CED-10/Rac1 regulates endocytic recycling through the RAB-5 GAP TBC-2

Rac1 is a founding member of the Rho-GTPase family and a key regulator of membrane remodeling. In the context of apoptotic cell corpse engulfment, CED-10/Rac1 acts with its bipartite guanine nucleotide exchange factor, CED-5/Dock180-CED-12/ELMO, in an evolutionarily conserved pathway to promote phagocytosis. Here we show that in the context of the Caenorhabditis elegans intestinal epithelium CED-10/Rac1, CED-5/Dock180, and CED-12/ELMO promote basolateral recycling. Furthermore, we show that CED-10 binds to the RAB-5 GTPase activating protein TBC-2, that CED-10 contributes to recruitment of TBC-2 to endosomes, and that recycling cargo is trapped in recycling endosomes in ced-12, ced-10, and tbc-2 mutants. Expression of GTPase defective RAB-5(Q78L) also traps recycling cargo. Our results indicate that down-regulation of early endosome regulator RAB-5/Rab5 by a CED-5, CED-12, CED-10, TBC-2 cascade is an important step in the transport of cargo through the basolateral recycling endosome for delivery to the plasma membrane.
Author Summary When cargo is internalized from the cell surface by endocytosis, it enters a series of intracellular organelles called endosomes. Endosomes sort cargo, such that some cargos are sent to the lysosome for degradation, while others are recycled to the plasma membrane. Small GTPase proteins (Rabs) are well-known master regulators of endosome function. As cargo moves through the endosomal system, it must pass from the domain controlled by one Rab-GTPase to the domain controlled by another. Little is known about how transitions along the recycling pathway are controlled, or if Rab transitions are necessary for cargo recycling. Here we identified a group of proteins that act on recycling endosomes to deactivate the early acting GTPase RAB-5. Disruption of any of these proteins interferes with recycling. Our work shows that RAB-5 deactivation is important for cargo recycling, and it provides some of the first mechanistic insight into how changes in Rabs can be controlled during endocytic recycling. Importantly, several proteins that we found contribute to this recycling function have roles in other cellular processes, such as cell migration and the removal of cell corpses. Therefore our work also suggests that endocytic recycling could contribute to these processes in previously unsuspected ways.