Bipolar mitotic spindles play a critical part in accurate chromosome segregation. During late mitosis, spindle microtubules undergo drastic elongation in a process called anaphase B. Two kinesin motors, Kinesin-5 and Kinesin-6, are thought to generate outward forces to drive spindle elongation, and the microtubule crosslinker Ase1/PRC1 maintains structural integrity of antiparallel microtubules. However, how these three proteins orchestrate this process remains unknown. Here we explore the functional interplay among fission yeast Kinesin-5/Cut7, Kinesin-6/Klp9 and Ase1. Using total internal reflection fluorescence microscopy, we show that Klp9 forms homotetramers and that Klp9 is a processive plus end-directed motor. klp9Δase1Δ is synthetically lethal. Surprisingly, this lethality is not ascribable to the defective motor activity of Klp9; instead, it is dependent upon a nuclear localisation signal and coiled coil domains within the non-motor region. We isolated a cut7 mutant ( cut7-122 ) that displays temperature sensitivity only in the absence of Klp9. Interestingly, cut7-122 alone is impaired in spindle elongation during anaphase B, and furthermore, cut7-122klp9Δ double mutants exhibit additive defects. We propose that Klp9 plays dual roles during anaphase B; one is motor-dependent that collaborates with Cut7 in force generation, while the other is motor-independent that ensures structural integrity of spindle microtubules together with Ase1.

The bipolar mitotic spindle drives accurate chromosome segregation by capturing the kinetochore and pulling each set of sister chromatids to the opposite poles. In this review, we describe recent findings on the multiple pathways leading to bipolar spindle formation in fission yeast and discuss these results from a broader perspective. The roles of three mitotic kinesins (Kinesin-5, Kinesin-6 and Kinesin-14) in spindle assembly are depicted, and how a group of microtubule-associated proteins, sister chromatid cohesion and the kinetochore collaborate with these motors is shown. We have paid special attention to the molecular pathways that render otherwise essential Kinesin-5 to become non-essential: how cells build bipolar mitotic spindles without the need for Kinesin-5 and where the alternate forces come from are considered. We highlight the force balance for bipolar spindle assembly and explain how outward and inward forces are generated by various ways, in which the proper fine-tuning of microtubule dynamics plays a crucial role. Overall, these new pathways have illuminated the remarkable plasticity and adaptability of spindle mechanics. Kinesin molecules are regarded as prospective targets for cancer chemotherapy and many specific inhibitors have been developed. However, several hurdles have arisen against their clinical implementation. This review provides insight into possible strategies to overcome these challenges.

Running head: Spindle elongation in anaphase BAbbreviations used: microtubule associated proteins, MAPs; nuclear localisation signal, NLS; spindle pole body, SPB; temperature sensitive, ts; total internal reflection fluorescence microscopy, TIRFM ABSTRACT Bipolar mitotic spindles play a critical part in accurate chromosome segregation. During late mitosis, spindle microtubules undergo drastic elongation towards the cell cortex in a process called anaphase B. Two kinesin motors, Kinesin-5 and Kinesin-6, are thought to generate outward forces to drive spindle elongation, and the microtubule crosslinker Ase1/PRC1 maintains structural integrity of antiparallel microtubules. However, how these three proteins orchestrate this process remains unknown. Here we explore the functional interplay among fission yeast Kinesin-5/Cut7, Kinesin-6/Klp9 and Ase1.Using total internal reflection fluorescence microscopy, we show that Klp9 is a processive plus end-directed motor. klp9Dase1D is synthetically lethal. Surprisingly, this lethality is not ascribable to the defective motor activity of Klp9; instead, it is dependent upon an NLS and coiled coil domains within the non-motor region. We isolated a cut7 mutant (cut7-122) that displays temperature sensitivity only in the absence of Klp9. Interestingly, cut7-122 is impaired specifically in late mitotic stages. cut7-122klp9D double mutant cells exhibit additive defects in spindle elongation. Together, we propose that Klp9 plays dual roles during anaphase B; one is motor-dependent that collaborates with Cut7 in force generation, while the other is motor-independent and ensures structural integrity of spindle microtubules together with Ase1.Bipolar spindle assembly is essential for proper sister chromatid segregation. Aberrations in this process result in chromosome missegregation and the emergence of aneuploid progenies, leading to miscarriages, birth defects and several human diseases, including cancer 1, 2 . The formation of bipolar spindle microtubules consists of multiple, sequential steps, in which a myriad of proteins, including motor molecules, non-motor microtubule associated proteins (MAPs) and regulatory protein-modifying enzymes, act in a coordinated spatiotemporal manner. The establishment of spindle bipolarity is in sync with the physical separation of duplicated centrosomes (the spindle pole bodies, SPBs, in fungi), which is driven by the Kienisn-5 motor (budding yeast Cin8 and Kip1, fission yeast Cut7, Aspergillus BimC, Drosophila Klp61F, Xenopus Eg5 and human Kif11) 3-9 .Accordingly, in many organisms, Kinesin-5s are essential for cell proliferation; their inactivation results in the formation of lethal monopolar spindles with unseparated centrosomes 10-12 . Kinesin-5s form homotetramers, thereby crosslinking and sliding apart antiparallel microtubules 13,14 .Interestingly, in many systems, outward forces generated by Kinesin-5s are antagonised by opposing inward forces elicited by Kinesin-14 motors; monopolar phenotypes resulting from inactivation of Kinesin-5s are re...

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SUMMARYProper nuclear positioning is essential for the execution of a wide variety of cellular processes in eukaryotic cells (Gundersen and Worman, 2013; Kopf et al., 2020; Lele et al., 2018). In proliferating mitotic cells, nuclear positioning is crucial for successful cell division. The bipolar spindle, which pulls sister chromatids towards two opposite poles, needs to assemble in the geometrical center of the cell. This ensures symmetrical positioning of the two nuclei that are reformed upon mitotic exit, by which two daughter cells inherit the identical set of the chromosomes upon cytokinesis. In fission yeast, the nucleus is positioned in the cell center during interphase; cytoplasmic microtubules interact with both the nucleus and the cell tips, thereby retaining the nucleus in the medial position of the cell (Daga et al., 2006; Tran et al., 2001). By contrast, how the nucleus is positioned during mitosis remains elusive. Here we show that several cell-cycle mutants that arrest in mitosis all displace the nucleus towards one end of the cell axis. Intriguingly, the actin cytoskeleton, not the microtubule counterpart, is responsible for the asymmetric movement of the nucleus. Time-lapse live imaging indicates that mitosis-specific F-actin cables interact with the nuclear membrane, thereby possibly generating an asymmetrical pushing force. In addition, constriction of the actomyosin ring further promotes nuclear displacement. This nuclear movement is beneficial, because if the nuclei were retained in the cell center, subsequent cell division would impose the lethal cut phenotype (Hirano et al., 1986; Yanagida, 1998), in which chromosomes are intersected by the contractile actin ring and the septum. Thus, fission yeast escapes from mitotic catastrophe by means of actin-dependent nuclear movement.