CYTOCHALASIN-D INTERFERES WITH CONTRACTILE ACTIN RING AND SEPTUM FORMATION IN SCHIZOSACCHAROMYCES JAPONICUS VAR. VERSATILIS

The cells of Schizosaccharomyces japonicus var. versatilis responded t o the presence of cytochalasin D (CD). an inhibitor of actin polymeriz ation, by the disappearance of contractile actin rings (ARs) that had already formed and by inhibition of new ring formation. Actin cables d isappeared. Actin patches remained preserved and became co-localized w ith regions of actual cell wall formation (at cell poles and at the si te of septum development). Removal of the AR arrested formation of the primary septum and led to the production of aberrant septum protrusio ns in that region. Nuclear division was accomplished in the presence o f CD but new ARs were not produced. The wall (septum) material was dep osited in the form of a wide band at the inner surface of the lateral cell wall in the cell centre. This layer showed a thin fibrillar struc ture. The removal of CD resulted in rapid formation of new ARs in the equatorial region of the cells. This implies that the signal for AR lo calization was not abolished either by CD effects or by removal of an AR already formed. Some of the newly developed ARs showed atypical loc alization and orientation. In addition, redundant, subcortically situa ted actin bundles were produced. The removal of CD was quickly followe d by the development of primary septa colocalized with ARs. Wall protr usions occurred co-localized with the redundant actin bundles. If thes e were completed in a circle, redundant septa developed. The AR is a m echanism which, in time and space, triggers cytokinesis by building a septum sequentially dependent on the AR. Aberrant septa were not capab le of separating daughter cells. However, non-separated daughter cells subsequently gave rise to normal cells.