Simultaneous but independent activation of adenylate cyclase and glycosylphosphatidylinositol-phospholipase C under stress conditions in Trypanosoma brucei.
Previous observations suggested a concomitant relationship between the release of the variant surface glycoprotein (VSG) and the activation of adenylate cyclase in the bloodstream form of the parasitic protozoan Trypanosoma brucei. In order to evaluate this hypothesis, adenylate cyclase activity was measured in live trypanosomes subjected to different treatments known to induce the shedding of the VSG coat, namely low pH and trypsin digestion. In both cases adenylate cyclase activation occurred in parallel with the release of the VSG. The latter was found to be mediated by the glycosylphosphatidylinositol-specific phospholipase C that cleaves the glycosylphosphatidylinositol anchor of the protein (VSG lipase). Furthermore, both adenylate cyclase and VSG release were activated by the incubation of trypanosomes with specific inhibitors of protein kinase C, suggesting a repressive role for protein kinase C on both VSG lipase and adenylate cyclase activities. Significantly, in mutant trypanosomes lacking VSG lipase, adenylate cyclase was activated under conditions where VSG release did not occur. Moreover,VSG release was also found to occur in the absence of activation of the cyclase, as observed in the presence of low concentration of the thiol modifying reagent p-chloromercuriphenylsulfonic acid. These observations provide the first demonstration that release of the VSG in response to cellular stress is mediated by the VSG lipase and that while both release of the VSG and activation of adenylate cyclase occur in response to the same stimuli they are not obligatorily coupled.