Quantitative encoding of the effect of a partial agonist on individual opioid receptors by multisite phosphorylation and threshold detection

EK Lau, M Trester-Zedlitz, JC Trinidad, SJ Kotowski… - Science …, 2011 - science.org
EK Lau, M Trester-Zedlitz, JC Trinidad, SJ Kotowski, AN Krutchinsky, AL Burlingame…
Science signaling, 2011science.org
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 …
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.
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