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  2. The chirality of the mitotic spindle provides a mechanical response to forces and depends on microtubule motors and augmin

The chirality of the mitotic spindle provides a mechanical response to forces and depends on microtubule motors and augmin

  • Curr Biol. 2022 Jun 6;32(11):2480-2493.e6. doi: 10.1016/j.cub.2022.04.035.
Monika Trupinić 1 Barbara Kokanović 1 Ivana Ponjavić 1 Ivan Barišić 2 Siniša Šegvić 2 Arian Ivec 3 Iva M Tolić 4
Affiliations

Affiliations

  • 1 Division of Molecular Biology, Laboratory of Cell Biophysics, Ruđer Bošković Institute, Bijenička cesta 54, Zagreb 10000, Croatia.
  • 2 Department of Electronics, Microelectronics, Computer and Intelligent Systems, Faculty of Electrical Engineering and Computing, University of Zagreb, Unska 3, Zagreb 10000, Croatia.
  • 3 Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, Zagreb 10000, Croatia.
  • 4 Division of Molecular Biology, Laboratory of Cell Biophysics, Ruđer Bošković Institute, Bijenička cesta 54, Zagreb 10000, Croatia. Electronic address: tolic@irb.hr.
Abstract

Forces produced by motor proteins and microtubule dynamics within the mitotic spindle are crucial for proper chromosome segregation. In addition to linear forces, rotational forces or torques are present in the spindle, which are reflected in the left-handed twisted shapes of microtubule bundles that make the spindle chiral. However, the biological role and molecular origins of spindle chirality are unknown. By developing methods for measuring the spindle twist, we show that spindles are most chiral near the metaphase-to-anaphase transition. To assess the role of chirality in spindle mechanics, we compressed the spindles along their axis. This resulted in a stronger left-handed twist, suggesting that the twisted shape allows for a mechanical response to forces. Inhibition or depletion of motor proteins that perform chiral stepping, Eg5/kinesin-5, Kif18A/kinesin-8, MKLP1/Kinesin-6, and dynein, decreased the left-handed twist or led to right-handed twist, implying that these motors regulate the twist by rotating microtubules within their antiparallel overlaps or at the spindle pole. A right-handed twist was also observed after the depletion of the microtubule nucleator augmin, indicating its contribution to the twist through the nucleation of antiparallel bridging microtubules. The uncovered switch from left-handed to right-handed twist reveals the existence of competing mechanisms that promote twisting in opposite directions. As round spindles are more twisted than the elongated ones are, we infer that bending and twisting moments are generated by similar molecular mechanisms and propose a physiological role for spindle chirality in allowing the spindle to absorb mechanical load. VIDEO ABSTRACT.

Keywords

augmin; chirality; kinesins; mitosis; mitotic spindle; motor proteins; rotation; spindle compression; torques; twist.

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