A Plasmodium actin-depolymerizing factor that binds exclusively to actin monomers

ADF/cofilins (AC) are essential F- and G-actin binding proteins that modulate microfilament turnover. The genome of Plasmodium falciparum, the parasite causing malaria, contains two members of the AC family. Interestingly, P. falciparum ADF1 lacks the F-actin binding residues of the AC consensus. Reverse genetics in the rodent malaria model system suggest that ADF1 performs vital functions during the pathogenic red blood cell stages, whereas ADF2 is not present in these stages. We show that recombinant PfADF1 interacts with monomeric actin but does not bind to actin polymers. Although other AC proteins inhibit nucleotide exchange on monomeric actin, the Plasmodium ortholog stimulates nucleotide exchange. Thus, PfADF1 differs in its biochemical properties from previously known AC proteins and seems to promote turnover exclusively by interaction with actin monomers. These findings provide important insights into the low cytosolic abundance and unique turnover characteristics of actin polymers in parasites of the phylum Apicomplexa.