Serine/threonine kinase that acts as a key regulator of ciliogenesis: controls the initiation of ciliogenesis by binding to the distal end of the basal body and promoting the removal of CCP110, which caps the mother centriole, leading to the recruitment of IFT proteins, which build the ciliary axoneme. Has some substrate preference for proteins that are already phosphorylated on a Tyr residue at the +2 position relative to the phosphorylation site. Able to phosphorylate tau on serines in vitro (PubMed:23141541). Phosphorylates MPHOSPH9 which promotes its ubiquitination and proteasomal degradation, loss of MPHOSPH9 facilitates the removal of the CP110-CEP97 complex (a negative regulator of ciliogenesis) from the mother centrioles, promoting the initiation of ciliogenesis (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with CEP164. Localizes to the transition zone in primary cilia in response to cell cycle signals that promote ciliogenesis. May also be present in cytosol and, at lower level in the nucleus. Defects in Ttbk2 are the cause of the bartleby (bby) phenotype characterized by defects in sonic hedgehog/SHH signaling and ciliogenesis. Embryos display morphological defects at midgestation similar to those seen in mutants that lack cilia, including holoprosencephaly, twisted body axis, abnormal limb development and randomized laterality of heart looping. Mutants die at midgestation (around 10.5 dpc). Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Extended N-terminus.