A high-resolution description of β₁-adrenergic receptor functional dynamics and allosteric coupling from backbone NMR

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 (C_i), a preactive conformation (C_p) and an active conformation (C_a), which becomes fully populated in a ternary complex with a G protein mimicking nanobody. The C_i ↔ C_p exchange occurs on the microsecond scale, the C_p ↔ C_a exchange is slower than ~5 ms and only occurs in the presence of two highly conserved tyrosines (Y^5.58, Y^7.53), which stabilize the active conformation of TM6. The C_p→C_a chemical shift changes indicate a pivoting motion of the entire TM6 that couples the effector site to the orthosteric ligand pocket.