The galactomannan (GM) is a polysaccharide produced by the human fungal pathogen Aspergillus fumigatus and is used as a biomarker for the diagnosis of invasive aspergillosis. The GM remains unique in the fungal kingdom since it is produced under three forms. This polysaccharide can be covalently bound to β- (1,3)-glucans in the cell wall, anchored to the plasma membrane by a glycosylphosphaditylinositol (GPI), or released into the extracellular environment. GM is composed of linear α-(1,2)/α-(1,6)-mannan chain with short side chains of β-galactofuran. Its biosynthesis remain poorly understood. Previous studies have shown that GM biosynthesis take place in the Golgi apparatus, into which sugar-donors (UDP-galactofuranose and GDP-mannose) are transported prior the polysaccharide polymerisation. The addition of galactofuran was due to the action of a specific galactofuranosyltransferase GfsA, but the mannosyltransferases responsible for the synthesis of the mannan chain remain unknown. Here, we describe two α-mannosyltransferases (Ktr family) and GH76 family members in A. fumigatus involved, respectively, in the polymerisation and the cross-linking of GM into the cell wall. Phenotypic analyses showed that the absence of cell wall GM in these mutants leads to the a strong vegetative growth defect, resulting in the formation of an hyperbranched mycelium. These mutants produce a low amount of conidia which are more permeable and less viable, with polarization defect during germination process. Our data showed that, in addition to chitin and β-(1,3)-glucan, the GM is the third cell call polymer required for a filamentous growth.
Full conference title:
- Fungal Genetics Conference 30th (2019)