Probable 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. Amplification-dependent oncogene; promotes cell proliferation and tumorigenesis by down-regulating expression of the tumor suppressor protein, PML. May play a role in the pathogenesis of the lung cancer development and progression. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Heterotetramer composed of two catalytic subunits (alpha chain and/or alpha' chain) and two regulatory subunits (beta chains) (By similarity). Interacts with PML. Detected in blood platelets and megakaryocyte cell lines. Poorly expressed in lung. Highly expressed in lung tumor tissues. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CK2 subfamily.