Role of the synthase domain of Ags1p in cell wall alfa-glucan biosynthesis in fission yeast

The cell wall is important for maintenance of the structural integrity and morphology of fungal cells. Besides -glucan and chitin, -glucan is a major polysaccharide in the cell wall of many fungi. In the fission yeast Schizosaccharomyces pombe, cell wall -glucan is an essential component, consisting mainly of (1,3)--glucan with 10% (1,4)-linked -glucose residues. The multidomain protein Ags1p is required for -glucan biosynthesis and is conserved among cell wall -glucan-containing fungi. One of its domains shares amino acid sequence motifs with (1,4)--glucan synthases such as bacterial glycogen synthases and plant starch synthases. Whether Ags1p is involved in the synthesis of the (1,4)--glucan constituent of cell wall -glucan had remained unclear. Here, we show that overexpression of Ags1p in S. pombe cells results in accumulation of (1,4)--glucan. To determine whether the synthase domain of Ags1p is responsible for this activity, we overexpressed Ags1p-E1526A, which carries a mutation in a putative catalytic residue of the synthase domain, but observed no accumulation of (1,4)--glucan. Compared with wild-type Ags1p, this mutant Ags1p showed a markedly reduced ability to complement the cell lysis phenotype of the temperature-sensitive ags1-1 mutant. Therefore, we conclude that, in S. pombe, the production of (1,4)--glucan by the synthase domain of Ags1p is important for the biosynthesis of cell wall -glucan.