Abstract text
Trypanosoma brucei is an extracellular parasite which lives in the bloodstream of its mammalian host. The bloodstream is a nutrient-rich environment but also means that the parasite is in continuous exposure to the host immune system. To counter this threat, the parasite has developed specific survival strategies. Its surface is covered in a dense glycoprotein coat that is continually endo- and exocytosed to remove bound antibodies. Remarkably, all endo- and exocytosis is restricted to the flagellar pocket, a small invagination of the plasma membrane. It is not clear whether antibodies or cargo that are bound to the trypanosome surface passively enter the flagellar pocket, or if a more active mechanism is involved. The neck of the flagellar pocket has a number of distinct cytoskeleton-associated protein complexes coiled around its cytoplasmic face. One of these complexes is the hook complex, which contains the proteins TbMORN1 and TbSmee1. Knockdown of TbMORN1 results in impaired access of larger cargo to the flagellar pocket, suggesting a size exclusion limit. Uptake assays with different reporters revealed no change in the size limit after knockdown of TbMORN1 and TbSmee1, however. To test whether endocytosis was required for cargo entry to the flagellar pocket, the endocytic effector clathrin was knocked down. Clathrin knockdown phenocopied TbSmee1 knockdown, suggesting that endocytosis is required for the entry of surface-bound cargo into the flagellar pocket.