Apicortin defines the Plasmodium apical conoid body during transmission but is dispensable for the parasite life cycle

Apicomplexan parasites such as Plasmodium spp. and Toxoplasma gondii possess unique, highly stabilized microtubule structures, including subpellicular microtubules and apical polar rings, which are essential for parasite motility, host cell invasion, and replication. However, the molecular mechanisms responsible for stabilizing these structures remain poorly understood. In this study, we investigated the localization and functional role of a novel microtubule-associated protein, Apicortin, uniquely conserved in apicomplexans and the placozoan Trichoplax adhaerens. Apicortin contains both a doublecortin (DCX) domain and a partial tubulin polymerisation-promoting protein (TPPP) domain, both implicated in microtubule binding and stabilization. Using live cell imaging of transgenic Plasmodium berghei lines expressing GFP-tagged Apicortin, we observed its expression at the apical end of invasive transmission stages during parasite development within the mosquito vector. High-resolution imaging, including super-resolution and expansion microscopy, further revealed that Apicortin forms a distinct ring-like structure at the apical end of ookinetes and sporozoites, coinciding with the apical complex region. Surprisingly, functional analysis through gene knockout demonstrated that Apicortin is not essential for parasite development or transmission through the mosquito, suggesting potential redundancy in apical complex stabilization mechanisms or compensatory functions.