Rab5 is a small GTPase involved in the control of intracellular trafficking, both at the level of receptor endocytosis and endosomal dynamics1. The finding that Rab5 can be activated by receptor tyrosine kinases (RTK)2 raised the question of whether it also participates in effector pathways emanating from these receptors. Here we show that Rab5 is indispensable for a form of RTK-induced actin remodelling, called circular ruffling. Three independent signals, originating from Rab5, phosphatidylinositol-3-OH kinase and Rac, respectively, are simultaneously required for the induction of circular ruffles. Rab5 signals to the actin cytoskeleton through RN-tre, a previously identified Rab5-specific GTPase-activating protein (GAP)3. Here we demonstrate that RN-tre has the dual function of Rab5-GAP and Rab5 effector. We also show that RN-tre is critical for macropinocytosis, a process previously connected to the formation of circular ruffles4,5. Finally, RN-tre interacts with both F-actin and actinin-4, an F-actin bundling protein. We propose that RN-tre establishes a three-pronged connection with Rab5, F-actin and actinin-4. This may aid crosslinking of actin fibres into actin networks at the plasma membrane. Thus, we have shown that Rab5 is a signalling GTPase and have elucidated the major molecular elements of its downstream pathway.
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This work was supported by grants from the Associazione Italiana per la Ricerca sul Cancro and Human Science Frontier Program to P.P.D.F. and G.S., and by grants from the Association for International Cancer Research, European Community (VI Framework), Italian Ministry for University and Research and Consiglio Nazionale delle Ricerche to P.P.D.F. We thank E. Frittoli for technical assistance. L.L. was supported in part by a fellowship from the Associazione Sviluppo Piemonte.
The authors declare that they have no competing financial interests.
Active Ras and Rac induce lamellipodia, but not circular ruffles. (PDF 2509 kb)
Coated pit-mediated endocytosis and PDGF-induced circular ruffling are independent events. (PDF 3610 kb)
Effects of Rab5 and various GTPases and their effectors on the actin cytoskeleton. (PDF 4816 kb)
Involvement of RN-tre in circular ruffling and macropinocytosis. (PDF 17070 kb)
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Lanzetti, L., Palamidessi, A., Areces, L. et al. Rab5 is a signalling GTPase involved in actin remodelling by receptor tyrosine kinases. Nature 429, 309–314 (2004) doi:10.1038/nature02542
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