protein kinase CK2 complex A protein complex that possesses protein serine/threonine kinase activity, and contains two catalytic alpha subunits and two regulatory beta subunits. Protein kinase CK2 complexes are found in nearly every subcellular compartment, and can phosphorylate many protein substrates in addition to casein. cellular_component Note that this term represents a location and not a function; the activity possessed by this complex is mentioned in the definition for the purpose of describing and distinguishing the complex. GO:0005956 casein kinase II complex intracellular protein-containing complex serine/threonine protein kinase complex