<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 Unc-51 from <taxon tax_id="6239">Caenorhabditis elegans</taxon>. Unc-51 is important for axonal elongation and axonal guidance [<cite idref="PUB00042769"/>]. It is required for either the maintenance of axons (membrane turnover) or for an unknown neuronal function. C elegans worms lacking Unc-51 exhibit various abnormalities in axonal elongation and axonal structures. Unc-51 could also help control cell size along with Bec-1, as mutations in their corresponding genes results in a reduction in small body size without affecting cell number [<cite idref="PUB00042770"/>].</p>