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Author(s): Antoine Koehl [sup.1] , Hongli Hu [sup.1] [sup.2] , Shoji Maeda [sup.2] , Yan Zhang [sup.1] [sup.2] , Qianhui Qu [sup.1] [sup.2] , Joseph M. Paggi [sup.1] [sup.2] [sup.3] [...], The [mu]-opioid receptor ([mu]OR) 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 [mu]OR signalling through the adenylyl cyclase-inhibiting heterotrimeric G protein G.sub.i. Here we present the 3.5 Å resolution cryo-electron microscopy structure of the [mu]OR bound to the agonist peptide DAMGO and nucleotide-free G.sub.i. 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 [mu]OR-G.sub.i complex to previously determined structures of other GPCRs bound to the stimulatory G protein G.sub.s reveals differences in the position of transmembrane receptor helix 6 and in the interactions between the G protein [alpha]-subunit and the receptor core. Together, these results shed light on the structural features that contribute to the G.sub.i protein-coupling specificity of the [micro]OR. A cryo-electron structure of the [micro]-opioid receptor in complex with the peptide agonist DAMGO and the inhibitory G protein G.sub.i reveals structural determinants of its G protein-binding specificity.

Intercellular signalling is an essential characteristic of multicellular organisms. Gap junctions, which consist of arrays of intercellular channels, permit the exchange of ions and small molecules between adjacent cells. Here, the structural determination of a gap junction channel composed of connexin 26 (Cx26) at 3.5 Å resolution is described. During each step of the purification process, the protein was examined using electron microscopy and/or dynamic light scattering. Dehydration of the crystals improved the resolution limits. Phase refinement using multi-crystal averaging in conjunction with noncrystallographic symmetry averaging based on strictly determined noncrystallographic symmetry operators resulted in an electron-density map for model building. The amino-acid sequence of a protomer structure consisting of the aminoterminal helix, four transmembrane helices and two extracellular loops was assigned to the electron-density map. The amino-acid assignment was confirmed using six selenomethionine (SeMet) sites in the difference Fourier map of the SeMet derivative and three intramolecular disulfide bonds in the anomalous difference Fourier map of the native crystal. [ABSTRACT FROM AUTHOR]