RNG2 tethers the conoid to the apical polar ring in Toxoplasma gondii: a key mechanism in parasite motility and invasion

In Toxoplasma gondii, the conoid is a dynamic organelle composed of spiraling tubulin fibers that extrudes during egress, gliding motility, and invasion. This organelle traverses the apical polar ring (APR) in response to calcium waves and plays a critical role in controlling parasite motility. While the actomyosin-dependent extrusion of the conoid is beginning to be understood, the mechanism by which it is anchored apically to the APR remains unclear. RNG2, a protein localized at both the conoid and the APR, has emerged as a key candidate for this function. By combining iterative ultrastructure expansion microscopy and immunoelectron microscopy we discovered that RNG2 forms 22 tethers between the APR and the conoid. The unique biochemical properties of RNG2, including several proteolytic processing events and its ability to form concatenations, enable it to function as a dynamic bridge between these structures. Conditional depletion of RNG2 resulted in the conoid organelle detaching from the APR without compromising the integrity of its structure, thereby confirming RNG2 essential tethering role. Although microneme secretion remains normal, parasites lacking RNG2 were unable to move and impaired in rhoptry discharge, highlighting the conoids crucial role in parasite motility and invasion. RNG2 is a pivotal protein that ensures conoid functionality in Coccidia.