A developmentally regulated rab11 homologue in Trypanosoma brucei is involved in recycling processes

Endocytosis in the parasitic protozoan Trypanosoma brucei, a deeply divergent eukaryote, is implicated as important in both general cellular function and virulence, and is strongly developmentally regulated. We report the characterisation of a previously undefined endosomal compartment in T. brucei based on identification of a new trypanosome gene (TbRAB11) homologous to Rabll/Ypt31. Northern and western analyses indicated that TbRAB11 expression was significantly upregulated in the bloodstream stage of the parasite, the first trypanosome Rab to be identified with a developmentally regulated expression profile. In procyclic form parasites TbRAB11 localised to a compartment positioned close to the basal body, similar to mammalian Rab11. By contrast, in bloodstream form parasites, TbRAB11-containing structures were more extensive and the TbRAB11 compartment extended towards the posterior face of the nucleus, was more elaborate and was not always adjacent to the basal body. Colocalisation studies by light and confocal microscopy demonstrated that TbRAB11 was located on a compartment that did not correspond to other established trypanosomal organelles or markers. Using concanavalin A internalisation and temperature block procedures, TbRAB11 was observed on endomembranes anterior to the flagellar pocket that are juxtaposed to the collecting tubules. TbRAB11 colocalised with the trypanosomal transferrin receptor and internalised anti-variant surface glycoprotein. Further, we show that the collecting tubules contain TbRAB5A, suggesting that they are the trypanosomatid early endosome. Hence, TbRAB11 is present on endosomal structures that contain recycling cargo molecules and is under developmental regulation, suggesting a role in stage-dependent endocytic processes.