The β2 adrenergic receptor(β2AR)constitutes the largest class of both human membrane proteins and drug targets.The conformational change from an inactive state to an active state enables the receptor to transmit a signal from the extracellular ligand-binding site to an intracellular G protein,thereby initiating diverse intracellular signaling processes.However,the activation mechanism has been not unclear.Thereby,we used target molecular dynamics simulations(TMD)to investigate the activation mechanism of β2AR from the inactive to the active state.Several simulations were performed from the inactive state to the active with different restraint forces.The results show that the transition begins from the conformational changes of the extracellular region close to the ligand-binding site then to the G-protein-binding site.The G-protein-binding site usually adopted an intermediate conformation substantially different from crystallographic conformations of active and inactive states.Upon activation,the Lys305-Asp192 salt bridge formed between ECL3 and ECL2 becomes unstable and is finally disrupted.Intracellular end of helix 7 adopts a distorted α-helical conformation,and helix 3 moves apart from helix 6 with Tyr326 losing contact with helix 6.Our results also indicate that the comprised multiple receptor conformations have different implications for signal transition.