Polo kinase and Asp are needed to promote the mitotic organizing activity of centrosomes

Abstract

Interfering with the activity of polo-like kinases can lead to the formation of monopolar spindles^1,2,3,4. Polo-like kinases also regulate mitotic entry, activation of the anaphase-promoting complex and the necessary preconditions for cytokinesis^5,6. Similarities between the phenotypes of the Drosophila mutants asp and polo point towards a common role in spindle pole function. The abnormal spindles of asp mutants are bipolar but have disorganized broad poles at which γ-tubulin has an abnormal distribution⁷. Moreover, the synergism or of polo¹ asp^E3 double mutants indicates a possible involvement of these genes in a common process⁸. Asp is a microtubule-associated protein of relative molecular mass 220,000 (M_r 220K) found at the face of the centrosome that contacts spindle microtubules. In partially purified centrosomes, it is required with γ-tubulin to organize microtubule asters⁷. Here, we show that Asp is the previously identified M_r 220K substrate of Polo kinase⁹. Polo phosphorylates Asp in vitro, converting it into an MPM2 epitope. Polo and Asp proteins immunoprecipitate together and exist as part of a 25–38S complex. Extracts of polo-derived embryos are unable to restore the ability of salt-stripped centrosomes to nucleate microtubule asters. This can be rescued by addition of phosphorylated Asp or active Polo kinase.