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 ϳ10% (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.

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