Toxoplasma gondii basal complex is essential for parasite motility, as it constitutes a force-transmission platform together with the apical complex, and also for division. The basal complex is assembled early during endodyogeny, close to the centrosomes and the apical complex. The polarization of the daughter cells is driven by the extension of each pole to an opposite side. Daughter cells are then individualized by the constriction of the basal pole, which involves the myosin heavy chain J (MyoJ). To identified new components of the basal complex, we performed a proximity labeling experiment using MyoJ as a bait. In this screen, we identified the basal complex component 1 (BCC1), which colocalizes with MyoJ at the posterior cup and is also involved in basal pole constriction and motility. The conditional knock-down of BCC1 impacts the localization of MyoJ at the basal pole and conversely, the absence of MyoJ impacts BCC1 localization. In addition, Centrin 2 (CEN2), a calcium-binding protein, was shown to localize at the posterior cup, and its depletion also leads to a defect in basal complex constriction. This draws the hypothesis of a contractile ring at the basal pole, consisting of the motor MyoJ, Cen2 as the light chain and BCC1, the cargo.

To characterize the interactions taking place within this complex, we set up complementation experiments in parasites depleted for MyoJ or knock-out for BCC1. In these strains, we follow the localization of BCC1 and MyoJ respectively, while adding different constructs of MyoJ and BCC1. By doing so, we found that the motor domain of MyoJ is necessary to drive the constriction of the basal complex while the neck and tail region is necessary for the localization of BCC1. For BCC1, we showed that the coiled-coil domain is essential for the localization of MyoJ but not the constriction. In parallel, we undertook yeast-two-hybrid experiments to test the direct interaction between the three components of the putative contractile ring. These experiments should allow us to decipher the interacting domains involved and get a better understanding of the function of this complex, which seems different from other eukaryotic organisms.