Structure of the µ-opioid receptor–Gi protein complex

Abstract

The μ-opioid receptor (μOR) 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 μOR signalling through the adenylyl cyclase-inhibiting heterotrimeric G protein G. Here we present the 3.5 Å resolution cryo-electron microscopy structure of the μOR bound to the agonist peptide DAMGO and nucleotide-free G. 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 μOR-G complex to previously determined structures of other GPCRs bound to the stimulatory G protein G reveals differences in the position of transmembrane receptor helix 6 and in the interactions between the G protein α-subunit and the receptor core. Together, these results shed light on the structural features that contribute to the G protein-coupling specificity of the µOR.

Receptor selectivity between the G proteins Gα 12 and Gα 13 is defined by a single leucine‐to‐isoleucine variation

“…Residue positions in this barcode represent the determinants of GPCR‐G protein selectivity, as deduced from evolutionary conservation (conserved in orthologs but not in paralogs) (36). Given the limited structural coverage of receptors and complexes to model a GPR35‐Gα 13 complex, we looked at the currently available receptor–G protein complexes of µ‐opioid receptor–Gα i1 (45), β 2 ‐adrenoceptor‐Gα s (8), adenosine A 1 receptor‐Gα i2 (46), and 5‐HT 1A –Gα o1 (47) to attempt to rationalize the importance of G.H5.23 for G 13 function. A comparison between these structures reveals differences in the position of transmembrane receptor helix VI and in the orientation between the G protein's α5 helix domain (∼20° rotation) (Fig.…”

Structural biology of G protein‐coupled receptor signaling complexes

“…The conformation of the ICL2, however, is not conserved in reported GPCR–G protein complex structures. While ICL2 can adopt helices in both the β2AR‐Gs and μOR–Gi complex structures, it formed loops in the two Class B GPCR–Gs complex structures . It is likely that the conformation of the receptor ICL2 in a GPCR–G protein complex is not defined by G protein subtype, but the distinct GPCR–G protein interface.…”

“…The first two cryo‐EM GPCR complex structures were determined for glucagon‐like peptide 1 receptor (GLP1R) and calcitonin receptor (CTR), both in complex with the stimulatory Gs protein . Entering 2018, a cryo‐EM structure of rhodopsin in complex with Gi, a homologue of the G protein transducin, and three other structures, μ‐opioid receptor (μOR) –Gi complex, adenosine A 1 receptor (A 1 R)‐Gi complex, and serotonin 5‐HT 1b R in complex with Go, have been reported . Most recently, a crystal structure of bovine rhodopsin in complex with a mini‐Go that includes the Ras domain of Gαo subunit, was published …”

How the ubiquitous GPCR receptor family selectively activates signalling pathways

“…GPCRs span the cell membrane and convert myriad extra cellular signals, including neurotransmitter molecules, hormones, and even light, into a cellular response by activating cellular G proteins and other transducer proteins. Four papers [2][3][4][5] in this issue help to unravel the mystery of how GPCRs selectively activate a particular group of G proteins known as G i/o , and provide clues that might aid the design of improved GPCR-targeting drugs.…”

“…The four papers in this issue report structures of G i/o -bound GPCRs obtained using cryoelectron microscopy: Koehl et al 2 (page 547) report the structure of the µ-opioid receptor bound to G i1 ; Draper-Joyce et al 3 (page 559) describe the adenosine A 1 receptor in complex with G i2 ; García-Nafría et al 4 (page 620) report the 5HT 1B receptor bound to G o ; and Kang et al 5 (page 553) reveal the structure of the light receptor rhodopsin in complex with G i1 . The G-protein activation cycle involves the binding and release of nucleotides to and from the G proteins, and all of the reported structures capture the receptors bound to the nucleotidefree state of their respective G proteins.…”