For intracellular, infectious agents that live inside a vacuole there are many benefits, but maybe even more challenges. On the one hand, they can be protected from detection by innate immune sensors and buffered from some types of immune attack. On the other hand, this isolation represents a barrier to actively engaging with the host cell sitting all around their “bubble,” e.g., for feeding and active coopting of host cell functions. The single-celled eukaryote, Toxoplasma gondii, is a remarkable example of an intracellular agent in that it can infect and grow within almost any mammalian or avian cell. Many studies have shown that despite growing within a parasitophorous vacuole (PV), Toxoplasma tachyzoites are in active dialogue with the host cell and that the parasites somehow introduce a large number of “effector” proteins into the infected cell cytoplasm; many of these eventually reach the host cell nucleus where they impact host gene expression. Toxoplasma effectors come in two flavors – ones that originate in the apical secretory organelles known as rhoptries and that are injected during invasion (ROPs); and a second set that are secreted into the PV from dense granules after invasion (GRAs). In my talk, I will first describe our recent efforts with cryo-electron tomography to understand the machinery that enables this remarkable organism to invade almost any host cell and inject the first wave of effectors, the ROPS. I will then discuss how they modify the parasitophorous vacuole membrane (PVM) to facilitate translocation of GRAs across the PVM, involving a completely novel translocon system. I will end with how GRAs and ROPs impact gene expression within the infected host.
A segment from cryo-ET of Toxoplasma's apical complex. This extraordinary machinery enables a tachyzoite to invade and co-opt almost any cell in a warm-blooded animal. Shown are preconoidal ring (PCR; red), conoid fibrils (CFs; cyan), apical polar ring (APR; golden), subpellicular microtubules (SPMTs; blue), intra-conoidal microtubules (red), micronemes (pink), rhoptries (yellow) and plasma membrane (pale pink). Scale bar, 100nm. Image credit: Stella Sun, Li-av Segev Zarko, Muyuan Chen et al.; Host protein MOSPD2 (green) is massively enriched at the parasitophorous vacuole membrane in human foreskin fibroblasts in which Toxoplasma tachyzoites are growing (4 tachyzoites are apparent in this image). This is one of many host and parasite proteins present at this literal interface between Toxoplasma and the host cell cytoplasm. Image credit: Abel Ferrel.
