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      Anaphase B

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          Abstract

          Anaphase B spindle elongation is characterized by the sliding apart of overlapping antiparallel interpolar (ip) microtubules (MTs) as the two opposite spindle poles separate, pulling along disjoined sister chromatids, thereby contributing to chromosome segregation and the propagation of all cellular life. The major biochemical “modules” that cooperate to mediate pole–pole separation include: (i) midzone pushing or (ii) braking by MT crosslinkers, such as kinesin-5 motors, which facilitate or restrict the outward sliding of antiparallel interpolar MTs (ipMTs); (iii) cortical pulling by disassembling astral MTs (aMTs) and/or dynein motors that pull aMTs outwards; (iv) ipMT plus end dynamics, notably net polymerization; and (v) ipMT minus end depolymerization manifest as poleward flux. The differential combination of these modules in different cell types produces diversity in the anaphase B mechanism. Combinations of antagonist modules can create a force balance that maintains the dynamic pre-anaphase B spindle at constant length. Tipping such a force balance at anaphase B onset can initiate and control the rate of spindle elongation. The activities of the basic motor filament components of the anaphase B machinery are controlled by a network of non-motor MT-associated proteins (MAPs), for example the key MT cross-linker, Ase1p/PRC1, and various cell-cycle kinases, phosphatases, and proteases. This review focuses on the molecular mechanisms of anaphase B spindle elongation in eukaryotic cells and briefly mentions bacterial DNA segregation systems that operate by spindle elongation.

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          Control of microtubule organization and dynamics: two ends in the limelight.

          Microtubules have fundamental roles in many essential biological processes, including cell division and intracellular transport. They assemble and disassemble from their two ends, denoted the plus end and the minus end. Significant advances have been made in our understanding of microtubule plus-end-tracking proteins (+TIPs) such as end-binding protein 1 (EB1), XMAP215, selected kinesins and dynein. By contrast, information on microtubule minus-end-targeting proteins (-TIPs), such as the calmodulin-regulated spectrin-associated proteins (CAMSAPs) and Patronin, has only recently started to emerge. Here, we review our current knowledge of factors, including microtubule-targeting agents, that associate with microtubule ends to control the dynamics and function of microtubules during the cell cycle and development.
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            Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation.

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              The bipolar mitotic kinesin Eg5 moves on both microtubules that it crosslinks.

              During cell division, mitotic spindles are assembled by microtubule-based motor proteins. The bipolar organization of spindles is essential for proper segregation of chromosomes, and requires plus-end-directed homotetrameric motor proteins of the widely conserved kinesin-5 (BimC) family. Hypotheses for bipolar spindle formation include the 'push-pull mitotic muscle' model, in which kinesin-5 and opposing motor proteins act between overlapping microtubules. However, the precise roles of kinesin-5 during this process are unknown. Here we show that the vertebrate kinesin-5 Eg5 drives the sliding of microtubules depending on their relative orientation. We found in controlled in vitro assays that Eg5 has the remarkable capability of simultaneously moving at approximately 20 nm s(-1) towards the plus-ends of each of the two microtubules it crosslinks. For anti-parallel microtubules, this results in relative sliding at approximately 40 nm s(-1), comparable to spindle pole separation rates in vivo. Furthermore, we found that Eg5 can tether microtubule plus-ends, suggesting an additional microtubule-binding mode for Eg5. Our results demonstrate how members of the kinesin-5 family are likely to function in mitosis, pushing apart interpolar microtubules as well as recruiting microtubules into bundles that are subsequently polarized by relative sliding.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                Biology (Basel)
                Biology (Basel)
                biology
                Biology
                MDPI
                2079-7737
                08 December 2016
                December 2016
                : 5
                : 4
                : 51
                Affiliations
                [1 ]Department of Molecular and Cell Biology, University of California, Davis, CA 95616, USA; egcivelekogluscholey@ 123456ucdavis.edu
                [2 ]Department of Anatomy, Physiology and Cell Biology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA
                Author notes
                [* ]Correspondence: jmscholey@ 123456ucdavis.edu (J.M.S.); ibrustmascher@ 123456ucdavis.edu (I.B.-M.); Tel.: +1-530-752-8854 (I.B.-M.)
                Article
                biology-05-00051
                10.3390/biology5040051
                5192431
                27941648
                ef7a3b61-4317-434e-93d0-6383c96cac25
                © 2016 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 28 September 2016
                : 01 December 2016
                Categories
                Review

                anaphase b,mitotic motors,spindle elongation,poleward flux

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