A Plasmodium falciparum protein kinase with two unusually large kinase inserts.

A protein kinase gene (PfPK1) has been isolated from the human parasite Plasmodium falciparum by using a mixed oligonucleotide pool which corresponds to a highly conserved region of serine/threonine protein kinases. The gene, which contains one intron, encodes a protein with a predicted length of 909 amino acids. The predicted protein contains all the conserved sequences characteristic of a protein kinase catalytic domain. These sequences are discontinuous, however, since they are separated by two large kinase inserts with 178 and 330 amino acids in size. Specific antisera were raised against recombinant fragments of the protein and a PfPK1-specific peptide. Using one of these antibodies, a functional protein kinase was precipitated from malarial lysates and this kinase recognized casein as an exogenous substrate. PfPK1 was expressed in a stage-specific fashion and also had a stage-specific cellular localization. During the intraerythrocytic life cycle, PfPK1 shifts from the parasite cytosol to the parasite membrane fraction. An unusual feature of PfPK1 is its electrophoretic mobility on SDS-PAGE. Whereas the predicted protein size is about 100 kDa, the apparent size is about 70 kDa. There are no indications for RNA processing and we could exclude proteolytic processing as an explanation.

Calcium-binding properties of a calcium-dependent protein kinase from Plasmodium falciparum and the significance of individual calcium-binding sites for kinase activation.

Calcium-dependent protein kinase from Plasmodium falciparum (PfCPK) is a multidomain protein composed of an N-terminal kinase domain connected via a linker region to a C-terminal CaM-like calcium-binding domain. The kinase can be activated by Ca2+ alone and associates with 45Ca2+. Here we describe the calcium-binding properties of the kinase and the significance of the individual calcium-binding sites with respect to enzymatic activation, as well as the Ca(2+)-induced conformational change as detected by circular dichroism. As predicted from the cDNA sequence, the kinase has four EF-hand calcium-binding sites in the C-terminal domain. To understand the roles of the individual calcium-binding sites, two series of mutations were generated at the individual EF-hand motifs. The highly conserved glutamic acid residue at position 12 in each calcium-binding loop was mutated to either lysine or glutamine, and therefore a total of eight mutants were generated. Either of these mutations (to lysine or glutamine) is sufficient to eliminate calcium binding at the mutated site. Sites I and II appear to be crucial for both Ca(2+)-induced conformational change and enzymatic activation. Whereas mutations at site II almost completely abolish kinase activity, mutations at site I are also deleterious and dramatically reduce the sensitivity of the Ca(2+)-induced conformational change and the Ca(2+)-dependent activation. Mutations at sites III and IV have minor effects.