Abstract
Signal transmission and regulation of G-protein-coupled receptors (GPCRs) by extra- and intracellular ligands occurs via modulation of complex conformational equilibria, but their exact kinetic details and underlying atomic mechanisms are unknown. Here we quantified these dynamic equilibria in the β1-adrenergic receptor in its apo form and seven ligand complexes using 1H/15N NMR spectroscopy. We observe three major exchanging conformations: an inactive conformation (Ci), a preactive conformation (Cp) and an active conformation (Ca), which becomes fully populated in a ternary complex with a G protein mimicking nanobody. The Ci ↔ Cp exchange occurs on the microsecond scale, the Cp ↔ Ca exchange is slower than ~5 ms and only occurs in the presence of two highly conserved tyrosines (Y5.58, Y7.53), which stabilize the active conformation of TM6. The Cp→Ca chemical shift changes indicate a pivoting motion of the entire TM6 that couples the effector site to the orthosteric ligand pocket.