Developmental and morphological regulation of clathrin-mediated endocytosis in Trypanosoma brucei

Essentially all macromolecular communication between Trypanosoma brucei and its host is confined to vesicular trafficking events occurring at or around the flagellar pocket. The vertebrate stage bloodstream form trypomastigote exhibits an extremely high rate of endocytosis required for nutrient uptake and probably also evasion of the host immune system. However, the rate of endocytosis is very low in the procyclic vector parasite, indicating that endocytosis is subject to a marked level of developmental regulation. Previous ultrastructural studies and crude biochemical fractionations have indicated the presence of coated pits and vesicles that are analogous to clathrin coats in the bloodstream form, but not in the procyclic. However, a definitive description of the components of this coat and its molecular function in T. brucei has remained elusive. We describe the molecular cloning and initial characterisation of components of the T. brucei endocytic coats: clathrin heavy chain (TbCLH) and a beta-adaptin (TbAPbeta1). TbCLH is markedly upregulated in the bloodstream form compared with the procyclic, whereas TbAPbeta1 is subject to more limited developmental regulation. We generated antisera against both proteins and show that the clathrin coat is tightly associated with the flagellar pocket in both major life stages. However, in bloodstream parasites TbCLH is also extensively distributed throughout the posterior end of the cell on numerous large vesicular and tubular structures. By cryo-immuno EM, clathrin is localised to collecting tubules at the flagellar pocket and is also associated with the trans-Golgi network. These EM data confirm that the electron dense coats reported on trypanosome vesicles and tubules contain clathrin. The TbAPbeta1 exhibits an atypical distribution relative to previously characterised adaptins, associating not only with the trans-Golgi but also with other tubular-vesicular elements. Localisation of TbAPbeta1 is also subject to developmental regulation. These data describe major endocytic coat proteins in T. brucei for the first time, and indicate stage-specific expression of the clathrin heavy chain. Modulation of clathrin expression is likely to be an important factor in the developmental regulation of endocytosis and recycling in the African trypanosome.