Biochemical characterization of FIKK8 – A unique protein kinase from the malaria parasite Plasmodium falciparum and other apicomplexans

FIKKs are protein kinases with distinctive sequence motifs found exclusively in Apicomplexa. Here, we report on the biochemical characterization of Plasmodium falciparum FIKK8 (PfFIKK8) and its Cryptosporidium parvum orthologue (CpFIKK) – the only member of the family predicted to be cytosolic and conserved amongst non-Plasmodium parasites. Recombinant protein samples of both were catalytically active. We characterized their phosphorylation ability using an enzymatic assay and substrate specificities using an arrayed positional scanning peptide library. Our results show that FIKK8 targets serine, preferably with arginine in the +3 and −3 positions. Furthermore, the soluble and active FIKK constructs in our experiments contained an N-terminal extension (NTE) conserved in FIKK8 orthologues from other apicomplexan species. Based on our results, we propose that this NTE is an integral feature of the FIKK subfamily.