Fig. 5.

An allosteric activation pathway of the M2 receptor derived from aMD simulations. As a constitutively active GPCR, the apo M2 receptor exists in a conformational equilibrium of inactive, intermediate, and active states. (A) The inactive state with the TM3, TM5, TM6, and TM7 helices shown in cartoons and key residues Trp400^6.48, Tyr430^7.43, Tyr206^5.58, and Tyr440^7.53 in CPK representation. (B) Trp400^6.48, Tyr430^7.43, and Tyr440^7.53 relocate their side chains during the receptor transition to the intermediate state. (C) Tyr206^5.58 reorients the side chain from the initial position between TM3 and TM6 to the lipid-exposed side of TM6, resulting in an alternative intermediate conformation. (D) During final transition to the active sate, Tyr206^5.58 and Tyr440^7.53 relocate their side chains toward each other, forming a hydrogen bond, and the cytoplasmic end of TM6 tilts ∼6 Å outward away from the TM bundle. Activation of the receptor significantly reduces the network strength of the intracellular domains in the G-protein-coupling site, which apparently facilitates association of the G protein and further stabilization of the receptor active conformation.