Cell
Volume 104, Issue 1, 12 January 2001, Pages 83-93
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Article
Ran Induces Spindle Assembly by Reversing the Inhibitory Effect of Importin α on TPX2 Activity

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Abstract

The small GTPase Ran, bound to GTP, is required for the induction of spindle formation by chromosomes in M phase. High concentrations of Ran.GTP are proposed to surround M phase chromatin. We show that the action of Ran.GTP in spindle formation requires TPX2, a microtubule-associated protein previously known to target a motor protein, Xklp2, to microtubules. TPX2 is normally inactivated by binding to the nuclear import factor, importin α, and is displaced from importin α by the action of Ran.GTP. TPX2 is required for Ran.GTP and chromatin-induced microtubule assembly in M phase extracts and mediates spontaneous microtubule assembly when present in excess over free importin α. Thus, components of the nuclear transport machinery serve to regulate spindle formation in M phase.

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