The μ-opioid receptor (μOR) is a G-protein-coupled receptor (GPCR) and the target of most clinically and recreationally used opioids. The induced positive effects of analgesia and euphoria are mediated by μOR signalling through the adenylyl cyclase-inhibiting heterotrimeric G protein G. Here we present the 3.5 Å resolution cryo-electron microscopy structure of the μOR bound to the agonist peptide DAMGO and nucleotide-free G. DAMGO occupies the morphinan ligand pocket, with its N terminus interacting with conserved receptor residues and its C terminus engaging regions important for opioid-ligand selectivity. Comparison of the μOR-G complex to previously determined structures of other GPCRs bound to the stimulatory G protein G reveals differences in the position of transmembrane receptor helix 6 and in the interactions between the G protein α-subunit and the receptor core. Together, these results shed light on the structural features that contribute to the G protein-coupling specificity of the µOR.

The function of G protein-coupled receptors (GPCRs)-which represent the largest class of both human membrane proteins and drug targets-depends critically on their ability to change shape, transitioning among distinct conformations. Determining the structural dynamics of GPCRs is thus essential both for understanding the physiology of these receptors and for the rational design of GPCR-targeted drugs. Here we review what is currently known about the flexibility and dynamics of GPCRs, as determined through crystallography, spectroscopy, and computer simulations. We first provide an overview of the types of motion exhibited by a GPCR and then discuss GPCR dynamics in the context of ligand binding, activation, allosteric modulation, and biased signaling. Finally, we discuss the implications of GPCR conformational plasticity for drug design.

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.

Summary Following agonist activation, G protein-coupled receptors (GPCRs) recruit β-arrestin, which desensitizes heterotrimeric G protein signaling and promotes receptor endocytosis 1 . Additionally, β-arrestin directly regulates many cell signaling pathways that can induce cellular responses distinct from that of G proteins 2 . Here we present a cryo-electron microscopy (cryoEM) structure of β-arrestin1 (βarr1) in complex with muscarinic acetylcholine-2-receptor (M2R) reconstituted in lipid nanodiscs. The M2R-βarr1 structure shows a multimodal network of flexible interactions, including binding of the βarr1 N-domain to phosphorylated receptor residues and βarr1 finger loop insertion into the M2R seven-transmembrane bundle, which adopts a conformation similar to that in the M2R-heterotrimeric G o protein structure 3 . Moreover, the cryoEM map reveals that the βarr1 C-domain edge engages the lipid bilayer. Through atomistic simulations, biophysical, biochemical, and cellular assays, we show that the C-edge is critical for stable complex formation, βarr1 recruitment, receptor internalization, and desensitization of G protein activation. Taken together, these data suggest the cooperative interactions of β-arrestin with both the receptor and phospholipid bilayer contribute to its functional versatility.

Graphical AbstractHighlights d 3 Å cryo-EM structure of the CB1-G i complex bound to potent agonist MDMB-Fubinaca d MDMB-Fubinaca locks ''toggle switch'' residues F200 3.36 / W356 6.48 in active conformation d Quantum mechanics calculations reveal the mechanism for the high affinity of Fubinaca d Molecular dynamic simulations reveal a path for ligand entry between TM1 and TM7 In BriefLooking at how a toxic, synthetic ligand locks cannabinoid receptor 1 into a signaling conformation points to ways to understand and modulate receptor activity. SUMMARYCannabis elicits its mood-enhancing and analgesic effects through the cannabinoid receptor 1 (CB1), a G protein-coupled receptor (GPCR) that signals primarily through the adenylyl cyclase-inhibiting heterotrimeric G protein G i . Activation of CB1-G i signaling pathways holds potential for treating a number of neurological disorders and is thus crucial to understand the mechanism of G i activation by CB1. Here, we present the structure of the CB1-G i signaling complex bound to the highly potent agonist MDMB-Fubinaca (FUB), a recently emerged illicit synthetic cannabinoid infused in street drugs that have been associated with numerous overdoses and fatalities. The structure illustrates how FUB stabilizes the receptor in an active state to facilitate nucleotide exchange in G i . The results compose the structural framework to explain CB1 activation by different classes of ligands and provide insights into the G protein coupling and selectivity mechanisms adopted by the receptor.