Protein Kinase CK2
One of the first members of the protein kinase superfamily ever discovered is CK2, formerly known as "casein kinase 2." The first report, dated 1954 by Burnett and Kennedy, described this enzyme isolated from liver extracts along with protein kinase CK1 (Burnett and Kennedy 1954). For their experiments, the authors utilized casein as an artificial substrate, and this is the reason for the early name casein kinase 2 (indeed, since casein is not a natural substrate for the enzyme, and since the real ones are still undefined, the name was moved to the more generic "CK2").
Protein kinase CK2 is an eukaryotic acidophilic Ser/Thr protein kinase. This ubiquitous enzyme is well conserved throughout evolution and, based on a recent classification of the human kinome (Manning et al. 2002b; see also the web-site at http://kinase.com/kinbase/), the catalytic subunit belongs to the so-called other kinase group, i.e., without strong similarities to other groups, giving origin to the CK2 family. Among the nearest relatives are the cyclin-dependent kinases, belonging to the CMGC group.
The holoenzyme comprises a tetrameric assembly formed by two catalytic (a) and two regulatory (b) subunits. Depending on the species, several catalytic and regulatory paralogs have been identified; in humans, two catalytic (a- and a') subunits and one regulatory (b) subunit are known.
Owing to some peculiar properties, CK2 is considered a quite anomalous protein kinase (Pinna 2002): (a) it is highly pleiotropic, (b) it can use ATP and guanosine triphosphate (GTP) as co-substrates, (c) the target serine or threonine must be surrounded by acidic residues and (d) the a-catalytic subunit is intrinsically active. Finally, most importantly, the precise function of this enzyme has not been deciphered. For recent reviews on the subject, see references (Pinna 2002; Litchfield 2003; Meggio and Pinna 2003).
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