<p>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 [<cite idref="PUB00005115"/>]:</p><p> <ul> <li>Serine/threonine-protein kinases</li><li>Tyrosine-protein kinases</li><li>Dual specific protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)</li> </ul> </p><p>Protein kinase function has been evolutionarily conserved from <taxon tax_id="562">Escherichia coli</taxon> to human [<cite idref="PUB00020114"/>]. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation [<cite idref="PUB00015362"/>]. 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 [<cite idref="PUB00034898"/>], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [<cite idref="PUB00034899"/>].</p><p>This entry represents serine/threonine-protein kinases (<db_xref db="EC" dbkey="2.7.11.1"/>) such as Rad53, which can phosphorylate proteins on serine, threonine, and tyrosine residues. Rad53 controls the S-phase checkpoint as well as G1 and G2 DNA damage checkpoints. It prevents entry into anaphase and mitotic exit after DNA damage via regulation of the Polo kinase Cdc5 [<cite idref="PUB00042765"/>]. Rad53 also seems to be involved in the phosphorylation of Rph1, a zinc finger protein that acts as a damage-responsive repressor of the photolyase gene Phr1 in <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) [<cite idref="PUB00042766"/>]. The 2C type phosphatase, Ptc2, which is required for DNA checkpoint inactivation after double-strand breaks, appears to dephosphorylate Rad53 [<cite idref="PUB00042767"/>].</p>