Spindle poles gain importins

The GTPase Ran and its effectors importins regulate not only nuclear import but also spindle organization, assembly of the nuclear envelope and other key cell-cycle events. Spatial control of the Ran system is critical in each case. Patrizia Lavia and co-workers have therefore studied the localization of importin β during mitosis (see p. 6511). They observe that, after the nuclear envelope breaks down, importin β migrates to the spindle poles, where it colocalizes with Ran. They then use cytoskeletal inhibitors and anti-dynein antibodies to demonstrate that this requires microtubules and that importin β is transported along these by the motor protein dynein. The authors go on to show that RNAi directed against the importin-binding spindle regulator TPX2 prevents importin β localizing to spindle poles. Furthermore, they demonstrate that importin β overexpression causes spindle abnormalities, which can be rescued by coexpression of TPX2. Lavia and co-workers conclude that the balance of importin and TPX2 activities is critical for assembly of normal mitotic spindles. They also propose that targeting of importin-TPX2 complexes to spindle poles is an essential aspect of the Ran system's mitotic role.