The dynamics of agonist-β 2 -adrenergic receptor activation induced by binding of GDP-bound Gs protein

There is considerable uncertainty about the mechanism by which the β 2 -adrenergic receptor (β 2 AR) is activated. Here we use molecular metadynamics computations to predict the mechanism by which an agonist induces the activation of the β 2 AR and its cognate Gs protein. We found that binding agonist alone to the inactive β 2 AR does not break the ionic lock and hence does not drive the β 2 AR towards the activated conformation. However, we found that attaching the inactive Gs protein to the agonist-bound inactive β 2 AR (containing the ionic lock) leads to partial insertion of Gαs-α5 into the core of β 2 AR, which breaks the ionic lock, leading to activation of the Gs protein coupled to β 2 AR. Upon activation, the Gαs protein undergoes a remarkable opening of the GDP binding pocket, making the GDP available for exchange or release. Concomitantly, Gαs-α5 undergoes a remarkable expansion in the β 2 AR cytoplasmic region after the ionic lock is broken, inducing TM6 to displace outward by ~5 Å from TM3.