Localization of saponin-sterol complexes and lectin-binding sites during interaction of Toxoplasma gondii with host cells.

The attachment of tachyzoites of Toxoplasma gondii to the surface of host cells induces the formation of a vacuole known as the parasitophorous vacuole (PV). In the present study we used lectins labeled with either fluorescein or colloidal gold particles to follow the fate of lectin-binding sites on the host-cell surface during the process of parasite/host-cell interaction. The results obtained indicate that few, if any, lectin-binding sites are internalized together with the tachyzoites to make part of the PV membrane. However, these sites are internalized and observed in small vesicles that are seen close to the PV. We also used the freeze-fracture technique in cells treated with saponin, which reveals cholesterol-containing sites as easily identifiable elongated sulculi and depressions. Intramembranous particles, which represent membrane integral proteins, were seen in the membrane lining the PV. However, saponin-cholesterol complexes were not observed in the membrane lining the PV, although they were seen in the parasite membranes. These observations, in association with data from previous studies, show that some, but not all, membrane components of the host cell contribute to the formation of the PV membrane.