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First published online 30 November 2004
doi: 10.1242/jcs.01569

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Importin ß is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells

Marilena Ciciarello¹, Rosamaria Mangiacasale¹, Catherine Thibier², Giulia Guarguaglini¹, Enzo Marchetti³, Barbara Di Fiore^1,* and Patrizia Lavia^1,

¹ Institute of Molecular Biology and Pathology, Section of Genetics, CNR Consiglio Nazionale delle Ricerche, Via degli Apuli 4, Rome 00185, Italy
² Laboratory of Developmental Biology, Université Pierre-et-Marie Curie, Paris VI, 4 place Jussieu, 75005, France
³ Department of Genetics and Molecular Biology, University `La Sapienza', Pz. A. Moro 5, Rome 00185, Italy

Author for correspondence (e-mail: patrizia.lavia{at}uniroma1.it)

Accepted 30 September 2004

Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established. In mammalian cells, in which centrosomes are major spindle organizers, a link is emerging between the Ran network, centrosomes and spindle poles. Here, we show that, after nuclear envelope breakdown, importin ß is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin ß dissociates from poles and is recruited back around reforming nuclei. Importin ß sediments with mitotic microtubules in vitro and its accumulation at poles requires microtubule integrity and dynamics in vivo. Furthermore, RNA interference-dependent inactivation of TPX2, the major Ran-dependent spindle organizer, abolishes importin ß accumulation at poles. Importin ß has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin ß mitigates these abnormalities. Together, these results indicate that the balance between importins and spindle regulators of the TPX2 type is crucial for spindle formation. Targeting of TPX2/importin-ß complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.

Key words: Importin ß, Mitosis, Spindle poles, GTPase Ran, Microtubules, TPX2

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