Abstract
Fungal cell walls frequently contain a polymer of mannose and galactose called galactomannan. In the pathogenic filamentous fungus Aspergillus fumigatus, this polysaccharide is made of a linear mannan backbone with side chains of galactofuran and is anchored to the plasma membrane via a glycosylphosphatidylinositol or is covalently linked to the cell wall. To date, the biosynthesis and significance of this polysaccharide are unknown. The present data demonstrate that deletion of the Golgi UDP-galactofuranose transporter GlfB or the GDP-mannose transporter GmtA leads to the absence of galactofuran or galactomannan, respectively. This indicates that the biosynthesis of galactomannan probably occurs in the lumen of the Golgi apparatus and thus contrasts with the biosynthesis of other fungal cell wall polysaccharides studied to date that takes place at the plasma membrane. Transglycosylation of galactomannan from the membrane to the cell wall is hypothesized because both the cell wall-bound and membrane-bound polysaccharide forms are affected in the generated mutants. Considering the severe growth defect of the A. fumigatus GmtA-deficient mutant, proving this paradigm might provide new targets for antifungal therapy.
Highlights
Understanding the mechanisms of cell wall biogenesis is important for development of antifungal strategies
The present data demonstrate that deletion of the Golgi UDP-galactofuranose transporter GlfB or the GDP-mannose transporter GmtA leads to the absence of galactofuran or galactomannan, respectively
This indicates that the biosynthesis of galactomannan probably occurs in the lumen of the Golgi apparatus and contrasts with the biosynthesis of other fungal cell wall polysaccharides studied to date that takes place at the plasma membrane
Summary
Understanding the mechanisms of cell wall biogenesis is important for development of antifungal strategies. The present data demonstrate that deletion of the Golgi UDP-galactofuranose transporter GlfB or the GDP-mannose transporter GmtA leads to the absence of galactofuran or galactomannan, respectively This indicates that the biosynthesis of galactomannan probably occurs in the lumen of the Golgi apparatus and contrasts with the biosynthesis of other fungal cell wall polysaccharides studied to date that takes place at the plasma membrane. Galactomannan, a linear ␣-1,2/␣-1,6mannan with short side chains of -1,5-galactofuran, substitutes the cell wall 1,3/1,6-glucan [6, 7] This polysaccharide can be anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI)4 [8] or be part of the alkalisoluble fraction of the cell wall, containing the non-covalently associated polysaccharides ␣-1,3-glucan and galactosaminogalactan as well as secreted glycoproteins [3].
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