GTPase proteins switch between
                    active / inactive forms

 GTPase-accelerating proteins (GAPs)
        hydrolyze GTP & convert active to inactive

 Regulators of G-proteins (RGSs)
        hydrolyze GTP & convert active to inactive

 Guanine nucleotide-exchange factors (GEFs)
        reactivate GAPs & RGSs

 Guanine nucleotide dissociation inhibitors (GDIs)
  inhibit hydrolysis of GTP, leave proteins inactive