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Papers In Press, published online ahead of print December 20, 2001
Imperial College, London SW7 2AY
Corresponding Author: mfield{at}ic.ac.uk
We here demonstrate the presence of a 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 distinct mechanisms involving sequestration into specific membrane microdomains, and in consequence 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 and developmentally-regulated endocytic network, vital for nutrient uptake and evasion of the immune response, exists in T. brucei, and in common with mammalian cells an early endosomal compartment is defined by the Rab5 small GTPase subfamily, which controls transport processes through the endosomal system. Here we investigate the function of two trypanosome (Tb) Rab5 homologues. TbRAB5A and TbRAB5B, which colocalise in the procyclic stage, are distinct in the bloodstream form of the parasite. TbRAB5A endosomes contain VSG and internalised transferrin, endocytosed by the T. brucei GPI-anchored transferrin receptor, whilst TbRAB5B endosomes contain the transmembrane protein ISG100 but neither VSG or 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 B indicates that TbRAB5A plays a role in LDL endocytosis whilst 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 specialised transport of GPI-anchored proteins is required for survival in the mammalian host.
J. Biol. Chem, 10.1074/jbc.M110055200
Submitted on October 18, 2001
Revised on December 17, 2001
Accepted on December 19, 2001
Differential endocytic functions of Trypanosoma brucei Rab5 isoforms reveal a GPI-specific endosomal pathway
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