Evolutionary repurposing of endosomal systems for apical organelle biogenesis in Toxoplasma gondii

It is very difficult to define an endocytic system in Toxoplasma gondii. The parasite does not appear to take up exogenous materials via classical endocytosis. The presence of Rab5 and Rab7, classical markers of endocytic compartments, and their decoration of endomembranous structures suggest, however, that an endosomal-like system may operate. Additionally, new findings reveal that dynamin and the transmembrane type-I receptor sortilin are involved in the biogenesis of T. gondii micronemes and rhoptries, unique apical secretory organelles required for parasite migration and host-cell invasion, manipulation and egress. Evidence suggests that the parasite uses an endosomal-like system to traffic and sort proteins to rhoptries and micronemes via the endoplasmic reticulum and Golgi. In this review, I discuss recent findings suggesting that T. gondii and other apicomplexans have reduced their endosomal system and repurposed the evolutionarily conserved regulators of the system to build the apical secretory organelles. This review is also intended to serve as a resource for future investigations of apicomplexan biology and evolution.