Cryo-EM structure of human rhodopsin bound to an inhibitory G protein

Abstract

G-protein-coupled receptors comprise the largest family of mammalian transmembrane receptors. They mediate numerous cellular pathways by coupling with downstream signalling transducers, including the hetrotrimeric G proteins G (stimulatory) and G (inhibitory) and several arrestin proteins. The structural mechanisms that define how G-protein-coupled receptors selectively couple to a specific type of G protein or arrestin remain unknown. Here, using cryo-electron microscopy, we show that the major interactions between activated rhodopsin and G are mediated by the C-terminal helix of the G α-subunit, which is wedged into the cytoplasmic cavity of the transmembrane helix bundle and directly contacts the amino terminus of helix 8 of rhodopsin. Structural comparisons of inactive, G-bound and arrestin-bound forms of rhodopsin with inactive and G-bound forms of the β-adrenergic receptor provide a foundation to understand the unique structural signatures that are associated with the recognition of G, G and arrestin by activated G-protein-coupled receptors.

Structural biology of G protein‐coupled receptor signaling complexes

“…(B)]. These polar interactions, noted as “capping interactions”, serve as one of the distinguished features for GPCR‐Gi or ‐Go coupling, are not observed in GPCR–Gs complexes …”

“…The conformational differences in TM6 helices of Gi‐ or Gs‐coupling GPCRs have been verified by all‐atom mollified adaptive biasing potential (mABP) simulation. The biased simulations indicated that the intracellular ends of TM6 helices of Gi‐coupling rhodopsin and μOR were considerably less dynamic and remained in a more straight conformation, whereas those of Gs‐coupling β2AR and A 2A R favored an outward tilted conformation …”

“…Later, crystal structures have been published for rhodopsin and its retinal‐free form opsin in active conformations, with or without the binding of a peptide derived from the C‐terminal helix α5 of the α subunit of G protein transducin (Gt) . The overall structural information of how rhodopsin recruits its downstream signaling partners, the G protein transducin and visual arrestin, however, had not been available until the structures of rhodopsin–visual arrestin and rhodopsin‐Gi complexes were determined . As rhodopsin serves as a prototype for the GPCR family, the discovery of the structural information of these complexes has significantly impacted the understanding of the signaling mechanisms of the entire GPCR superfamily.…”

How the ubiquitous GPCR receptor family selectively activates signalling pathways

“…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.…”

“…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.…”