Integrity of Extracellular Loop 1 of the Human Cannabinoid Receptor 1 is Critical for High-affinity Binding of the Ligand CP 55,940 but Not SR 141716A

Like other G-protein coupled receptors with hydrophobic ligands, the human cannabinoid receptor 1 (CB1) is thought to bind its ligands within the transmembrane region of the receptor. However, for some of these receptors the extracellular loops (ECs) have also been shown to play a role in ligand recognition and selectivity. We have taken a mutagenesis approach to examine the role of the amino terminus, EC1, and EC3 of CB1 in ligand binding. Eight mutant receptors, each with a dipeptide insertion, were constructed, expressed, and evaluated for binding to the cannabinoid ligands (-)-cis-3[2-hydroxy-4-(1',1'-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP 55,940) and N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR 141716A). Mutants with insertions in the membrane distal region of the amino terminus or EC3 maintained affinity for both ligands. Those with insertions in the membrane proximal region of the amino terminus or EC1 exhibited a loss of affinity for CP 55,940 while retaining wild-type affinity for SR 141716A. Representative mutants were analyzed for agonist-induced inhibition of cyclic AMP accumulation, and the results indicated that G-protein coupling remained intact. Alanine substitution mutants were made to address whether it was the perturbation of the overall structure of the region or the displacement of particular side chains that was responsible for the loss of CP 55,940 binding. We conclude that a structurally intact EC1, but not the comparably short EC3, is essential for high-affinity CP 55,940 binding.