FIGURE 15-4 GTPase switch proteins cycle between active and inactive forms. The switch protein is active when it has bound GTP and inactive when it has bound GDP. Conversion of the active into the inactive form by hydrolysis of the bound GTP is accelerated by GAPs (GTPase-activating proteins), RGSs (regulators of G protein signaling), and other types of proteins. Reactivation is promoted by GEFs (guanine nucleotide exchange factors), which catalyze the dissociation of the bound GDP and its replacement by GTP.