Fig. 2

Activation cycle of G proteins/G protein-coupled receptor (GPCR) upon ligand binding. The receptor in an unbound state is inactive (a), and its coupled G protein is bound to GDP. Ligand binding to its GPCR (b) induces a change in GPCR conformation that promotes GDP exchange for GTP on the heterotrimeric complex α subunit (c, d). Both active, GTP-bound Gα and the Gβγ dimer then stimulate downstream effectors (e). When the ligand is no longer bound to the GPCR and the GTP on Gα is hydrolyzed to GDP (f), a new inactive GDP-bound heterotrimeric G protein can couple to the GPCR, and the original receptor is restored