Rabs switch between two conformations, an inactive form bound to GDP (guanosine diphosphate), and an active form bound to GTP (guanosine triphosphate). A guanine nucleotide exchange factor (GEF) catalyzes the conversion from GDP-bound to GTP-bound form, thereby activating the Rab. The inherent GTP hydrolysis of Rabs can be enhanced by a GTPase-activating protein (GAP) leading to Rab inactivation. REPs carry only the GDP-bound form of Rab, and Rab effectors, proteins with which Rab interacts and through which it functions, only bind the GTP-bound form of Rab. Rab effectors are very heterogeneous, and each Rab isoform has many effectors through which it carries out multiple functions. The specific binding of the effector to the Rab protein allows the Rab protein to be effective, and conversely, the conformation shift of the Rab protein to the inactive state leads to effector dissociation from the Rab protein.