title: Quantitative encoding of the effect of a partial agonist on individual opioid receptors by multisite phosphorylation and threshold detection.
abstract: In comparison to endogenous ligands of seven-transmembrane receptors, which typically act as full agonists, many drugs act as partial agonists. Partial agonism is best described as a "macroscopic" property that is manifest at the level of physiological systems or cell populations; however, whether partial agonists also encode discrete regulatory information at the "microscopic" level of individual receptors is not known. Here, we addressed this question by focusing on morphine, a partial agonist drug for μ-type opioid peptide receptors (MORs), and by combining quantitative mass spectrometry with cell biological analysis to investigate the reduced efficacy of morphine, compared to that of a peptide full agonist, in promoting receptor endocytosis. We showed that these chemically distinct ligands produced a complex and qualitatively similar mixture of phosphorylated opioid receptor forms in intact cells. Quantitatively, however, the different agonists promoted disproportionate multisite phosphorylation of a specific serine and threonine motif, and we found that modification at more than one residue was essential for the efficient recruitment of the adaptor protein β-arrestin that mediated subsequent endocytosis of MORs. Thus, quantitative encoding of agonist-selective endocytosis at the level of individual opioid receptors was based on the conserved biochemical principles of multisite phosphorylation and threshold detection.
authors.affiliation: Department of Psychiatry, University of California, San Francisco, CA 94158, USA.
authors.forename: Elaine K
authors.initials: EK
authors.lastname: Lau
authors.forename: Michelle
authors.initials: M
authors.lastname: Trester-Zedlitz
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Trinidad
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Kotowski
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Krutchinsky
authors.forename: Alma L
authors.initials: AL
authors.lastname: Burlingame
authors.forename: Mark
authors.initials: M
authors.lastname: von Zastrow
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D004705:Endocytosis / Q000187:drug effects*; D006801:Humans; D051379:Mice; D009020:Morphine / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology*; D010766:Phosphorylation / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000235:genetics / Q000378:metabolism*
chemical_list: D009294:Narcotics; D017450:Receptors, Opioid, mu; D009020:Morphine