Caenorhabditis elegans RME-6 is a novel regulator of RAB-5 at the clathrin-coated pit

Using C. elegans genetics we identified a new regulator of endocytosis called RME-6. RME-6 is evolutionarily conserved among metazoans and contains RasGAP-like and Vsp9 domains. Consistent with the known catalytic function of Vps9 domains in Rab5 GDP/GTP exchange, we found that RME-6 binds specifically to C. elegans Rab5 in the GDP-bound conformation, and rme-6 mutants display phenotypes that indicate low Rab5 activity. Furthermore, rme-6 interacts genetically with the worm homologue of the known Rab5 exchange factor Rabex-5. However, unlike other Rab5 associated proteins, a rescuing GFP::RME-6 fusion protein primarily localizes to clathrin-coated pits, physically interacts with α-adaptin, a clathrin adaptor protein, and requires clathrin to achieve its cortical localization. In rme-6 mutants transport from the plasma membrane to endosomes is defective, and small 100 nm endocytic vesicles accumulate just below the plasma membrane. These results suggest a mechanism for the activation of Rab5 in clathrin-coated pits or clathrin coated vesicles that is essential for the delivery of endocytic cargo to early endosomes.