Activation of a Plasmodium falciparum cdc2-related kinase by heterologous p25 and cyclin H - Functional characterization of a P. falciparum cyclin homologue

Several Plasmodium falciparum genes encoding cdc2-related protein kinases h ave been identified, but the modalities of their regulation remains largely unexplored. Zn the present study, we investigated the regulation in vitro of PfPK5, a putative homologue of Cdk1 (cdc2) in P. falciparum. We show tha t (i) PfPK5 is efficiently activated by heterologous (human) cyclin H and p 25, a cyclin-like molecule that specifically activates human Cdk5; (ii) the activated enzyme can be inhibited by chemical Cdk inhibitors; (iii) Pfmrk, a putative P. falciparum homologue of the Cdk-activating kinase, does neit her activate nor phosphorylate PfPK5; and (iv) PfPK5 is able to autophospho rylate in the presence of a cyclin, Taken together, these results suggest t hat the regulation of Plasmodium Cdks may differ in important aspects from that of their human counterparts. Furthermore, we cloned an open reading fr ame encoding a novel P. falciparum protein possessing maximal homology to c yclin H from various organisms, and we show that this protein, called Pfcyc -1, is able to activate recombinant PfPK5 in vitro with an efficiency simil ar to that of human cyclin H and p25, This work opens the way to the develo pment of screening procedures aimed at identifying compounds that specifica lly target the parasite Cdks.