Characterization of Plasmodium falciparum protein kinase 2

A sustained elevation of free Ca2+ is observed on the rupture and release of merozoites of Plasm odium falciparum from the erythrocytes. The immunoelectron micrographs demonstrate that calmodulin is localized in merozoites. To elucidate the Ca2+ signal of P. falciparum invasion, we attempted to characterize P. falciparum protein kinase 2 (PfPK2), which is homologous to human calcium calmodulin-dependent protein kinase (CaMK). PfPK2 was purified as a fusion protein that was labeled with [gamma-P-32]ATP; this labeling was then eliminated by phosphatase. This phosphorylation was eliminated when the putative catalytic lysine residue of PfPK2 was replaced with alanine. PfPK2 phosphorylated histone IIAS as a representative substrate in a Ca2+- and calmodulin-de pendent manner. Calmodulin antagonists inhibited the phosphorylation of PfPK2 in vitro and markedly decreased the parasitemia of ring forms in an invasion assay, whereas CaMKII-specific inhibitors had no effect. PfPK2 was localized in the merozoites in the culture of P. falciparum. Thus, purified PfPK2 possesses protein kinase activity in a Ca2+- and calmodulin-dependent manner and the catalytic lysine of this protein was determined. These data suggest that PfPK2 is the Plasmodium protein kinase expressed in the merozoites during the invasion stage. (C) 2008 Elsevier B.V. All rights reserved.