Conformational Heterogeneity Underlying Divergent Signaling in Class A G Protein-Coupled Receptors

Class A G protein-coupled receptors (GPCRs) are targets for ∼36% of commercial drugs. GPCRs in their apo-forms exhibit conformational heterogeneity, and more than a single active and inactive conformation exists in equilibrium. Distinct transient conformational states can be significantly populated and can be coupled with different agonists, transducers, and effectors, giving rise to divergent signaling pathways. The characterization of such transient conformational states, which may have eluded identification by X-ray crystallography and cryogenic electron microscopy, can be achieved through a combination of biophysical techniques, such as nuclear magnetic resonance, double electron-electron resonance spectroscopy, single-molecule fluorescence microscopy, molecular dynamics simulations, and mass spectrometry. We review findings about the functional, conformational states of four class A GPCRs, including detailed results for the adenosine A_2A and β₂ adrenergic receptors and important observations for the β₁ and μ opioid receptors. The identification of ligands that can bind to distinct conformations, e.g., agonists that activate favorable pathways while inhibiting deleterious ones, represents an important goal in drug development.