Differential binding domains of peptide and non-peptide ligands in the cloned rat kappa opioid receptor

This study was to identify specific regions in kappa opioid receptors that accounted for binding selectivity of kappa ligands. Six chimeric mu/kappa receptors were constructed from cloned rat kappa and mu opioid receptors and transiently expressed in COS-1 cells. All six chimeric mu/kappa receptors bound [3H] diprenorphine with high affinities, indicating that these chimeras retain opioid receptor conformation. Binding affinities of three peptide ligands (dynorphin A, alpha-neo-endorphin, and dynorphin B) and three nonpeptide ligands (norbinaltorphimine, U50,488H, and U69,593) for chimeras were determined and compared to those for mu and kappa opioid receptors. The second extracellular loop and the adjoining C-terminal portion of the fourth transmembrane helix were essential for the high affinity binding of dynorphin A, alpha-neo-endorphin, and dynorphin B to the kappa receptor. The third extracellular loop and the sixth and seventh transmembrane helices played an important role in determining the selectivity of nor-binaltorphimine for the kappa over the mu receptor. U50,488H and U69,593 appeared to require the whole kappa receptor except the second extracellular loop to attain high affinity binding. Thus, the kappa opioid receptor has differential binding domains for peptide and non-peptide ligands.