Fig. 4. Orientation of GPCR-G protein complexes in a phospholipid membrane.

Semi-empirical models of the complex formed between light-activated rhodopsin dimer and a G_t heterotrimer (A and B) [52] and the T4L-β₂AR-G_s-nanobody complex (C) [48] were fitted into a 3D molecular envelope calculated from projections of negatively stained, bis[succinimidyl] 2,2,4,4-glutarate (DSG) crosslinked rhodopsin*-G_t complexes purified in lauryl maltose neopentyl glycol (LMNG). Fitting of the rhodopsin*-G_t model generated with the structure of inactive G_t (PDB code: 1GOT) leaves a significant unoccupied density above one of the rhodopsin molecules (A), which becomes occupied after a 30^o hinge-like motion of the α-helical domain is applied (B). Though fitting the T4L-β₂-adrenergic receptor-G_s-nanobody structure into our EM 3D map leaves sufficient space to accommodate a second molecule of this receptor, conformation of the G_sα helical domain is inconsistent with our EM-density (C). Thus, the favored structure of the rhodopsin-G_t complex appears to be that shown in (B). Photoactivated rhodopsin (Rho*) that binds the C-terminal peptide derived from G_tα and the T4L-β₂-adrenergic receptor-nanobody molecule are both depicted in yellow. The second rhodopsin molecule in (A) and (B) is shown in gray. G_tα, G_tβ, G_tγ are colored pink, dark cyan and dark blue, respectively.