Molecular organization of the alkali-insoluble fraction of Aspergillus fumigatus cell wall.

Abstract Physical and biological properties of the fungal cell wall are determined by the composition and arrangement of the structural polysaccharides. Cell wall polymers of fungi are classically divided into two groups depending on their solubility in hot alkali. We have analyzed the alkali-insoluble fraction of the Aspergillus fumigatus cell wall, which is the fraction believed to be responsible for fungal cell wall rigidity. Using enzymatic digestions with recombinant endo-β-1,3-glucanase and chitinase, fractionation by gel filtration, affinity chromatography with immobilized lectins, and high performance liquid chromatography, several fractions that contained specific interpolysaccharide covalent linkages were isolated. Unique features of the A. fumigatuscell wall are (i) the absence of β-1,6-glucan and (ii) the presence of a linear β-1,3/1,4-glucan, never previously described in fungi. Galactomannan, chitin, and β-1,3-glucan were also found in the alkali-insoluble fraction. The β-1,3-glucan is a branched polymer with 4% of β-1,6 branch points. Chitin, galactomannan, and the linear β-1,3/1,4-glucan were covalently linked to the nonreducing end of β-1,3-glucan side chains. As in Saccharomyces cerevisiae, chitin was linked via a β-1,4 linkage to β-1,3-glucan. The data obtained suggested that the branching of β-1,3-glucan is an early event in the construction of the cell wall, resulting in an increase of potential acceptor sites for chitin, galactomannan, and the linear β-1,3/1,4-glucan.