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Alp7/TACC is a crucial target in Ran-GTPase-dependent spindle formation in fission yeast

Abstract

Microtubules are essential intracellular structures involved in several cellular phenomena, including polarity establishment and chromosome segregation1. Because the nuclear envelope persists during mitosis (closed mitosis) in fission yeast (Schizosaccharomyces pombe), cytoplasmic microtubules must be reorganized into the spindle in the compartmentalized nucleus on mitotic entry2. An ideal mechanism might be to take advantage of an evolutionarily conserved microtubule formation system that uses the Ran-GTPase nuclear transport machinery3,4,5, but no targets of Ran for spindle formation have been identified in yeast. Here we show that a microtubule-associated protein, Alp7, which forms a complex with Alp14, is a target of Ran in yeast for spindle formation. The Ran-deficient pim1 mutant (pim1-F201S) failed to show mitosis-specific nuclear accumulation of Alp7. Moreover, this mutant exhibited compromised spindle formation and early mitotic delay. Importantly, these defects were suppressed by Alp7 that was artificially targeted to the nucleus by a Ran-independent and importin-α-mediated system. Thus, Ran targets Alp7–Alp14 to achieve nuclear spindle formation, and might differentiate its targets depending on whether the organism undergoes closed or open mitosis.

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Figure 1: Defects in bipolar spindle formation in the pim1 mutant.
Figure 2: Alp7 is a target of the Ran-dependent transport machinery.
Figure 3: Establishment of a Ran-independent nuclear import and unloading system by importin-α and TEV protease.
Figure 4: The Imp1*–tev–Alp7 system restores spindle formation and mitotic progression of the pim1-F201S mutant.

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Acknowledgements

We thank E. Karsenti for comments and R. E. Carazo-Salas and M. Toya for critical reading of the manuscript, discussion and technical support for microscopy. We also thank M. Yoshida, H. Maekawa, E. Schiebel, I. Hagan, R. Y. Tsien, S. Yokobayashi and Y. Watanabe for plasmids and/or information. We are grateful to members of the Cell Regulation Laboratory and M. Yamamoto for support. This work is supported by Cancer Research UK. M.S. is a recipient of a Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Research Abroad.

Author Contributions The experiments were designed by M.S with the support of T.T. and performed by M.S. M.S. and T.T. wrote the paper.

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Correspondence to Masamitsu Sato or Takashi Toda.

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Sato, M., Toda, T. Alp7/TACC is a crucial target in Ran-GTPase-dependent spindle formation in fission yeast. Nature 447, 334–337 (2007). https://doi.org/10.1038/nature05773

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