Research
Virtual breakdown of nuclear envelope
| Journal | Curr Biol 20, 1919-25 (2010) |
|---|---|
| Authors | Haruhiko Asakawa (1), Tomoko Kojidani (2), Chie Mori (2), Hiroko Osakada (2), Mamiko Sato (3), Da-Qiao Ding (2), Yasushi Hiraoka (1, 2, 4), Tokuko Haraguchi (1, 2, 4)
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| Title | Virtual breakdown of the nuclear envelope in fission yeast meiosis |
| PubMed | 20970342 |
| Laboratory |
Nuclear Dynamics Group |
| Abstract | Background In eukaryotic cells, nuclear and cytoplasmic compartmentalization is maintained by the activity of Ran, a small Ras-like GTPase, regulated by asymmetric localization of Ran guanine nucleotide exchange factor (RanGEF/RCC1) in the nucleus and Ran activating protein1 (RanGAP1) in the cytoplasm. The balance between the opposing activities of RanGEF/RCC1 and RanGAP1 generates a gradient of Ran-GTP across the NE, and this gradient controls the directionality of nucleocytoplasmic transport. In open mitosis, NEBD leads to collapse of the Ran-GTP gradient and diffusion of nuclear and cytoplasmic macromolecules and allows microtubules to attach to the centromeres for chromosome segregation. On the other hand, in closed mitosis, the nuclear envelope remains intact throughout the mitotic cell cycle. The fission yeast Schizosaccharomyces pombe is one of the closed mitosis organisms: the nuclear envelope and NPC remain intact throughout the meiotic and mitotic cell cycles and the mitotic spindle forms within the nucleus during mitosis; in closed mitosis, tubulins are transported into the nucleus by importin. Results We found that in S. pombe nuclear proteins diffuse into the cytoplasm transiently in anaphase of the second meiotic division (anaphase II) as if NEBD had occurred in this closed mitosis organism (Figure 1): The nuclear membrane and NPCs remain intact throughout meiosis (Figure 2, 3), but the permeability of the nuclear envelope during anaphase II indicates that the nuclear envelope is structurally closed but functionally open. Thus, we named this phenomenon virtual nuclear envelope breakdown (V-NEBD). Strikingly, our study also revealed that RanGAP1 changes its localization from the cytoplasm to the nucleus at the onset of anaphase II in exact correlation with the timing of nuclear protein diffusion into the cytoplasm (Figure 1). Because the timing of the nuclear localization of RanGAP1 coincides with the diffusion of nuclear proteins out of the nucleus and because ectopic expression of NLS-conjugated RanGAP1 can induce diffusion of nuclear proteins to the cytoplasm, we speculate that the nuclear localization of RanGAP1 triggers diffusion of nuclear proteins out of the nucleus during anaphase II, although, we currently have no direct evidence for this hypothesis. Both closed mitosis and V-NEBD proceed with structurally intact nuclear envelopes and NPCs. Except for the nuclear localization of RanGAP1, the molecular basis generating the difference between closed mitosis and V-NEBD remain unclear, however, there are a number of other possible factors. For example, a meiosis-specific modification of NPC, such as phosphorylation, may be involved in changing the permeability barrier function of the NPC. Modification and/or alteration of transport machinery, such as importin β-s, may also be a factor in the permeability barrier change of the NPC. A third possible factor in V-NEBD is the formation of forespore membranes. Since the forespore membranes form during this period, the nuclear envelope may be weakened by sequestration of membrane components into forespore membranes during their assembly. Another type of fungi, Aspergillus nidulans, undergoes “semi-open” mitosis. During this type of mitosis, some of the nucleoporins disassemble from the NPC, and partial disassembly of the NPCs changes RanGAP1 localization from the cytoplasm into the nucleus (Figure 4, Semi-open mitosis). Considering these facts, the translocation of RanGAP1 seems a common strategy in closed mitosis and semi-open mitosis organisms; and in closed mitosis organisms, this translocation abates the Ran-GTP gradient and leads to virtual breakdown of the nuclear envelope without physical breakdown of the nuclear envelope. Thus, localization of RanGAP1 may act as a molecular switch to control the barrier function of the nuclear envelope. |
Figure 1
Nuclear proteins are reduced in the nucleus and RanGAP1 enters the nucleus in S. pombe anaphase II.
Figure 2
(A) When a live cell reached anaphase II, as judged by the behavior of RanGAP1, the cell was fixed on site. (B) The fixed cell was subjected to electron microscopy.
Figure 3
Nucleoporins are structurally intact in meiosis.
Live cell imaging of GFP-tagged nucleoporins. Cut11, human Ndc1 homologue; Nup45, human Nup58 homologue; Nup211, human Tpr homologue.
Figure 4
Four modes of nuclear envelope morphology during mitosis in eukaryotes.
In higher eukaryotes, both the nuclear membrane and the NPCs disassemble at the beginning of mitosis, and subsequently chromosomes are segregated by the mitotic spindle during mitosis (Open mitosis). In a filamentous fungus Aspergillus nidulans, the nuclear membrane does not disassemble, but the NPCs are partially disassembled and RanGAP1 enters into the nucleus (Semi-open mitosis). In the fission yeast Schizosaccharomyces pombe, both the nuclear membrane and the NPCs remain intact but RanGAP1 enters into the nucleus, resulting in V-NEBD during meiosis II (Virtual nuclear envelope breakdown). Above three types of mitosis, the collapse of the Ran-GTP gradient results in scrambling of nuclear and cytoplasmic materials. On the other hand, many lower eukaryotes, like fungi, undergo closed mitosis. During closed mitosis both the nuclear membrane and the NPCs remain intact, RanGAP1 remains localized in the cytoplasm, and the Ran-GTP gradient across the NE remains intact.
