THE ROLE OF GPI-PLC IN TRYPANOSOMA-BRUCEI

The glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) fr om Trypanosoma brucei exhibits exquisite specificity for the GPI-ancho r of the variant specific glycoprotein (VSG). However the evidence tha t it is involved in VSG metabolism in the living trypanosome is circum stantial; it shows the same life cycle stage regulated expression as t he VSG,no feasible alternative substrate has been identified, and it m etabolises the VSG efficiently in vitro and in vivo on hypotonic lysis . Against these considerations are the observations that the GPI-PLC i s found on the cytoplasmic face of vesicles so it could not gain acces s to the VSG through normal vesicle fusion and that the accelerated lo ss of VSG from bloodstream forms on differentiation to procyclic forms occurs through the action of a protease. To try to determine the role of the GPI-PLC, a homozygous null mutant T. brucei has been construct ed. The null mutant was created by replacement of the entire gene at b oth alleles with selectable antibiotic resistance markers in procyclic form trypanosomes. The GPI-PLC gene is not usually expressed in procy clic forms and so, as would be expected, the null procyclics display n o obvious phenotype. The null procyclics have been used to infect tset se flies and it remains to be seen whether it is possible for them to differentiate to bloodstream forms and, if so, what the antigenic vari ation phenotype of the null bloodstream forms would be.