Intersection of endocytic and exocytic systems in Toxoplasma gondii

Host cytosolic proteins are endocytosed by Toxoplasma gondii and degraded in its lysosome-like compartment, the vacuolar compartment (VAC), but the dynamics and route of endocytic trafficking remain undefined. Conserved endocytic components and plant-like features suggest T. gondii endocytic trafficking involves transit through early and late endosome-like compartments (ELCs) and potentially the trans-Golgi network (TGN) as in plants. However, exocytic trafficking to regulated secretory organelles, micronemes and rhoptries, also proceeds through ELCs and requires classical endocytic components, including a dynamin-related protein, DrpB. Here, we show that host cytosolic proteins are endocytosed within 7 minutes post-invasion, trafficked through ELCs en route to the VAC, and degraded within 30 minutes. We could not definitively interpret if ingested protein is trafficked through the TGN. We also found that parasites ingest material from the host cytosol throughout the parasite cell cycle. Ingested host proteins colocalize with immature microneme proteins, proM2AP and proMIC5, in transit to the micronemes, but not with the immature rhoptry protein proRON4, indicating that endocytic trafficking of ingested protein intersects with exocytic trafficking of microneme proteins. Finally, we show that conditional expression of a DrpB dominant negative mutant increases T. gondii ingestion of host-derived proteins, suggesting that DrpB is not required for parasite endocytosis.