EXPRESSION OF BLOOD-STREAM VARIANT SURFACE GLYCOPROTEINS IN PROCYCLICSTAGE TRYPANOSOMA-BRUCEI - ROLE OF GPI ANCHORS IN SECRETION

Using transformed procyclic trypanosomes, the synthesis, intracellular transport and secretion of wildtype and mutant variant surface glycop rotein (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-resist ant glycosyl phosphatidylinositol (GPI) anchor, dimerizes and is N-gly cosylated. It is transported to the plasma membrane with rapid kinetic s (t(1/2) similar to 1 h) and then released by a cell surface zinc-dep endent metalloendoprotease activity, a possible homolog of leishmanial gp63. Deletion of the C-terminal GPI addition signal generates a solu ble form of VSG that is exported with greatly reduced kinetics (t(1/2) similar to 5 h). Fusion of the procyclic acidic repetitive protein (P ARP) 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 transpo rt of newly synthesized VSG, The VSG-PARP fusion is also processed nea r the C-terminus by events that do not involve N-linked oligosaccharid es and which are consistent with GPI side chain modification, This une xpected result suggests that GPI processing may be influenced by adjac ent peptide sequence or conformation.