Fig. 3: ICL2 helix orientation affects G protein binding.

a Model of active FFAR1 with bound AP8 and heterotrimeric Gq constructed from homology modeling and alignment with other complexes (see Methods). Zoomed image shows that ICL2 in the AP8-stabilized conformation (PR state) forms a tight interface with Gqα (red). In the lower image, ICL2 modeled in the NR state has poor shape complementarity with the Gq surface. b AP8 and Gq both independently stabilize the PR state of ICL2, and do so to an even greater degree together, indicating a cooperative effect. Data presented as mean with 68% CI (N = 5 independent simulations for each condition). The simulation trace below shows ICL2 angle vs. time for the receptor-only condition (grey) and receptor-Gq condition (red). When Gq is bound to the receptor, the ICL2 PR state is stabilized relative to the receptor alone. The dashed horizontal line at 0 degrees is the distance in the AP8-bound crystal structure. c ICL2 conformation is also coupled to the orientation of Gq relative to the receptor, in particular Gα helix 5. Representative simulation frames (left) and traces (middle) of the FFAR1-Gq model are shown with and without AP8 bound. In images, the starting structure is shown in grey, and the simulation frame in color. The displacement of the Gα helix 5 was calculated by aligning simulation frames on the receptor and calculating the root mean square displacement (RMSD) of helix 5 (terminal 10 residues) relative to the starting structure. Bars (right) show mean displacement with 68% CI (N = 5 independent simulations for each condition).