Abstract
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 (β(2)AR), 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 β(2)AR by G(s) protein recruitment, in line with experimental observations. We also captured the partial dissociation of the ligand from β(2)AR and the conformational interconversions of G(s) between closed and open conformations in the NE(+)-β(2)AR-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(s)α) conformational transitions. Our simulations showed that the interdomain movement and the stacking of G(s)α α1 and α5 helices are significant for increasing the distance between the G(s)α and β(2)AR, which may indicate a partial dissociation of G(s)α The distance increase commences when G(s)α is predominantly in an open state and can be triggered by the intracellular loop 3 (ICL3) of β(2)AR interacting with G(s)α, causing conformational changes of the α5 helix. Our results help explain molecular mechanisms of ligand and GPCR-mediated modulation of G protein activation.