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

A cDNA encoding the rat mu-opioid receptor was expressed stably in a Rat-1 fibroblast cell line. Expression of this receptor was demonstrated with specific binding of the mu-opioid selective ligand [H-3][D-Ala2,N-MePhe4,Gly5-ol]-enkephalin ([H-3]DAMGO). In membranes of clone mu 11 cells DAMGO produced a robust, concentration-dependent stimulation of basal high affinity GTPase activity. Cholera toxin-catalyzed [P-32]ADP-ribosylation in membranes of this clone labelled a 40 kDa G(i) family polypeptide(s) that was markedly enhanced by the addition of DAMGO, Antisera against G(i)2 alpha and G(i)3 alpha were both able to immunoprecipitate a [P-32]-radiolabelled 40 kDa polypeptide(s) from DAMGO and cholera-toxin treated membranes of clone mu 11, indicating that the mu-opioid receptor was able to interact effectively with both G(i)2 and G,(i)3 in Rat-1 fibroblasts. A series of peptides derived from the delta-opioid receptor sequence were assessed for their ability to modify agonist-stimulated G protein activation and [H-3] agonist binding to the receptor. In membranes from the clone mu 11, specific binding of [H-3]DAMGO was reduced by peptides corresponding to the NH2-terminal region of the third intracellular loop (i3.1) and the carboxyl-terminal tail (i4) of this receptor. Agonist stimulated GTPase activity and DAMGO dependent cholera toxin-catalyzed [P-32]ADP-ribosylation were inhibited by peptides derived from the proximal (i3.1) and the distal portion (i3.3) of the third intracellular loop. Peptide i3.1 also inhibited DAMGO-stimulated [S-35]guanosine-5'-O-(3-thio)triphosphate ([S-35]GTP gamma S) binding in the same membranes. In contrast, peptides derived from the second intracellular loop were without any effect. (C) 1997 Elsevier Science B.V.