DISTINCT RAB-BINDING DOMAINS MEDIATE THE INTERACTION OF RABAPTIN-5 WITH GTP-BOUND RAB4 AND RAB5

Rabaptin-5 functions as an effector for the small GTPase Rab5, a regul ator of endocytosis and early endosome fusion, We have searched for st ructural determinants that confer functional specificity on Rabaptin-5 . Here we report that native cytosolic Rabaptin-5 is present in a homo dimeric state and dimerization depends upon the presence of its coiled -coil predicted sequences. A 73 residue C-terminal region of Rabaptin- 5 is necessary and sufficient both for the interaction with Rab5 and f or Rab5-dependent recruitment of the protein on early endosomes. Surpr isingly, we uncovered the presence of an additional Rab-binding domain at the N-terminus of Rabaptin-5, This domain mediates the direct inte raction with the GTP-bound form of Rab4, a small GTPase that has been implicated in recycling from early endosomes to the cell surface. Base d on these results, we propose that Rabaptin-5 functions as a molecula r linker between two sequentially acting GTPases to coordinate endocyt ic and recycling traffic.