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 opioi d receptors and transiently expressed in COS-1 cells. All six chimeric mu/kappa receptors bound [H-3] diprenorphine with high affinities, in dicating that these chimeras retain opioid receptor conformation. Bind ing affinities of three peptide ligands (dynorphin A, alpha-neo-endorp hin, and dynorphin B) and three nonpeptide ligands (nor-binaltorphimin e, U50,488H, and U69,593) for chimeras were determined and compared to those for mu and kappa opioid receptors. The second extracellular loo p and the adjoining C-terminal portion of the fourth transmembrane hel ix were essential for the high affinity binding of dynorphin A, alpha- neo-endorphin, and dynorphin B to the kappa receptor. The third extrac ellular 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 r equire the whole kappa receptor except the second extracellular loop t o attain high affinity binding. Thus, the kappa opioid receptor has di fferential binding domains for peptide and non-peptide ligands.