The Second Intracellular Loop of the Human Cannabinoid CB2 Receptor Governs G Protein Coupling in Coordination with the Carboxyl Terminal Domain

The major effects of cannabinoids and endocannabinoids are mediated via two G protein-coupled receptors, CB1 and CB2, elucidation of the mechanism and structural determinants of the CB2 receptor coupling with G proteins will have a significant impact on drug discovery. In the present study, we systematically investigated the role of the intracellular loops in the interaction of the CB2 receptor with G proteins using chimeric receptors alongside the characterization of cAMP accumulation and ERK1/2 phosphorylation. We provided evidence that ICL2 was significantly involved in G protein coupling in coordination with the C-terminal end. Moreover, a single alanine substitution of the Pro-139 in the CB2 receptor that corresponds to Leu-222 in the CB1 receptor resulted in a moderate impairment in the inhibition of cAMP accumulation, whereas mutants P139F, P139M and P139L were able to couple to the G(s) protein in a CRE-driven luciferase assay. With the ERK activation experiments, we further found that P139L has the ability to activate ERK through both G(i)- and G(s)-mediated pathways. Our findings defined an essential role of the second intracellular loop of the CB2 receptor in coordination with the C-terminal tail in G protein coupling and receptor activation.