HURP is part of a ran-dependent complex involved in spindle formation

Background: GTP-loaded Ran induces the assembly of microtubules into aster-like and spindle-like structures in Xenopus egg extract. The microtubule-associated protein (MAP), TPX2, can mediate Ran's role in aster formation, but factors responsible for the transition from aster-like to spindle-like structures have not been described. Results: Here we identify a complex that is required for the conversion of aster-like to spindle-like structures. The complex consists of two characterized MAPs (TPX2, XMAP215), a plus end-directed motor (Eg5), a mitotic kinase (Aurora A), and HURP, a protein associated with hepatocellular carcinoma. Formation and function of the complex is dependent on Aurora A activity. HURP protein was further characterized and shown to bind microtubules and affect their organization both in vitro and in vivo. In egg extract, anti-HURP antibodies disrupt the formation of both Ran-dependent and chromatin and centrosome-induced spindles. HURP is also required for the proper formation and function of mitotic spindles in HeLa cells. Conclusions: HURP is a new and essential component of the mitotic apparatus. HURP acts as part of a multicomponent complex that affects the growth or stability of spindle MTs and is required for spindle MT organization.