Identification of two distinct inactive conformations of the β 2 -adrenergic receptor reconciles structural and biochemical observations

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Abstract
Fully understanding the mechanisms of signaling proteins such as G protein-coupled receptors (GPCRs) will require the characterization of their conformational states and the pathways connecting those states. The recent crystal structures of the β2- and β1-adrenergic receptors in a nominally inactive state constituted a major advance toward this goal, but also raised new questions. Although earlier biochemical observations had suggested that these receptors possessed a set of contacts between helices 3 and 6, known as the ionic lock, which was believed to form a molecular switch for receptor activation, the crystal structures lacked these contacts. The unexpectedly broken ionic lock has raised questions about the true conformation(s) of the inactive state and the role of the ionic lock in receptor activation and signaling. To address these questions, we performed microsecond-timescale molecular dynamics simulations of the β2-adrenergic receptor (β2AR) in multiple wild-type and mutant forms. In wild-type simulations, the ionic lock formed reproducibly, bringing the intracellular ends of helices 3 and 6 together to adopt a conformation similar to that found in inactive rhodopsin. Our results suggest that inactive β2AR exists in equilibrium between conformations with the lock formed and the lock broken, whether or not the cocrystallized ligand is present. These findings, along with the formation of several secondary structural elements in the β2AR loops during our simulations, may provide a more comprehensive picture of the inactive state of the β-adrenergic receptors, reconciling the crystal structures with biochemical studies.