Polymyxin B inhibits the chaperone activity of Plasmodium falciparum Hsp70

Heat shock protein 70 (Hsp70) is a molecular chaperone that plays an important role in cellular proteostasis. Hsp70s are also implicated in the survival and pathogenicity of malaria parasites. The main agent of malaria, Plasmodium falciparum, expresses six Hsp70s. Of these, two (PfHsp70-1 and PfHsp70-z) localize to the parasite cytosol. Previously conducted gene knockout studies suggested that PfHsp70-z is essential, and it has been demonstrated that small-molecule inhibitors targeting PfHsp70-1 cause parasite death. For this reason, both PfHsp70-1 and PfHsp70-z are potential antimalarial targets. Two cyclic lipopeptides, colistin and polymyxin B (PMB), have been shown to bind another heat shock protein, Hsp90, inhibiting its chaperone function. In the current study, we investigated the effect of PMB on the structure-function features of PfHsp70-1 and PfHsp70-z. Using surface plasmon resonance analysis, we observed that PMB directly interacts with both PfHsp70-1 and PfHsp70-z. In addition, using circular dichroism spectrometric analysis combined with tryptophan fluorescence measurements, we observed that PMB modulated the secondary and tertiary structures of Hsp70. Furthermore, PMB inhibited the basal ATPase activity and chaperone function of the two Hsp70s. Our findings suggest that PMB associates with Hsp70 to inhibit its function. In light of the central role of Hsp70 in cellular proteostasis and its essential role in the development of malaria parasites in particular, our findings expand the library of small-molecule inhibitors that target this medically important class of molecular chaperones.