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.