G-Protein Activation1. When not activated, the alpha subunit binds GDP.2. When the ligand binds to the GPCR, it changes conformation and, in turn,

alters the conformation of the G protein.3. Altering the conformation of the alpha subunit allows it to exchange GDP

for GTP.4. The binding of GTP to the alpha subunit results in dissociation from the

beta subunit.5. These are now two independent entities: the alpha subunit and the beta-

gamma (bg) subunit.

6. GTP binding and dissociation of the bg subunit causes the alpha subunit to adopt a new conformation enabling it to now bind to intracellular signaling proteins.

7. The bg subunit does not change conformation, but its dissociation from the alpha exposes this face of the beta subunit. It can now also interact with signaling molecules.

8. The cycle is repeated by the hydrolysis of  subunit-bound GTP to GDP, resulting in the re-association of the  and  subunits.