The anti‐fungal β‐sitosterol targets the yeast oxysterol‐binding protein Osh4

BACKGROUND: beta-Sitosterol is a plant metabolite with a broad range of anti-fungal activity, however, this compound is not toxic against a few fungal species. The target of beta-sitosterol and the nature of its selective toxicity are not yet clear. Using a yeast model system and taking advantage of molecular biology and computational approaches, we identify the target and explain why beta-sitosterol is not toxic against some fungal pathogens. RESULTS: beta-Sitosterol (200 mug mL(-1) ) is toxic against yeast cells expressing only Osh4 (an oxysterol-binding protein) and harbouring a upc2-1 mutation (which enables sterol uptake), but not against yeast strains expressing all seven Osh proteins and harbouring a upc2-1 mutation. Furthermore, beta-sitosterol is not toxic against yeast strains without the upc2-1 mutation irrespective of the number of Osh proteins being expressed. The deletion of COQ1 (a gene known to be highly induced upon deletion of OSH4) enhances the toxicity of beta-sitosterol in yeast cells expressing only Osh4 and harbouring the upc2-1 mutation. Molecular modelling suggests that beta-sitosterol binds to Osh4 and the binding mode is similar to the binding of cholesterol to Osh4. CONCLUSION: Our results indicate that the concentrations of beta-sitosterol, and Osh4, as well as its homologues within cells, are most likely the main determinants of beta-sitosterol toxicity. Furthermore, some fungal species do not take up sterols, e.g. Saccharomyces cerevisiae, under aerobic conditions. Therefore, sterol uptake may also contribute to the beta-sitosterol anti-fungal effect. These findings enable predicting the toxicity of beta-sitosterol against plant fungal pathogens. (c) 2019 Society of Chemical Industry.