The role of the tryptophan-rich allosteric network and sodium egress in GPCR activation

The human adenosine A_2A receptor (A_2AR) is a prototypical member of the class A family of G Protein-Coupled Receptors (GPCRs), which are engaged by over one-third of FDA-approved drugs. Here, we used ¹⁹F NMR to simultaneously evaluate functional states from the perspective of a CF₃-tag on transmembrane helix-6 (TM6) and all 5-fluorotryptophan reporters. Whereas spectra from the TM6 tag reveal a dynamic conformational ensemble, the tryptophan spectra exhibit more discrete ligand-dependent states. These ligand-dependent signatures provide insights into microswitches including a universal toggle switch which interfaces with a sodium binding pocket. Over 560 GPCRs share this toggle switch - sodium pocket cluster, implying a fundamental role in activation. Computational rigidity-theory reveals tryptophan reporters reside along prominent allosteric activation pathways, underscoring their roles in mechanical signal propagation and corroborating NMR observations. Among them, W246^6.48, associated with the toggle switch, plays a critical role in regulation of allosteric networks spanning the orthosteric pocket and extending via tryptophan-rich pathways through the receptor to the A_2AR-Gβ interface and the nucleotide pocket in G_sα. While higher sodium concentrations (~100 mM) predictably stabilize the inactive conformation of the receptor, lower concentrations (below 40 mM) greatly enhanced the presence of the activation ensemble and in particular, the precoupled state, suggesting that the release of sodium from the conserved pocket-either through basal conditions for the apo receptor, or upon binding of the agonist-enables sampling of a precoupled state, ultimately needed to initiate activation and coupling.