Deacetylation of Fungal Exopolysaccharide Mediates Adhesion and Biofilm Formation

The mold Aspergillus fumigatus causes invasive infection in immunocompromised patients. Recently, galactosaminogalactan (GAG), an exopolysaccharide composed of galactose and N-acetylgalactosamine (GalNAc), was identified as a virulence factor required for biofilm formation. The molecular mechanisms underlying GAG biosynthesis and GAG-mediated biofilm formation were unknown. We identified a cluster of five coregulated genes that were dysregulated in GAG-deficient mutants and whose gene products share functional similarity with proteins that mediate the synthesis of the bacterial biofilm exopolysaccharide poly-(β1-6)-N-acetyl-d-glucosamine (PNAG). Bioinformatic analyses suggested that the GAG cluster gene agd3 encodes a protein containing a deacetylase domain. Because deacetylation of N-acetylglucosamine residues is critical for the function of PNAG, we investigated the role of GAG deacetylation in fungal biofilm formation. Agd3 was found to mediate deacetylation of GalNAc residues within GAG and render the polysaccharide polycationic. As with PNAG, deacetylation is required for the adherence of GAG to hyphae and for biofilm formation. Growth of the Δagd3 mutant in the presence of culture supernatants of the GAG-deficient Δuge3 mutant rescued the biofilm defect of the Δagd3 mutant and restored the adhesive properties of GAG, suggesting that deacetylation is an extracellular process. The GAG biosynthetic gene cluster is present in the genomes of members of the Pezizomycotina subphylum of the Ascomycota including a number of plant-pathogenic fungi and a single basidiomycete species, Trichosporon asahii, likely a result of recent horizontal gene transfer. The current study demonstrates that the production of cationic, deacetylated exopolysaccharides is a strategy used by both fungi and bacteria for biofilm formation.