Multiple ligand-specific conformations of the β2-adrenergic receptor

A quantitative covalent labeling strategy reveals that multiple ligand-specific conformational states are present in the G protein–coupled β 2 -adrenergic receptor. Their existence may underlie 'biased agonism', which describes the differential abilities of agonists to activate distinct signaling mechanisms downstream of GPCRs. Seven-transmembrane receptors (7TMRs), also called G protein–coupled receptors (GPCRs), represent the largest class of drug targets, and they can signal through several distinct mechanisms including those mediated by G proteins and the multifunctional adaptor proteins β-arrestins. Moreover, several receptor ligands with differential efficacies toward these distinct signaling pathways have been identified. However, the structural basis and mechanism underlying this 'biased agonism' remains largely unknown. Here, we develop a quantitative mass spectrometry strategy that measures specific reactivities of individual side chains to investigate dynamic conformational changes in the β 2 -adrenergic receptor occupied by nine functionally distinct ligands. Unexpectedly, only a minority of residues showed reactivity patterns consistent with classical agonism, whereas the majority showed distinct patterns of reactivity even between functionally similar ligands. These findings demonstrate, contrary to two-state models for receptor activity, that there is significant variability in receptor conformations induced by different ligands, which has significant implications for the design of new therapeutic agents.