Investigation of a cryptic ligand binding site on Plasmodium falciparum Hsp90

The molecular chaperone heat shock protein 90 (Hsp90) has an important role in maintaining proteostasis in Plasmodium parasites, the causative agents of malaria, and is of interest as a potential antimalarial drug target. Inhibitors targeting its well-characterized N-terminal ATP-binding site are lethal, but the development of high-affinity binders with selectivity for the Plasmodium over the human homolog has been challenging given the high conservation of this domain. A binding site in the less conserved Hsp90 C-terminus has been reported to interact with nucleotides and inhibitors in other eukaryotic systems, which could offer an alternative route for antimalarial design. Herein, we characterize the potential ligandability of the C-terminus in the Plasmodium falciparum chaperone PfHsp90 with in silico and in vitro methods. We conducted affinity experiments with both a lysine-reactive nucleotide analog and an ATP resin that support a specific interaction between ATP and a C-terminal truncation of PfHsp90. We further explored the nucleotide structural requirements for this interaction with limited proteolysis experiments, which suggest association with ATP, dATP, and ADP, but not AMP or GTP. Lastly, we employed computational analyses and mutagenesis studies to interrogate the molecular basis for the interaction. Our findings provide the foundation for future studies to assess and develop C-terminal Hsp90 inhibitors against Plasmodium parasites.