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NMR spectroscopy reveals the conformational changes of the μ-opioid receptor that are associated with receptor activation, helping to explain why the allosteric coupling between the agonist-binding pocket and the cytoplasmic G-protein-coupling interface of this receptor is relatively weak. Activation of the μ-opioid receptor The μ-opioid receptor is a G-protein-coupled receptor (GPCR) activated by various analgesics, endogenous endorphins and drugs of abuse such as heroin and opium. Our understanding of the mechanism by which agonist binding leads to recognition, coupling, and activation of a particular G protein subtype is incomplete. In two papers in this issue of Nature , the authors used X-ray crystallography, molecular dynamics simulations, and NMR spectroscopy to probe the structural basis for receptor activation. As well as revealing the conformational changes in the extracellular and intracellular domains of this GPCR associated with receptor activation, these studies help explain why the allosteric coupling between the agonist-binding pocket and the cytoplasmic G-protein-coupling interface of this receptor is relatively weak. µ-Opioid receptors (µORs) are G-protein-coupled receptors that are activated by a structurally diverse spectrum of natural and synthetic agonists including endogenous endorphin peptides, morphine and methadone. The recent structures of the μOR in inactive 1 and agonist-induced active states (Huang et al. , ref. 2 ) provide snapshots of the receptor at the beginning and end of a signalling event, but little is known about the dynamic sequence of events that span these two states. Here we use solution-state NMR to examine the process of μOR activation using a purified receptor (mouse sequence) preparation in an amphiphile membrane-like environment. We obtain spectra of the μOR in the absence of ligand, and in the presence of the high-affinity agonist BU72 alone, or with BU72 and a G protein mimetic nanobody. Our results show that conformational changes in transmembrane segments 5 and 6 (TM5 and TM6), which are required for the full engagement of a G protein, are almost completely dependent on the presence of both the agonist and the G protein mimetic nanobody, revealing a weak allosteric coupling between the agonist-binding pocket and the G-protein-coupling interface (TM5 and TM6), similar to that observed for the β2-adrenergic receptor 3 . Unexpectedly, in the presence of agonist alone, we find larger spectral changes involving intracellular loop 1 and helix 8 compared to changes in TM5 and TM6. These results suggest that one or both of these domains may play a role in the initial interaction with the G protein, and that TM5 and TM6 are only engaged later in the process of complex formation. The initial interactions between the G protein and intracellular loop 1 and/or helix 8 may be involved in G-protein coupling specificity, as has been suggested for other family A G-protein-coupled receptors.

The X-ray crystal structure of the mouse δ-opioid receptor in complex with the subtype-selective antagonist naltrindole is reported. Where opiates hit home 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 understood 1 . The structures of the µ-OR and κ-OR have recently been solved 2 , 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 pharmacology 4 , 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.

Structures of the adiponectin receptors ADIPOR1 and ADIPOR2 combined with molecular dynamics simulations and enzymatic assays suggest that both receptors have intrinsic ceramidase activity. Enzymatic activity of ADIPORs Adiponectin receptors (ADIPORs) are seven-transmembrane proteins that control glucose and lipid metabolism. By unknown molecular mechanisms, they mediate cellular ceramidase activity, which catalyses the hydrolysis of ceramide into sphingosine and a free fatty acid (FFA). In this study, the crystal structure of ADIPOR2 bound to a FFA molecule is presented. Also presented is a revised ADIPOR1 crystal structure, which is shown to be clearly distinct, with no FFA observed. Because both ADIPOR1 and ADIPOR2 have intrinsic basal ceramidase activity, the authors suggest that these distinct structures may represent different key steps in the enzymatic activity of ADIPORs. However, the observed ceramidase activity is low, and further studies will be required to fully characterize ADIPORs enzymatic parameters and their substrate specificity. Adiponectin receptors (ADIPORs) are integral membrane proteins that control glucose and lipid metabolism by mediating, at least in part, a cellular ceramidase activity 1 that catalyses the hydrolysis of ceramide to produce sphingosine and a free fatty acid (FFA). The crystal structures of the two receptor subtypes, ADIPOR1 and ADIPOR2, show a similar overall seven-transmembrane-domain architecture with large unoccupied cavities and a zinc binding site within the seven transmembrane domain 2 . However, the molecular mechanisms by which ADIPORs function are not known. Here we describe the crystal structure of ADIPOR2 bound to a FFA molecule and show that ADIPOR2 possesses intrinsic basal ceramidase activity that is enhanced by adiponectin. We also identify a ceramide binding pose and propose a possible mechanism for the hydrolytic activity of ADIPOR2 using computational approaches. In molecular dynamics simulations, the side chains of residues coordinating the zinc rearrange quickly to promote the nucleophilic attack of a zinc-bound hydroxide ion onto the ceramide amide carbonyl. Furthermore, we present a revised ADIPOR1 crystal structure exhibiting a seven-transmembrane-domain architecture that is clearly distinct from that of ADIPOR2. In this structure, no FFA is observed and the ceramide binding pocket and putative zinc catalytic site are exposed to the inner membrane leaflet. ADIPOR1 also possesses intrinsic ceramidase activity, so we suspect that the two distinct structures may represent key steps in the enzymatic activity of ADIPORs. The ceramidase activity is low, however, and further studies will be required to characterize fully the enzymatic parameters and substrate specificity of ADIPORs. These insights into ADIPOR function will enable the structure-based design of potent modulators of these clinically relevant enzymes.

Opium is one of the world’s oldest drugs, and its derivatives morphine and codeine are among the most used clinical drugs to relieve severe pain. These prototypical opioids produce analgesia as well as many undesirable side effects (sedation, apnoea and dependence) by binding to and activating the G-protein-coupled µ-opioid receptor (µ-OR) in the central nervous system. Here we describe the 2.8 Å crystal structure of the mouse µ-OR in complex with an irreversible morphinan antagonist. Compared to the buried binding pocket observed in most G-protein-coupled receptors published so far, the morphinan ligand binds deeply within a large solvent-exposed pocket. Of particular interest, the µ-OR crystallizes as a two-fold symmetrical dimer through a four-helix bundle motif formed by transmembrane segments 5 and 6. These high-resolution insights into opioid receptor structure will enable the application of structure-based approaches to develop better drugs for the management of pain and addiction. The crystal structure of the mouse μ-opioid receptor bound to an antagonist is described, with possible implications for the future development of analgesics. Where opiates hit home 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.

Activation of the μ-opioid receptor (μOR) is responsible for the efficacy of the most effective analgesics. To shed light on the structural basis for μOR activation, here we report a 2.1 Å X-ray crystal structure of the murine μOR bound to the morphinan agonist BU72 and a G protein mimetic camelid antibody fragment. The BU72-stabilized changes in the μOR binding pocket are subtle and differ from those observed for agonist-bound structures of the β 2 -adrenergic receptor (β 2 AR) and the M2 muscarinic receptor. Comparison with active β 2 AR reveals a common rearrangement in the packing of three conserved amino acids in the core of the μOR, and molecular dynamics simulations illustrate how the ligand-binding pocket is conformationally linked to this conserved triad. Additionally, an extensive polar network between the ligand-binding pocket and the cytoplasmic domains appears to play a similar role in signal propagation for all three G-protein-coupled receptors. X-ray crystallography and molecular dynamics simulations of the μ-opioid receptor reveal the conformational changes in the extracellular and intracellular domains of this G-protein-coupled receptor that are associated with its activation. Activation of the μ-opioid receptor The μ-opioid receptor is a G-protein-coupled receptor (GPCR) activated by various analgesics, endogenous endorphins and drugs of abuse such as heroin and opium. Our understanding of the mechanism by which agonist binding leads to recognition, coupling, and activation of a particular G protein subtype is incomplete. In two papers in this issue of Nature , the authors used X-ray crystallography, molecular dynamics simulations, and NMR spectroscopy to probe the structural basis for receptor activation. As well as revealing the conformational changes in the extracellular and intracellular domains of this GPCR associated with receptor activation, these studies help explain why the allosteric coupling between the agonist-binding pocket and the cytoplasmic G-protein-coupling interface of this receptor is relatively weak.

Atypical chemokine receptor 1 (ACKR1) is a G protein–coupled receptor (GPCR) targeted by Staphylococcus aureus bicomponent pore-forming leukotoxins to promote bacterial growth and immune evasion. Here, we have developed an integrative molecular pharmacology and structural biology approach in order to characterize the effect of leukotoxins HlgA and HlgB on ACKR1 structure and function. Interestingly, using cell-based assays and native mass spectrometry, we found that both components HlgA and HlgB compete with endogenous chemokines through a direct binding with the extracellular domain of ACKR1. Unexpectedly, hydrogen/deuterium exchange mass spectrometry analysis revealed that toxin binding allosterically modulates the intracellular G protein–binding domain of the receptor, resulting in dissociation and/or changes in the architecture of ACKR1–Gαi1 protein complexes observed in living cells. Altogether, our study brings important molecular insights into the initial steps of leukotoxins targeting a host GPCR.

G protein–coupled receptors (GPCRs) are versatile molecular machines that regulate the majority of physiological responses to chemically diverse hormones and neurotransmitters. Recent breakthroughs in structural studies have advanced our understanding of GPCR signaling, particularly the selectivity of ligand recognition and receptor activation of G proteins.

We have designed a microfluidic device in which we can manipulate, lyse, label, separate, and quantify the protein contents of a single cell using single-molecule fluorescence counting. Generic labeling of proteins is achieved through fluorescent-antibody binding. The use of cylindrical optics enables high-efficiency ([almost equal to]60%) counting of molecules in micrometer-sized channels. We used this microfluidic device to quantify β₂ adrenergic receptors expressed in insect cells (SF9). We also analyzed phycobiliprotein contents in individual cyanobacterial cells (Synechococcus sp. PCC 7942) and observed marked differences in the levels of specific complexes in cell populations that were grown under nitrogen-depleted conditions.

The phosphoprotein (P) is the main and essential cofactor of the RNA polymerase (L) of non-segmented, negative‐strand RNA viruses. P positions the viral polymerase onto its nucleoprotein–RNA template and acts as a chaperone of the nucleoprotein (N), thereby preventing nonspecific encapsidation of cellular RNAs. The phosphoprotein of human metapneumovirus (HMPV) forms homotetramers composed of a stable oligomerization domain (P core ) flanked by large intrinsically disordered regions (IDRs). Here we combined x-ray crystallography of P core with small angle x-ray scattering (SAXS)-based ensemble modeling of the full-length P protein and several of its fragments to provide a structural description of P that captures its dynamic character, and highlights the presence of varyingly stable structural elements within the IDRs. We discuss the implications of the structural properties of HMPV P for the assembly and functioning of the viral transcription/replication machinery.