DISRUPTION OF THE ACTIN CYTOSKELETON IN BUDDING YEAST RESULTS IN FORMATION OF AN ABERRANT CELL-WALL

A temperature-sensitive, conditionally lethal actin mutant of Saccharomyces cerevisiae, DBY 1693, was used to study, using light and electron microscopy, dysfunction of the actin cytoskeleton in the morphogenesis of the cell wall, Cells of this mutant strain survived at least 24 h at the restrictive temperature (37 degrees C). These cells showed isodiametric growth. Mutant cells accumulated vesicles, probably as a consequence of chaotic secretory transport caused by loss of polarity, A conspicuous morphological response to the dysfunction of actin was the formation of an aberrant wall over the whole surface of the isodiametrically-growing cell. This wall was of loose texture with protruding glucan microfibrils incompletely masked with amorphous matrix. It resembled the regenerating cell wall on the surfaces of yeast protoplasts. The localization of wall synthesis over the whole surface of temperature sensitive actin mutant cells was in accordance with an even distribution of submembranous actin in the form of patches (similarly to regenerating protoplasts). Delocalization of finger-like invaginations of the plasma membrane from the bud region to the whole surface of the growing cell was also found in mutant cells.