Casein kinase I (also designated CKI) and casein kinase II (CKII) compose a
family of serine/threonine protein kinases which are present in all eukaryotes
examined to date. Casein kinase I family members, which include casein kinase
Iα, Iγ, Iδ and Iε, have been implicated in the control of cytoplasmic and
nuclear processes, including DNA replication and repair. CKII is usually expressed
as a tetrameric complex consisting of either an α2β2 or an αα‘β2
structure. The a catalytic subunit is stimulated by the β regulatory subunit,
which undergoes autophosphorylation. Casein kinase II activity is high in the
cytosol and nucleus of proliferating and differentiating cells. Casein kinase
II is known to phosphorylate more than 100 different substrates including
nuclear oncoproteins, transcription factors and enzymes involved in DNA
metabolism.