Changes in cell wall composition of deformedras1 − cells ofSchizosaccharomyces pombe

Disruption of theSchizosaccharomyces pombe ras1 gene results in a morphological transformation to large spheres, in contrast to wild-type cells which grow as rods. Chemical analysis of isolated cell walls showed no significant changes in saccharide content but an increase in protein and phosphate contents inras1 ⁻ walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treatment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-mass species, a component moving as a band at 31 kDa was conspicuous inras1 ⁻ cell walls. It was solubilized by heating in Tris-HCl buffer and shown to have a β-1,3-glucanase activity against laminarin. The level of the enzyme was by 30% higher in theras1 ⁻ cell wall than in the wild-type cell wall. This enzyme may participate in the remodelling of the rigid glucan network and account (at least partially) for the aberrant cell shape. Theras1 ⁻ cell wall contained a high level of charged polymers, especially phosphoproteins, raising the appealing possibility thatras1 ⁻ is involved in a putative kinase cascade required to sense and respond to external stimuli destined for the cell wall. Although the present study shows thatras1 loss of function and altered cell wall composition are closely linked defects, it has still to be shown that theras1 protein is directly involved in alterations found in the mutant cell walls.