Abstract

A large body of evidence implicates the second and third intracellular loops and the carboxyl-terminal portion of many G protein-coupled receptors as sites responsible for the interaction to G proteins. We synthesized a number of peptides from selected sites of the murine delta-opioid receptor and measured their ability to modify ligand-stimulated G protein activation and 3H agonist binding to the receptor. In membranes from Rat-1 fibroblasts transfected to express the murine delta-opioid receptor stably (clone D2 cells), the delta-opioid agonist [D-Ser2-Leu5-Thr6]enkephalin (DSLET) stimulated high affinity GTPase activity, which was inhibited by peptides that are derived from the proximal (i3.1) and the distal portions (i3.3) of the third intracellular loop with IC50 values of 15 +/- 5 and 50 +/- 4 microM, respectively. Peptides i3.1 and i3.3 inhibited DSLET-stimulated [35S]guanosine 5'-O-thiotriphosphate binding in the same membranes. However, a peptide designated i4, which was derived from a juxtamembranous region of the carboxyl-terminal tail of the delta-opioid receptor, failed to alter agonist-mediated high affinity GTPase activity or agonist-driven [35S]guanosine 5'-O-thiotriphosphate binding. Specific binding of [3H]DSLET to membrane preparations from clone D2 was reduced by peptides i3.1 and i4. Combinations of these peptides abolished detectable [3H]DSLET binding in the same membranes. Peptides i3.1 and i3.3 also destabilized the high affinity state of the receptor as assessed in 3H agonist binding on membranes from neuroblastoma X glioma (NG108-15) hybrid cells, which express the delta-opioid receptor endogenously; furthermore, delta-opioid receptor-stimulated GTPase activity in the same membranes was inhibited by peptides i3.1 and i3.3 but i4 was inactive. In contrast, peptides derived from the second intracellular loop (i2.1 and i2.2), an intermediate portion of the third intracellular loop (i3.2), and the extreme amino-terminal region of the receptor were without effect in these assays. These observations indicate that although peptides i3.1, i3.3, and i4 act via different mechanisms, they provide evidence that at least two sites of the third intracellular loop and part of the carboxyl-terminal tail of the delta-opioid receptor are important in the interaction between this receptor and cellular G proteins. Collectively, these results provide novel information about regions of the delta-opioid receptor that are involved in G protein coupling and high affinity agonist binding.