Importin beta is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells.
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 beta is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin beta dissociates from poles and is recruited back around reforming nuclei. Importin beta 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 beta accumulation at poles. Importin beta has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin beta 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-beta complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.
Ciciarello
Marilena
M
Institute of Molecular Biology and Pathology, Section of Genetics, CNR Consiglio Nazionale delle Ricerche, Via degli Apuli 4, Rome 00185, Italy.
Mangiacasale
Rosamaria
R
Thibier
Catherine
C
Guarguaglini
Giulia
G
Marchetti
Enzo
E
Di Fiore
Barbara
B
Lavia
Patrizia
P
D000818:Animals; D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D006367:HeLa Cells; D006801:Humans; D007739:L Cells; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008870:Microtubules / Q000378:metabolism; D008938:Mitosis; D041681:NIH 3T3 Cells; D009363:Neoplasm Proteins / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D034622:RNA Interference; D008941:Spindle Apparatus / Q000378:metabolism*; D028961:beta Karyopherins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism*
D018797:Cell Cycle Proteins; D008869:Microtubule-Associated Proteins; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D010750:Phosphoproteins; C122167:TPX2 protein, human; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein