A Role for the Distal Carboxyl Tails in Generating the Novel Pharmacology and G Protein Activation Profile of μ and δ Opioid Receptor Hetero-oligomers

Opioid receptor pharmacology in vivo has predicted a greater number of receptor subtypes than explained by the profiles of the three cloned opioid receptors, and the functional dependence of the receptors on each other shown in gene-deleted animal models remains unexplained. One mechanism for such findings is the generation of novel signaling complexes by receptor hetero-oligomerization, which we previously showed results in significantly different pharmacology for μ and δ receptor hetero-oligomers compared with the individual receptors. In the present study, we show that deltorphin-II is a fully functional agonist of the μ-δ heteromer, which induced desensitization and inhibited adenylyl cyclase through a pertussis toxin-insensitive G protein. Activation of the μ-δ receptor heteromer resulted in preferential activation of Gα z , illustrated by incorporation of GTPγ 35 S, whereas activation of the individually expressed μ and δ receptors preferentially activated Gα i . The unique pharmacology of the μ-δ heteromer was dependent on the reciprocal involvement of the distal carboxyl tails of both receptors, so that truncation of the distal μ receptor carboxyl tail modified the δ-selective ligand-binding pocket, and truncation of the δ receptor distal carboxyl tail modified the μ-selective binding pocket. The distal carboxyl tails of both receptors also had a significant role in receptor interaction, as evidenced by the reduced ability to co-immunoprecipitate when the carboxyl tails were truncated. The interaction between μ and δ receptors occurred constitutively when the receptors were co-expressed, but did not occur when receptor expression was temporally separated, indicating that the hetero-oligomers were generated by a co-translational mechanism.