Fig. 2

Conoid diagram and EM images of conoids isolated from wild-type, mCherryFP-TgDCX knock-in and TgDCX knockout parasites. a CryoEM image of disassembled apical complexes from wild-type (“WT”) parasites. Several groups of conoid fibers (“CF”, arrowheads) and fragments of cortical microtubules (“MT”, arrows) are seen. Note that the cortical microtubules are straight, whereas the conoid fibers are uniformly curved. The conoid fibers appear to become wider along their length, and their protofilaments become clearer, indicating a twist in the fibers, as diagrammed in the cartoon. The cartoon represents the cluster of fibers in the upper right of the cryoEM image. The hollow arrow in the cartoon shows the direction of view in the EM image, and the boxes contain cross-sections of the fiber at the indicated locations. Near the apical end of the fibers (towards the bottom in the cartoon and the image), the direction of view yields a narrow fiber with protofilaments obscured by superposition. As the fiber twists along its length, its profile becomes wider, and there is decreasing superposition of protofilaments, giving the splayed appearance at the basal region of the fibers. b End on (left) and side views (right) of negatively stained isolated conoids from mCherryFP-TgDCX knock-in parasites (“K-in mCh-TgDCX”). In the left image, the apical polar ring with stumps of broken cortical microtubules encircles the conoid. A second conoid, almost completely disassembled, is also seen. In the right image, the two intra-conoid microtubules are seen projecting through the conoid, which is detached from the apical polar ring. c Three examples of disassembled conoids isolated from mCherryFP-TgDCX knock-in parasites (“K-in mCh-TgDCX”). All 14 of the fibers that formed each conoid are seen. Arrows: preconoidal rings, which often remain attached to the apical ends of the fibers. d End on views of conoids isolated from TgDCX knockout parasites (“ΔTgDCX”). The conoids are encircled by the apical polar ring with attached fragments of cortical microtubules. Isolated conoid fibers or conoids detached from the apical polar ring were never observed in preparations from the TgDCX knockout parasite. e Diagram illustrating the changing geometry of the TgDCX-containing fibers of the conoid. Extension of the conoid through the apical polar ring, which occurs as the parasites reactivate motility and exit their lysed host cell, is accompanied by a change in conoid shape from more conical to more cylindrical. The structural implications, for the conoid fibers, that follow from this change in overall shape are described in the Discussion. For clarity, the change in fiber orientation has been exaggerated in the diagram. Note also that the diagrams here are oversimplified for clarity: in an untilted sagittal section as diagrammed, the fiber profiles cannot be clearly seen. In order to make the profiles visible, the section must be tilted in the microscope by plus (to see the profiles on one side) or minus (to see the profiles on the other side) the pitch angle of the fibers. See Fig. 6 in [5] for a demonstration