Cell density-dependent ectopic expression in bloodstream form Trypanosoma brucei

The role of genomic location and flanking 3'UTR in the generation of functional levels of variant surface glycoprotein in Trypanosoma brucei

Trypanosoma brucei faces relentless immune attack in the mammalian bloodstream, where it is protected by an essential coat of Variant Surface Glycoprotein (VSG) comprising ∼10% total protein. The active VSG gene is in a Pol I‐transcribed telomeric expression site (ES). We investigated factors mediating these extremely high levels of VSG expression by inserting ectopic VSG117 into VSG221 expressing T. brucei. Mutational analysis of the ectopic VSG 3′UTR demonstrated the essentiality of a conserved 16‐mer for mRNA stability. Expressing ectopic VSG117 from different genomic locations showed that functional VSG levels could be produced from a gene 60 kb upstream of its normal telomeric location. High, but very heterogeneous levels of VSG117 were obtained from the Pol I‐transcribed rDNA. Blocking VSG synthesis normally triggers a precise precytokinesis cell‐cycle checkpoint. VSG117 expression from the rDNA was not adequate for functional complementation, and the stalled cells arrested prior to cytokinesis. However, VSG levels were not consistently low enough to trigger a characteristic ‘VSG synthesis block’ cell‐cycle checkpoint, as some cells reinitiated S phase. This demonstrates the essentiality of a Pol I‐transcribed ES, as well as conserved VSG 3′UTR 16‐mer sequences for the generation of functional levels of VSG expression in bloodstream form T. brucei.

The ancient small GTPase Rab21 functions in intermediate endocytic steps in trypanosomes

Endocytosis is an essential process in nearly all eukaryotic cells, including the African trypanosome Trypanosoma brucei. Endocytosis in these organisms is exclusively clathrin mediated, although several lineage-specific features indicate that precise mechanisms are distinct from those of higher eukaryotes. T. brucei Rab21 is a member of an ancient, pan-eukaryotic, endocytic Rab clade that is retained by trypanosomes. We show that T. brucei Rab21 (TbRab21) localizes to endosomes, partially colocalizing with TbRab5A, TbRab28, and TbVps23, the latter two being present at late endosomes. TbRab21 expression is essential for cellular proliferation, and its suppression results in a partial block in traffic to the lysosome. RNA interference (RNAi)-mediated knockdown of TbRab21 had no effect on TbRab5A expression or location but did result in decreased in trans expression of ESCRT (trypanosome endosomal sorting complex required for transport) components and TbRab28, while knockdown of ESCRT subunit TbVps23 resulted in decreased TbRab21 expression. These data suggest that TbRab21 acts downstream of TbRab5A and functions in intimate connection with the trypanosome ESCRT system.