Fig. 2.

Role of Rab and Arf proteins in directing vesicle transport. The small GTPases are held in the cytosol as inactive GDP-forms by GDP dissociation inhibitor (GDI) proteins. Translocation to membranes requires membrane-bound GDI displacement factors (GDFs) which release the GTPase and expose a C-terminal lipid group for membrane binding. Guanine nucleotide exchange factors (GEFs) exchange GDP for GTP and allow the activated GTPase to bind effector proteins which facilitate vesicle budding and fusion. The cycle is completed by GTP-hydrolysis, which requires GTPase activating proteins (GAPs), followed by extraction of the GDP-bound enzyme from the membrane by GDI.