Differential helical orientations among related G protein-coupled receptors provide a novel mechanism for selectivity -: Studies with salvinorin A and the κ-opioid receptor

Salvinorin A, the active component of the hallucinogenic sage Salvia divinorum, is an apparently selective and highly potent kappa-opioid receptor (KOR) agonist. Salvinorin A is unique among ligands for peptidergic G protein-coupled receptors in being nonnitrogenous and lipid-like in character. To examine the molecular basis for the subtype-selective binding of salvinorin A, we utilized an integrated approach using chimeric opioid receptors, site-directed mutagenesis, the substituted cysteine accessibility method, and molecular modeling and dynamics studies. We discovered that helix 2 is required for salvinorin A binding to KOR and that two residues (Val-108(2.53) and Val-118(2.63)) confer subtype selectivity. Intriguingly, molecular modeling studies predicted that these loci exhibit an indirect effect on salvinorin A binding, presumably through rotation of helix 2. Significantly, and in agreement with our in silico predictions, substituted cysteine accessibility method analysis of helix 2 comparing KOR and the delta-opioid receptor, which has negligible affinity for salvinorin A, revealed that residues known to be important for salvinorin A binding exhibit a differential pattern of water accessibility. These findings imply that differences in the helical orientation of helix 2 are critical for the selectivity of salvinorin A binding to KOR and provide a structurally novel basis for ligand selectivity.