Imaging and quantifying cytoplasmic tail dynamics of membrane receptors in living cells

The cytoplasmic tails (C-tails) of membrane receptors are fundamental hubs for signal transductions. Yet directly investigating their dynamics in living cells is challenging. We developed a single-molecule technique to sense C-tail's conformational transitions. It was applied to investigate the ligand-induced activation of {beta}2AR, a prototypical G protein-coupled receptor (GPCR). Our data elucidate how the receptor transduces ligand binding to cell-level outcomes, with the ligand efficacy being interpreted in terms of the association and disassociation rates of the G protein. Moreover, upon G protein binding, not only the C-tail is displaced from the intracellular surface of the receptor but also the eighth helix H8 is dissociated from the plasma membrane. This work establishes a robust framework to directly decipher the activation of membrane receptors without interferences from overexpressed downstream transducers.