Fig. 4 |. The rhoptry secretion machine includes an apical vesicle tightly connected with the rosette.

a, A slice through a tomogram showing a side view of the apical complex – conoid (brown), micronemes (yellow), plasma membrane (PM; light blue) and the rhoptry secretion system consisting of the rosette (dark blue), apical vesicle (AV; magenta), rhoptry (orange) and rhoptry tip density (cyan). Original image (right) is annotated with color overlays (left). b, Left: magnified image of the boxed region in (a) showing the connections between the rhoptry, tip density, AV, rosette and the PM. The rhoptry tip is 9 nm distant from the AV. Right: 3-dimensional (3D) segmentation of the scene above. The PM is rendered transparent on the right to reveal the rosette. c, Magnified image of the boxed region in (b) showing the side view of the rosette. The AV is 14 nm distant from the PM. d) Top view of the rosette from a horizontal tomogram section, perpendicular to the plane in (c), showing 8-fold rotational symmetry and a diameter of ~67 nm. e) AV connected with the PM via a rosette in the absence of a rhoptry. All measurements are made in 3D. Images in (b-e) are low pass filtered to boost contrast. The images in (c) and (d) are from two different cells that were initially oriented perpendicular to each other on the EM grid. f, Quantification of apical rosettes in TgARO-iKD mutants ± ATc 72h. g, Left: Ultrastructure of wild type (RH strain type I) tachyzoites with the apical vesicle positioned beneath the plasma membrane, where the rosette is located, and above the tip of the rhoptry neck. Right: In TgARO-iKD ATc-treated tachyzoites (72h), the apical vesicle is still properly positioned under the rosette, while rhoptries are not engaged in the conoid. co: conoid; m: micronemes; Rh: rhoptry; APR: apical polar ring; PCR: pre-conoidal apical rings; AV: apical vesicle; CM: pair of intraconoidal microtubules. h, Schematic of similarities and differences of the exocytic machinery between ciliates and Apicomplexa. Exocytosis in Alveolata (Ciliate, Dinoflagellate and Apicomplexa) is outlined by the presence of a rosette of particles embedded in the outer membrane, defining the site of exocytosis. In ciliates, organelles discharge can have a defensive or predatory function. Rhoptry exocytosis in Apicomplexa is one of the critical steps of host cell invasion and therefore fundamental for parasitism. In Apicomplexa, but not in ciliates, an apical vesicle (Av) of unknown function is present between the tip of the rhoptry and the plasmalemma, plausibly involved in the injection of rhoptry proteins into the host cell. PM, plasma membrane, PVM, parasitophorous vacuole membrane, AS, alveolar sac, which is homologous to the IMC (inner membrane complex) of Apicomplexa.