The expression level determines the surface distribution of the transferrin receptor in Trypanosoma brucei

The transferrin receptor (TfR) of Trypanosoma brucei is a heterodimer attached to the surface membrane by a glycosylphosphatidlylinositol (GPI) anchor. The TfR is restricted to the flagellar pocket, a deep invagination of the plasma membrane. The membrane of the flagellar pocket and the rest of the cell surface are continuous, and the mechanism that selectively retains the TfR in the pocket is unknown. Here, we report that the TfR is retained in the flagellar pocket by a specific and saturable mechanism. In bloodstream-form trypanosomes transfected with the TfR genes, TfR molecules escaped flagellar pocket retention and accumulated on the entire surface, even at modest (threefold) overproduction levels. Similar surface accumulation was observed when the TfR levels were physiologically upregulated threefold when trypanosomes were starved for transferrin. These results suggest that the TfR flagellar pocket retention mechanism is easily saturated and that control of the expression level is critical to maintain the restricted surface distribution of the receptor.