Crystal structure of opsin in its G-protein-interacting conformation

Opsin, the ligand-free form of the G-protein-coupled receptor rhodopsin, at low pH adopts a conformationally distinct, active G-protein-binding state known as Ops*. A synthetic peptide derived from the main binding site of the heterotrimeric G protein - the carboxy terminus of the α-subunit (GαCT) - stabilizes Ops*. Here we present the 3.2 Å crystal structure of the bovine Ops*-GαCT peptide complex. GαCT binds to a site in opsin that is opened by an outward tilt of transmembrane helix (TM) 6, a pairing of TM5 and TM6, and a restructured TM7-helix 8 kink. Contacts along the inner surface of TM5 and TM6 induce an α-helical conformation in GαCT with a C-terminal reverse turn. Main-chain carbonyl groups in the reverse turn constitute the centre of a hydrogen-bonded network, which links the two receptor regions containing the conserved E(D)RY and NPxxY(x) _5,6F motifs. On the basis of the Ops*-GαCT structure and known conformational changes in Gα, we discuss signal transfer from the receptor to the G protein nucleotide-binding site.