A calpain unique to alveolates is essential in Plasmodium falciparum and its knockdown reveals an involvement in pre-S-phase development

Plasmodium falciparum encodes a single calpain that has a distinct domain composition restricted to alveolates. To evaluate the potential of this protein as a drug target, we assessed its essentiality. Both gene disruption by double cross-over and gene truncation by single cross-over recombination failed. We were also unable to achieve allelic replacement by using a missense mutation at the catalytic cysteine codon, although we could obtain synonymous allelic replacement parasites. These results suggested that the calpain gene and its proteolytic activity are important for optimal parasite growth. To gain further insight into its biological role, we used the FKBP degradation domain system to generate a fusion protein whose stability in transfected parasites could be modulated by a small FKBP ligand, Shield1 (Shld1). We made a calpain-GFP-FKBP fusion through single cross-over integration at the endogenous calpain locus. Calpain levels were knocked down and parasite growth was greatly impaired in the absence of Shld1. Parasites were delayed in their ability to transition out of the ring stage and in their ability to progress to the S phase. Calpain is required for cell cycle progression in Plasmodium parasites and appears to be an attractive drug target. We have shown that regulated knockdowns are possible in P. falciparum and can be useful for evaluating essentiality and function.