Regulation of mitotic spindle assembly factor NuMA by Importin-β.

Ran-guanosine triphosphatase orchestrates mitotic spindle assembly by modulation of the interaction between Importin-alpha/-beta and spindle assembly factors (SAFs). The inhibition of SAFs performed by importins needs to be done without much sequestration from abundant nuclear localization signal (NLS) -containing proteins. However, the molecular mechanisms that determine NLS-binding selectivity and that inhibit activity of Importin-beta-regulated SAFs (e.g., nuclear mitotic apparatus protein [NuMA]) remain undefined. Here, we present a crystal structure of the Importin-alpha-NuMA C terminus complex showing a novel binding pattern that accounts for selective NLS recognition. We demonstrate that, in the presence of Importin-alpha, Importin-beta inhibits the microtubule-binding function of NuMA. Further, we have identified a high-affinity microtubule-binding region that lies carboxyl-terminal to the NLS, which is sterically masked by Importin-beta on being bound by Importin-alpha. Our study provides mechanistic evidence of how Importin-alpha/-beta regulates the NuMA functioning required for assembly of higher-order microtubule structures, further illuminating how Ran-governed transport factors regulate diverse SAFs and accommodate various cell demands.