ABSTRACT
TgDCX is a doublecortin-domain protein that binds to the conoid fibers, a set of strongly curved tubulin-based non-tubular polymers in Toxoplasma gondii. Deletion of TgDCX impairs conoid structure and parasite invasion. TgDCX contains two tubulin-binding domains: P25-alpha and the DCX (doublecortin) domain. Orthologues are found in all Apicomplexans, their free-living marine relatives Chromera and Vitrella, and an early branching metazoan, Trichoplax. Here we report that isolated TgDCX-containing conoid fibers retain their pronounced curvature, but loss of TgDCX destabilizes the fibers. We crystallized and determined the 3D structure of the DCX domain of TgDCX, which is highly similar to those of human doublecortin. Doublecortin’s ubiquitin-like folds are maintained and surface residues at the predicted tubulin contact sites are similar, and well-conserved among TgDCX orthologues. However, the orthologues vary widely in targeting to the conoid in Toxoplasma and in modulating the organization of microtubule arrays in Xenopus S3 cells. Several orthologues bind to microtubules in Xenopus cells, but only TgDCX generates short, strongly curved microtubule arcs. EM analysis shows microtubules decorated with TgDCX bundled into rafts, often bordered on one edge by a “C”-shaped incomplete tube. A Chromera orthologue closely mimics TgDCX targeting in Toxoplasma and binds to microtubules in Xenopus, but does not generate arced bundles or “C”-shaped incomplete tubes, and fails to rescue the defects of the TgDCX knockout parasite. These observations indicate that species-specific features of TgDCX are important for its function, and likely related to its unique ability to generate the strongly curved tubulin polymers in the cytoskeletal structure critical for efficient host-cell invasion.