Many serine threonine (ser/thr) protein kinases are activated by specific small signalling molecules such as calcium, cAMP, cGMP while others appear to be cofactor independent. The casein kinases belong to this group, and how their activity is regulated is still something of a mystery (1,2). There are two broad families of these kinases called CKI and CKII (or CK1 and CK2). They were originally isolated since extracts of most cells contain two distinct fractions of kinase activity both of which are able to phosphorylate the milk protein casein. It is now known that the CKI and CKII activities represent two different families of distinct kinases, each consisting of several different gene products. The different proteins clearly represent an ancient and unusually conserved family of kinases, sharing some unique sequence characteristics not found in other protein kinases. The CKI family consists of several different genes, divided in α, β, γ, δ and ε (alpha, beta, gamma, delta and epsilon), some of which are encoded by more than one gene. The single mammalian CK1α gene can generate four different proteins by altering transcription. The CK1 alpha proteins are involved in the control of protein degradation, and phosphorylate acidic regions of their substrate molecules. Typically the target Ser/Thr residues closely C-terminal to two or three Asp or Glu acid residues. Alternately, phosphorylation on residues 3 or 4 amino acids N-terminal to a Ser/Thr residue can also favor CKI phosphorylation. This means that CKI family enzymes often phosphorylate one or more Ser/Thr residues C-terminal to a phosphorylation site generated by the activity of another protein kinase.
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