Specific proteolysis of surface antigen as a cell response to stress in Paramecium primaurelia. Relation to antigenic variation

The cell-coat of Paramecium is mainly composed of glycoproteins called surface antigens (SAgs), which are anchored in the plasma membrane via a glycosyl phosphatidylinositol (GPI). SAgs are encoded by a family of genes whose mutually exclusive expression is controlled by environmental parameters such as temperature. In order to study the involvement of the SAg molecule in the regulation of its own expression, we established the membrane characteristics of the SAg turn-over in cell populations in which antigenic variation had been induced. We show that the turn-over is significantly increased when cells are submitted to a shift in temperature, and two pathways of SAg elimination are used: release into the external medium, and cellular proteolysis. SAg proteolysis is triggered by cell transfer into culture medium in different temperature conditions, but further disappearance of the SAg proteolytic fragments observed in control cells appeared to be delayed in temperature-shifted cells. NH4Cl and monensine were both unable to prevent the SAg proteolysis. But, remarkably, NH4Cl was shown to inhibit cellular elimination of the proteolytic peptides, and to induce the downregulation of the expressed antigen. We looked also for the previously described endogenous GPI-hydrolase activity on SAg in temperature-shifted cells, and found no evidence of SAg anchor cleavage.