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Sensing Centromere Tension: Aurora B and the Regulation of Kinetochore Function

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Sensing Centromere Tension: Aurora B and the Regulation of Kinetochore Function TICB-760; No. of Pages 8 Review Sensing centromere tension: Aurora B and the regulation of kinetochore function Michael A. Lampson1 and Iain M. Cheeseman2 1 Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA 2 Whitehead Institute for Biomedical Research and Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02142, USA Maintaining genome integrity during cell division mechanism that selectively stabilizes only correct attach- requires regulated interactions between chromosomes ments is crucial to understanding proper chromosome and spindle microtubules. To ensure that daughter cells segregation. Here, we review recent work in an attempt inherit the correct chromosomes, the sister kinetochores to understand the molecular mechanisms by which erro- must attach to opposite spindle poles. Tension across neous attachments are detected and corrected, focusing on the centromere stabilizes correct attachments, whereas the role of Aurora B kinase in this process. We discuss the phosphorylation of kinetochore substrates by the con- processes that act upstream to control the activity of served Ipl1/Aurora B kinase selectively eliminates incor- Aurora B and its phosphorylation of kinetochore sub- rect attachments. Here, we review our current strates, and the downstream consequences of Aurora B understanding of how mechanical forces acting on the phosphorylation for kinetochore activity and function. kinetochore are linked to biochemical changes to control chromosome segregation. We discuss models for ten- Regulating attachments: reconciling mechanical and sion sensing and regulation of kinetochore function molecular mechanisms downstream of Aurora B, and mechanisms that specify Classic experiments by Bruce Nicklas using micromanipu- Aurora B localization to the inner centromere and lation in insect spermatocyes provided direct experimental determine its interactions with substrates at distinct evidence that attachments are stabilized through tension locations. across the centromere. In cells, this tension is established as spindle microtubules pull bi-oriented kinetochores in oppo- Introduction site directions. Experimentally induced tension, applied The accurate segregation of chromosomes during cell divi- with a glass microneedle, stabilizes unipolar attachments sion is essential to maintain genomic stability. In eukary- that are otherwise unstable [6,7]. These experiments laid otic cells, the microtubule-based mitotic spindle generates the foundation for a model to explain the general principle of forces to align the sister chromatids at the metaphase how bi-orientation can be achieved before any molecular plate, and then to pull the sister chromatids in opposite details of this regulation had been defined. directions to segregate them to the two daughter cells. The One of the first pieces to the molecular puzzle of tension- kinetochore assembles at the centromere of each chromo- dependent regulation was the identification of the Ipl1 some to mediate interactions with spindle microtubules. kinase in the budding yeast Saccharomyces cerevisiae in Kinetochores can initially bind to microtubules in any a screen for mutants that display an increase-in-ploidy (ipl) configuration, but accurate chromosome segregation phenotype [8]. Ipl1 was subsequently shown to be required requires that each pair of sister kinetochores ultimately for accurate chromosome segregation and to phosphorylate attach to microtubules from opposite spindle poles (bi- kinetochore substrates regulating microtubule binding orientation). Although there is a bias towards bi-orienta- [9–11]. Furthermore, Ipl1 promotes the turnover of attach- tion owing to geometric constraints imposed by chromo- ments in the absence of tension [12], suggesting that it some structure [1,2], frequent errors in kinetochore– might function in the pathway described by Nicklas. Par- microtubule attachments do occur [3,4] and would lead allel work in Drosophila melanogaster, Caenorhabditis to unequal segregation if uncorrected. Therefore, kineto- elegans and vertebrates identified Aurora kinases, the chore–microtubule attachments must be carefully regu- Ipl1 homologs, as key regulators of cell division (reviewed lated: incorrect attachments are destabilized and correct in [13]). The functional homolog of Ipl1 is Aurora B, which attachments are stabilized. In this way, all kinetochores localizes to the inner centromere as the enzymatic compo- eventually reach the correct attachment state in a trial- nent of the chromosome passenger complex (CPC), which and-error process, with destabilization providing a fresh also includes the inner centromere protein (INCENP), opportunity to bi-orient (reviewed in [5]). Defining the Survivin and Borealin (also known as Dasra or CSC-1) (reviewed in [14]). The CPC remains at the inner centro- Corresponding authors: Lampson, M.A. ([email protected]); mere until anaphase onset and then redistributes to the Cheeseman, I.M. ([email protected]). midzone of the anaphase spindle and the equatorial cell 0962-8924/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2010.10.007 Trends in Cell Biology xx (2010) 1–8 1 TICB-760; No. of Pages 8 Review Trends in Cell Biology Vol.xxx No.x cortex.AlthoughwefocushereontheroleofAuroraB regulate Ipl1 (Aurora B) activity through a tension-depen- and the CPC in kinetochore function, the CPC also dent change in the complex [26]. regulates cytokinesis. In vertebrates, Aurora B inhibi- Several other mechanisms that regulate Aurora B tion using small molecules or inhibitory antibodies leads activity have been described, such as interactions with to stabilization of incorrect attachments, for example the protein TD-60, binding of the CPC to microtubules, with both sister kinetochores attached to a single spindle activation by other kinases such as Chk1, Tousled-like pole [15–17]. Activation of Aurora B by removing an kinase (TLK-1) and Mps1, or inactivation by phosphatases inhibitor leads to correction of these attachment errors including PP1 and PP2A (reviewed in [27]). Mps1 is par- by selectively destabilizing incorrect attachments [18]. ticularly intriguing because, like Aurora B, it is required Together, these studies demonstrate that Ipl1/Aurora B for the correction of attachment errors in both budding phosphorylates kinetochore substrates in the absence of yeast and mammalian cells [28,29]. Mps1 phosphorylates tension, which destabilizes incorrect attachments and Borealin, but the effect of this phosphorylation on Aurora B allows re-orientation. activity is controversial [29–35]. Overall, it is not clear Aurora B belongs to a family of serine/threonine protein whether Aurora B kinase activity is regulated on the short kinases that includes Aurora A, and has strong structural time scales associated with the error correction process. similarity to protein kinase A (PKA; cAMP-dependent Instead, these upstream regulators appear to act primarily protein kinase). Thus, while the cellular localizations to generally ‘license’ Aurora B activity during mitosis, and functions of Aurora A, Aurora B and PKA are distinct, rather than to modulate its function at a specific kineto- their substrate preference is extremely similar. The pre- chore. Any factor that is required for full Aurora B activity ferred phosphorylation consensus sequence for each of is also likely to be required for proper regulation of kineto- these kinases is [RK]x[TS][ILV] [10,19]. This consensus chore–microtubule attachments. Attachments can form site provides a good approximation for those sequences when Aurora B activity is low, but they are unlikely to targeted by Aurora B, although some established sub- fully achieve correct bi-orientation. strates lack the downstream hydrophobic residue. In ad- Rather than changes in intrinsic kinase activity, recent dition, extra upstream positively charged residues appear work has suggested that the ability to sense tension to increase the probability of phosphorylation. Aurora B depends on the localization of Aurora B relative to its substrates often contain multiple, closely clustered phos- substrates at the outer kinetochore. Under this hypothesis, phorylation sites. These multiple sites might allow a the force exerted on bi-oriented kinetochores separates the switch-like behavior for the regulation of a given substrate, kinase at the inner centromere from its outer kinetochore as described for the CDK target Sic1 [20], or might be substrates, making Aurora B less able to access these required to generate charge effects that alter certain bio- substrates [12,36,37]. Evidence for this ‘spatial separation’ chemical functions. Unlike kinases such as Polo-like ki- model was provided in two ways. First, the phosphoryla- nase (Plk1), additional targeting regions in Aurora B tion level of an Aurora B substrate is strongly related to its substrates have not been defined outside of the consensus position within the kinetochore. When positioned close to phosphorylation site. Thus, a primary determinant of the kinase, a FRET-based biosensor that reports on phos- whether a potential substrate will be targeted by Aurora phorylation by Aurora B is constitutively phosphorylated, B is its proximity to the kinase. Aurora B localizes to the independent of tension, suggesting that kinase activity inner centromere, close to the kinetochore and thus proxi- itself is not force-dependent. However, if the same sensor mal to the site where regulation is required to ensure that
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