An Amylase-Like Protein, AmyD, Is the Major Negative Regulator for α-Glucan Synthesis in Aspergillus nidulans during the Asexual Life Cycle

Abstract

α-Glucan affects fungal cell–cell interactions and is important for the virulence of pathogenic fungi. Interfering with production of α-glucan could help to prevent fungal infection. In our previous study, we reported that an amylase-like protein, AmyD, could repress α-glucan accumulation in Aspergillus nidulans. However, the underlying molecular mechanism was not clear. Here, we examined the localization of AmyD and found it was a membrane-associated protein. We studied AmyD function in α-glucan degradation, as well as with other predicted amylase-like proteins and three annotated α-glucanases. AmyC and AmyE share a substantial sequence identity with AmyD, however, neither affects α-glucan synthesis. In contrast, AgnB and MutA (but not AgnE) are functional α-glucanases that also repress α-glucan accumulation. Nevertheless, the functions of AmyD and these glucanases were independent from each other. The dynamics of α-glucan accumulation showed different patterns between the AmyD overexpression strain and the α-glucanase overexpression strains, suggesting AmyD may not be involved in the α-glucan degradation process. These results suggest the function of AmyD is to directly suppress α-glucan synthesis, but not to facilitate its degradation.

Function and Biosynthesis of Cell Wall α-1,3-Glucan in Fungi

“…Unlike AmyG, AmyD of A. nidulans has a signal peptide and a GPI-anchor site, and is a membrane-associated protein [ 33 , 37 ]. Carbon starvation did not affect the transcript levels of amyD [ 63 ] Treatment with calcofluor white increased the amyD transcript level [ 33 ].…”

“…The transcript level was markedly higher at 24 h than at 14 h in the absence of calcofluor white in both shaken and static liquid cultures [ 33 ]. The amount of cell wall α-1,3-glucan was higher in the ∆ amyD strain than in the wild-type strain, but was decreased by the overexpression of amyD [ 33 , 37 ]. From these results, He et al [ 37 ] predicted that AmyD directly suppresses α-1,3-glucan synthesis in A. nidulans .…”

“…Recently, He et al [ 37 ] reported the function of AmyD, AgnB, and MutA in A. nidulans . Since AgtA of A. niger does not hydrolyze α-1,3-glucan and has low starch hydrolysis ability, it was thought that AmyD of A. nidulans hydrolyzes α-1,4-glucan linked to the end of α-1,3-glucan, thus accelerating α-1,3-glucan degradation by α-1,3-glucanase [ 33 ].…”

“…After 24 h, the content of cell wall α-1,3-glucan was only about half the wild-type level in all three overexpressing strains. He et al [ 37 ] predicted that AmyD, AgnB, and MutA decrease cell wall α-1,3-glucan content through different mechanisms, although the mechanisms remain unclear. AmyC and AmyE share a considerable sequence identity with AmyD; however, the overexpression of amyC and amyE did not affect α-1,3-glucan synthesis in A. nidulans [ 37 ].…”

“…He et al [ 37 ] predicted that AmyD, AgnB, and MutA decrease cell wall α-1,3-glucan content through different mechanisms, although the mechanisms remain unclear. AmyC and AmyE share a considerable sequence identity with AmyD; however, the overexpression of amyC and amyE did not affect α-1,3-glucan synthesis in A. nidulans [ 37 ]. Nitsche et al [ 64 ] identified some carbon starvation–induced glycoside hydrolases by transcriptome and secretome analysis in A. niger .…”