Inhibition mechanism of the fungal β-1,3-glucan synthases by triterpenoid antifungal drugs.

β-1,3-glucan synthase is the molecular target for triterpenoid and echinocandin antifungal drugs in clinical. It catalyzes the formation of β-1,3-glucan, which is the primary component of the fungal cell wall. However, the inhibition mechanism of β-1,3-glucan synthase by triterpenoid drugs remains unclear. In this study, we report cryo-electron microscopy (cryo-EM) structures of Saccharomyces cerevisiae β-1,3-glucan synthase Fks1 and Fks2 in the apo state, the triterpenoid drug enfumafungin-bound state, and an open state. Structural analysis along with mutagenesis reveals the enfumafungin binding site, and the mechanism of the clinical drug-resistant mutations of the β-1,3-glucan synthases. Remarkably, the enfumafungin attaches on a single transmembrane helix TM5 of the β-1,3-glucan synthases, reorganizes its nearby lipid environment, and stabilizes the enzyme in a specific basal state with intact active site. Moreover, we elucidate that both the basal state and the open state are essential for FKS's glycosyltransferase activity. Our research also shows that Fks2 is highly conserved with Fks1 in terms of structure, activity, and drug inhibition. These findings provide deep insights into the fungal cell wall synthesis, and will facilitate the development of antifungal drugs targeting β-1,3-glucan synthase.