The first and third intracellular loops together with the carboxy terminal tail of the δ-opioid receptor contribute toward functional interaction with Gα₁₆

Opioid peptides exert their regulatory effects on both central and pheripheral nervous systems via multiple opioid receptors that are linked to seemingly identical sets of guanine nucleotide-binding regulatory proteins (G proteins). In contrast to the p-opioid receptor, the δ-opioid receptor can efficiently stimulate phospholipase C via G₁₆. We used a series of μ/δ-opioid receptor chimeras to examine the involvement of intracellular receptor domains in the recognition of G₁₆. After ascertaining that the chimeras can bind opioid ligands with high affinity and elicit inhibition of adenylyl cyclase, COS-7 cells were cotransfected with cDNAs encoding Gα₁₆ and a μ/δ-opioid receptor chimera and assayed for [D-Ala²,D-Leu⁵]enkephalin-induced stimulation of phospholipase C. Our results indicate that (i) the carboxy terminal tail of the δ-opioid receptor is necessary but insufficient for conferring coupling to Gα₁₆, (ii) the third inner loop together with the carboxy terminal tail of the δ-opioid receptor can provide sufficient contact domains for Gα₁₆, and (iii) the first inner loop of the δ-opioid receptor, in particular Leu ⁸⁰, as well as the fifth transmembrane domain and/or the third extracellular loop may also contribute in defining the fidelity of interaction between the δ-opioid receptor and Gα₁₆. These results indicate that efficient coupling of the δ-opioid receptor to Gα ₁₆ requires the participation of most of the intracellular regions, including the first intracellular loop.