Rab8 has a drastic effect on cell shape, but the membrane trafficking route it regulates is poorly defined. Here, we show that endogenous and ectopically expressed Rab8 is associated with macropinosomes generated at ruffling membrane domains. These macropinosomes fuse or transform into tubules that move toward the cell center, from where they are recycled back to the leading edge. The biogenesis of these tubules is dependent on actin and microtubular dynamics. Expression of dominant-negative Rab8 mutants or depletion of Rab8 by RNA interference inhibit protrusion formation, but promote cell-cell adhesion and actin stress fiber formation, whereas expression of the constitutively active Rab8-Q67L has the opposite effect. Rab8 localization overlaps with both Rab11 and Arf6, and is functionally linked to Arf6. We also demonstrate that Rab8 activity is needed for the transport of transferrin and the transferrin receptor to the pericentriolar region and to cell protrusions, and that Rab8 controls the traffic of cholera toxin B to the Golgi compartment. Finally, Rab8 colocalizes and binds specifically to a synaptotagmin-like protein (Slp1/JFC1), which is involved in controlling Rab8 membrane dynamics. We propose that Rab8 regulates a membrane-recycling pathway that mediates protrusion formation.

The mechanisms mediating polarized delivery of vesicles to cell surface domains are poorly understood in animal cells. We have previously shown that expression of Rab8 promotes the formation of new cell surface domains through reorganization of actin and microtubules. To unravel the function of Rab8, we used the yeast two-hybrid system to search for potential Rab8-specific activators. We identified a coil-coiled protein (Rabin8), homologous to the rat Rabin3 that stimulated nucleotide exchange on Rab8 but not on Rab3A and Rab5. Furthermore, we show that rat Rabin3 has exchange activity on Rab8 but not on Rab3A, supporting the view that rat Rabin3 is the rat equivalent of human Rabin8. Rabin8 localized to the cortical actin and expression of Rabin8 resulted in remodeling of actin and the formation of polarized cell surface domains. Activation of PKC by phorbol esters enhanced translocation of both Rabin8 and Rab8-specific vesicles to the outer edge of lamellipodial structures. Moreover, coexpression of Rabin8 with dominant negative Rab8 (T22N) redistributes Rabin8 from cortical actin to Rab8-specific vesicles and promotes their polarized transport to cell protrusions. The C-terminal region of Rabin8 plays an essential role in this transport. We propose that Rabin8 is a Rab8-specific activator that is connected to processes that mediate polarized membrane traffic to dynamic cell surface structures.

Huntington's disease is characterised by the death of cortical and striatal neurons, and is the result of an expanded polyglutamine tract in the Huntingtin protein [1]. Huntingtin is present on both endocytic and secretory membrane organelles but its function is unclear [2,3]. Rab GTPases regulate both of these transport pathways [4]. We have previously shown that Rab8 controls polarised membrane transport by modulating cell morphogenesis [5]. To understand Rab8-mediated processes, we searched for Rab8-interacting proteins by the yeast two-hybrid system. Here, we report that Huntingtin is linked to the Rab8 protein through FIP-2, a tumour necrosis factor-alpha (TNF-alpha)-inducible coiled-coil protein related to the NEMO protein [6,7]. The activated form of Rab8 interacted with the amino-terminal region of FIP-2, whereas dominant-negative Rab8 did not. Huntingtin bound to the carboxy-terminal region of FIP-2. Coexpressed FIP-2 and Huntingtin enhanced the recruitment of Huntingtin to Rab8-positive vesicular structures, and FIP-2 promoted cell polarisation in a similar way to Rab8. We propose a model in which Huntingtin, together with FIP-2 and Rab8, are part of a protein network that regulates membrane trafficking and cellular morphogenesis.

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