Structural basis for inhibition and regulation of a chitin synthase from Candida albicans.

Abstract

Chitin is one of the major components of the fungal cell wall. Chitin synthases (Chs's) catalyze chitin formation and translocation across the membrane. The detailed mechanism of chitin synthesis, translocation and assembly remains unknown due to the lack of high-resolution structures of chitin synthases. Chs's are targets of antifungal agents, including nikkomycin Z and polyoxin D. Nikkomycin Z has been clinically investigated to treat endemic fungal infectious disease (coccidioidomycosis). Polyoxin D has been widely used as a safe fungicide in organic agriculture. Lack of structural insights into the action of these inhibitors on Chs has hampered their further development for clinical usage because many fungal pathogens contain Chs's that are less sensitive to nikkomycins and polyoxins. We present the cryo-EM structures of Chs2 from the pathological fungi, Candida albicans (CaChs2), in the apo, substrate-bound, nikkomycin Z-bound, and polyoxin D-bound states. CaChs2 adopts a unique domain-swapped dimer configuration where a conserved motif in the domain swapped region controls enzyme activity. CaChs2 is also regulated by lipids. The chitin translocation tunnel is closed by the extracellular gate and plugged by a lipid molecule in the apo state to prevent non-specific leak. Analyses of substrate and inhibitor binding provide insights into the chemical logic of Chs inhibition, which can guide the optimization of Chs-targeted antifungal drugs.