Catalytic subunit of a constitutively active serine/threonine-protein kinase complex that phosphorylates a large number of substrates containing acidic residues C-terminal to the phosphorylated serine or threonine (PubMed : 11239457, PubMed : 11704824, PubMed : 16193064, PubMed : 18411307, PubMed : 18583988, PubMed : 18678890, PubMed : 19188443, PubMed : 20545769, PubMed : 20625391, PubMed : 22017874, PubMed : 22406621, PubMed : 24962073, PubMed : 30898438, PubMed : 31439799). Regulates numerous cellular processes, such as cell cycle progression, apoptosis and transcription, as well as viral infection (PubMed : 12631575, PubMed : 19387551, PubMed : 19387552). May act as a regulatory node which integrates and coordinates numerous signals leading to an appropriate cellular response (PubMed : 12631575, PubMed : 19387551, PubMed : 19387552). During mitosis, functions as a component of the p53/TP53-dependent spindle assembly checkpoint (SAC) that maintains cyclin-B-CDK1 activity and G2 arrest in response to spindle damage (PubMed : 11704824, PubMed : 19188443). Also required for p53/TP53-mediated apoptosis, phosphorylating 'Ser-392' of p53/TP53 following UV irradiation (PubMed : 11239457). Phosphorylates a number of DNA repair proteins in response to DNA damage, such as MDC1, MRE11, RAD9A, RAD51 and HTATSF1, promoting their recruitment to DNA damage sites (PubMed : 18411307, PubMed : 18583988, PubMed : 18678890, PubMed : 20545769, PubMed : 21482717, PubMed : 22325354, PubMed : 26811421, PubMed : 28512243, PubMed : 30898438, PubMed : 35597237). Can also negatively regulate apoptosis (PubMed : 16193064, PubMed : 22184066). Phosphorylates the caspases CASP9 and CASP2 and the apoptotic regulator NOL3 (PubMed : 16193064). Phosphorylation protects CASP9 from cleavage and activation by CASP8, and inhibits the dimerization of CASP2 and activation of CASP8 (PubMed : 16193064). Phosphorylates YY1, protecting YY1 from cleavage by CASP7 during apoptosis (PubMed : 22184066). Regulates transcription by direct phosphorylation of RNA polymerases I, II, III and IV (PubMed : 12631575, PubMed : 19387550, PubMed : 19387551, PubMed : 19387552, PubMed : 23123191). Also phosphorylates and regulates numerous transcription factors including NF-kappa-B, STAT1, CREB1, IRF1, IRF2, ATF1, ATF4, SRF, MAX, JUN, FOS, MYC and MYB (PubMed : 12631575, PubMed : 19387550, PubMed : 19387551, PubMed : 19387552, PubMed : 23123191). Phosphorylates Hsp90 and its co-chaperones FKBP4 and CDC37, which is essential for chaperone function (PubMed : 19387550). Mediates sequential phosphorylation of FNIP1, promoting its gradual interaction with Hsp90, leading to activate both kinase and non-kinase client proteins of Hsp90 (PubMed : 30699359). Regulates Wnt signaling by phosphorylating CTNNB1 and the transcription factor LEF1 (PubMed : 19387549). Acts as an ectokinase that phosphorylates several extracellular proteins (PubMed : 12631575, PubMed : 19387550, PubMed : 19387551, PubMed : 19387552). During viral infection, phosphorylates various proteins involved in the viral life cycles of EBV, HSV, HBV, HCV, HIV, CMV and HPV (PubMed : 12631575, PubMed : 19387550, PubMed : 19387551, PubMed : 19387552). Phosphorylates PML at 'Ser-565' and primes it for ubiquitin-mediated degradation (PubMed : 20625391, PubMed : 22406621). Plays an important role in the circadian clock function by phosphorylating BMAL1 at 'Ser-90' which is pivotal for its interaction with CLOCK and which controls CLOCK nuclear entry (By similarity). Phosphorylates CCAR2 at 'Thr-454' in gastric carcinoma tissue (PubMed : 24962073). Phosphorylates FMR1, promoting FMR1-dependent formation of a membraneless compartment (PubMed : 30765518, PubMed : 31439799). May phosphorylate histone H2A on 'Ser-1' (PubMed : 38334665).