Rab GTPases control membrane traffic and organelle identity. They are activated and inactivated by Rab effectors, guanine exchange factors (GEFs) and GTPase-activating proteins (GAPs), respectively. Rab5 is involved in early endosome fusion, and is thought to maintain a high activity at these endosomes by recruiting its own effector rabaptin5 (Rbpt5) and its GEF Rabex5; this cascade is considered a prime example of a positive feedback Rab activation loop. Because Rbpt5 also interacts with Rab4 and Rabex5, Martin Spiess and colleagues (p. ) set out to analyse its function in greater detail by assessing the effects of deleting individual interaction domains. Unexpectedly, they find that Rbpt5 is not recruited to early endosomes by Rab5, but by Rab4 and through the ubiquitin-interaction domain of Rabex5. This raises the possibility that Rbpt5–Rabex5 is released from the maturing endosome upon deubiquitylation and internalisation of its cargo or when Rab4-GTP levels decrease, thus providing an attractive mechanism for terminating Rab5 activation. Furthermore, lack of the Rab5-binding site in Rbpt5 or silencing of Rab5 expression results in large endosomes that contain early and late endosomal markers, which suggests that endosome maturation requires the transition from Rab4 to Rab5. Taken together, the data presented here challenge the commonly held view of a simple feedback cascade for Rab activation, and provide new important insights into the role of Rab4 and Rab5 in endosome maturation.
