GTPase activity of Rab5 acts as a timer for endocytic membrane fusion

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Abstract
THE GTPase cycle is a versatile regulatory mechanism directing many cell functions1, and Rab family members use it to regulate intracellular transport2–4. Current models propose that GTP hydrolysis by Rab proteins is either required for membrane fusion or occurs afterwards to allow recycling of the protein1–4.To measure the GTPase activity of Rab5 in endocytic membrane fusion5, we engineered a mutant that preferentially binds xantho-sine 5′-triphosphate (XTP), Rab5D136N and monitored the kinetics of [α32P]-XTP hydrolysis in situ during endosome fusion in vitro. Surprisingly, nucleotide hydrolysis occurred even in the absence of membrane fusion, indicating that membrane-bound Rab5 undergoes futile cycles of GTP (XTP) binding and hydrolysis. Nucleotide triphosphate hydrolysis by Rab5 is not conditional on membrane fusion and is reduced by its effector Rabaptin-5 (ref. 6). Our data reveal that the GTP cycle of Rab proteins differs from that of other GTPases (for example, EF-Tu) and indicate that GTP hydrolysis acts as a timer that determines the frequency of membrane docking/fusion events.