Phosphorylation of maskin by Aurora-A is regulated by RanGTP and importin beta.

Mitotic spindle assembly in Xenopus egg extracts is regulated at least in part by importin beta and its regulator, the small GTPase, Ran. RanGTP stabilizes microtubules near the chromosomes during spindle assembly by selectively releasing spindle assembly factors from inhibition by importin alpha/beta in the vicinity of the chromosomes. Several spindle assembly factors are regulated in this manner. We identified maskin, the Xenopus member of the transforming acidic coiled coil family of proteins, as a potential candidate in a two-step affinity chromatography approach designed to uncover additional downstream targets of importin alpha/beta in mitosis. Here, we show that although maskin lacks a canonical nuclear localization sequence, it binds importin beta in a RanGTP-regulated manner. We further show that importin beta inhibits the regulatory phosphorylation of maskin by Aurora-A. This suggests a novel mechanism by which importin beta regulates the activity of a spindle assembly factor.