Heterodimerization of somatostatin and opioid receptors cross-modulates phosphorylation, internalization, and desensitization

Abstract

Heterodimerization has been shown to modulate the ligand binding, signaling, and trafficking properties of G protein-coupled receptors. However, to what extent heterodimerization may alter agonist-induced phosphorylation and desensitization of these receptors has not been documented. We have recently shown that heterodimerization of sst2A and sst3 somatostatin receptors results in inactivation of sst3 receptor function (Pfeiffer, M., Koch, T., Schröder, H., Klutzny, M., Kirscht S., Kreienkamp, H. J., Hdllt, V., and Schulz, S. (2001) J. Biol. Chem. 276, 14027-14036). Here we examine dimerization of the sst2A somatostatin receptor and the μ-opioid receptor, members of closely related G protein-coupled receptor families. In coimmunoprecipitation studies using differentially epitope-tagged receptors, we provide direct evidence for heterodimerization of sst2A and MOR1 in human embryonic kidney 293 cells. Unlike heteromeric assembly of sst2A and sst3, sst2A-MOR1 heterodimerization did not substantially alter the ligand binding or coupling properties of these receptors. However, exposure of the sst2A-MOR1 heterodimer to the sst2A-selective ligand L-779,976 induced phosphorylation, internalization, and desensitization of sst2A as well as MOR1. Similarly, exposure of the sst2A-MOR1 heterodimer to the μ-selective ligand [ D.Ala2,Me-Phe4,Gly5-ol]enkephalin induced phosphorylation and desensitization of both MOR1 and sst2A but not internalization of sst2A. Cross-phosphorylation and cross-desensitization of the sst2A-MOR1 SSt2A-MoRl heterodimer were selective; they were neither observed with the sst2A-sst3 heterodimer nor with the endogenously expressed lysophosphatidic acid receptor. Heterodimerization may thus represent a novel regulatory mechanism that could either restrict or enhance phosphorylation and desensitization of G protein-coupled receptors.