Kinetics and pattern of organelle exocytosis during Toxoplasma gondii/host-cell interaction

The fate of Toxoplasma gondii dense-granule (GRA2, GRA3), rhoptry (ROP1), and surface (SAG1) proteins was followed by immunofluorescence assay (IFA) and immunoelectron microscopy at different stages after infection. Dense-granule exocytosis occurred in the apical area of the tachyzoite within minutes of invasion. Several exocytic events were found simultaneously in the same organism, both by serial sectioning and by freeze-fracture studies. Dense-granule contents were first found as a dense material trapped between parasite and vacuole membranes before either the vacuolar network or the vacuole membrane could be immunolabeled with specific antibodies. The vacuolar network was strongly labeled with dense-granule antibodies but not with the SAG1-specific probe, which suggests that the network is not enriched in membrane proteins. In addition to strongly labeling the vacuole membrane, GRA3 antibodies also labeled strands extending from the parasitophorous vacuoles into the host-cell cytoplasm.