Aspergillus nidulans AmyG Functions as an Intracellular α-Amylase to Promote α-Glucan Synthesis

alpha-Glucan is a major cell wall component and a virulence and adhesion factor for fungal cells. However, the biosynthetic pathway of alpha-glucan was still unclear. alpha-Glucan was shown to be composed mainly of 1,3-glycosidically linked glucose, with trace amounts of 1,4-glycosidically linked glucose. Besides the alpha-glucan synthetases, amylase-like proteins were also important for alpha-glucan synthesis. In our previous work, we showed that Aspergillus nidulans AmyG was an intracellular protein and was crucial for the proper formation of alpha-glucan. In the present study, we expressed and purified AmyG in an Escherichia coli system. Enzymatic characterization found that AmyG mainly functioned as an alpha-amylase that degraded starch into maltose. AmyG also showed weak glucanotransferase activity. Most intriguingly, supplementation with maltose in shaken liquid medium could restore the alpha-glucan content and the phenotypic defect of a Delta amyG strain. These data suggested that AmyG functions mainly as an intracellular alpha-amylase to provide maltose during alpha-glucan synthesis in A. nidulans. IMPORTANCE Short alpha-1,4-glucan was suggested as the primer structure for alpha-glucan synthesis. However, the exact structure and its source remain elusive. Delta myG was essential to promote alpha-glucan synthesis and had a major impact on the structure of alpha-glucan in the cell wall. Data presented here revealed that AmyG belongs to the GH13_5 family and showed strong amylase function, digesting starch into maltose. Supplementation with maltose efficiently rescued the phenotypic defect and alpha-glucan deficiency in an Delta amyG strain but not in an Delta agsB strain. These results provide the first piece of evidence for the primer structure of alpha-glucan in fungal cells, although it might be specific to A. nidulans.

Automated summary

Research has found that AmyG belongs to the GH13_5 family and functions mainly as an intracellular alpha-amylase that degrades starch into maltose in Aspergillus nidulans. The addition of maltose can rescue the phenotypic defect and alpha-glucan deficiency in a Delta amyG strain, providing evidence for the primer structure of alpha-glucan in fungal cells.