It stands to reason that BB assembly and cell division are linked to guard against such outcomes. Premature cell division before an adequate number of BBs are produced would lead to a reductional loss of BBs and would therefore impair motility. However, it is not understood how new BB assembly is coordinated with the timing of cell division. During each cell cycle, new BBs are assembled and, like DNA replication, the number of BBs must double before cell division completes to ensure that both daughter cells have enough cilia for motility ( Nanney et al., 1978 Galati et al., 2016). The ciliate Tetrahymena thermophila harbors hundreds of BBs per cell organized into linear rows along the anterior-posterior cell axis to position motile cilia for cellular movement. The conservation of these structures underscores the importance of their microtubule organizing functions in diverse organisms. As BBs, these structures are positioned at the cell cortex to nucleate cilia that can function in both signaling and motility ( Pala et al., 2017 Haimo and Rosenbaum, 1981). As centrioles, they act in pairs to recruit pericentriolar material that nucleates cytoplasmic microtubules and the mitotic spindle apparatus ( Brinkley, 1985). Thus, Sas4 links BBs with an ancient signaling pathway known to promote the accurate and symmetric segregation of the genome.Ĭentrioles and basal bodies (BBs) are microtubule-organizing centers that are conserved across the eukaryotic lineage ( Carvalho-Santos et al., 2011 Wickstead and Gull, 2011). We find that Sas4 loss disrupts localization of the Hippo activator, Mob1, suggesting that Sas4 mediates Hippo activity by promoting scaffolds for Mob1 localization to the cell cortex. The Hippo signaling pathway is known to regulate cell division furrow position, and Hippo molecules localize to BBs and BB-appendages. Sas4 is necessary for BB assembly and cortical microtubule organization, and Sas4 loss disrupts cell division furrow positioning and DNA segregation. The Tetrahymena Sas4/CPAP protein is enriched at assembling BBs, localizing to the core BB structure and to the base of BB-appendage microtubules and striated fiber. Both BB assembly and DNA replication are tightly coordinated with the cell cycle to ensure their accurate segregation and propagation to daughter cells, but the mechanisms ensuring coordination are unclear. Basal bodies (BBs) are macromolecular complexes required for the formation and cortical positioning of cilia.
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