provinces in China announced the public health emergency. Psychological distress in the general population has been reported in China during the COVID-19 quarantine [1]. The healthcare system in Wuhan was quickly overwhelmed as tens of thousands of people with flu-like symptoms swarmed the hospitals. Frontline healthcare workers (HCWs) in Wuhan have been under tremendous pressure and risk of contracting COVID-19 since the beginning of the quarantine. As of February 12, 2020, 21,569 HCWs from other cities in China have been deployed to support emergency response efforts in Wuhan [2], while 1716 HCWs have contracted COVID-19 and 5 have died [3]. Two nurses Note. * p < .05, ** p < .01, *** p < .001.

SUMMARY G protein-coupled receptors (GPCRs) mediate diverse signaling in part through interaction with arrestins, whose binding promotes receptor internalization and signaling through G protein-independent pathways. High-affinity arrestin binding requires receptor phosphorylation, often at the receptor’s C-terminal tail. Here we report an X-ray free electron laser (XFEL) crystal structure of the rhodopsin–arrestin complex, in which the phosphorylated C-terminus of rhodopsin forms an extended intermolecular β-sheet with the N-terminal β-strands of arrestin. Phosphorylation was detected at rhodopsin C-terminal tail residues T336 and S338. These two phospho-residues, together with E341, form an extensive network of electrostatic interactions with three positively charged pockets in arrestin in a mode that resembles binding of the phosphorylated vasopressin-2 receptor tail to β-arrestin-1. Based on these observations, we derived and validated a set of phosphorylation codes that serve as a common mechanism for phosphorylation-dependent recruitment of arrestins by GPCRs.

G-protein-coupled receptors comprise the largest family of mammalian transmembrane receptors. They mediate numerous cellular pathways by coupling with downstream signalling transducers, including the hetrotrimeric G proteins G (stimulatory) and G (inhibitory) and several arrestin proteins. The structural mechanisms that define how G-protein-coupled receptors selectively couple to a specific type of G protein or arrestin remain unknown. Here, using cryo-electron microscopy, we show that the major interactions between activated rhodopsin and G are mediated by the C-terminal helix of the G α-subunit, which is wedged into the cytoplasmic cavity of the transmembrane helix bundle and directly contacts the amino terminus of helix 8 of rhodopsin. Structural comparisons of inactive, G-bound and arrestin-bound forms of rhodopsin with inactive and G-bound forms of the β-adrenergic receptor provide a foundation to understand the unique structural signatures that are associated with the recognition of G, G and arrestin by activated G-protein-coupled receptors.

Arrestins comprise a family of signal regulators of G-protein-coupled receptors (GPCRs), which include arrestins 1 to 4. While arrestins 1 and 4 are visual arrestins dedicated to rhodopsin, arrestins 2 and 3 (Arr2 and Arr3) are β-arrestins known to regulate many nonvisual GPCRs. The dynamic and promiscuous coupling of Arr2 to nonvisual GPCRs has posed technical challenges to tackle the basis of arrestin binding to GPCRs. Here we report the structure of Arr2 in complex with neurotensin receptor 1 (NTSR1), which reveals an overall assembly that is strikingly different from the visual arrestin-rhodopsin complex by a 90°rotation of Arr2 relative to the receptor. In this new configuration, intracellular loop 3 (ICL3) and transmembrane helix 6 (TM6) of the receptor are oriented toward the N-terminal domain of the arrestin, making it possible for GPCRs that lack the C-terminal tail to couple Arr2 through their ICL3. Molecular dynamics simulation and crosslinking data further support the assembly of the Arr2-NTSR1 complex. Sequence analysis and homology modeling suggest that the Arr2-NTSR1 complex structure may provide an alternative template for modeling arrestin-GPCR interactions.

G protein-coupled receptors (GPCRs) from the secretin-like (class B) family are key players in hormonal homeostasis and are important drug targets for the treatment of metabolic disorders and neuronal diseases. They consist of a large N-terminal extracellular domain (ECD) and a transmembrane domain (TMD) with the GPCR signature of seven transmembrane helices. Class B GPCRs are activated by peptide hormones with their C termini bound to the receptor ECD and their N termini bound to the TMD. It is thought that the ECD functions as an affinity trap to bind and localize the hormone to the receptor. This in turn would allow the hormone N terminus to insert into the TMD and induce conformational changes of the TMD to activate downstream signaling. In contrast to this prevailing model, we demonstrate that human class B GPCRs vary widely in their requirement of the ECD for activation. In one group, represented by corticotrophin-releasing factor receptor 1 (CRF 1 R), parathyroid hormone receptor (PTH1R), and pituitary adenylate cyclase activating polypeptide type 1 receptor (PAC1R), the ECD requirement for high affinity hormone binding can be bypassed by induced proximity and mass action effects, whereas in the other group, represented by glucagon receptor (GCGR) and glucagon-like peptide-1 receptor (GLP-1R), the ECD is required for signaling even when the hormone is covalently linked to the TMD. Furthermore, the activation of GLP-1R by small molecules that interact with the intracellular side of the receptor is dependent on the presence of its ECD, suggesting a direct role of the ECD in GLP-1R activation.The class B or secretin family of GPCRs consists of 15 receptors for peptide hormones that include glucagon, glucagon-like peptides, parathyroid hormone, and calcitonin (Fig. 1A). These receptors are important drug targets for many human diseases including diabetes, neurodegeneration, cardiovascular diseases, and psychiatric disorders. The full-length receptors consist of two modular domains: a globular extracellular domain (ECD) 3 defined by three conserved disulfide bonds and a TMD that contains seven transmembrane helices (1-4). The ECD is responsible for the high affinity and specificity of hormone binding, and the TMD is required for receptor activation and signal coupling to downstream G proteins and other signaling effectors (4). Peptide hormone binding is thought to proceed through fast binding of its C terminus to the ECD followed by a slower association of the peptide N terminus with the receptor TMD (5), which leads to conformational changes in the receptor TMD and the receptor activation. The activated receptors are coupled primarily to stimulatory G proteins, resulting in elevation of the intracellular cAMP level.Structures of the ECDs of class B GPCRs in complex with their peptide ligands have been determined by x-ray crystallography (1, 6 -9) and NMR (10) and have provided useful information about structural mechanisms of ligand recognition and selectivity (2, 11). Only recently, crystal structures of t...

Arrestins comprise a family of signal regulators of G-protein-coupled receptors (GPCRs), which include arrestins 1 to 4. While arrestins 1 and 4 are visual arrestins dedicated to rhodopsin, arrestins 2 and 3 (Arr2 and Arr3) are β-arrestins known to regulate many nonvisual GPCRs. The dynamic and promiscuous coupling of Arr2 to nonvisual GPCRs has posed technical challenges to tackle the basis of arrestin binding to GPCRs. Here we report the structure of Arr2 in complex with neurotensin receptor 1 (NTSR1), which reveals an overall assembly that is strikingly different from the visual arrestin-rhodopsin complex by a 90° rotation of Arr2 relative to the receptor. In this new configuration, intracellular loop 3 (ICL3) and transmembrane helix 6 (TM6) of the receptor are oriented toward the N-terminal domain of the arrestin, making it possible for GPCRs that lack the C-terminal tail to couple Arr2 through their ICL3. Molecular dynamics simulation and crosslinking data further support the assembly of the Arr2-NTSR1 complex. Sequence analysis and homology modeling suggest that the Arr2-NTSR1 complex structure may provide an alternative template for modeling arrestin-GPCR interactions.

5-hydroxytryptamine (5-HT, also known as serotonin) regulates many physiological processes through the 5-HT receptor family. Here we report the crystal structure of 5-HT1B subtype receptor (5-HT1BR) bound to the psychotropic serotonin receptor inverse agonist methiothepin (MT). Crystallization was facilitated by replacing ICL3 with a novel optimized variant of BRIL (OB1) that enhances the formation of intermolecular polar interactions, making OB1 a potential useful tool for structural studies of membrane proteins. Unlike the agonist ergotamine (ERG), MT occupies only the conserved orthosteric binding pocket, explaining the wide spectrum effect of MT on serotonin receptors. Compared with ERG, MT shifts toward TM6 and sterically pushes residues W3276.48, F3306.50 and F3316.51 from inside the orthosteric binding pocket, leading to an outward movement of the extracellular end and a corresponding inward shift of the intracellular end of TM6, a feature shared by other reported inactive G protein-coupled receptor (GPCR) structures. Together with the previous agonist-bound serotonin receptor structures, the inverse agonist-bound 5-HT1BR structure identifies a basis for the ligand-mediated switch of 5-HT1BR activity and provides a structural understanding of the inactivation mechanism of 5-HT1BR and some other class A GPCRs, characterized by ligand-induced outward movement of the extracellular end of TM6 that is coupled with inward movement of the cytoplasmic end of this helix.

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