UC Davis

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 (β₂AR), with its endogenous agonist, norepinephrine (NE), and the stimulatory G protein (G_s). Using molecular dynamics (MD) simulations, we demonstrated the stabilization of cationic NE, NE(+), binding to β₂AR by G_s protein recruitment, in line with experimental observations. We also captured the partial dissociation of the ligand from β₂AR and the conformational interconversions of G_s between closed and open conformations in the NE(+)-β₂AR-G_s ternary complex while it is still bound to the receptor. The variation of NE(+) binding poses was found to alter G_s α subunit (G_sα) conformational transitions. Our simulations showed that the interdomain movement and the stacking of G_sα α1 and α5 helices are significant for increasing the distance between the G_sα and β₂AR, which may indicate a partial dissociation of G_sα The distance increase commences when G_sα is predominantly in an open state and can be triggered by the intracellular loop 3 (ICL3) of β₂AR interacting with G_sα, causing conformational changes of the α5 helix. Our results help explain molecular mechanisms of ligand and GPCR-mediated modulation of G protein activation.