Differential endocytic functions of Trypanosoma brucei Rab5 isoforms reveal a glycosylphosphatidylinositol-specific endosomal pathway

Abstract

We demonstrate the presence of a glycosylphosphatidylinositol (GPI) anchor-specific endosomal pathway in the protozoan pathogen Trypanosoma brucei. In higher eukaryotes evidence indicates that GPI-anchored proteins are transported in both the endocytic and exocytic systems by mechanisms involving sequestration into specific membrane microdomains and consequently sorting into distinct compartments. This is potentially extremely important in trypanosomatids as the GPI anchor is the predominant mechanism for membrane attachment of surface macromolecules, including the variant surface glycoprotein (VSG). A highly complex developmentally regulated endocytic network, vital for nutrient uptake and evasion of the immune response, exists in T. brucei. In common with mammalian cells an early endosomal compartment is defined by Rab5 small GTPases, which control transport processes through the endosomal system. We investigate the function of two trypanosome Rab5 homologues. TbRAB5A and TbRAB5B, which colocalize in the procyclic stage, are distinct in the bloodstream form of the parasite. TbRAB5A endosomes contain VSG and transferrin, endocytosed by the T. brucei GPI-anchored transferrin receptor, whereas TbRAB5B endosomes contain the transmembrane protein ISG 100 but neither VSG nor transferrin. These findings indicate the presence of trypanosome endosomal pathways trafficking proteins through specific routes depending on the mode of membrane attachment. Ectopic expression of mutant TbRAB5A or -5B indicates that TbRAB5A plays a role in LDL endocytosis, whereas TbRAB5B does not, but both have a role in fluid phase endocytosis. Hence TbRAB5A and TbRAB5B have distinct functions in the endosomal system of T. brucei. A developmentally regulated GPI-specific endosomal pathway in the bloodstream form suggests that specialized transport of GPI-anchored proteins is required for survival in the mammalian host.