Osw2 is required for proper assembly of glucan and/or mannan layers of the yeast spore wall

OSW2 is a meiotically-induced gene required for spore wall formation. osw2 Delta spores are sensitive to ether treatment. Except for this phenotype, the mutants do not show obvious sporulation defects; thus, its function remains elusive. We found that deletion of both OSW2 and CHS3 results in a synthetic sporulation defect. The spore wall is composed of four layers, and chs3 Delta spores lack the outer two (chitosan and dityrosine) layers. Thus, Osw2 is involved in the assembly of the inner (glucan and mannan) layers. In agreement with this notion, a glycosylphosphatidylinositol-anchored protein reporter mislocalizes in osw2 Delta spores. The osw2 Delta mutation also exhibited a severe synthetic sporulation defect when combined with the deletion of a beta-1,6-glucan synthesis-related gene, BIG1. Osw2 is localized to the prospore membrane during sporulation. However, it disappears in mature spores, indicating that it is not a structural component of the spore wall. Given that Osw2 contains a probable 2-dehydropantoate 2-reductase domain, it may mediate an enzymatic reaction. Osw2 shows a weak similarity to other 2-dehydropantoate 2-reductase domain-containing proteins, Svl3 and Pam1. A pam1 Delta svl3 Delta mutant exhibits vegetative cell and spore wall defects. Thus, the 2-dehydropantoate 2-reductase domain-containing proteins may have a similar function in glucan and/or mannan layer assembly.