A quantitative covalent labeling strategy reveals that multiple ligand-specific conformational states are present in the G protein–coupled β2-adrenergic receptor. Their existence may underlie 'biased agonism', which describes the differential abilities of agonists to activate distinct signaling mechanisms downstream of GPCRs.
- Alem W Kahsai
- Kunhong Xiao
- Robert J Lefkowitz
