InterPro domain: IPR012198

Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity [ 1 ]:

• Serine/threonine-protein kinases
• Tyrosine-protein kinases
• Dual specificity protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)

Protein kinase function is evolutionarily conserved from Escherichia coli to human [ 2 ]. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation [ 3 ]. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [ 4 ], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [ 5 ].

In the absence of cAMP, protein kinase A (PKA) exists as an equimolar tetramer of regulatory (R) and catalytic (C) subunits. In addition to its role as an inhibitor of the C subunit, the R subunit anchors the holoenzyme to specific intracellular locations and prevents the C subunit from entering the nucleus. Typical R subunits have a conserved domain structure, consisting of the N-terminal dimerisation domain, inhibitory region, cAMP-binding domain A and cAMP-binding domain B. R subunits interact with C subunits primarily through the inhibitory site. The cAMP-binding domains show extensive sequence similarity and bind cAMP cooperatively.

On the basis of phylogenetic trees generated from multiple sequence alignment of complete sequences, this family was divided into four sub-families, types I to IV [ 6 ]. Types I and II, found in animals, differ in molecular weight, sequence, autophosphorylation capability, cellular location and tissue distribution. Types I and II are further sub-divided into alpha and beta subtypes, based mainly on sequence similarity. Type III are from fungi and type IV are from alveolates.

1. The protein kinase family: conserved features and deduced phylogeny of the catalytic domains. Science 241, 42-52
2. The protein kinase complement of the human genome. Science 298, 1912-34
3. Evolution of protein kinase signaling from yeast to man. Trends Biochem. Sci. 27, 514-20
4. High-throughput structural biology in drug discovery: protein kinases. Curr. Pharm. Des. 10, 1069-82
5. Creating chemical diversity to target protein kinases. Comb. Chem. High Throughput Screen. 7, 453-72
6. Classification and phylogenetic analysis of the cAMP-dependent protein kinase regulatory subunit family. J. Mol. Evol. 54, 17-29