CHANGES IN CELL-WALL COMPOSITION OF DEFORMED RAS1(-) CELLS OF SCHIZOSACCHAROMYCES-POMBE

Disruption of the Schizosaccharomyces pombe ras1 gene results in a mor phological transformation to large spheres, in contrast to wild-type c ells which grow as rods. Chemical analysis of isolated cell walls show ed no significant changes in saccharide content but an increase in pro tein and phosphate contents in ras1(-) walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treat ment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-ma ss species, a component moving as a band at 31 kDa was conspicuous in ras1(-) cell walls. It was solubilized by heating in Tris-HCl buffer a nd shown to have a beta-1,3-glucanase activity against laminarin. The level of the enzyme was by 30 % higher in the ras1(-) cell wall than i n the wild-type cell wall. This enzyme may participate in the remodell ing of the rigid glucan network and account (at least partially) for t he aberrant cell shape. The ras1(-) cell wall contained a high level o f charged polymers, especially phosphoproteins, raising the appealing possibility that ras1(-) is involved in a putative kinase cascade requ ired to sense and respond to external stimuli destined for the cell wa ll. Although the present study shows that ras1 loss of function and al tered cell wall composition are closely linked defects, it has still t o be shown that the ras1 protein is directly involved in alterations f ound in the mutant cell walls.