Rhoptry secretion system structures and regulatory mechanisms in Apicomplexan parasites revealed using cryo-ET

Parasites of the phylum Apicomplexa cause important diseases including cryptosporidiosis, toxoplasmosis and malaria. These intracellular pathogens inject the contents of an essential organelle, the rhoptry, into host cells to facilitate invasion and infection. However, the structure and mechanism of this eukaryotic secretion system remain elusive. Here we used cryo-electron tomography to image the supramolecular architecture of this secretion system directly inside intact, frozen-hydrated Cryptosporidium parvum sporozoites, Toxoplasma gondii tachyzoites, and Plasmodium falciparum merozoites at a resolution of few nanometers in three dimensions. We identified helical filaments which appear to shape and compartmentalize the rhoptries, and an apical vesicle (AV) which facilitates docking of the rhoptry tip(s) at the parasite's apical region with the help of a remarkable multi-component molecular machine named the rhoptry secretory apparatus (RSA). We further resolved different conformations of the RSA in P. falciparum corresponding to different rhoptry morphological states, supporting RSA and AV's roles in mediating rhoptry secretion. Moreover, we showed that T. gondii contains a line of AV-like vesicles which interact with a pair of microtubules and accumulate towards the AV, leading to a working model for AV-reloading and discharging of multiple rhoptries in some apicomplexan parasites. Together, our analyses provide a structural framework to elucidate how these important parasites regulate and conduct rhoptry discharge for their infection.