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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