Salmeterol is a partial agonist for the β2 adrenergic receptor (β2AR) and the first long-acting β2AR agonist to be widely used clinically for the treatment of asthma and chronic obstructive pulmonary disease. Salmeterol’s safety and mechanism of action have both been controversial. To understand its unusual pharmacological action and partial agonism, we obtained the crystal structure of salmeterol-bound β2AR in complex with an active-state-stabilizing nanobody. The structure reveals the location of the salmeterol exosite, where sequence differences between β1AR and β2AR explain the high receptor-subtype selectivity. A structural comparison with the β2AR bound to the full agonist epinephrine reveals differences in the hydrogen-bond network involving residues Ser2045.43 and Asn2936.55. Mutagenesis and biophysical studies suggested that these interactions lead to a distinct active-state conformation that is responsible for the partial efficacy of G-protein activation and the limited β-arrestin recruitment for salmeterol.
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Atomic coordinates and structure factors for the crystal structure have been deposited in the Protein Data Bank under accession code PDB 6CSY. Other data and results are available upon request.
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This work was supported by National Institutes of Health grant R01NS028471 (B.K.K.), Canadian Institute for Health Research foundation grant FDN-148431 (M.B.), an American Heart Association Postdoctoral fellowship (17POST33410958; M.M.) and Predoctoral Fellowship (13PRE17110027; J.P.M.), a studentship from the FRQ-S (L.-P.P.), the NIH Pharmacological Sciences Training Program (T32GM007767; J.P.M.) and the National Institutes of Health MIRA 1R35GM128641-01 (C.Z.). B.K.K. is supported by the Chan Zuckerberg Biohub. M.B. is supported as a Canada Research Chair in Signal Transduction and Molecular Pharmacology. The authors thank J. Gullingsrud for assistance with MD software.
The BRET-based biosensors used in the present study are licensed to Domain Therapeutics but are freely available from M.B. for noncommercial academic use. M.B. is the chair of the Scientific Advisory Board of Domain Therapeutics. B.K.K. is a cofounder of and consultant for ConfometRx.
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Masureel, M., Zou, Y., Picard, L. et al. Structural insights into binding specificity, efficacy and bias of a β2AR partial agonist. Nat Chem Biol 14, 1059–1066 (2018). https://doi.org/10.1038/s41589-018-0145-x
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