The apical annuli of Toxoplasma gondii are composed of coiled-coil and signalling proteins embedded in the inner membrane complex sutures.

The apical annuli are among the most intriguing and understudied structures in the cytoskeleton of the apicomplexan parasite Toxoplasma gondii. We mapped the proteome of the annuli in Toxoplasma by reciprocal proximity biotinylation (BioID), and validated five apical annuli proteins (AAP1-5), Centrin2, and an apical annuli methyltransferase. Moreover, inner membrane complex (IMC) suture proteins connecting the alveolar vesicles were also detected and support annuli residence within the sutures. Super-resolution microscopy identified a concentric organisation comprising four rings with diameters ranging from 200 to 400 nm. The high prevalence of domain signatures shared with centrosomal proteins in the AAPs together with Centrin2 suggests that the annuli are related and/or derived from the centrosomes. Phylogenetic analysis revealed that the AAPs are conserved narrowly in coccidian, apicomplexan parasites that multiply by an internal budding mechanism. This suggests a role in replication, for example, to provide pores in the mother IMC permitting exchange of building blocks and waste products. However, presence of multiple signalling domains and proteins are suggestive of additional functions. Knockout of AAP4, the most conserved compound forming the largest ring-like structure, modestly decreased parasite fitness in vitro but had no significant impact on acute virulence in vivo. In conclusion, the apical annuli are composed of coiled-coil and signalling proteins assembled in a pore-like structure crossing the IMC barrier maintained during internal budding.

Toxoplasma gondii's Basal Complex: The Other Apicomplexan Business End Is Multifunctional.

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in 'import', such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.