Heterotrimeric G proteins (1) release GDP upon binding to a GDP Exchange Factor (GEF), which are G protein-coupled receptors, (2) bind GTP and recruit downstream effectors, and (3) hydrolyze GTP promoted by GTPase activating protein (GAP), leading to (4) the inactive, GDP-bound state. Structures of the Gα subunit (blue) bound to GDP (1got; inactive state; top) and bound to the β2-AR (grey) (3sn6; active state; bottom) are shown.