Antimalarial mechanism of action of the natural product 9-methoxystrobilurin G

The natural product 9-methoxystrobilurin G (9MG) from <i>Favolaschia</i> spp basidiomycetes is a potent and selective antimalarial. The mechanism of action of 9MG is unknown. We induced 9MG resistance in <i>Plasmodium falciparum</i> 3D7 and Dd2 strains and identified mutations associated with resistance by genome sequencing. All 9MG-resistant clones possessed missense mutations in the cytochrome b (CYTB) gene, a key component of mitochondrial complex III. The mutations map to the quinol oxidation site of CYTB, which is also the target of antimalarials such as atovaquone. In a complementary approach to identify protein targets of 9MG, a photoactivatable derivative of 9MG was synthesized and applied in chemoproteomic-based target profiling. Three components of mitochondrial complex III (QCR7, QCR9, and COX15) were specifically enriched consistent with 9MG targeting CYTB and complex III function in <i>P. falciparum</i>. Inhibition of complex III activity by 9MG was confirmed by ubiquinone cytochrome <i>c</i> reductase assay using <i>P. falciparum</i> extract. The findings from this study may be useful for developing novel antimalarials targeting CYTB.