<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 fungi MAPK-activated protein kinases that have similarity to calcium/calmodulin protein kinases, but for whom experimental evidence supporting regulation by Ca2+/CaM is lacking [<cite idref="PUB00052343"/>]. It includes Rck1 and Rck2 (Clk1) [<cite idref="PUB00055513"/>], the latter being involved in a signal transduction pathway that is activated by changes in the osmolarity of the extracellular environment [<cite idref="PUB00052344"/>]. Srk1 has a role in the regulation of meiosis via the Sty1 stress-activated MAPK pathway and has been shown to inhibit nitrogen-limitation induced arrest at G1 [<cite idref="PUB00052345"/>]. This entry also includes Schizosaccharomyces pombe Cmk2 (homologous to Rck2) which acts downstream of Sty1 and is an essential for oxidative stress responses [<cite idref="PUB00055514"/>].</p>