Fig. 8. Model illustrating the role of the β2AR-[CT] dynamics in G-protein activation.

In the inactive state, the disordered CT of the β₂AR adopts multiple conformations and directly interacts with intracellular loops of the TM core. The solid lines represent conformations of the CT directly interacting with the TM core, while the dashed lines represent conformations of the CT that have spontaneously dissociated from the TM core. The agonist and positive allosteric modulator promote the outward movement of TM6 and the transition of ICL2 to a helix. These changes result in weakened interactions between the CT and the cytoplasmic surface and facilitate spontaneous dissociation thereby decreasing the high-FRET dwell time and increasing the accessibility of receptors to Gs. Coupling with Gs prevents interactions between the CT and the TM core, stabilizing the low-FRET state. The stability of the β₂AR-Gs complex is dependent on the concentration of GDP and GTP in cells. Apyrase treatment hydrolyses GDP and GTP resulting in the formation of a stable nucleotide-free complex.