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

Abstract

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.

The BCC7 Protein Contributes to the Toxoplasma Basal Pole by Interfacing between the MyoC Motor and the IMC Membrane Network

“…With the imaging at a few tens of nanometers of the BCC7 protein recently identified [ 12 , 13 ] but not yet resolved in situ, we now document that this remarkably large protein of about 500 kDa organizes as a ribbed ring around the conic basal pole. Interestingly, one can distinguish some kind of claws that extend from the ring-like base and could promote better stabilization of the molecular pieces lying on top for optimal biomechanical coordination.…”

“…However, that BCC7 was found dispensable in vitro for T. gondii tachyzoite perpetuation based on plaque formation capacity was surprising. A detailed study by Gubbels et al published in the course of our manuscript evaluation [ 12 ] points out the subtle effect of BCC7 loss following gene knock out strategy on the rosette organization of the progeny within vacuole which appears to be less consistent than in wild type, and which do not impact on the general in vitro fitness score. This defect suggests that BCC7 could assist in the formation and maintenance of the cytoplasmic bridge between progeny after division within the parasitophorous vacuole which would translate into a rosette disorganization.…”

“…Beyond the long-lasting interest for the structural organization and functional contribution of the zoite apical complex to the motile and invasive behaviors of extracellular zoites [ 9 , 10 ], an increasing number of studies have started to highlight the specific composition and architecture of the basal pole and to model its progressive fabrication during the development of new zoites inside the host cells [ 7 , 11 , 12 , 13 ]. Zoites multiply by endodyogeny, a process through which two daughter cells assemble their cortex and cytoskeleton de novo inside an intact mother prior to cytokinesis.…”

“…This in situ observation prompted us to map the protein position with respect to the posteriorly located myosins. In the course of our study, a BioID-based map of the basal complex (BC) proteome also picked up this protein as a basal complex component and named it as BCC7 [ 12 , 13 ]. Using a combination of biochemistry, co-immunoprecipitation coupled to mass spectrometry (MS)-based proteomics, and live and super resolution laser scanning microscopy, we now bring evidence for BCC7 positioned at the interface between a set of IMC proteins including two newly identified herein, and the MyoC motor.…”

Proteomic characterization of the Toxoplasma gondii cytokinesis machinery portrays an expanded hierarchy of its assembly and function

“…A last transition in BC composition occurs upon daughter parasite emergence when proteins such as FIKK 17 and MSC1a 15 are recruited. The role of these proteins, which are only found in the BC of the mother cell, is currently not well understood 18 . The set of known BC markers in combination with electron microscopy resolves into three substructures within the BC 7 , 16 , 19 (Fig.…”