Complexes of α2A-ARs (α2A-adrenergic receptors) and MORs (μ-opioid receptors), probably hetero-oligomers, were detected by co-immunoisolation after extraction from HEK-293 cells (human embryonic kidney 293 cells). Functional communication between these receptors is revealed by α2A-AR activation of a pertussis toxin-insensitive Giα subunit (termed as Gi1) when fused with the MOR and evaluated in membranes from pertussis toxin-treated cells. However, the α2A-AR does not require transactivation through MOR, since quantitatively indistinguishable results were observed in cells co-expressing α2A-AR and a fusion protein of Gi1 with the first transmembrane span of MOR (myc–MOR-TM1). Functional cross-talk among these α2A-AR–MOR complexes does not occur for internalization profiles; incubation with adrenaline (epinephrine) leads to endocytosis of α2A-AR but not MOR, while incubation with DAMGO ([D-Ala,NMe-Phe,Gly-ol]enkephalin) leads to endocytosis of MOR but not α2A-AR in cells co-expressing both the receptors. Hence, α2A-AR and MOR hetero-oligomers, although they occur, do not have an obligatory functional influence on one another in the paradigms studied.
Selective interactions of μ-opioid receptors with pertussis toxin-sensitive G proteins: involvement of the third intracellular loop and the c-terminal tail in coupling
