Discussion

In African trypanosomes, trafficking of GPI-anchored Variant Surface Glycoprotein (VSG) to the cell surface is critical to parasite survival and pathogenesis. VSG is initially synthesized in the Endoplasmic Reticulum (ER), and is then trafficked to the Golgi apparatus in Coat Protein II (COPII)-coated vesicles. Efficient ER exit of VSG requires its GPI anchor and is mediated by specific COPII subunits, suggesting a sorting receptor that simultaneously recognizes the GPI anchor on VSG (lumenal-facing) and the assembling COPII subunits at ER exit sites (cytoplasm-facing). In other systems, GPI recognition during ER exit is carried out by members of the p24 family of transmembrane proteins. We have identified 8 putative p24s in the T. brucei genome: TbERP1-8 (EMP24-related protein). Of these, only TbERP1,2,3, and 8 are expressed during bloodstream-form growth, and RNAi silencing of each caused a significant delay in the trafficking of VSG to the cell surface. We confirmed TbERP localization to ERES, consistent with function during ER exit. TbERP1,2,3 and 8 show inter-dependence, and the protein stability of a given TbERP depends on the expression of the others. The shared RNAi phenotypes, co-localization, and coordinate expression suggest a TbERP complex with novel specificity. Experiments are underway to confirm direct physical interactions amongst the TbERPs and with VSG cargo.