Revealing the Mechanism of Agonist-Mediated Cannabinoid Receptor 1 (CB1) Activation and Phospholipid-Mediated Allosteric Modulation.

Cannabinoid receptor 1 (CB1) mediates the functional responses of Δ⁹-tetrahydrocannabinol. Although progress has been made in understanding cannabinoid binding and receptor activation, detailed knowledge of the dynamics involved in the activation mechanism of CB1 is lacking. Here, we use recently determined CB1 crystal structures to analyze its transition from inactive to active state by performing unbiased microsecond-length molecular dynamics (MD) simulations, totaling 32 μs, with and without bound potent cannabinoid agonist CP-55940. CB1 activation is characterized by an upward axial movement of transmembrane (TM) helix 3, inward movement of TM7, and outward movement of TM6. These conformational changes collectively allow G_i protein docking, although fully active states of the receptor occur only transiently during MD simulations. Additionally, positive allosteric modulation of CB1 by anionic phospholipids is found to increase action of the bound agonist. Specifically, this involves protein-lipid interactions at intracellular loop 3, TM6, and ionic lock residue Arg214^3.50.