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Filamentous fungi are used as production hosts for various commercially valuable enzymes and chemicals including organic acids and secondary metabolites. We previously revealed that α-1,3-glucan and galactosaminogalactan (GAG) contribute to hyphal aggregation in the industrial fungus Aspergillus oryzae, and that production of recombinant protein in shake-flask culture is higher in a mutant lacking both α-1,3-glucan and GAG (AGΔ-GAGΔ) than in the parental strain. Here, we compared the productivity of the wild type, AGΔ-GAGΔ, and mutants lacking α-1,3-glucan (AGΔ) or GAG (GAGΔ) in batch culture with intermittent addition of glucose in a 5-L lab-scale bioreactor. The hyphae of the wild type and all mutants were dispersed by agitation, although the wild type and AGΔ formed small amounts of aggregates. Although mycelial weight was similar among the strains, the concentration of a secreted recombinant protein (CutL1) was the highest in AGΔ-GAGΔ. Evaluation of fluid properties revealed that the apparent viscosities of mycelial cultures of the wild type and AGΔ-GAGΔ decreased as the agitation speed was increased. The apparent viscosity of the AGΔ-GAGΔ culture tended to be lower than that of the wild-type strain at each agitation speed, and was significantly lower at 600 rpm. Overall, the lack of α-1,3-glucan and GAG in the hyphae improved culture rheology, resulting in an increase in recombinant protein production in AGΔ-GAGΔ. This is the first report of flow behavior improvement by a cell-surface component defect in a filamentous fungus.

α-1,3-Glucan is one of the main polysaccharides in the cell wall of Aspergillus nidulans. We previously revealed that it plays a role in hyphal aggregation in liquid culture, and that its molecular mass (MM) in an agsA-overexpressing (agsAOE) strain was larger than that in an agsB-overexpressing (agsBOE) strain. The mechanism that regulates the MM of α-1,3-glucan is poorly understood. Although the gene amyD, which encodes glycosyl-phosphatidylinositol (GPI)-anchored α-amylase (AmyD), is involved in the biosynthesis of α-1,3-glucan in A. nidulans, how it regulates this biosynthesis remains unclear. Here we constructed strains with disrupted amyD (ΔamyD) or overexpressed amyD (amyDOE) in the genetic background of the ABPU1 (wild-type), agsAOE, or agsBOE strain, and characterized the chemical structure of α-1,3-glucans in the cell wall of each strain, focusing on their MM. The MM of α-1,3-glucan from the agsBOE amyDOE strain was smaller than that in the parental agsBOE strain. In addition, the MM of α-1,3-glucan from the agsAOE ΔamyD strain was greater than that in the agsAOE strain. These results suggest that AmyD is involved in decreasing the MM of α-1,3-glucan. We also found that the C-terminal GPI-anchoring region is important for these functions.