Elucidation of a dynamic interplay between a beta-2 adrenergic receptor, its agonist, and stimulatory G protein.

G protein-coupled receptors (GPCRs) represent the largest group of membrane receptors for transmembrane signal transduction. Ligand-induced activation of GPCRs triggers G protein activation followed by various signaling cascades. Understanding the structural and energetic determinants of ligand binding to GPCRs and GPCRs to G proteins is crucial to the design of pharmacological treatments targeting specific conformations of these proteins to precisely control their signaling properties. In this study, we focused on interactions of a prototypical GPCR, beta-2 adrenergic receptor (βAR), with its endogenous agonist, norepinephrine (NE), and the stimulatory G protein (G). Using molecular dynamics (MD) simulations, we demonstrated the stabilization of cationic NE, NE(+), binding to βAR by G protein recruitment, in line with experimental observations. We also captured the partial dissociation of the ligand from βAR and the conformational interconversions of G between closed and open conformations in the NE(+)-βAR-G ternary complex while it is still bound to the receptor. The variation of NE(+) binding poses was found to alter G α subunit (Gα) conformational transitions. Our simulations showed that the interdomain movement and the stacking of Gα α1 and α5 helices are significant for increasing the distance between the Gα and βAR, which may indicate a partial dissociation of Gα The distance increase commences when Gα is predominantly in an open state and can be triggered by the intracellular loop 3 (ICL3) of βAR interacting with Gα, causing conformational changes of the α5 helix. Our results help explain molecular mechanisms of ligand and GPCR-mediated modulation of G protein activation.