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Fig. 3 | Fungal Biology and Biotechnology

Fig. 3

From: The mechanisms of hyphal pellet formation mediated by polysaccharides, α-1,3-glucan and galactosaminogalactan, in Aspergillus species

Fig. 3

Speculative model for biosynthesis of (A) α-1,3-glucan and (B) galactosaminogalactan (GAG) in Aspergillus species. A AgsB, an α-1,3-glucan synthase, has three domains: extracellular, intracellular, and multitransmembrane. The likely substrate for α-1,3-glucan synthesis, UDP-glucose, is produced from glucose-6-phosphate and UTP by the UTP-glucose-1-phosphate uridylyltransferase GalF, which is encoded by a gene orthologous to H. capsulatum UGP1. Maltooligosaccharide produced by intracellular α-amylase, AmyG, might act as a primer for polymerization of glucose from UDP-glucose as the sugar donor; polymerization is performed by the intracellular domain of AgsB, resulting in a subunit composed of approximately 200 residues of 1,3-linked α-glucose with a short 1,4-linked α-glucose primer at its reducing end. The polymer synthesized by the intracellular domain is exported through a pore-like structure of the multitransmembrane domain. Then the extracellular domain catalyzes interconnection of several subunits, resulting in a mature α-1,3-glucan chain. A GPI-anchored α-amylase, AmyD, has a repressive effect on α-1,3-glucan biosynthesis, but the detailed mechanism of this effect remains unclear. G6P, glucose-6-phosphate. Protein names are for A. nidulans unless otherwise noted. B The UDP-glucose 4-epimerase Uge3 epimerizes UDP-glucose to UDP-galactopyranose and UDP-N-acetylglucosamine to UDP-N-acetylgalactosamine (GalNAc). UDP-galactopyranose and UDP-GalNAc are expected to be polymerized by Gtb3. The polymers are thought to be exported from the cell through a pore formed by Gtb3, and then the GalNAc residues are partially deacetylated by Agd3. The deacetylated polymer is mature GAG and either associates with the cell wall or is dissolved into the culture supernatant. Sph3 hydrolyzes the GalNAc residues owing to its endo-α-1,4-N-acetylgalactosaminidase activity. Ega3 might be released by digestion of its N-terminal transmembrane region, and might then hydrolyze the deacetylated GAG, but no direct evidence has been reported. Protein names are for A. fumigatus

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