BACKGROUND: Primary cilia facilitate cellular signalling and play critical roles in development, homeostasis, and disease. Their assembly is under the control of Tau-Tubulin Kinase 2 (TTBK2), a key enzyme mutated in patients with spinocerebellar ataxia. Recent work has implicated TTBK2 in the regulation of cilia maintenance and function, but the underlying molecular mechanisms are not understood. METHODS: To dissect the role of TTBK2 during cilia growth and maintenance in human cells, we examined disease-related TTBK2 truncations. We used biochemical approaches, proteomics, genetic engineering, and advanced microscopy techniques to unveil molecular events triggered by TTBK2. RESULTS: We demonstrate that truncated TTBK2 protein moieties, unable to localize to the mother centriole, create unique semi-permissive conditions for cilia assembly, under which cilia begin to form but fail to elongate. Subsequently, we link the defects in cilia growth to aberrant turnover of a microtubule-depolymerizing kinesin KIF2A, which we find restrained by TTBK2 phosphorylation. CONCLUSIONS: Together, our data imply that the regulation of KIF2A by TTBK2 represents an important mechanism governing cilia elongation and maintenance. Further, the requirement for concentrating TTBK2 activity to the mother centriole to initiate ciliogenesis can be under specific conditions bypassed, revealing TTBK2 recruitment-independent functions of its key partner, CEP164.

Department of Cell Biology Faculty of Science Charles University Viničná 7 Prague 12800 Czech Republic

Department of Experimental Biology Faculty of Science Masaryk University Kamenice 5 Brno 62500 Czech Republic

Institute of Biotechnology Czech Academy of Sciences BIOCEV Průmyslová 595 Vestec Prague 252 50 Czech Republic

Laboratory of Cilia and Centrosome Biology Department of Histology and Embryology Faculty of Medicine Masaryk University Kamenice 3 Brno 62500 Czech Republic

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Cilia at the crossroad: convergence of regulatory mechanisms to govern cilia dynamics during cell signaling and the cell cycle