An Aromatic Microdomain at the Cannabinoid CB1 Receptor Constitutes an Agonist/Inverse Agonist Binding Region

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The cannabinoid CB1 receptor transmembrane helix (TMH) 3−4−5−6 region includes an aromatic microdomain comprised of residues F3.25, F3.36, W4.64, Y5.39, W5.43, and W6.48. In previous work, we have demonstrated that aromaticity at position 5.39 in CB1 is crucial for proper function of CB1. Modeling studies reported here suggest that in the inactive state of CB1, the binding site of the CB1 inverse agonist/antagonist SR141716A is within the TMH3−4−5−6 aromatic microdomain and involves direct aromatic stacking interactions with F3.36, Y5.39, and W5.43, as well as hydrogen bonding with K3.28. Further, modeling studies suggest that in the active state of CB1, the CB agonist WIN55,212−2 binds in this same aromatic microdomain, with direct aromatic stacking interactions with F3.36, W5.43, and W6.48. In contrast, in the binding pocket model, the CB agonist anandamide binds in the TMH2−3−6−7 region in which hydrogen bonding and C−H···π interactions appear to be important. Only one TMH3 aromatic residue, F3.25, was found to be part of the anandamide binding pocket. To probe the importance of the TMH3−4−5−6 aromatic microdomain to ligand binding, stable transfected cell lines were created for single-point mutations of each aromatic microdomain residue to alanine. Improper cellular expression of the W4.64A was observed and precluded further characterization of this mutation. The affinity of the cannabinoid agonist CP55,940 was unaffected by the F3.25A, F3.36A, W5.43A, or W6.48A mutations, making CP55,940 an appropriate choice as the radioligand for binding studies. The binding of SR141716A and WIN55,212−2 were found to be affected by the F3.36A, W5.43A, and W6.48A mutations, suggesting that these residues are part of the binding site for these two ligands. Only the F3.25A mutation was found to affect the binding of anandamide, suggesting a divergence in binding site regions for anandamide from WIN55,212−2, as well as SR141716A. Taken together, these results support modeling studies that identify the TMH3−4−5−6 aromatic microdomain as the binding region of SR141716A and WIN55,212−2, but not of anandamide.