Abnormal formation of the glucan network from regenerating protoplasts in Schizosaccharomyces pombe cps8 actin point mutant

To study the close relationship between the actin cytoskeleton and cell wal l formation, the process of cell wall formation in reverting protoplasts of the fission yeast, Schizosaccharomyces pombe, cps8 actin point mutant was investigated by ultra-high-resolution low-voltage scanning electron microsc opy (UHR-LVSEM) and transmission electron microscopy (TEM). The protoplast of the cps8 mutant began to form a glucan network in a unipolar manner and to secrete alpha-galactomannan. The site of cell wall formation grew in a c ylindrical shape in the wild-type protoplast. The alpha-galactomannan did n ot fill in the intrafibrillar spaces completely, however, and the fibrils w ere exposed on the cell surface. UHR-LVSEM images indicated that the glucan fibrils were thin and rope-shaped, forming a looser network than the wild- type. TEM images indicated the finest fibrils were similar to 1.5 nm in dia meter, the same diameter as the wild-type. These results suggest that the c ps8 mutant was insufficient in developing cross-linkage with the glucan fib rils up to the wide ribbon shape as found in the wild-type [Osumi M et al. (1989) J. Electron Microsc. 38: 457-468; Osumi M (1998) Micron 29: 207-233] . These findings appear to indicate that the actin cytoskeleton controls fo rmation of the glucan network and secretion of beta-1,6-glucan, and confirm the close relationship of the actin cytoskeleton and glucan formation.