MOD_CK2_1

The casein kinases (CK) belong to the serine/threonine kinases and are subdivided further into either casein kinase 1 (CK1) or casein kinase 2 (CK2) families due to their high homology in their catalytic domains. CK2 is the most pleiotropic serine/threonine kinase constitutively active in all eukaryotic tissues and resides in various cellular compartments but particularly the nucleus and the cytosol. The CK2 holoenzyme is a tetrameric complex comprising two regulatory β-subunits and two catalytic subunits (Niefind,2001). The regulatory subunits mediate interactions between the catalytic subunits and many of its substrates. In cells, the subunits can exist individually or as the holoenzyme. The CK2 kinase is constitutively active and its basal catalytic activity is not influenced by any external stimuli.
Studies have shown that CK2 phosphorylates hundreds of substrates in various cellular compartments and many of its substrates are signaling proteins, including membrane receptors, adaptors, protein kinases, phosphatases, transcription factors, modulators of DNA and RNA structure, and proteins involved in RNA and protein biosynthesis, etc. (Meggio,2003). Although CK2 is present in all cell types, it is highly expressed in cancer cells and counteracts apoptosis, which makes it a valuable target in anticancer therapies (Ahmad,2008). Most of the CK2 phosphorylation sites have acidic amino acids surrounding the phosphoacceptor residue, especially at the +3 position.