Banerjee, S.; Minshall, N.; Cook, A. D.; Macleod, O.; Webb, H.; Higgins, M. K.; Carrington, M.

Trypanosoma brucei, the causal agent of Human and Animal African trypanosomiasis proliferates in the extracellular milieu of mammals. It acquires host macromolecular nutrients, by receptor mediated endocytosis. The best characterised receptor is for transferrin (TfR) and it has been reported to be sequestered in the flagellar pocket, the sole site of endocytosis. In this location the TfR may be inaccessible to adaptive immune system effectors. The T. brucei genome encodes ~15 TfR variants, and here we compared two, the first attached to the plasma membrane by a single glycosylphosphoinositol (GPI)-anchor and the other by two. Transferrin uptake kinetics were similar and rapid for both. Unexpectedly, initial binding of transferrin occurred over the whole cell surface suggesting the TfR was not sequestered in the flagellar pocket. This localisation was confirmed by immunofluorescence assays and was independent of the number of GPI-anchors. Two other GPI-anchored receptors were investigated to determine whether localisation to the whole cell surface was a general property of GPI-anchored receptors. Haptoglobin-haemoglobin uptake assays and immunofluorescence localisation of complement factor H receptor showed both were also whole cell surface localised. The mechanisms by which trypanosome receptors are protected from antibody-mediated attack are more subtle than hiding in a pocket.