<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>The divalent cation calcium (Ca2+) is one of the most widely utilised second messengers in cellular signaling. Many of the second messenger effects of Ca2+ aremediated through the ubiquitous Ca2+ sensing protein, calmodulin (CaM). CaM has no enzymatic activity as such, and its function is to integrate the Ca2+ signal and transduce it to other downstream enzymes, like the calmodulin-dependent kinases [<cite idref="PUB00055510"/>].</p><p>This entry represent calcium/calmodulin-dependent protein kinase (CMK) from fungi. Saccharomyces cerevisiae has two kinds of genes encoding CaM kinase isozymes (CMK1 and CMK2) whose structural and functional properties are closely related to those of mammalian CaM kinase II [<cite idref="PUB00055511"/>], while in Aspergillus nidulans CMK (CMKA) is encoded by a single-copy gene [<cite idref="PUB00055512"/>]. CaM kinase II has been implicated in cellular signal transduction and the control of cell division. </p>