Regulates the GDP/GTP exchange reaction of most Rab proteins by inhibiting the dissociation of GDP from them, and the subsequent binding of GTP to them. Promotes the dissociation of GDP-bound Rab proteins from the membrane and inhibits their activation. Promotes the dissociation of RAB1A, RAB3A, RAB5A and RAB10 from membranes. Interacts with RHOH (By similarity). Interacts with the non-phosphorylated forms of RAB1A, RAB3A, RAB5A, RAB5B, RAB5C, RAB8A, RAB8B, RAB12, RAB35, and RAB43 (By similarity). Interacts with RAB10 (PubMed:19570034). High expression in brain, lower in other tissues. Belongs to the Rab GDI family.