The Rab GTPase cycle. The Rab GTPase switches between GDP- and GTP-bound forms, which have different conformations. Conversion from the GDP- to the GTP-bound form is caused by nucleotide exchange, catalyzed by a GDP/GTP exchange factor (GEF). Conversion from the GTP-to the GDP-bound form occurs by GTP hydrolysis, facilitated by a GTPase-activating protein (GAP). The GTP-bound form interacts with effector molecules, whereas the GDP-bound form interacts with Rab escort protein (REP) and GDP dissociation inhibitor (GDI). Pi, inorganic phosphate.