In search of an optimal ring to couple microtubule depolymerization to processive chromosome motions

Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309-0347, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 12/2007; 104(48):19017-22. DOI: 10.1073/pnas.0709524104
Source: PubMed


Mitotic chromosome motions are driven by microtubules (MTs) and associated proteins that couple kinetochores to MT ends. A good coupler should ensure a high stability of attachment, even when the chromosome changes direction or experiences a large opposing force. The optimal coupler is also expected to be efficient in converting the energy of MT depolymerization into chromosome motility. As was shown years ago, a "sleeve"-based, chromosome-associated structure could, in principle, couple MT dynamics to chromosome motion. A recently identified kinetochore complex from yeast, the "Dam1" or "DASH" complex, may function as an encircling coupler in vivo. Some features of the Dam1 ring differ from those of the "sleeve," but whether these differences are significant has not been examined. Here, we analyze theoretically the biomechanical properties of encircling couplers that have properties of the Dam1/DASH complex, such as its large diameter and inward-directed extensions. We demonstrate that, if the coupler is modeled as a wide ring with links that bind the MT wall, its optimal performance is achieved when the linkers are flexible and their binding to tubulin dimers is strong. The diffusive movement of such a coupler is limited, but MT depolymerization can drive its motion via a "forced walk," whose features differ significantly from those of the mechanisms based on biased diffusion. Our analysis identifies key experimental parameters whose values should determine whether the Dam1/DASH ring moves via diffusion or a forced walk.

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Available from: Ekaterina L Grishchuk
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    • "omplexes from budding yeast have also been shown to synergize in forming load - bearing attachments to depolymerizing microtubules ( Lampert et al . , 2010 ; Tien et al . , 2010 ) . The models previously described for the Ndc80 and Dam1 complexes were under the implicit assumption that each complex interacts with the straight microtubule lattice ( Efremov et al . , 2007 ; Hill , 1985 ) . Here , we demonstrated that the human Ska1 complex autonomously tracks with depolymerizing micro - tubules . We propose that the Ska1 complex remains associ - ated with depolymerizing microtubules by not only associating with and diffusing along the straight microtubule lattice , but also by interacting with the curved"
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    ABSTRACT: To ensure equal chromosome segregation during mitosis, the macromolecular kinetochore must remain attached to depolymerizing microtubules, which drive chromosome movements. How kinetochores associate with depolymerizing microtubules, which undergo dramatic structural changes forming curved protofilaments, has yet to be defined in vertebrates. Here, we demonstrate that the conserved kinetochore-localized Ska1 complex tracks with depolymerizing microtubule ends and associates with both the microtubule lattice and curved protofilaments. In contrast, the Ndc80 complex, a central player in the kinetochore-microtubule interface, binds only to the straight microtubule lattice and lacks tracking activity. We demonstrate that the Ska1 complex imparts its tracking capability to the Ndc80 complex. Finally, we present a structure of the Ska1 microtubule-binding domain that reveals its interaction with microtubules and its regulation by Aurora B. This work defines an integrated kinetochore-microtubule interface formed by the Ska1 and Ndc80 complexes that associates with depolymerizing microtubules, potentially by interacting with curved microtubule protofilaments.
    Full-text · Article · Oct 2012 · Developmental Cell
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    • "The discovery that DASH forms rings around microtubules captured the imagination of researchers interested in the microtubule-kinetochore interface (Salmon 2005). Rings provide an excellent model for a microtubule plus end tracker that could couple depolymerisation to movement (Efremov et al. 2007; Armond and Turner 2010). A ringbased coupler bound weakly to the plus end would effectively track the depolymerising tip as the splayed microtubule end would push on the ring as the microtubule depolymerises (Fig. 1a). "
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    ABSTRACT: Regulated interaction between kinetochores and the mitotic spindle is essential for the fidelity of chromosome segregation. Potentially deleterious attachments are corrected during prometaphase and metaphase. Correct attachments must persist during anaphase, when spindle-generated forces separate chromosomes to opposite poles. In yeast, the heterodecameric DASH complex plays a vital pole in maintaining this link. In vitro DASH forms both oligomeric patches and rings that can form load-bearing attachments with the tips of polymerising and depolymerising microtubules. In vivo, DASH localises primarily at the kinetochore, and has a role maintaining correct attachment between spindles and chromosomes in both Saccharomyces cerevisiae and Schizosaccharomyces pombe. Recent work has begun to describe how DASH acts alongside other components of the outer kinetochore to create a dynamic, regulated kinetochore-microtubule interface. Here, we review some of the key experiments into DASH function and discuss their implications for the nature of kinetochore-microtubule attachments in yeast and other organisms.
    Full-text · Article · Apr 2011 · Chromosome Research
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    • "We were, therefore, able to produce an accurate snapshot of Dam1 ring–microtubule orientations. The distribution of angles we see here would come, according to the modeling of Ataullakhanov and colleagues (Efremov et al., 2007), from two binding sites on the microtubule wall. However, we would also expect to see a similar distribution from Dam1–microtubule binding involving flexible elements, for example, interactions between the E-hook of tubulin and extended elements from the Dam1 complex. "
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    ABSTRACT: There has been much effort in recent years aimed at understanding the molecular mechanism by which the Dam1 kinetochore complex is able to couple microtubule depolymerization to poleward movement. Both a biased diffusion and a forced walk model have been proposed, and several key functional aspects of Dam1-microtubule binding are disputed. Here, we investigate the elements involved in tubulin-Dam1 complex interactions and directly visualize Dam1 rings on microtubules in order to infer their dynamic behavior on the microtubule lattice and its likely relevance at the kinetochore. We find that the Dam1 complex has a preference for native tubulin over tubulin that is lacking its acidic C-terminal tail. Statistical mechanical analysis of images of Dam1 rings on microtubules, applied to both the distance between rings and the tilt angle of the rings with respect to the microtubule axis, supports a diffusive ring model. We also present a cryo-EM reconstruction of the Dam1 ring, likely the relevant assembly form of the complex for energy coupling during microtubule depolymerization in budding yeast. The present studies constitute a significant step forward by linking structural and biochemical observations toward a comprehensive understanding of the Dam1 complex.
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