Antibody-induced caps in Entamoeba histolytica: isolation and electrophoretic analysis.

Polyspecific antibodies bound to the cell surface of Entamoeba histolytica are polarized toward the uroid region and spontaneously released by the amoeba as supramolecular aggregate or cap. We purified these exfoliated structures and analyzed them by electron microscopy and sodium dodecyl sulfate electrophoresis. Isolated caps were mainly composed of membranes and contained five major [35S]methionine-labeled bands; many more bands appeared (together with immunoglobulins) after silver staining. Surface immunoprecipitates contained about twelve major [35S]methionine-labeled polypeptides, five of which had molecular weights similar to those of radiolabeled polypeptides in the cap. Some proteins in the caps had the same electrophoretic migration pattern as that from iodinated cell-surface proteins and polypeptides from isolated plasma membranes. Results suggest that only half of the surface-immunoprecipitated antigens are enriched in the cap.