The Dynamic Process of b 2 -Adrenergic Receptor Activation

SUMMARY G-protein-coupled receptors (GPCRs) can modulate diverse signaling pathways, often in a ligand-specific manner. The full range of functionally relevant GPCR conformations is poorly understood. Here, we use NMR spectroscopy to characterize the conformational dynamics of the transmembrane core of the b2-adrenergic receptor (b2AR), a prototypical GPCR. We labeled b2AR with 13 CH3e-methionine and obtained HSQC spectra of unliganded receptor as well as receptor bound to an inverse agonist, an agonist, and a G-protein-mimetic nanobody. These studies provide evidence for conformational states not observed in crystal structures, as well as substantial conformational heterogeneity in agonistand inverse-agonist-bound preparations. They also show that for b2AR, unlike rhodopsin, an agonist alone does not stabilize a fully active conformation, suggesting that the conformational link between the agonist-binding pocket and the G-proteincoupling surface is not rigid. The observed heterogeneity may be important for b2AR’s ability to engage multiple signaling and regulatory proteins.