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The Apicomplexa phylum is composed of numerous intracellular parasites of human and veterinary importance. Plasmodium species responsible of malaria and Toxoplasma gondii the causative agent of toxoplasmosis are both part of the phylum. Apicomplexan parasites possess at their apical pole an arsenal of cytoskeletal components as well as secretory organelles grouped as the “apical complex” that gives its name to the phylum. Micronemes contains adhesins, proteases and perforins essential for attachment, invasion, and egress of host cells while the rhoptries contain proteins essential for invasion and the establishment of the intracellular niche in which the parasite replicates. The cytoskeletal components of the apical complex are organized around the conoid, a cone made of atypical tubulin fibers, able to extrude through the apical polar ring (APR) that presumably serves as microtubule organizing center (MTOC) for the 22 subpellicular microtubules (SPMTs). In addition to the SPMTs, the inner membrane complex (IMC), a patchwork of flattened vesicles, and a meshwork of intermediate-like filaments called alveolin network (SPN) confers its strength and shape to the parasite. We identified two proteins of the alveolin network called AC9 and AC10, essential for the stability of the whole apical complex. In their absence, the APR is fragmented, the SPMTs are disorganized and the conoid is lost, leading to lethal defects in microneme secretion and invasion. We also scrutinized a protein called ERK7 reported to be associated to AC9-10. Depletion of ERK7 phenocopied the depletion of AC9 or AC10 with a complete destruction of the apical complex and clear abrogation of microneme secretion. The conoid is topped by two small preconoidal rings (PCRs) of unknown function. Thanks to ultrastructure expansion microscopy (U-ExM) we were able to localize five novel PCRs proteins (PCRPs) and essential gliding factors such as Formin 1. Using this technique, we also proved that conoid extrusion is an actomyosin dependent process powered by MyoH. The apical complex is a site of choice bringing together key gliding motility components. The machinery powering motility and invasion called glideosome lies inside the pellicle, between the plasma membrane and the IMC. To initiate motility, actin filament generated by Formin 1 at the PCRs are translocated backward by MyoH at the conoid level and MyoA at the IMC level. When this apico-basal flux of actin is linked to secreted micronemal adhesins by the glideosome associated connector (GAC), the parasite is propelled forward. Interestingly, we showed that GAC also localizes at the PCRs. Finally, two short intraconoidal microtubules (ICMTs) lie inside the conoid. In close proximity with rhoptries neck, the ICMTs are thought to be important for rhoptry discharge. Here we have identified and functionally characterized the second ICMT protein called ICMAP2, essential for the stability of the ICMTs and important rhoptry secretion. ICMAP1, previously described at the ICMTs, do not colocalize perfectly with ICMAP2, indicating that the biology and role of those short microtubules might be more complex than previously anticipated. |
