bgs2(+), a sporulation-specific glucan synthase homologue is required for proper ascospore wall maturation in fission yeast

The formation of the ascospore cell wall of Schizosaccharomyces pombe requi res the co-ordinated activity of enzymes involved in the biosynthesis of it s components, such as glucans. We have cloned the bgs2(+) gene. bgs2(+) bel ongs to the glucan synthase family of S. pombe and is homologous to the Sac charomyces cerevisiae FKS1 and FKS2 genes. Deletion or overexpression of th is gene does not lead to any apparent defect during vegetative growth, but homozygous bgs2 Delta diploids do show a sporulation defect. Although meios is takes place normally, ascospores are unable to mature, and their wall di ffers from that of wild-type ascospores. Moreover, bgs2 Delta zygotes were not able to release ascospores spontaneously, and the ascospores were unabl e to germinate. We show that expression of bgs2(+) is restricted to sporula tion and that a bgs2-green fluorescent protein (GFP) fusion protein localiz es to the ascospore envelope. The glucan synthase activity in sporulating d iploids bearing a bgs2 deletion was diminished in comparison with that of t he wild-type diploids, a fact that underscores the importance of the bgs2() gene and glucan synthesis for the proper formation and maturation of the ascospore wall.