Structural basis of caspofungin inhibition of Nakaseomyces glabratus Fks1

The Fks1 catalytic subunit of fungal glucan synthase crosslinks UDP-glucose into {beta}-1,3-D-glucan in the fungal cell wall and is a target for the echinocandin class of antifungal drugs. Here, we present the resting state of Fks1 from Nakaseomyces glabratus (NgFks1) in both apo and caspofungin-bound states. Caspofungin binds the extracellular glucan exit portal of Fks1 adjacent to the product {beta}-1,3-D-glucan in a position similar to that reported for S. cerevisiae Fks1. Unlike S. cerevisiae Fks1, no Rho1 binding is present, but the glucan channel opening is in an open-like state at the resistance hotspot regions. However, the lower gate is closed, suggesting glucan is trapped from a previous round of catalysis. Our results show that caspofungin can bind in a mode in the absence of Rho1 in N. glabratus and point to possible species-specific differences in substrate translocation, inhibition, and resistance mechanisms.