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α-1,3-Glucan is one of the main polysaccharides in the cell wall of filamentous fungi.  Aspergillus nidulans  has two α-1,3-glucan synthase genes,  agsA  and  agsB . We previously revealed that AgsB is a major α-1,3-glucan synthase in vegetative hyphae, but the function of AgsA remained unknown because of its low expression level and lack of phenotypic alteration upon gene disruption. To clarify the role of α-1,3-glucan in hyphal aggregation, we constructed strains overexpressing  agsA  ( agsA OE  ) or  agsB  ( agsB OE  ), in which the other α-1,3-glucan synthase gene was disrupted. In liquid culture, the wild-type and  agsB OE   strains formed tightly aggregated hyphal pellets, whereas  agsA OE   hyphae aggregated weakly. We analyzed the chemical properties of cell wall α-1,3-glucan from the  agsA OE   and  agsB OE   strains. The peak molecular mass of α-1,3-glucan from the  agsA OE   strain (1,480 ± 80 kDa) was much larger than that from the wild type (147 ± 52 kDa) and  agsB OE   (372 ± 47 kDa); however, the peak molecular mass of repeating subunits in α-1,3-glucan was almost the same (after Smith degradation:  agsA OE  , 41.6 ± 5.8 kDa;  agsB OE  , 38.3 ± 3.0 kDa). We also analyzed localization of α-1,3-glucan in the cell wall of the two strains by fluorescent labeling with α-1,3-glucan-binding domain – fused GFP (AGBD-GFP). α-1,3-Glucan of the  agsB OE   cells was clearly located in the outermost layer, whereas weak labeling was detected in the  agsA OE   cells. However, the  agsA OE   cells treated with β-1,3-glucanase were clearly labeled with AGBD-GFP. These observations suggest that β-1,3-glucan covered most of α-1,3-glucan synthesized by AgsA, although a small amount of α-1,3-glucan was still present in the outer layer. We also constructed a strain with disruption of the  amyG  gene, which encodes an intracellular α-amylase that synthesizes α-1,4-glucooligosaccharide as a primer for α-1,3-glucan biosynthesis. In this strain, the hyphal pellets and peak molecular mass of α-1,3-glucan (94.5 ± 1.4 kDa) were smaller than in the wild-type strain, and α-1,3-glucan was still labeled with AGBD-GFP in the outermost layer. Overall, these results suggest that hyphal pellet formation depends on the molecular mass and spatial localization of α-1,3-glucan as well as the amount of α-1,3-glucan in the cell wall of  A. nidulans .

Molecular Mass and Localization of α-1,3-Glucan in Cell Wall Control the Degree of Hyphal Aggregation in Liquid Culture of Aspergillus nidulans