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Agonist‐induced signaling and trafficking of the μ‐opioid receptor: role of serine and threonine residues in the third cytoplasmic loop and C‐terminal domain
Author(s) -
Capeyrou Régine,
Riond Joëlle,
Corbani Maité,
Lepage Jean-François,
Bertin Brigitte,
Emorine Laurent J
Publication year - 1997
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/s0014-5793(97)01124-1
Subject(s) - adenylyl cyclase , receptor , etorphine , agonist , chemistry , μ opioid receptor , enzyme linked receptor , microbiology and biotechnology , g protein coupled receptor , forskolin , opioid , endocrinology , biology , biochemistry , (+) naloxone
The human μ‐opioid receptor and a mutant form, μS/T[i3+Cter]A, in which all Ser and Thr residues from the third cytoplasmic loop and C‐terminal domain were changed to Ala, were studied after expression in CHO‐K1 cells. Although the mutant receptors had similar affinities for agonists and EC 50 values for inhibition of adenylyl cyclase as compared to wild‐type receptors, the E max were almost 2‐fold decreased, suggesting a role of the mutated residues in G‐protein coupling. After chronic morphine or etorphine, the EC 50 values of the agonists were about 5‐fold increased at both receptors but the E max values were not altered; upon agonist withdrawal forskolin‐stimulated cAMP levels were increased to almost 200% of control levels. Sequestration and rapid down‐regulation of the μ‐opioid receptor were induced by DAGO and etorphine but not morphine. In contrast, the μS/T[i3+Cter]A receptor was not sequestered and was up‐regulated (150–380%) after treatment with agonists. The results indicate that the Ser and Thr residues in the third cytoplasmic loop and C‐terminus of the μ‐opioid receptor are not involved in the limited desensitization or in the adenylyl cyclase superactivation promoted by agonists but that their integrity and/or their phosphorylation is required in the intricate and coordinately regulated pathways involved in receptor signaling and trafficking.