Protein kinase which plays an important role in mitotic cell cycle progression (PubMed:14563848). Required for chromosome segregation at metaphase-anaphase transition, robust mitotic spindle formation and cytokinesis (PubMed:19414596). Phosphorylates ATF4, CIR1, PTN, RAD26L, RBBP6, RPS7, RPS6KB1, TRIP4, STAT3 and histones H1 and H3 (PubMed:12054534, PubMed:20873783). Phosphorylates KIF11 to promote mitotic spindle formation (PubMed:19001501). Involved in G2/M phase cell cycle arrest induced by DNA damage (PubMed:18728393). Inhibition of activity results in apoptosis. May contribute to tumorigenesis by suppressing p53/TP53-induced cancer cell senescence (PubMed:21099361). Phosphorylates EML4 at 'Ser-144', promoting its dissociation from microtubules during mitosis which is required for efficient chromosome congression (PubMed:31409757). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binding to NEK9 stimulates its activity by releasing the autoinhibitory function of Tyr-108. Interacts with STAT3 and RPS6KB1 (By similarity). Interacts with NEK9, predominantly in mitosis. Interacts with KIF11 (via C-terminus). Interacts with APBB1 (via WW domain). Interacts with ANKRA2, ATF4, ARHGAP33, CDC42, CIR1, PRAM1, PTN, PRDX3, PIN1, RAD26L, RBBP6, RPS7 and TRIP4. Colocalizes with APBB1 at the nuclear speckles. Colocalizes with PIN1 in the nucleus. Colocalizes with ATF4, CIR1, ARHGAP33, ANKRA2, CDC42, NEK9, RAD26L, RBBP6, RPS7, TRIP4, RELB and PHF1 in the centrosome. Localizes to spindle microtubules in metaphase and anaphase and to the midbody during cytokinesis. Ubiquitous, with highest expression in heart and skeletal muscle. Up-regulated during the M phase of cell cycle progression. Down-regulated in both replicative and premature senescence of cancer cells. Displays an autoinhibited conformation: Tyr-108 side chain points into the active site, interacts with the activation loop, and blocks the alphaC helix. The autoinhibitory conformation is released upon binding with NEK9. Autophosphorylated. Phosphorylation at Ser-206 is required for its activation. Phosphorylated upon IR or UV-induced DNA damage. Phosphorylated by CHEK1 and CHEK2. Interaction with APBB1 down-regulates phosphorylation at Thr-210. Belongs to the protein kinase superfamily. NEK Ser/Thr protein kinase family. NIMA subfamily. A paper showing a role in tumorigenesis has been retracted due to panel duplication in several figures. Contaminating sequence. Sequence of unknown origin in the N-terminal part. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.