MOVING FROM THE ORPHANIN FQ RECEPTOR TO AN OPIOID RECEPTOR USING 4 POINT MUTATIONS

It is unclear how receptor/ligand families that are evolutionarily clo sely related achieve functional separation. To address this question, we focus here on the newly discovered Orphanin FQ, a peptide homologou s to the opioid peptide Dynorphin, and its receptor, the Orphanin FQ r eceptor, which is highly homologous to the opioid receptors. In spite of this high degree of homology in terms of both ligands and receptors , there is little direct cross-talk between the Orphanin FQ system and the endogenous opioid system. Thus, the opioid peptides show either r elatively low affinity or no affinity toward the Orphanin FQ receptor; conversely, Orphanin FQ has no affinity toward any of the opioid rece ptors. We sought to investigate the molecular basis of such discrimina tion by attempting to reverse it and endowing the Orphanin FQ receptor with the ability to bind opioids. We report that by mutating as few a s four amino acids, we can produce a receptor that recognizes pro Dyno rphin products with very high affinity and yet still binds Orphanin F- Q as well as the wild-type receptor. This suggests that the Orphanin F Q receptor has developed features that specifically exclude the opioid s and that these features are distinct from those required for the hig h affinity binding of its own endogenous ligand.