Rab5 is a 24 kDa GTP-binding protein that is localized to the cytoplasmic side of the plasma membrane, clathrin-coated vesicles, and early endosomes. Rab5 is believed to regulate vesicle fusion throμgh a cycle of GDP/GTP exchange and GTP hydrolysis. The different guanine nucleotide binding states of rab5 is postulated to affect its ability to associate or dissociate with membranes during endocytotic membrane traffic. Its GTP-bound form, which represents the active form of Rab5, associates with membrane and regulates vesicle docking and fusion.