Plasmodium falciparum putative Hsp90 functionally complements yeast Hsp90 and is inhibited in an isoform-specific manner

Plasmodium falciparum Hsp90α has been proven to be an antimalarial target. However, high sequence conservation between PfHsp90α and HsHsp90α is an obstacle to developing selective inhibitors against the parasite chaperone. The parasite harbours an uncharacterised paralogue of Hsp90α, namely putative PfHsp90 (PF14_0417), which has low sequence conservation with PfHsp90α. A piggyBac transposon insertional mutagenesis study has indicated that putative PfHsp90 is essential for parasite survival. However, its poor sequence conservation with PfHsp90α raises concerns about its functionality. Here, we show that the putative isoform can complement a temperature-sensitive Schsp82 mutant, confirming its Hsp90-like activity. To identify selective inhibitors against this isoform, we developed a yeast-based assay system using isogenic Schsp82 temperature-sensitive Δpdr5 yeast strains, each harbouring Plasmodium Hsp90 isoforms. Utilising bona fide Hsp90 inhibitors, we measured their in silico affinities towards the available structure of putative PfHsp90 and categorised them in increasing order of binding affinities towards the putative isoform. The top-scoring inhibitors were tested within surrogate yeast systems. Our study indicates that only one compound, CCT018159, manifests selective inhibition of the putative PfHsp90 but remains ineffective against PfHsp90α. When tested in the asexual growth of the drug-sensitive and drug-resistant parasites, CCT018159 showed antiparasitic activity in the micromolar range. Together, our study underlines the importance of putative PfHsp90 in parasite survival and merits additional research.