Implication of the first and third extracellular loops of the mu-opioid receptor in the formation of the ligand binding site: a study using chimeric mu-opioid/angiotensin receptors.
Recent studies on chimeric mu/delta-, mu/kappa- and delta/kappa-opioid receptors have suggested that extracellular loops of the receptors were involved in the discriminatory binding of selective ligands by controlling their entry into the transmembrane binding site. Since homochimeric opioid receptors are mostly informative in terms of selectivity, the role of extracellular loops was examined here by studying heterochimeric mu receptors where the totality or parts of extracellular loops were replaced by the corresponding regions of the receptor for angiotensin II. Chimeric mu receptors with extracellular loop EL1 or EL3 originating from the angiotensin receptor had 100-fold decreased affinities for opioids; the length of the first extracellular loop, which is one residue longer in angiotensin than mu receptors, was shown to be responsible for this situation. Substitution of the mu receptor second extracellular loop by that of the angiotensin receptor diminished by approximately 10-fold the affinities for opioids. Since all chimeras had altered affinities for selective and nonselective ligands, we propose that extracellular domains of the mu receptor, particularly the first and third loops, constrain the relative positioning of the connected transmembrane domains where selective as well as nonselective contact points form the opioid binding site.