ORGANELLE MARKERS - Endosome

Rab5

Rab5 is a 24 kDa GTP-binding protein that regulates the fusion of plasma membrane-derived clathrin-coated vesicles with early endosomes and homotypic fusion among early endosomes. It is localized to the cytoplasmic side of the plasma membrane, clathrincoated vesicles, and early endosomes. Rab5 is believed to regulate vesicle fusion through 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. Studies using Rab5 mutant that hydrolysed xanthosine 5.-triphosphate (XTP) indicated that nucleotide hydrolysis occurs even in the absence of membrane fusion. GTP hydrolysis by Rab5 is postulated to determine the frequency of membrane docking and fusion events.

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