A role for the dynamic acylation of a cluster of cysteine residues in regulating the activity of the glycosylphosphatidylinositol-specific phospholipase C of Trypanosoma brucei

The glycosylphosphatidylinositol-specific phospholipase C or VSG lipase is the enzyme responsible for the cleavage of the glycosylphosphatidylinositol anchor of the variant surface glycoprotein (VSG) and concommitant release of the surface coat in Trypanosoma brucei during osmotic shock or extracell ular acidic stress. In Xenopus laevis oocytes the VSG lipase was expressed as a nonacylated and a thioacylated form. This thioacylation occurred withi n a cluster of three cysteine residues but was not essential for catalytic activity per se. These two forms were also detected in trypanosomes and app eared to be present at roughly equivalent amounts. A reversible shift to th e acylated form occurred when cells were triggered to release the VSG by ei ther nonlytic acid stress or osmotic lysis, A wild type VSG lipase or a gen e mutated in the three codons for the acylated cysteines were reinserted in the genome of a trypanosome null mutant for this gene. A comparative analy sis of these revertant trypanosomes indicated that thioacylation might be i nvolved in regulating enzyme access to the VSG substrate.