Guanosine triphosphate- (GTP-) binding proteins constitute a supergene family of proteins that utilize GTP binding and hydrolysis as a chemical switch. These proteins are activated by exchanging bound guanosine diphosphate (GDP) with GTP and are subsequently inactivated by hydrolyzing bound GTP to GDP.1–3 Hence, by shuttling between an inactive, GDP-bound to an active, GTP-bound conformation, these proteins couple ligand binding to effector activation. This superfamily includes several subclasses of proteins, which share structural and functional homology in the core structure of their GTP-binding domains, but maintain distinct functions.