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 μ-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 Leu80, 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 Gα16 requires the participation of most of the intracellular regions, including the first intracellular loop.