Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. The alpha chain contains the catalytic site. The tetrameric holoenzyme CK2 is composed of two alpha and two beta subunits (By similarity). Acts as circadian clock component that maintains the correct period length through phosphorylation of CCA1 (PubMed:21900482). 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. 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.