Gene structure and expression of an unusual protein kinase from Plasmodium falciparum homologous at its carboxyl terminus with the EF hand calcium-binding proteins.

An unusual protein kinase gene, termed PfCPK, was isolated from Plasmodium falciparum. The gene, which contains five exons and four introns, encodes a product with a predicted length of 524 amino acids. The amino-terminal segment of the predicted protein contains all of the conserved sequences characteristic of a protein kinase catalytic domain and has a high homology to several protein serine-threonine kinase subfamilies (30-41% amino acid identities). These subfamilies include calcium/calmodulin-dependent protein kinases, calcium-dependent protein kinase, ribosomal S6 protein kinase, cyclic nucleotide-dependent protein kinases, protein kinase C, and the yeast SNF1 subfamily. All of these protein kinases are relatively close in the phylogeny tree and within the kinase catalytic domains have about 35% amino acid identities to each other, suggesting that PfCPK is also in this region of the phylogeny tree. An unusual feature of PfCPK is that its carboxyl-terminal segment displays homology to the EF hand calcium-binding proteins, for example 34% amino acid identity to chicken fast skeletal muscle troponin C and 35% amino acid identity to human calmodulin. Like troponin Cs and calmodulins, PfCPK also contains four EF hand calcium-binding motifs. Furthermore, the four introns in the PfCPK gene are all located in the carboxyl-terminal putative EF hand calcium-binding region (EF hand calcium-binding proteins from higher eukaryotes generally contain multiple introns). This combination of a protein kinase and an EF hand calcium-binding protein in a single polypeptide implies that PfCPK may be directly activated by calcium. Constructs containing the full-length PfCPK cDNA have been expressed in Escherichia coli at a high level to generate a 60-kDa recombinant protein. Compared with similar fractions from control cells, the fraction containing PfCPK recombinant protein exhibited an elevated protein kinase activity which was Ca(2+)-dependent.