Features of the cell periphery in the deformed ras1 − cells of Schizosaccharomyces pombe

The rounding-up phenomenon in the sterile ras1 − mutant of Schizosaccharomyces pombe was examined by fluorescence microscopy of Calcofluor- and rhodamine-phalloidin-labeled cells and by freeze-etching. In cells that entered the stationary phase, rapid and dramatic changes occurred in cell shape and isotropic cell expansion, with the cells becoming round and increasing in size. This was accompanied by a collapse of the actin-containing cytoskeleton and a transition of actin dot to transcellular filaments. A considerable part of the plasma membrane of the mutant was smooth and uninvaginated. Notable was a gradual delocalization of the cell wall deposition and a special kind of cell wall rearrangement which altered the preexisting cell wall architecture. This is the first report that describes the morphological consequences induced by the deletion of the single ras gene in S. pombe.