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Dopamine D1 receptor (D1R) is an important drug target implicated in many psychiatric and neurological disorders. Selective agonism of D1R are sought to be the therapeutic strategy for these disorders. Most selective D1R agonists share a dopamine-like catechol moiety in their molecular structure, and their therapeutic potential is therefore limited by poor pharmacological properties in vivo. Recently, a class of non-catechol D1R selective agonists with a distinct scaffold and pharmacological properties were reported. Here, we report the crystal structure of D1R in complex with stimulatory G protein (Gs) and a non-catechol agonist Compound 1 at 3.8 Å resolution. The structure reveals the ligand bound to D1R in an extended conformation, spanning from the orthosteric site to extracellular loop 2 (ECL2). Structural analysis reveals that the unique features of D1R ligand binding pocket explains the remarkable selectivity of this scaffold for D1R over other aminergic receptors, and sheds light on the mechanism for D1R activation by the non-catechol agonist., Recently, a class of non-catechol Dopamine D1 receptor (D1R) selective agonists with novel scaffold and improved pharmacological properties were reported. Here, authors report the crystal structure of D1R in complex with stimulatory G protein (Gs) and a non-catechol agonist Compound 1 which explains the selectivity of this scaffold for D1R over other aminergic receptors and the mechanism of activating D1R.

Glucagon-like peptide 1 (GLP-1) is a hormone with essential roles in regulating insulin secretion, carbohydrate metabolism and appetite. GLP-1 effects are mediated through binding to the GLP-1 receptor (GLP-1R), a class B G-protein-coupled receptor (GPCR) that signals primarily through the stimulatory G protein Gs. Class B GPCRs are important therapeutic targets; however, our understanding of their mechanism of action is limited by the lack of structural information on activated and full-length receptors. Here we report the cryo-electron microscopy structure of the peptide-activated GLP-1R-Gs complex at near atomic resolution. The peptide is clasped between the N-terminal domain and the transmembrane core of the receptor, and further stabilized by extracellular loops. Conformational changes in the transmembrane domain result in a sharp kink in the middle of transmembrane helix 6, which pivots its intracellular half outward to accommodate the α5-helix of the Ras-like domain of Gs. These results provide a structural framework for understanding class B GPCR activation through hormone binding.

The X-ray crystal structure of the mouse δ-opioid receptor in complex with the subtype-selective antagonist naltrindole is reported. Four papers in this issue of Nature present the long-awaited high-resolution crystal structures of the four known opioid receptors in ligand-bound conformations. These G-protein-coupled receptors are the targets of a broad range of drugs, including painkillers, antidepressants, anti-anxiety agents and anti-addiction medications. Brian Kobilka’s group reports the crystal structure of the µ-opioid receptor bound to a morphinan antagonist and the δ-opioid receptor bound to naltrindole. Raymond Stevens’ group reports on the κ-opioid receptor bound to the selective antagonist JDTic, and the nociceptin/orphanin FQ receptor bound to a peptide mimetic. In an associated News and Views, Marta Filizola and Lakshmi Devi discuss the implications of these landmark papers for research on the mechanisms underlying receptor function and drug development. The opioid receptor family comprises three members, the µ-, δ- and κ-opioid receptors, which respond to classical opioid alkaloids such as morphine and heroin as well as to endogenous peptide ligands like endorphins. They belong to the G-protein-coupled receptor (GPCR) superfamily, and are excellent therapeutic targets for pain control. The δ-opioid receptor (δ-OR) has a role in analgesia, as well as in other neurological functions that remain poorly understood1. The structures of the µ-OR and κ-OR have recently been solved2,3. Here we report the crystal structure of the mouse δ-OR, bound to the subtype-selective antagonist naltrindole. Together with the structures of the µ-OR and κ-OR, the δ-OR structure provides insights into conserved elements of opioid ligand recognition while also revealing structural features associated with ligand-subtype selectivity. The binding pocket of opioid receptors can be divided into two distinct regions. Whereas the lower part of this pocket is highly conserved among opioid receptors, the upper part contains divergent residues that confer subtype selectivity. This provides a structural explanation and validation for the ‘message–address’ model of opioid receptor pharmacology4,5, in which distinct ‘message’ (efficacy) and ‘address’ (selectivity) determinants are contained within a single ligand. Comparison of the address region of the δ-OR with other GPCRs reveals that this structural organization may be a more general phenomenon, extending to other GPCR families as well.

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β(2) adrenergic receptor (β(2)AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β(2)AR and nucleotide-free Gs heterotrimer. The principal interactions between the β(2)AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β(2)AR include a 14 Å outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR.

G-protein-coupled receptors (GPCRs) transduce signals from the extracellular environment to intracellular proteins. To gain structural insight into the regulation of receptor cytoplasmic conformations by extracellular ligands during signaling, we examine the structural dynamics of the cytoplasmic domain of the β2-adrenergic receptor (β2AR) using (19)F-fluorine NMR and double electron-electron resonance spectroscopy. These studies show that unliganded and inverse-agonist-bound β2AR exists predominantly in two inactive conformations that exchange within hundreds of microseconds. Although agonists shiftthe equilibrium toward a conformation capable of engaging cytoplasmic G proteins, they do so incompletely, resulting in increased conformational heterogeneity and the coexistence of inactive, intermediate, and active states. Complete transition to the active conformation requires subsequent interaction with a G protein or an intracellular G protein mimetic. These studies demonstrate a loose allosteric coupling of the agonist-binding site and G-protein-coupling interface that may generally be responsible for the complex signaling behavior observed for many GPCRs.