Endomorphin-1 discriminates the mu-opioid receptor from the delta- and kappa-opioid receptors by recognizing the difference in multiple regions

Endomorphin-1 is a novel endogenous peptide that is highly selective for th e mu-opioid receptor over the delta- and kappa-opioid receptors. The struct ural basis of high selectivity of endomorphin-1 to the mu-opioid receptor w as examined using chimeric receptors between mu- and delta-opioid receptors and those between mu- and k-opioid receptors. The chimeric receptors were constructed by using restriction enzyme sites intrinsically possessed by or introduced to the mu-, delta- and kappa-opioid receptor cDNAs. The junctio ns for the construction were located at the first intracellular loop (Bbs I site), third transmembrane domain (Afl III site) and fifth transmembrane d omain (Bgl II site). The competitive binding assay using chimeric receptors revealed that the region from the Bbs I site to the Afl III site, includin g the first extracellular loop, contributes to the discrimination between m u- and delta-opioid receptors by endomorphin-1 more than any other regions. However, the region from the Afl III site to the Bgl II site and that from the Bgl II site to the carboxy terminal also somewhat contribute to the di scrimination between mu- and delta-opioid receptors. For the discrimination between mu- and K-opioid receptors, two regions, that is, the region from the Bbs I site to the Afl III site and that from the Bgl II site to the car boxy terminal, were shown to be important. The present results show that en domorphin-1 discriminates the mu-opioid receptor from the other two types o f opioid receptors by recognizing the differences in several amino acid res idues widely distributed through the receptor structure. We previously repo rted that DAMGO, a synthetic highly mu-selective peptide, discriminates bet ween mu- and delta-opioid receptors by recognizing the difference in only o ne amino acid residue and discriminates between mu- and kappa-opioid recept ors by recognizing the difference in four residues localized in the restric ted region. Although both endomorphin-1 and DAMGO are mu-opioid receptor se lective peptides, molecular mechanisms for mu-selectivity are different bet ween these peptides.