Figure 2. Activation cycle of small GTPases of the Ras superfamily. Small GTPases of the Ras superfamily cycle between a GTP-bound active and a GDP-bound inactive state. GTPase Activating Proteins (GAPs) enhance the slow intrinsic GTPase activity of small GTPases. Guanosine Dissociation Inhibitors (GDIs) inhibit dissociation of GDP from the GDP-bound form of these proteins, whereas Guanosine Exchange Factors (GEFs) enhance this process.