Recombinant <i>Leishmania mexicana</i> CRK3:CYCA has protein kinase activity in the absence of phosphorylation on the T-loop residue Thr178

The activity of cyclin-dependent kinases (CDKs), which are key regulators of the eukaryotic cell cycle, is regulated through post-translational mechanisms, including binding of a cyclin and phosphorylation. Previously studies have shown that &lt;i&gt;Leishmania mexicana&lt;/i&gt; CRK3 is an essential CDK that is a functional homologue of human CDK1. In this study, recombinant histidine tagged &lt;i&gt;L. mexicana&lt;/i&gt; CRK3 and the cyclin CYCA were combined &lt;i&gt;in vitro&lt;/i&gt; to produce an active histone H1 kinase that was inhibited by the CDK inhibitors, flavopiridol and indirubin-3′-monoxime. Protein kinase activity was observed in the absence of phosphorylation of the T-loop residue Thr178, but increased 5-fold upon phosphorylation by the CDK activating kinase Civ1 of &lt;i&gt;Saccharomyces cerevisiae&lt;/i&gt;. Seven recombinant &lt;i&gt;L. major&lt;/i&gt; CRKs (1, 2, 3, 4, 6, 7 and 8) were also expressed and purified, none of which were active as monomers. Moreover, only CRK3 was phosphorylated by Civ1. HA-tagged CYCA expressed in &lt;i&gt;L. major&lt;/i&gt; procyclic promastigotes was co-precipitated with CRK3 and exhibited histone H1 kinase activity. These data indicate that in &lt;i&gt;Leishmania&lt;/i&gt; CYCA interacts with CRK3 to form an active protein kinase, confirm the conservation of the regulatory mechanisms that control CDK activity in other eukaryotes, but identifies biochemical differences to human CDK1