A third
calcium-dependent protein kinase (CDPK) gene has been isolated from the human
malaria parasite Plasmodium falciparum by vectorette technology. The gene consists of five exons and four introns. The open reading frame resulting from removal of the four introns encodes a
protein of 562
amino acid residues with a predicted molecular mass of 65.3 kDa. The encoded
protein, termed PfCDPK3, consists of four distinct domains characteristic of a member of the CDPK family and displays the highest homology (46% identity and 69% similarity) to PfCDPK2, the second CDPK of P. falciparum. The N-terminal variable domain is rich in
serine/
threonine and
lysine and contains multiple consensus phosphorylation sites for a range of
protein kinases. The catalytic domain possesses all conserved motifs of the
protein kinase family except for the highly conserved
glutamic acid residue in subdomain VIII, which is replaced by a
glutamine residue. The sequence of the junction domain comprising 31
amino acid residues is less conserved. The
calmodulin-like regulatory domain contains four EF-hand
calcium-binding motifs, each consisting of a loop of 12
amino acid residues which is flanked by two alpha-helices. Southern blotting of genomic
DNA digests showed that the Pfcdpk3 gene is present as a single copy per haploid genome. A 2900
nucleotide transcript of this gene is expressed specifically in the sexual erythrocytic stage, indicating that PfCDPK3 is involved in sexual stage-specific events. It is proposed that PfCDPK3 may serve as a link between
calcium and gametogenesis of P. falciparum.