Structural and biochemical characterization of the exopolysaccharide deacetylase Agd3 required for Aspergillus fumigatus biofilm formation

The exopolysaccharide galactosaminogalactan (GAG) is an important virulence factor of the fungal pathogen Aspergillus fumigatus. Deletion of a gene encoding a putative deacetylase, Agd3, leads to defects in GAG deacetylation, biofilm formation, and virulence. Here, we show that Agd3 deacetylates GAG in a metal-dependent manner, and is the founding member of carbohydrate esterase family CE18. The active site is formed by four catalytic motifs that are essential for activity. The structure of Agd3 includes an elongated substrate-binding cleft formed by a carbohydrate binding module (CBM) that is the founding member of CBM family 87. Agd3 homologues are encoded in previously unidentified putative bacterial exopolysaccharide biosynthetic operons and in other fungal genomes. The exopolysaccharide galactosaminogalactan (GAG) is an important virulence factor of the fungal pathogen Aspergillus fumigatus. Here, the authors study an A. fumigatus enzyme that deacetylates GAG in a metal-dependent manner and constitutes a founding member of a new carbohydrate esterase family.Bio-organic Synthesi

The Fungal Exopolysaccharide Galactosaminogalactan Mediates Virulence by Enhancing Resistance to Neutrophil Extracellular Traps.

International audienceOf the over 250 Aspergillus species, Aspergillus fumigatus accounts for up to 80% of invasive human infections. A. fumigatus produces galactosaminogalactan (GAG), an exopolysaccharide composed of galactose and N-acetyl-galactosamine (GalNAc) that mediates adherence and is required for full virulence. Less pathogenic Aspergillus species were found to produce GAG with a lower GalNAc content than A. fumigatus and expressed minimal amounts of cell wall-bound GAG. Increasing the GalNAc content of GAG of the minimally pathogenic A. nidulans, either through overexpression of the A. nidulans epimerase UgeB or by heterologous expression of the A. fumigatus epimerase Uge3 increased the amount of cell wall bound GAG, augmented adherence in vitro and enhanced virulence in corticosteroid-treated mice to levels similar to A. fumigatus. The enhanced virulence of the overexpression strain of A. nidulans was associated with increased resistance to NADPH oxidase-dependent neutrophil extracellular traps (NETs) in vitro, and was not observed in neutropenic mice or mice deficient in NADPH-oxidase that are unable to form NETs. Collectively, these data suggest that cell wall-bound GAG enhances virulence through mediating resistance to NETs