Distribution of tubulin and actin through the cell division cycle of the fission yeast Schizosaccharomyces japonicus var. versatilis: a comparison with Schizosaccharomyces pombe.

Changes in the distribution of microtubules and F-actin through the cell division cycle of the fission yeast Schizosaccharomyces japonicus var. versatilis were investigated by fluorescence microscopy. The fluorescence images obtained with S. japonicus were markedly superior to those previously reported for S. pombe and revealed new details of cytoskeletal organization in this important genus. As in S. pombe, F-actin in S. japonicus was present as a concentration of 'dots' at the growing poles of interphase cells and as a filamentous equatorial ring directing the deposition of the cytokinetic septum. The transition between these two states occurred at late anaphase, in contrast to the situation in S. pombe where the appearance of the equatorial actin ring is tightly coupled to the early events of mitosis. During the course of cytokinesis in S. japonicus the actin ring constricted and broadened, suggesting that it is contractile. Microtubule organization in S. japonicus also revealed interesting differences from S. pombe. Whereas in S. pombe cytoplasmic microtubules are reinitiated from a pair of microtubule organizing centres (MTOCs) at the cell equator, in S. japonicus they arise by extensive microtubule growth from the spindle poles. Western blots of cell extracts enriched for tubulin by DEAE-Sephadex chromatography showed that, like S. pombe, S. japonicus contains two alpha-tubulins and a single beta-tubulin. Whilst the alpha 1- and beta-tubulins from the two species comigrated on one-dimensional polyacrylamide gels, the alpha 2 species were electrophoretically distinct. Although fundamental differences clearly exist between the two species, S. japonicus could prove to be a useful tool in basic studies of fission yeast cell biology.