MAPPING THE RECEPTOR DOMAINS CRITICAL FOR THE BINDING SELECTIVITY OF DELTA-OPIOID RECEPTOR LIGANDS

While a good deal has been learned about determinants of high affinity ligand/receptor interactions in G-protein-coupled receptors, less is known about mechanisms of ligand selectivity. The opioid receptors off er an excellent opportunity to study the mechanisms whereby structural ly very similar receptors discriminate between different but structura lly highly related ligands. In the current study, we use a series of c himeric constructs between the delta-opioid receptor and either the mu - or the kappa-opioid receptors to investigate the structural basis of binding selectivity of multiple classes of delta-opioid receptor sele ctive ligands. Our results demonstrate that a region containing the si xth transmembrane domain (TM6) and the third extracellular loop (EL3) in the delta-opioid receptor is absolutely critical for delta-opioid r eceptor selectivity. The introduction of this region into the kappa-op ioid receptor is sufficient to impart a delta profile for delta-opioid receptor selective alkaloids such as naltrindole and naltriben. In or der to locate the amino acid residues that may be involved in ligand s electivity in TM6 and EL3 of the delta-opioid receptor, several mutati ons were introduced into that region. These mutations showed different ial effects on peptide and alkaloid ligands. In addition, none of the individual mutations alone could account for the changes exhibited by the chimeric receptors. We conclude that the selectivity of most delta -opioid ligands is achieved through their interaction with many differ ent residues in the TM6/EL3 region. Our results also support a view th at the extracellular domains of peptide receptors may provide the basi s of a sorting mechanism for ligand selectivity.