Rab11 function in Trypanosoma brucei: identification of conserved and novel interaction partners

The Ras-like GTPase Rab11 is implicated in multiple aspects of intracellular transport, including maintenance of plasma membrane composition and cytokinesis. In metazoans, these functions are mediated in part via coiled-coil Rab11-interacting proteins (FIPs) acting as Rab11 effectors. Additional interaction between Rab11 and the exocyst subunit Sec15 connects Rab11 with exocytosis. We find that FIPs are metazoan specific, suggesting that other factors mediate Rab11 functions in nonmetazoans. We examined Rab11 interactions in Trypanosoma brucei, where endocytosis is well studied and the role of Rab11 in recycling well documented. TbSec15 and TbRab11 interact, demonstrating evolutionary conservation. By yeast two-hybrid screening, we identified additional Rab11 interaction partners. Tb927.5.1640 (designated RBP74) interacted with both Rab11 and Rab5. RBP74 shares a coiled-coil architecture with metazoan FIPs but is unrelated by sequence and appears to play a role in coordinating endocytosis and recycling. A second coiled-coil protein, Tb09.211.4830 (TbAZI1), orthologous to AZI1 in Homo sapiens, interacts exclusively with Rab11. AZI1 is restricted to taxa with motile cilia/flagella. These data suggest that Rab11 functions are mediated by evolutionarily conserved (i.e., AZI1 and Sec15) and potentially lineage-specific (RBP74) interactions essential for the integration of the endomembrane system.