Turnover of the GPI-anchored surface antigen in Paramecium: Partial release of its acylated form into the culture medium.

Surface antigens of Paramecium are high molecular weight proteins encoded by a multigene family, and their mutually exclusive expression at the cell surface is elicited by environmental conditions: changes in external factors trigger antigenic variation. The surface antigens are anchored in the plasma membrane by a glycosyl phosphatidylinositol moiety which can be removed by an endogenous phospholipase C-like hydrolase, releasing a lipid-lacking form of the molecules. In order to understand the mechanisms of the antigenic variation and the physiological involvement of the endogenous enzyme, I studied the turnover of the G antigen stably expressed at 23°C in Paramecium primaurelia. By (35)S labeling and chase experiments, I demonstrate that the turnover occurs at a slow rate (half-life beyond 45 hours), and is concomitant with a release of the molecule into the external medium: 16% of the initial cell-associated antigen is found in the medium after 45 hours. Analysis by immunolabeling and [(3)H]myristate radiolabeling of the released antigen showed that it is acylated, indicating that the release phenomenon does not involve the endogenous phospholipase C. Furthermore, the results indicate that in addition to releasing, an internal degradative pathway intervenes in the surface antigen turnover.