Faculty Collaboration Database

Viewing rare conformations of the β₂ adrenergic receptor with pressure-resolved DEER spectroscopy. Proc Natl Acad Sci U S A 2020 Dec 15;117(50):31824-31831

Date

12/02/2020

Pubmed ID

33257561

Pubmed Central ID

PMC7749303

DOI

10.1073/pnas.2013904117

Scopus ID

2-s2.0-85098468007 (requires institutional sign-in at Scopus site)   27 Citations

Abstract

The β₂ adrenergic receptor (β₂AR) is an archetypal G protein coupled receptor (GPCR). One structural signature of GPCR activation is a large-scale movement (ca. 6 to 14 Å) of transmembrane helix 6 (TM6) to a conformation which binds and activates a cognate G protein. The β₂AR exhibits a low level of agonist-independent G protein activation. The structural origin of this basal activity and its suppression by inverse agonists is unknown but could involve a unique receptor conformation that promotes G protein activation. Alternatively, a conformational selection model proposes that a minor population of the canonical active receptor conformation exists in equilibrium with inactive forms, thus giving rise to basal activity of the ligand-free receptor. Previous spin-labeling and fluorescence resonance energy transfer experiments designed to monitor the positional distribution of TM6 did not detect the presence of the active conformation of ligand-free β₂AR. Here we employ spin-labeling and pressure-resolved double electron-electron resonance spectroscopy to reveal the presence of a minor population of unliganded receptor, with the signature outward TM6 displacement, in equilibrium with inactive conformations. Binding of inverse agonists suppresses this population. These results provide direct structural evidence in favor of a conformational selection model for basal activity in β₂AR and provide a mechanism for inverse agonism. In addition, they emphasize 1) the importance of minor populations in GPCR catalytic function; 2) the use of spin-labeling and variable-pressure electron paramagnetic resonance to reveal them in a membrane protein; and 3) the quantitative evaluation of their thermodynamic properties relative to the inactive forms, including free energy, partial molar volume, and compressibility.

Author List

Lerch MT, Matt RA, Masureel M, Elgeti M, Kumar KK, Hilger D, Foys B, Kobilka BK, Hubbell WL

Author

Michael Lerch PhD Assistant Professor in the Biophysics department at Medical College of Wisconsin

MESH terms used to index this publication - Major topics in bold

Magnetic Resonance Spectroscopy
Models, Molecular
Pressure
Protein Conformation, alpha-Helical
Receptors, Adrenergic, beta-2
Thermodynamics