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 c ells induces the formation of a vacuole known as the parasitophorous vacuol e (PV). In the present study we used lectins labeled with either fluorescei n 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 cel ls treated with saponin, which reveals cholesterol-containing sites as easi ly identifiable elongated sulculi and depressions. Intramembranous particle s, which represent membrane integral proteins, were seen in the membrane li ning the PV. However, saponin-cholesterol complexes were not observed in th e 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.