Abstract
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.
Highlights
The exopolysaccharide galactosaminogalactan (GAG) is an important virulence factor of the fungal pathogen Aspergillus fumigatus
Deacetylation is important in exopolysaccharide processing and production in multiple bacterial biofilm systems including Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Bordetella bronchiseptica, Listeria monocytogenes, and Pseudomonas aeruginosa[10,11,12,13,14,15,16,17]
Agd[3] has very low sequence similarity to the carbohydrate esterase family 4 (CE4) family and has a predicted carbohydrate esterase (CE) domain encompassing at least 370 residues
Summary
The exopolysaccharide galactosaminogalactan (GAG) is an important virulence factor of the fungal pathogen Aspergillus fumigatus. Deletion of a gene encoding a putative deacetylase, Agd[3], leads to defects in GAG deacetylation, biofilm formation, and virulence. Galactosaminogalactan (GAG), an α-1,4-linked linear exopolysaccharide of galactose (Gal) and N-acetylgalactosamine (GalNAc), is essential for A. fumigatus biofilm formation and a key virulence factor[4]. The synthesis of GAG is dependent on a cluster of genes encoding five carbohydrate-active enzymes (Fig. 1)[5,6]. De-N-acetylation of biofilm exopolysaccharide has been linked to cell aggregation, surface attachment, exopolysaccharide secretion, and biofilm maturation depending on the organism[13,14,15,16,17] In each of these systems, de-Nacetylation is mediated by a protein that belongs to carbohydrate esterase (CE) family 4. Agd[3] has low sequence homology to the CE4 family and does not belong to any of the currently defined Carbohydrate Active enZyme database (CAZy) CE families
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