Toxoplasma gondii is an obligate intracellular parasite capable of invading a wide range of host cells where it resides in a specially modified compartment termed the parasitophorous vacuole. This compartment provides a protected environment for the parasite which enters a rapid growth phase shortly after invasion. To identify functional components of this interface, we have used immunolabeling and cell fractionation to localize the potent nucleoside triphosphate hydrolase (NTPase) produced by Toxoplasma. In extracellular tachyzoites, NTPase was not exposed on the cell surface but was localized in small vesicles, called dense granules, scattered throughout the cell cytoplasm. Shortly following invasion, NTPase was secreted from dense granules and occupied the lumen of the parasitophorous vacuole where it was often associated with the intravacuolar network. NTPase was exclusively found in the supernatant in cell fractionation studies of both extracellular tachyzoites lysed by freeze-thaw and following secretion into the parasitophorous vacuole. These studies provide evidence for the rapid secretion of NTPase into the parasitophorous vacuole, where it may play a key role in processing of nucleotides for purine salvage by the parasite.
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