ABSTRACT
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 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 ß2-adrenergic receptor. 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.
Subject(s)
Receptors, Opioid, mu/chemistry , Receptors, Opioid, mu/metabolism , Allosteric Regulation , Animals , Binding Sites , Heterotrimeric GTP-Binding Proteins/metabolism , Lysine/metabolism , Mice , Models, Molecular , Morphinans/chemistry , Morphinans/metabolism , Morphinans/pharmacology , Nuclear Magnetic Resonance, Biomolecular , Protein Binding , Protein Conformation/drug effects , Pyrroles/chemistry , Pyrroles/metabolism , Pyrroles/pharmacology , Receptors, Adrenergic, beta-2/chemistry , Single-Chain Antibodies/chemistry , Single-Chain Antibodies/metabolism , Single-Chain Antibodies/pharmacology , Structure-Activity Relationship , Substrate SpecificityABSTRACT
By sequencing of the FGD4 coding sequence in a cohort of 101 patients affected by autosomal recessive demyelinating Charcot-Marie-Tooth disease (CMT), we have identified two novel missense mutations in FGD4 in two patients from consanguineous descent: p.Arg442His in an Algerian patient and p.Met566Ile in a Lebanese girl. The patients present early onset, slowly progressive CMT, with drastic reduction of nerve conduction velocities. These mutations are the second and third missense mutations characterized in FGD4. They are likely to lead to conformational changes in the PH1 and FYVE domains.
