Explore the vast range of titles available.

Class B G-protein-coupled receptors are major targets for the treatment of chronic diseases, such as osteoporosis, diabetes and obesity. Here we report the structure of a full-length class B receptor, the calcitonin receptor, in complex with peptide ligand and heterotrimeric Gα s βγ protein determined by Volta phase-plate single-particle cryo-electron microscopy. The peptide agonist engages the receptor by binding to an extended hydrophobic pocket facilitated by the large outward movement of the extracellular ends of transmembrane helices 6 and 7. This conformation is accompanied by a 60° kink in helix 6 and a large outward movement of the intracellular end of this helix, opening the bundle to accommodate interactions with the α5-helix of Gα s . Also observed is an extended intracellular helix 8 that contributes to both receptor stability and functional G-protein coupling via an interaction with the Gβ subunit. This structure provides a new framework for understanding G-protein-coupled receptor function. Volta phase-plate cryo-electron microscopy reveals the structure of the full-length calcitonin receptor in complex with its peptide ligand and Gα s βγ. GPCR structure solved by cryo-electron microscopy The use of cryo-electron microscopy (cryo-EM) in structural biology has exploded in recent years as it provides structural information at near atomic resolution without the need for crystallization. However, cryo-EM has typically been limited to proteins larger than 200 kDa because of issues with low contrast. Patrick Sexton and colleagues report the structure of the full-length calcitonin receptor in complex with its peptide ligand and Gα s βγ protein by Volta phase-plate single-particle cryo-EM. This is the first G-protein-coupled receptor (GPCR) structure to be solved at high resolution by cryo-EM, the first full-length class B GPCR reported and only the second in complex with the full heterotrimeric G protein. The structure shows the GPCR in the active state and reveals key information about the conformational changes associated with peptide agonist binding and G-protein coupling in class B receptors.

Drugs that allosterically target the human calcium-sensing receptor （CaSR） have substantial therapeutic potential, but are currently limited. Given the absence of high-resolution structures of the CaSR, we combined mutagenesis with a novel analytical approach and molecular modeling to develop an ＂enriched＂ picture of structure-function requirements for interaction between CaZ＋o and allosteric modulators within the CaSR＇s 7 transmembrane （7TM） domain. An extended cavity that accommodates multiple binding sites for structurally diverse ligands was identified. Phenylalkylamines bind to a site that overlaps with a putative Ca2＋o-binding site and extends towards an extracellu- lar vestibule. In contrast, the structurally and pharmacologically distinct AC-265347 binds deeper within the 7TM domains. Furthermore, distinct amino acid networks were found to mediate cooperativity by different modulators. These findings may facilitate the rational design of aliosteric modulators with distinct and potentially pathway-biased pharmacological effects.

Allosteric modulation and biased agonism at GPCRs could be manifestations of the same underlying 'conformational selection' mechanism, and these can be harmonized by considering the influence of ligand–receptor residence time and kinetic context. G-protein-coupled receptors (GPCRs) are one of the most tractable classes of drug targets. These dynamic proteins can adopt multiple active states that are linked to distinct functional outcomes. Such states can be differentially stabilized by ligands interacting with the endogenous agonist-binding orthosteric site and/or by ligands acting via spatially distinct allosteric sites, leading to the phenomena of 'biased agonism' or 'biased modulation'. These paradigms are having a major impact on modern drug discovery, but it is becoming increasingly apparent that 'kinetic context', at the level of both ligand–receptor and receptor—signal pathway kinetics, can have a profound impact on the observation and quantification of these phenomena. The concept of kinetic context thus represents an important new consideration that should be routinely incorporated into contemporary chemical biology and drug discovery studies of GPCR bias and allostery.

The drug discovery landscape has been transformed over the past decade by the discovery of allosteric modulators of all major mammalian receptor superfamilies. Allosteric ligands are a rich potential source of drugs and drug targets with clear therapeutic advantages. G protein–coupled receptors, ligand-gated ion channels and intracellular nuclear hormone receptors have all been targeted by allosteric modulators. More recently, a receptor tyrosine kinase (RTK) has been targeted by an extracellular small-molecule allosteric modulator. Allosteric mechanisms of structurally distinct molecules that target the various receptor families are more alike than originally anticipated and include selectivity, orthosteric probe dependence and pathway-biased signaling.

The structure of the glucagon-like peptide 1 receptor (GLP-1R) with its biased agonist exendin-P5 sheds light on both receptor activation and the mechanism of biased agonism. The basis of bias in class B GPCRs The glucagon-like peptide 1 receptor (GLP-1R) is a key target for the treatment of type 2 diabetes and obesity. Recently, biased agonists—ligands that selectively activate primarily one signalling pathway—have been identified. One of these, exendin-P5, is biased for G-protein signalling, increasing adipogenesis—the generation of fat cells—and is more effective at correcting hyperglycaemia than other agonists. Here, Patrick Sexton and colleagues report the structure of the human GLP-1R in complex with exendin-P5 and the G-protein heterotrimer determined from cryo-electron microscopy. Not only does this work reveal more detailed structural information about the receptor complex and its activation, it also proposes a basis for biased agonism, in the interactions with both the ligand and the G protein. The class B glucagon-like peptide-1 (GLP-1) G protein-coupled receptor is a major target for the treatment of type 2 diabetes and obesity 1 . Endogenous and mimetic GLP-1 peptides exhibit biased agonism—a difference in functional selectivity—that may provide improved therapeutic outcomes 1 . Here we describe the structure of the human GLP-1 receptor in complex with the G protein-biased peptide exendin-P5 and a Gα s heterotrimer, determined at a global resolution of 3.3 Å. At the extracellular surface, the organization of extracellular loop 3 and proximal transmembrane segments differs between our exendin-P5-bound structure and previous GLP-1-bound GLP-1 receptor structure 2 . At the intracellular face, there was a six-degree difference in the angle of the Gαs–α5 helix engagement between structures, which was propagated across the G protein heterotrimer. In addition, the structures differed in the rate and extent of conformational reorganization of the Gα s protein. Our structure provides insights into the molecular basis of biased agonism.

Calcitonin gene-related peptide (CGRP) is a widely expressed neuropeptide that has a major role in sensory neurotransmission. The CGRP receptor is a heterodimer of the calcitonin receptor-like receptor (CLR) class B G-protein-coupled receptor and a type 1 transmembrane domain protein, receptor activity-modifying protein 1 (RAMP1). Here we report the structure of the human CGRP receptor in complex with CGRP and the G s -protein heterotrimer at 3.3 Å global resolution, determined by Volta phase-plate cryo-electron microscopy. The receptor activity-modifying protein transmembrane domain sits at the interface between transmembrane domains 3, 4 and 5 of CLR, and stabilizes CLR extracellular loop 2. RAMP1 makes only limited direct contact with CGRP, consistent with its function in allosteric modulation of CLR. Molecular dynamics simulations indicate that RAMP1 provides stability to the receptor complex, particularly in the positioning of the extracellular domain of CLR. This work provides insights into the control of G-protein-coupled receptor function. The structure of a complex containing calcitonin gene-related peptide, the human calcitonin gene-related peptide receptor and the G s heterotrimer, determined using Volta phase-plate cryo-electron microscopy, provides structural insight into the regulation of G-protein-coupled receptors by receptor activity modifying protein 1.

Muscarinic M1–M5 acetylcholine receptors are G-protein-coupled receptors that regulate many vital functions of the central and peripheral nervous systems. In particular, the M1 and M4 receptor subtypes have emerged as attractive drug targets for treatments of neurological disorders, such as Alzheimer’s disease and schizophrenia, but the high conservation of the acetylcholine-binding pocket has spurred current research into targeting allosteric sites on these receptors. Here we report the crystal structures of the M1 and M4 muscarinic receptors bound to the inverse agonist, tiotropium. Comparison of these structures with each other, as well as with the previously reported M2 and M3 receptor structures, reveals differences in the orthosteric and allosteric binding sites that contribute to a role in drug selectivity at this important receptor family. We also report identification of a cluster of residues that form a network linking the orthosteric and allosteric sites of the M4 receptor, which provides new insight into how allosteric modulation may be transmitted between the two spatially distinct domains. X-ray crystal structures of the M1 and M4 muscarinic acetylcholine receptors, revealing differences in the orthosteric and allosteric binding sites that help to explain the subtype selectivity of drugs targeting this family of receptors. M1 and M4 muscarinic acetylcholine receptor structures Arthur Christopoulos and colleagues present the first X-ray crystal structures of the M1 and M4 muscarinic acetylcholine receptors, G-protein-coupled receptors (GPCRs) that regulate many vital functions of the central and peripheral nervous systems. The structures reveal differences in the orthosteric and allosteric binding sites that help to explain the subtype selectivity of drugs targeting this family of receptors. The M1 and M4 receptor subtypes are potential drug targets for treatments of neurological disorders, such as Alzheimer's disease and schizophrenia.

Despite recent advances in crystallography and the availability of G-protein-coupled receptor (GPCR) structures, little is known about the mechanism of their activation process, as only the β 2 adrenergic receptor (β 2 AR) and rhodopsin have been crystallized in fully active conformations. Here we report the structure of an agonist-bound, active state of the human M2 muscarinic acetylcholine receptor stabilized by a G-protein mimetic camelid antibody fragment isolated by conformational selection using yeast surface display. In addition to the expected changes in the intracellular surface, the structure reveals larger conformational changes in the extracellular region and orthosteric binding site than observed in the active states of the β 2 AR and rhodopsin. We also report the structure of the M2 receptor simultaneously bound to the orthosteric agonist iperoxo and the positive allosteric modulator LY2119620. This structure reveals that LY2119620 recognizes a largely pre-formed binding site in the extracellular vestibule of the iperoxo-bound receptor, inducing a slight contraction of this outer binding pocket. These structures offer important insights into the activation mechanism and allosteric modulation of muscarinic receptors. Very little is known about how a G-protein-coupled receptor (GPCR) transitions from an inactive to an active state, but this study has solved the X-ray crystal structures of the human M2 muscarinic acetylcholine receptor bound to a high-affinity agonist in an active state and to a high-affinity agonist and a small-molecule allosteric modulator in an active state; the structures provide insights into the activation mechanism and allosteric modulation of muscarinic receptors. Allostery in the M2 muscarinic acetylcholine receptor The structures of many G-protein-coupled receptors (GPCRs), including members of the class B and class F families, are now available but little is known about the transitions from the inactive to active states. In this study the authors solve the X-ray crystal structures of the human M2 muscarinic acetylcholine receptor in the active state bound to the agonist iperoxo alone and in combination with LY2119620, a positive allosteric modulator. The structures reveal that the activated M2 receptor has an extremely small orthosteric binding site, with LY2119620 'sitting' right on top of the agonist. The authors also note that the region that makes up the allosteric site in the inactive conformation of the M2 receptor is too large to bind to LY2119620; this means that the extracellular region needs to contract (by binding to the high-affinity agonist) before LY2119620 can bind to the allosteric site. This GPCR is essential for the physiological control of cardiovascular function, cognition, and pain perception, and since allosteric sites are less conserved in sequence and structure than the orthosteric binding site, the hope is that ligands that bind to allosteric sites could be turned into drugs that selectively interact with only one of the five muscarinic receptor subtypes.

Key Points Tremendous advances have been made in the discovery of novel ligands for G-protein-coupled receptors (GPCRs) that act at allosteric sites to regulate receptor function. Small molecules can act at allosteric sites to directly activate the receptor (allosteric agonists) or to potentiate (positive allosteric modulators) or inhibit (negative allosteric modulators) responses to traditional GPCR agonists that act at the orthosteric site. Allosteric modulators of GPCRs often provide higher selectivity for individual GPCR subtypes than has been achieved with traditional orthosteric-site ligands. Allosteric ligands can provide novel modes of efficacy that are not possible with orthosteric-site ligands and may provide advantages as therapeutic agents, such as allosteric potentiator and partial antagonist activity. Two allosteric modulators that are not marketed for treatment of human disorders and multiple allosteric modulators are now advancing in discovery and clinical development programmes. Allosteric modulators may lead to novel therapeutic agents for treatment of multiple psychiatric and neurological disorders, including anxiety disorders, schizophrenia and Alzheimer's disease. G protein–coupled receptors (GPCRs) represent one of the most targeted protein families in pharmaceutical research. Traditionally, drug discovery programmes have searched for ligands that act at endogenous orthosteric sites. Here, Conn and colleagues discuss recent advances in the identification of novel GPCR ligands that act at allosteric sites, highlighting their potential in the treatment of psychiatric and neurological disorders. Despite G-protein-coupled receptors (GPCRs) being among the most fruitful targets for marketed drugs, intense discovery efforts for several GPCR subtypes have failed to deliver selective drug candidates. Historically, drug discovery programmes for GPCR ligands have been dominated by efforts to develop agonists and antagonists that act at orthosteric sites for endogenous ligands. However, in recent years, there have been tremendous advances in the discovery of novel ligands for GPCRs that act at allosteric sites to regulate receptor function. These compounds provide high selectivity, novel modes of efficacy and may lead to novel therapeutic agents for the treatment of multiple psychiatric and neurological human disorders.

The adenosine A 1 receptor (A 1 R) is a promising therapeutic target for non-opioid analgesic agents to treat neuropathic pain 1 , 2 . However, development of analgesic orthosteric A 1 R agonists has failed because of a lack of sufficient on-target selectivity as well as off-tissue adverse effects 3 . Here we show that [2-amino-4-(3,5-bis(trifluoromethyl)phenyl)thiophen-3-yl)(4-chlorophenyl)methanone] (MIPS521), a positive allosteric modulator of the A 1 R, exhibits analgesic efficacy in rats in vivo through modulation of the increased levels of endogenous adenosine that occur in the spinal cord of rats with neuropathic pain. We also report the structure of the A 1 R co-bound to adenosine, MIPS521 and a G i2 heterotrimer, revealing an extrahelical lipid–detergent-facing allosteric binding pocket that involves transmembrane helixes 1, 6 and 7. Molecular dynamics simulations and ligand kinetic binding experiments support a mechanism whereby MIPS521 stabilizes the adenosine–receptor–G protein complex. This study provides proof of concept for structure-based allosteric drug design of non-opioid analgesic agents that are specific to disease contexts. MIPS521, a positive allosteric modulator of the adenosine A 1 receptor, has analgesic properties in a rat model of neuropathic pain through a mechanism by which MIPS521 stabilizes the complex between adenosine, receptor and G protein.