Ciliary transition zone activation of phosphorylated Tctex-1 controls ciliary resorption, S-phase entry and fate of neural progenitors.

Primary cilia are displayed during the G(0)/G(1) phase of many cell types. Cilia are resorbed as cells prepare to re-enter the cell cycle, but the causal and molecular link between these two cellular events remains unclear. We show that Tctex-1 phosphorylated at Thr 94 is recruited to ciliary transition zones before S-phase entry and has a pivotal role in both ciliary disassembly and cell cycle progression. However, the role of Tctex-1 in S-phase entry is dispensable in non-ciliated cells. Exogenously adding a phospho-mimic Tctex-1(T94E) mutant accelerates cilium disassembly and S-phase entry. These results support a model in which the cilia act as a brake to prevent cell cycle progression. Mechanistic studies show the involvement of actin dynamics in Tctex-1-regulated cilium resorption. Tctex-1 phosphorylated at Thr 94 is also selectively enriched at the ciliary transition zones of cortical neural progenitors, and has a key role in controlling G(1) length, cell cycle entry and fate determination of these cells during corticogenesis.