Expression of bloodstream variant surface glycoproteins in procyclic stage Trypanosoma brucei: role of GPI anchors in secretion

Using transformed procyclic trypanosomes, the synthesis, intracellular transport and secretion of wildtype and mutant variant surface glycoprotein (VSG) is characterized. We find no impediment to the expression of this bloodstream stage protein in insect stage cells, VSG receives a procyclic-type phosphatidylinositol-specific phospholipase C-resistant glycosyl phosphatidylinositol (GPI) anchor, dimerizes and is N-glycosylated. It is transported to the plasma membrane with rapid kinetics (t(1/2) similar to 1 h) and then released by a cell surface zinc-dependent metalloendoprotease activity, a possible homolog of leishmanial gp63. Deletion of the C-terminal GPI addition signal generates a soluble form of VSG that is exported with greatly reduced kinetics (t(1/2) similar to 5 h). Fusion of the procyclic acidic repetitive protein (PARP) GPI anchor signal to the C-terminus of the truncated VSG reporter restores both GPI addition and transport competence, suggesting that GPI anchors play a critical role in the folding and/or forward transport of newly synthesized VSG, The VSG-PARP fusion is also processed near the C-terminus by events that do not involve N-linked oligosaccharides and which are consistent with GPI side chain modification, This unexpected result suggests that GPI processing may be influenced by adjacent peptide sequence or conformation.