The Xenopus laevis aurora/Ip11p-related kinase pEg2 participates in the stability of the bipolar mitotic spindle

The Xenopus Laevis aurora/Ip11p-related kinase pEg2 is required for centros ome separation, which is a prerequisite for bipolar mitotic spindle formati on. Here, we report that the inhibition of pEg2 by addition of either an in active kinase or a monoclonal antibody destabilizes bipolar spindles previo usly assembled in Xenopus egg extracts. The bipolar spindles collapse to fo rm structures such as microtubule asters with chromosome rosettes, monopola r spindles, and multipolar spindles. In collapsed spindles, chromosomes rem ain attached to the microtubules plus ends. The destabilization of the bipo lar spindle is reminiscent of the destabilization observed after inhibition of cross-linking activities which maintain parallel and anti-parallel micr otubules linked together, We have previously reported that pEg2 phosphoryla tes the kinesin-related protein XlEg5 which is involved in centrosome separ ation but which was also reported to be involved in spindle stability. The collapse of the bipolar spindle observed after inhibition of pEg2 suggests that the kinase might regulate the cross-linking activity of XlEg5. We do n ot exclude the possibility that pEg2 also regulates other microtubule-based motor proteins involved in bipolar spindle stability. To our knowledge, th is is the first evidence that aurora/Ip11p-related kinase activity actually participates not only in mitotic spindle formation by regulating centrosom e separation but also in mitotic spindle stabilization, (C) 2000 Academic P ress.