Protein kinase that seems to act exclusively upon threonine residues (By similarity). Required for normal entry into proliferative arrest after a limited number of cell divisions, also called replicative senescence. Required for normal cell cycle arrest in response to double-stranded DNA damage. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with RPGRIP1 and RPGRIP1L. Highest expression in adult heart, followed by pancreas, skeletal muscle, brain, testis, retina, liver, kidney, lung and placenta. Present in most primary carcinomas. Belongs to the protein kinase superfamily. NEK Ser/Thr protein kinase family. NIMA subfamily.