TPX2 activation by GM130 controls astral microtubule formation and spindle orientation

Spindle orientation is important in multiple developmental processes as it determines cell fate and function. The correct orientation of the mitotic spindle depends on the assembly of a proper astral microtubule network. Here, we report that the spindle assembly factor TPX2 regulates the astral microtubule network. TPX2 in the spindle pole area is activated by GM130 on mitotic Golgi membranes to promote astral microtubule growth. GM130 relieves TPX2 inhibition by competing for importin binding. During mitosis, phosphorylation of importin at Serine 62 by Cdk1 switches its substrate preference from TPX2 to GM130, thereby enabling competition-based activation. Importin S62A impedes local TPX2 activation and compromises astral microtubule formation, ultimately resulting in misoriented spindles. Blocking the GM130-importin -TPX2 pathway impairs astral microtubule growth. Our results reveal a novel role for TPX2 in the organization of astral microtubules. Furthermore, we show that the substrate preference of the important mitotic modulator importin is regulated by Cdk1 phosphorylation.