Abstract

Lys192 in the third transmembrane domain of the human CB1 cannabinoid receptor was converted to an alanine to study its role in receptor recognition and activation by agonists. HU-210, CP-55940, WIN55212-2, and anandamide, four cannabinoid agonists with distinct chemical structures, were used to characterize the wild-type and the mutant receptors. In human embryonal kidney 293 cells stably expressing the wild-type receptor, specific binding to [3H]WIN55212-2 and inhibition of cAMP accumulation by cannabinoid agonists were demonstrated, with different ligands exhibiting the expected rank orders of potency and stereoselectivity in competition binding and functional assays. In cells expressing the mutant receptor, the binding affinity of the receptor for [3H]WIN55212-2 was only slightly affected (the Kd for the mutant receptor was twice that of the wild-type), and the ability of WIN55212-2 to inhibit cAMP accumulation was unchanged. However, HU-210, CP-55940, and anandamide were unable to compete for [3H]WIN55212-2 binding to the mutant receptor. In addition, the potencies of HU-210, CP-55940, and anandamide in inhibiting cAMP accumulation were reduced by > 100-fold. These results demonstrate that Lys192 is critical for receptor binding by HU-210, CP-55940, and anandamide. Because Lys192 is not important for receptor binding and activation by WIN55212-2, WIN55212-2 must interact with the cannabinoid receptor through at least one point of interaction that is distinct from those of the three other agonists.