Induction of F-actin rings in regenerating protoplasts of the cdc16 mutant of Schizosaccharomyces pombe

Formation of continuous F-actin rings was induced in regenerating protoplasts of the temperature-sensitive mutant of Schizosaccharomyces pombe defective in cdcI6 gene. The proportion of ring-containing protoplasts freshly prepared from cdc!6 mutant cells exponentially growing at permissive temperature (10%) corresponded to the proportion of mitotic cells in the starting culture. This suggests that the actin rings survived protoplast formation as was found in the wild type strain of S. pombe (Jochova et al. 1991). In 87% of the cdcI6 mutant protoplasts prepared under permissive conditions and then cultivated at 37°C in the absence of digesting enzyme, F-actin bundle assembly was initiated within 3 h. F-actin formed either abundant cables (in 63% of these spheroplasts) or distinct continuous rings (in 37% of the spheroplasts). In some spheroplasts two rings or partially split single rings were formed. These results indicate that F-actin rings not only persist during protoplast formation but also can be induced when the protoplasts are further cultivated under reversion conditions.