Heartless and Smog as part of a common regulatory pathway that functions to restrict Fog signaling in the embryonic CNS and highlights the context-specific role for Fog receptors during development.

Targeting G protein-coupled receptor kinases (GRKs) to G protein-coupled receptors

G protein-coupled receptors (GPCRs) interact with three protein families following agonist binding: heterotrimeric G proteins, G protein-coupled receptor kinases (GRKs) and arrestins. GRK-mediated phosphorylation of GPCRs promotes arrestin binding to uncouple the receptor from G protein, a process called desensitization, and for many GPCRs, arrestin binding also promotes receptor endocytosis and intracellular signaling. Thus, GRKs play a central role in modulating GPCR signaling and localization. Here we review recent advances in this field which include additional insight into how GRKs target GPCRs and bias signaling, and the development of specific inhibitors to dissect GRK function in model systems.

Ginsenoside compound K- a potential drug for rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory and autoimmune disease, if prescription of effective delayed, the articular disturbances may lead to disability. Ginsenoside compound K (GCK) is the main degradation product of oral ginsenosides in the human intestine. Numerous researches in vitro and in vivo have recorded the anti-arthritic effect of GCK, we discuss the mechanisms from the following three aspects, including anti-inflammatory, immune-regulatory, and bone-protective, respectively, in this review, and the anti-arthritic mechanism of GCK may be related to the effect on TNF-α-TNFR2, glucocorticoid receptor (GR) and β-arrestin1/2. We also describe the anti-anemia effect of GCK to open the possibility that GCK can be used as an effective disease-modifying anti-rheumatic drug (DMARD).

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.

The α-arrestin family of ubiquitin ligase adaptors links metabolism with selective endocytosis

The regulation of nutrient uptake into cells is important, as it allows to either increase biomass for cell growth or to preserve homoeostasis. A key strategy to adjust cellular nutrient uptake is the reconfiguration of the nutrient transporter repertoire at the plasma membrane by the addition of nutrient transporters through the secretory pathway and by their endocytic removal. In this review, we focus on the mechanisms that regulate selective nutrient transporter endocytosis, which is mediated by the α-arrestin protein family. In the budding yeast Saccharomyces cerevisiae, 14 different α-arrestins (also named arrestin-related trafficking adaptors, ARTs) function as adaptors for the ubiquitin ligase Rsp5. They instruct Rsp5 to ubiquitinate subsets of nutrient transporters to orchestrate their endocytosis. The ART proteins are under multilevel control of the major nutrient sensing systems, including amino acid sensing by the general amino acid control and target of rapamycin pathways, and energy sensing by 5'-adenosine-monophosphate-dependent kinase. The function of the six human α-arrestins is comparably under-characterised. Here, we summarise the current knowledge about the function, regulation and substrates of yeast ARTs and human α-arrestins, and highlight emerging communalities and general principles.

Dissecting the structural features of β-arrestins as multifunctional proteins

β-arrestins bind active G protein-coupled receptors (GPCRs) and play a crucial role in receptor desensitization and internalization. The classical paradigm of arrestin function has been expanded with the identification of many non-receptor-binding partners, which indicated the multifunctional role of β-arrestins in cellular functions. To elucidate the molecular mechanism of β-arrestin-mediated signaling, the structural features of β-arrestins were investigated using X-ray crystallography and cryogenic electron microscopy (cryo-EM). However, the intrinsic conformational flexibility of β-arrestins hampers the elucidation of structural interactions between β-arrestins and their binding partners using conventional structure determination tools. Therefore, structural information obtained using complementary structure analysis techniques would be necessary in combination with X-ray crystallography and cryo-EM data. In this review, we describe how β-arrestins interact with their binding partners from a structural point of view, as elucidated by both traditional methods (X-ray crystallography and cryo-EM) and complementary structure analysis techniques.

GPCR Activation States Induced by Nanobodies and Mini-G Proteins Compared by NMR Spectroscopy

In this work, we examine methyl nuclear magnetic resonance (NMR) spectra of the methionine ε-[¹³CH₃] labelled thermostabilized β₁ adrenergic receptor from turkey in association with a variety of different effectors, including mini-G_s and nanobody 60 (Nb60), which have not been previously studied in complex with β₁ adrenergic receptor (β₁AR) by NMR. Complexes with pindolol and Nb60 induce highly similar inactive states of the receptor, closely resembling the resting state conformational ensemble. We show that, upon binding of mini-Gs or nanobody 80 (Nb80), large allosteric changes throughout the receptor take place. The conformation of tβ₁AR stabilized by the native-like mini-G_s protein is highly similar to the conformation induced by the currently used surrogate Nb80. Interestingly, in both cases residual dynamics are present, which were not observed in the resting states. Finally, we reproduce a pharmaceutically relevant situation, where an antagonist abolishes the interaction of the receptor with the mini-G protein in a competitive manner, validating the functional integrity of our preparation. The presented system is therefore well suited for reproducing the individual steps of the activation cycle of a G protein-coupled receptor (GPCR) in vitro and serves as a basis for functional and pharmacological characterizations of more native-like systems in the future.

How GPCR Phosphorylation Patterns Orchestrate Arrestin-Mediated Signaling

Binding of arrestin to phosphorylated G-protein-coupled receptors (GPCRs) controls many aspects of cell signaling. The number and arrangement of phosphates may vary substantially for a given GPCR, and different phosphorylation patterns trigger different arrestin-mediated effects. Here, we determine how GPCR phosphorylation influences arrestin behavior by using atomic-level simulations and site-directed spectroscopy to reveal the effects of phosphorylation patterns on arrestin binding and conformation. We find that patterns favoring binding differ from those favoring activation-associated conformational change. Both binding and conformation depend more on arrangement of phosphates than on their total number, with phosphorylation at different positions sometimes exerting opposite effects. Phosphorylation patterns selectively favor a wide variety of arrestin conformations, differently affecting arrestin sites implicated in scaffolding distinct signaling proteins. We also reveal molecular mechanisms of these phenomena. Our work reveals the structural basis for the long-standing "barcode" hypothesis and has important implications for design of functionally selective GPCR-targeted drugs.

Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time

G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.