Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates (By similarity). The alpha chain contains the catalytic site. The tetrameric holoenzyme CK2, composed of two alpha and two beta subunits, phosphorylates the transcription factor PIF1 after an exposure to light, resulting in a proteasome-dependent degradation of PIF1 and promotion of photomorphogenesis (PubMed:21330376). CK2 phosphorylates translation initiation factors. May participate in the regulation of the initiation of translation (PubMed:19509278). Acts as circadian clock component that maintains the correct period length through phosphorylation of CCA1 (PubMed:21900482). May act as an ectokinase that phosphorylates several extracellular proteins. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Heterotetramer of two catalytic alpha subunits and two regulatory beta subunits. Enriched in the nucleolus. Seems to be present in all plant organs. But seems to be more expressed than CKA1. No visible phenotype under normal growth conditions, but the triple mutant cka1, cka2 and cka3 show altered circadian rhythms and delayed flowering under long day conditions. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CK2 subfamily.