TPX2 is phosphorylated by Aurora A and is essential for normal microtubule flux on the metaphase spindle.

Background: Aurora kinases show different localizations and play distinct roles, yet the mechanisms remain largely unknown. Results: Different localization leads to functional divergence of the Auroras and their N termini also contribute to the localization. Conclusion: Both N/C termini of Aurora A/B contribute to their spatial compartmentalization and function. Significance: Functional divergence of Aurora kinases is largely determined by their localizations through binding with partners/substrates.

Kinetochore capture by dynamic kinetochore microtubule fibers (K fibers) is essential for proper chromosome alignment and accurate distribution of the replicated genome during cell division. Although this capture process has been extensively studied, the mechanisms underlying the initiation of this process and the proper formation of the K fibers remain largely unknown. Here we show that transforming acidic coiled-coil-containing protein 3 (TACC3) is essential for kinetochore capture and proper K-fiber formation in HeLa cells. To observe the assembly of acentrosomal microtubules more clearly, the cells were released from higher concentrations of nocodazole into zero or lower concentrations. We find that small acentrosomal TACC3-microtubule aster formation near the kinetochores and binding of the asters with the kinetochores are the initial steps of the kinetochore capture by the acentrosomal microtubules, and that the sorting of kinetochore-captured acentrosomal microtubules with centrosomal microtubules leads to the capture of kinetochore by centrosomal microtubules from both spindle poles. We demonstrate that the sorting of the TACC3-associated microtubules with the centrosomal microtubules is a crucial process for spindle assembly and chromosome movement. These findings, which are also supported in the unperturbed mitosis without nocodazole, reveal a critical TACC3-dependent acentrosomal microtubule nucleation and sorting process to regulate kinetochore-microtubule connections and provide deep insight into the mechanisms of mitotic spindle assembly and chromosome alignment.T o ensure proper segregation of the chromosomes into its two daughter cells during proliferation, the chromosomes of a mother cell must be captured by its assembling mitotic spindle through attachment of the chromosome kinetochores and the dynamic spindle microtubules (1). A "search-and-capture" model was proposed long ago, in which the dynamic spindle microtubules nucleated from the centrosomes search for and capture the chromosome kinetochores (2). Previous studies showed that the kinetochores are initially captured by the spindle-polenucleated microtubules with their lateral side (3, 4). Once captured, the kinetochores with their chromosomes are transported along the microtubules toward a spindle pole, and the microtubules shrink at their plus ends until the establishment of the end-on attachment (4, 5). However, this model is insufficient to explain the initial connection of the kinetochore and the spindle microtubules in the centrosome-independent spindle assembly process. Recent studies in Xenopus extracts indicated that microtubules are nucleated near the chromosomes and self-organize into a spindle (6). A new model for acentrosomal spindle assembly has been raised in mouse oocytes, in which self-organized microtubule organizing centers (MTOCs) replace the centrosome function (7). The somatic cells may also use the centrosome-independent pathway for their spindle assembly (8-10). In Drosophila cells, the centrosome-independent assem...

The mitotic kinase Aurora B regulates the condensation of chromatin into chromosomes by phosphorylating chromatin proteins during early mitosis, whereas the phosphatase PP1γ performs the opposite function. The roles of Aurora B and PP1γ must be tightly coordinated to maintain chromosomes at a high phosphorylation state, but the precise mechanisms regulating their function remain largely unclear. Here, mainly through immunofluorescence microscopy and co-immunoprecipitation assays, we find that dissociation of PP1γ from chromosomes is essential for maintaining chromosome phosphorylation. We uncover that PP1γ is recruited to mitotic chromosomes by its regulatory subunit Repo-Man in the absence of Aurora B activity, and that Aurora B regulates dissociation of PP1γ by phosphorylating and disrupting PP1γ-Repo-Man interactions on chromatin. Overexpression of Repo-Man mutants that cannot be phosphorylated or inhibition of Aurora B kinase activity resulted in the retention of PP1γ on chromatin and prolonged the chromatin condensation process; a similar outcome was caused by the ectopic targeting of PP1γ to chromatin. Together, our findings reveal a novel regulation mechanism of chromatin condensation in which Aurora B counteracts PP1γ activity by releasing PP1γ from Repo-Man and provide important implications for understanding the regulations of dynamic structural changes of the chromosomes in mitosis.