Figure 1. Regulation of RhoGTPase activity by three classes of proteins: GEFs, GAPs, and GDIs. Rho GTPases are basic switches that oscillate between an inactive GDP-bound state and an active GTP-bound state. The family of GDIs sequesters the GTPases in the cytosol before they are targeted to the plasma membrane where they are activated by GEFs. GEFs, which receive activation by upstream signals, promote GDP/GTP exchange on the GTPases. Once bound to GTP, GTPases specifically bind to numerous effectors that trigger various cellular responses. GAPs catalyze the hydrolysis of GTP, rendering the GTPase inactive.