Late endosomal Rab7 regulates lysosomal trafficking of endocytic but not biosynthetic cargo in Trypanosoma brucei

We present the first functional analysis of the small GTPase, TbRab7, in Trypanosoma brucei. TbRab7 defines discrete late endosomes closely juxtaposed to the terminal p67(+) lysosome. RNAi indicates that TbRab7 is essential in bloodstream trypanosomes. Initial rates of endocytosis were unaffected, but lysosomal delivery of cargo, including tomato lectin (TL) and trypanolytic factor (TLF) were blocked. These accumulate in a dispersed internal compartment of elevated pH, likely derived from the late endosome. Surface binding of TL but not TLF was reduced, suggesting that cellular distribution of flagellar pocket receptors is differentially regulated by TbRab7. TLF activity was reduced approximately threefold confirming that lysosomal delivery is critical for trypanotoxicity. Unexpectedly, delivery of endogenous proteins, p67 and TbCatL, were unaffected indicating that TbRab7 does not regulate biosynthetic lysosomal trafficking. Thus, unlike mammalian cells and yeast, lysosomal trafficking of endocytosed and endogenous proteins occur via different routes and/or are regulated differentially. TbRab7 silencing had no effect on a cryptic default pathway to the lysosome, suggesting that the default lysosomal reporters p67 Delta TM, p67 Delta CD and VSG Delta GPI do not utilize the endocytic pathway as previously proposed. Surprisingly, conditional knockout indicates that TbRab7 may be non-essential in procyclic insect form trypanosomes.