The NPXXY Motif Regulates beta-Arrestin Recruitment by the CB1 Cannabinoid Receptor

Background: Activation of signaling effectors by G-protein coupled receptors (GPCRs) depends on different molecular mechanisms triggered by conserved amino acid residues. Although studies have focused on the G-protein signaling state, the mechanism for beta-arrestin signaling by CB1 is not yet well defined. Studies have indicated that transmembrane helix 7 (TMH7) and the highly conserved NPXXY motif can be subject to different conformational changes in response to biased ligands and could therefore participate in a molecular mechanism to trigger beta-arrestin recruitment.Objective: To investigate the effect of mutations in the NPXXY motif on different signaling pathways activated by the CB1 receptor.Materials and Methods: Point mutations of the NPXXY motif and associated residues were generated in the CB1 receptor using site-directed mutagenesis and transfection into HEK-293 cells. Signaling by wild-type and mutant receptors was analyzed by quantifying inhibition of cAMP, and by beta-arrestin recruitment assays.Results: We found that N7.49 and Y7.53 are essential for beta-arrestin recruitment by CB1. N7.49A and Y7.53F impair beta-arrestin signaling, with no effect on G-protein signaling. We found a regulatory role for residue I2.43; I2.43 interacts with Y7.53, affecting its positioning. Reducing steric bulk at I2.43 (I2.43A) enhances beta-arrestin1 recruitment, while introducing a polar residue (I2.43T) reduces beta-arrestin recruitment.Conclusions: These findings point to a novel mechanism for beta-arrestin recruitment, implicating amino acids in the NPXXY motif as critical for the putative beta-arrestin biased conformational state of Class A GPCRs.