Molecular mechanism of allosteric modulation for the cannabinoid receptor CB1.

Given the promising clinical value of allosteric modulators of G protein-coupled-receptors (GPCRs), mechanistic understanding of how these modulators alter GPCR function is of significance. Here, we report the crystallographic and cryo-electron microscopy structures of the cannabinoid receptor CB1 bound to the positive allosteric modulator (PAM) ZCZ011. These structures show that ZCZ011 binds to an extrahelical site in the transmembrane 2 (TM2)-TM3-TM4 surface. Through (un)biased molecular dynamics simulations and mutagenesis experiments, we show that TM2 rearrangement is critical for the propagation of allosteric signals. ZCZ011 exerts a PAM effect by promoting TM2 rearrangement in favor of receptor activation and increasing the population of receptors that adopt an active conformation. In contrast, ORG27569, a negative allosteric modulator (NAM) of CB1, also binds to the TM2-TM3-TM4 surface and exerts a NAM effect by impeding the TM2 rearrangement. Our findings fill a gap in the understanding of CB1 allosteric regulation and could guide the rational design of CB1 allosteric modulators.