Although α-1,3-glucan (AG) is one of major polysaccharides in the cell wall of Aspergillus species, the biological function of AG remains unclear, except for the role as a virulence factor in some pathogenic fungi. Previously, we carried out functional analysis of two α-1,3-glucan synthase (AGS) genes (i.e. agsA and agsB) in the model filamentous fungus Aspergillus nidulans. The agsBΔ strain lost most cell wall AG, suggesting that a main AGS in this fungus is AgsB. Interestingly, the hyphae of the agsBΔ strain was dispersed under liquid culture conditions, whereas the wild-type strain formed hyphal pellets under same culture conditions. These results suggest that AG has the role as an adhesive factor for hyphal cells. On the other hand, because the expression of agsA gene was scarcely detected under normal growth conditions and the agsAΔ strain did not show phenotypic defects, the role of agsA remains unclear. In this study, in order to investigate the roles of AgsA and AgsB in cell wall AG synthesis, we comparatively analyzed cell wall polysaccharides synthesized by AgsA and AgsB. First, we constructed agsA or agsB gene overexpression (O/E) strain by replacing the promoter region of agsA or agsB with tef1 promoter under the genetic background of the other AGS gene disruption, and confirmed the high expression of either of the AGS genes in the O/E strains. The O/E of agsA restored the growth characteristics of the agsBΔ strain under liquid culture conditions: the O/E agsA strain formed the hyphal pellets. This suggests that the agsA gene encodes a functional AGS. To elucidate the differences of cell wall structure between these two strains, we performed the alkaline-fractionation of cell wall and analyzed the sugar composition of the fractions. The carbohydrate analyses revealed that the sugar compositions of the AS2 fraction were similar in these two strains, but the texture of the AS2 fraction derived from these two strains was markedly different from each other, suggesting that the detailed chemical structure of AG obtained from the O/E agsA differs from that of AG derived from the O/E agsB strains.
Full conference title:
- Asperfest 14 (2017)