TgCep250 is dynamically processed through the division cycle and essential for structural integrity of the Toxoplasma centrosome.

The apicomplexan centrosome has a unique bipartite structure comprising an inner- and outer-core responsible for the nuclear cycle (mitosis) and budding cycles (cytokinesis), respectively. Although these two cores are always associated, they function independently to facilitate polyploid intermediates in the production of many progeny per replication round. Here, we describe the function of a large coiled-coil protein in Toxoplasma gondii, TgCep250, in connecting the two centrosomal cores and promoting their structural integrity. Throughout the cell cycle TgCep250 localizes to the inner-core but, associated with proteolytic processing, is also present on the outer-core during the onset of cell division. In absence of TgCep250, stray centrosome inner- and outer-core foci were observed. The detachment between centrosome inner- and outer-core was found in only one of the centrosomes during cell division, indicating distinct states of mother and daughter centrosomes. In mammals Cep250 processing is required for centrosome splitting and is mediated by Nek phopsphorylation. However, we show that neither the non-overlapping spatiotemporal localization of TgNek1 and TgCep250 nor the distinct phenotypes upon their respective depletion support conservation of this mechanism in Toxoplasma. In conclusion, TgCep250 has a tethering function tailored to the unique bipartite centrosome in the Apicomplexa.