The Rab cycle.

Rab GTPases cycle between an active GTP-bound form and an inactive GDP-bound form. Rab activation is mediated by a guanine exchange factor (GEF). The hydrolysis of bound GTP is catalyzed by a GTPase activating protein (GAP) resulting in the inactivation of the Rab protein. In its active form, the Rab is associated with membranes and can interact with a variety of effector proteins. In its inactive form the protein is cytosolic and is in complex with a GDP dissociation inhibitor (GDI) protein.