The differences between fungal α-glucan synthase determining pullulan synthesis and that controlling cell wall α-1,3 glucan synthesis.

The fungal α-glucan synthases (Agss) are multi-domain proteins catalyzing biosynthesis of cell wall α-1,3-glucan which determines cell wall integrity or fungal pathogenicity and pullulan which is a maltotriosyl polymer made of α-1,4 and α-1,6 bound glucose units. The Agss family can be divided into 11 groups, some of which lost the original functions due to accumulation of harmful mutations or gene loss. Schizosaccharomyces pombe kept five kinds of Agss in the genome while Aspergillus spp. and Penicillium spp. lost one or two or three kinds of Agss. All the human, animal and plant pathogens kept only one single kind of Ags or only one active Ags for synthesis of cell wall α-1,3-glucan, a virulence factor. While the genus Aureobasidium spp. contained three kinds of Agss, of which only some of the Ags2 was involved in pullulan biosynthesis. Although many Agss contained Big_5 domain, only the Big_5 domain with conserved amino acids LQS from some strains of A. melanogenum could catalyze pullulan biosynthesis. This whole amino acid sequence and phylogenetic differences may cause non-α-1,3-glucan synthesizing activity of some fungal Agss.

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