Model organisms using diverse modes of mitosis, including ‘open’, ‘closed’ or the intermediates, are useful to study to understand how the dynamics of the nuclear envelope play a role in mitosis, and why diverse forms of mitosis have evolved. The nuclear envelope is once removed during mitosis, displaying open mitosis ( Straube et al. In addition, dynamic rearrangement of nucleoporins occurs in the corn smut fungus Ustilago maydis. There is practically no restriction of transport between the nucleus and the cytoplasm, and therefore semi-open mitosis occurs in A. nidulans ( De Souza et al. After phosphorylation by NIMA and Cdk1 kinases, nuclear pore complex factors Nup98 and Gle2 are disassembled from the nuclear envelope during mitosis. Moreover, changes in the composition of nuclear pore complexes lead to increased permeability of the nuclear envelope during mitosis in Aspergillus nidulans ( De Souza et al. An intact nuclear envelope guarantees faithful mitosis of the fission yeast. Concomitantly, localization of nuclear pore complexes is disturbed in pim1 mutants, and the viability of mutant cells is remarkably decreased. Whereas the nuclear envelope remains intact throughout the cell cycle of Sz. pombe, a defect in guanine nucleotide exchange activity by a mutation of pim1 causes fragmentation of the nuclear envelope ( Demeter et al.
The nucleus of Sz. pombe is constricted at the equatorial plane during anaphase and then separated into daughter cells, synchronizing with chromosomal segregation ( Nabeshima et al. Indeed, the fission yeast Sz. pombe, in which closed mitosis occurs, has considerably contributed to the study of fundamental mechanisms of mitosis and cell division ( Yanagida 2005). However, there are many conserved mechanisms of chromosome segregation between open and closed mitosis. This mode of mitosis is referred to as closed mitosis ( Sazer 2005 De Souza & Osmani 2007). In contrast, it is known that the nuclear envelope does not break down during mitosis in lower eukaryotes, including fungi and yeasts. This process is often referred to as open mitosis, which is the typical means of inheritance of chromosomes to daughter cells in higher eukaryotes ( Guttinger et al. The nuclear envelope is regenerated around the chromosomes in telophase and encapsulates them to form new nuclei. Thus, cytoplasmic factors are allowed to interact with chromosomes directly. As a result of fragmentation of the nuclear envelope, nuclear and cytoplasmic factors are unified in the cell, and chromosomes are exposed in the cytoplasm. 2008) and sequential lamina depolymerization ( Gerace & Blobel 1980). Nuclear envelope breakdown is triggered by both the disassembly of nuclear pore complexes from the nuclear envelope ( Terasaki et al. In higher eukaryotic cells, nuclear envelope breakdown (NEBD), which is the fragmentation of the nuclear envelope, occurs during prophase ( Hetzer et al. Thus, transport of both proteins and RNAs between the nucleus and cytoplasm is tightly linked with the progression of the cell cycle. Defects in the components of nuclear pores cause cell cycle deficiency and cell death ( Adachi & Yanagida 1989 Fukuda et al. Many nuclear pore complexes are embedded in the nuclear envelope and control the transport of proteins or RNAs between the nucleus and cytoplasm in collaboration with importins for import and exportins for export ( Gorlich & Kutay 1999). The nuclear envelope is a lipid barrier membrane that compartmentalizes the nucleus and cytoplasm in eukaryotic cells ( Sazer 2005 Kutay & Hetzer 2008 Guttinger et al. These findings provide insight into the diversity of mitosis and the biological significance of breakdown of the nuclear envelope. We found that a similar tear of the nuclear envelope was also observed in late mitosis of the Caenorhabditis elegans embryo. It has been known that nuclear membrane remains intact in some metazoan mitosis. At the same time, a polarized-biased localization of nuclear pores was seen in the fusiform-shaped nuclear envelope, suggesting a compromise in the mechanical integrity of the lipid membrane. Finally, a tear in the nuclear envelope occurred in late anaphase. The mitotic nucleus of Sz. japonicus adopted a fusiform shape in anaphase, and its following extension caused separation. Here, we describe a form of mitosis in which the nuclear envelope is torn by elongation of the nucleus in the fission yeast Schizosaccharomyces japonicus. The mechanism of mitosis has been studied extensively in yeast, a closed mitosis organism.
By contrast, mitosis in lower eukaryotes, including fungi, proceeds with the nucleus enclosed in an intact nuclear envelope. During open mitosis in higher eukaryotic cells, the nuclear envelope completely breaks down and then mitotic chromosomes are exposed in the cytoplasm.
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