Calcium-dependent protein kinases play a crucial role in intracellular calcium signaling in plants, some algae and protozoa. In Plasmodium falciparum, calcium-dependent protein kinase 1 (PfCDPK1) is expressed during schizogony in the erythrocytic stage as well as in the sporozoite stage. It is coexpressed with genes that encode the parasite motor complex, a cellular component required for parasite invasion of host cells, parasite motility and potentially cytokinesis. A targeted gene-disruption approach demonstrated that pfcdpk1 seems to be essential for parasite viability. An in vitro biochemical screen using recombinant PfCDPK1 against a library of 20,000 compounds resulted in the identification of a series of structurally related 2,6,9-trisubstituted purines. Compound treatment caused sudden developmental arrest at the late schizont stage in P. falciparum and a large reduction in intracellular parasites in Toxoplasma gondii, which suggests a possible role for PfCDPK1 in regulation of parasite motility during egress and invasion.

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The activation of protein kinase B/Akt is thought to be a critical step in the phosphoinositide 3-kinase pathway that regulates cell growth and differentiation. Because insulin-like growth factor 1 stimulates the resumption of meiosis in Xenopus laevis oocytes via phosphoinositide 3-kinase activation, we investigated the Akt involvement in this process. Injection of mRNA coding for a constitutively active Akt in Xenopus oocytes induced germinal vesicle breakdown (GVBD) to the same extent as progesterone or insulin treatment. Injection of mRNA coding for the wild type Akt kinase was less effective in stimulating GVBD, whereas Akt bearing a lysine mutation in the catalytic domain that abolishes the kinase activity had no effect. A mutant Akt lacking a membrane-targeting sequence did not induce GVBD, despite high levels of expression and activity. As previously reported for insulin, induction of GVBD by Akt was prevented by incubating the oocytes with cilostamide, an inhibitor specific for the type 3 phosphodiesterase (PDE3), suggesting that the activity of a PDE is required for Akt action. That an increase in PDE activity in the oocyte is sufficient to induce meiotic resumption was demonstrated by expression of an active PDE protein. In addition, the constitutively active Akt caused a 2-fold increase in the activity of the endogenous PDE. These data demonstrate that Akt is in the pathway controlling resumption of meiosis in the Xenopus oocyte and that regulation of the activity of a PDE3 is a step distal to the kinase activation.A critical step in growth factor stimulation of the target cell is the activation of the phosphoinositide 3-kinase (PI3-K) 1 pathway. This signaling pathway has been implicated in a wide array of cellular events including mitogenesis, transformation, differentiation, and regulation of metabolism (1). Recently Akt, also known as protein kinase B or related to the A and protein kinase C (2-4), was identified as a kinase distal to PI3-K (1). Three Akt isoforms (␣, ␤, and ␥) with closely related properties have been identified. These are proteins of approximately 60 kDa containing a pleckstrin homology (PH) domain (5) and a serine/threonine kinase domain structurally related to the catalytic domains of protein kinases A and C. Phosphatidylinositol 3,4-bisphosphate and 3,4,5-trisphosphate, the products of PI3-K, bind to the PH domain of Akt and serve to anchor the enzyme to the membrane as well as to induce a conformational change in the enzyme (6). The subsequent phosphorylation of Akt, at Thr-308 in the catalytic domain and Ser-473 of the C terminus of Akt␣, is crucial for the activation of Akt (7,8). At least one of the kinases phosphorylating Akt has been identified as the 3-phosphoinositide-dependent kinase-1 (9 -11).Akt is activated by insulin and IGF-1 (12), and this regulation mediates the activation of glucose uptake (13), the phosphorylation and deactivation of GSK3 (14), and activation of p70 S6 kinases (15). Akt also phosphorylates the proapoptotic protein Bad, promoting its interac...