Distinct Rab-binding domains mediate the interaction of Rabaptin-5 with GTP-bound rab4 and rab5

Abstract

Rabaptin‐5 functions as an effector for the small GTPase Rab5, a regulator of endocytosis and early endosome fusion. We have searched for structural determinants that confer functional specificity on Rabaptin‐5. Here we report that native cytosolic Rabaptin‐5 is present in a homodimeric 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 for Rab5‐dependent recruitment of the protein on early endosomes. Surprisingly, we uncovered the presence of an additional Rab‐binding domain at the N‐terminus of Rabaptin‐5. This domain mediates the direct interaction with the GTP‐bound form of Rab4, a small GTPase that has been implicated in recycling from early endosomes to the cell surface. Based on these results, we propose that Rabaptin‐5 functions as a molecular linker between two sequentially acting GTPases to coordinate endocytic and recycling traffic.