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First published online July 1, 2009
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In the fission yeast Schizosaccharomyces pombe, mitosis is closed – that is, the nuclear envelope (NE) remains intact, instead of disassembling as it does in animal cells. Before dividing symmetrically, the S. pombe nucleus elongates rapidly and the NE expands – but how are these processes controlled? To address this question, Shelley Sazer and colleagues (p. 2464) now investigate whether the small GTPase Ran – which modulates NE structure in animals and yeast – has a specific role during S. pombe mitosis. The authors first show that, in pim1-d1 fission yeast (which have a temperature-sensitive mutation in the Ran activator Pim1), nuclear division is asymmetric, and the newly synthesised spindle pole body (SPB; the equivalent of a centrosome in animal cells) localises to the smaller daughter nucleus. pim1-d1 cells also lose NE integrity as the nucleus elongates. Importantly, though, NE breakdown is partially rescued in pim1-d1 cells with additional mutations that retard nuclear elongation, increase the amount of ER membrane (which is contiguous with the NE), or cause changes in ER morphology. The authors conclude that a Ran-dependent SPB defect leads to asymmetric nuclear division, and that NE integrity during mitotic expansion relies on the redistribution of lipids from the ER. This redistribution might, they propose, be a common feature of NE changes during open and closed mitosis.
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