The molecular function of Ase1p: evidence for a MAP-dependent midzone-specific spindle matrix. Microtubule-associated proteins

Meiosis II spindle disassembly requires two distinct pathways

During exit from meiosis II, cells undergo several structural rearrangements, including disassembly of the meiosis II spindles and cytokinesis. Each of these changes is regulated to ensure that they occur at the proper time. Previous studies have demonstrated that both SPS1, which encodes a STE20-family GCKIII kinase, and AMA1, which encodes a meiosis-specific activator of the Anaphase Promoting Complex, are required for both meiosis II spindle disassembly and cytokinesis in the budding yeast Saccharomyces cerevisiae. We examine the relationship between meiosis II spindle disassembly and cytokinesis and find that the meiosis II spindle disassembly failure in sps1Δ and ama1∆ cells is not the cause of the cytokinesis defect. We also see that the spindle disassembly defects in sps1Δ and ama1∆ cells are phenotypically distinct. We examined known microtubule-associated proteins Ase1, Cin8, and Bim1, and found that AMA1 is required for the proper loss of Ase1 and Cin8 on meiosis II spindles while SPS1 is required for Bim1 loss in meiosis II. Taken together, these data indicate that SPS1 and AMA1 promote distinct aspects of meiosis II spindle disassembly, and that both pathways are required for the successful completion of meiosis.

Actin–Microtubule Crosstalk Imparts Stiffness to the Contractile Ring in Fission Yeast

Actin–microtubule interactions are critical for cell division, yet how these networks of polymers mutually influence their mechanical properties and functions in live cells remains unknown. In fission yeast, the post-anaphase array (PAA) of microtubules assembles in the plane of the contractile ring, and its assembly relies on the Myp2p-dependent recruitment of Mto1p, a component of equatorial microtubule organizing centers (eMTOCs). The general organization of this array of microtubules and the impact on their physical attachment to the contractile ring remain unclear. We found that Myp2p facilitates the recruitment of Mto1p to the inner face of the contractile ring, where the eMTOCs polymerize microtubules without their direct interaction. The PAA microtubules form a dynamic polygon of Ase1p crosslinked microtubules inside the contractile ring. The specific loss of PAA microtubules affects the mechanical properties of the contractile ring of actin by lowering its stiffness. This change in the mechanical properties of the ring has no measurable impact on cytokinesis or on the anchoring of the ring. Our work proposes that the PAA microtubules exploit the contractile ring for their assembly and function during cell division, while the contractile ring may receive no benefit from these interactions.

Actin-microtubule crosstalk imparts stiffness to the contractile ring in fission yeast

Actin-microtubule interactions are critical for cell division yet how these networks of polymers mutually influence their mechanical properties and functions in live cells remains unknown. In fission yeast, the post-anaphase array (PAA) of microtubules assembles in the plane of the contractile ring and its assembly relies on the Myp2p-dependent recruitment of Mto1p, a component of equatorial microtubule organizing centers (eMTOCs). The general organization of this array of microtubule and the impact on their physical attachment to the contractile ring remain unclear. We found that Myp2p facilitates the recruitment of Mto1p to the inner face of the contractile ring where the eMTOCs polymerize microtubules without their direct interaction. The PAA microtubules form a dynamic polygon of Ase1p crosslinked microtubules inside the contractile ring. The specific loss of PAA microtubules affects the mechanical properties of the contractile ring of actin by lowering its stiffness. This change in the mechanical properties of the ring has no measurable impact on cytokinesis or on the anchoring of the ring. Our work proposes that the PAA microtubules exploit the contractile ring for their assembly and function during cell division while the contractile ring may receive no benefit from these interactions.

The midbody component Prc1-like is required for microtubule reorganization during cytokinesis and dorsal determinant segregation in the early zebrafish embryo

We show that the zebrafish maternal-effect mutation too much information (tmi) corresponds to zebrafish prc1-like (prc1l), which encodes a member of the MAP65/Ase1/PRC1 family of microtubule-associated proteins. Embryos from tmi homozygous mutant mothers display cytokinesis defects in meiotic and mitotic divisions in the early embryo, indicating that Prc1l has a role in midbody formation during cell division at the egg-to-embryo transition. Unexpectedly, maternal Prc1l function is also essential for the reorganization of vegetal pole microtubules required for the segregation of dorsal determinants. Whereas Prc1 is widely regarded to crosslink microtubules in an antiparallel conformation, our studies provide evidence for an additional function of Prc1l in the bundling of parallel microtubules in the vegetal cortex of the early embryo during cortical rotation and prior to mitotic cycling. These findings highlight common yet distinct aspects of microtubule reorganization that occur during the egg-to-embryo transition, driven by maternal product for the midbody component Prc1l and required for embryonic cell division and pattern formation.

A phylogenetically-restricted essential cell cycle progression factor in the human pathogen Candida albicans

Chromosomal instability caused by cell division errors is associated with antifungal drug resistance in fungal pathogens. Here, we identify potential mechanisms underlying such instability by conducting an overexpression screen monitoring chromosomal stability in the human fungal pathogen Candida albicans. Analysis of ~1000 genes uncovers six chromosomal stability (CSA) genes, five of which are related to cell division genes of other organisms. The sixth gene, CSA6, appears to be present only in species belonging to the CUG-Ser clade, which includes C. albicans and other human fungal pathogens. The protein encoded by CSA6 localizes to the spindle pole bodies, is required for exit from mitosis, and induces a checkpoint-dependent metaphase arrest upon overexpression. Thus, Csa6 is an essential cell cycle progression factor that is restricted to the CUG-Ser fungal clade, and could therefore be explored as a potential antifungal target.

Chromosomal instability caused by cell division errors is associated with antifungal drug resistance in fungal pathogens. Here, Jaitly et al. identify several genes involved in chromosomal stability in Candida albicans, including a phylogenetically restricted gene encoding an essential cell-cycle progression factor.

Low grain weight, a new allele of BRITTLE CULM12, affects grain size through regulating GW7 expression in rice

Grain weight is a major determinant in rice yield, which is tightly associated with grain size. However, the underlying molecular mechanisms that control this trait remain unclear. Here, we report a rice (Oryza sativa) mutant, low grain weight (lgw), which shows that reduced grain length is caused by decreased cell elongation and proliferation. Map-based cloning revealed that all mutant phenotypes resulted from a nine-base pair (bp) deletion in LGW, which encodes the kinesin-like protein BRITTLE CULM12 (BC12). Protein sequence alignment analysis revealed that the mutation site was located at the nuclear localization signal (NLS) of LGW/BC12, resulting in the lgw protein not being located in the nucleus. LGW is preferentially expressed in both culms and roots, as well as in the early developing panicles. Overexpression of LGW increased the grain length, indicating that LGW is a positive regulator for regulating grain length. In addition, LGW/BC12 is directly bound to the promoter of GW7 and activates its expression. Elevating the GW7 expression levels in lgw plants rescued the small grain size phenotype. We conclude that LGW regulates grain development by directly binding to the GW7 promoter and activating its expression. Our findings revealed that LGW plays an important role in regulating grain size, and manipulation of this gene provides a new strategy for regulating grain weight in rice.

Astral microtubule cross-linking safeguards uniform nuclear distribution in the Drosophila syncytium

The early insect embryo develops as a multinucleated cell distributing the genome uniformly to the cell cortex. Mechanistic insight for nuclear positioning beyond cytoskeletal requirements is missing. Contemporary hypotheses propose actomyosin-driven cytoplasmic movement transporting nuclei or repulsion of neighbor nuclei driven by microtubule motors. Here, we show that microtubule cross-linking by Feo and Klp3A is essential for nuclear distribution and internuclear distance maintenance in Drosophila. Germline knockdown causes irregular, less-dense nuclear delivery to the cell cortex and smaller distribution in ex vivo embryo explants. A minimal internuclear distance is maintained in explants from control embryos but not from Feo-inhibited embryos, following micromanipulation-assisted repositioning. A dimerization-deficient Feo abolishes nuclear separation in embryo explants, while the full-length protein rescues the genetic knockdown. We conclude that Feo and Klp3A cross-linking of antiparallel microtubule overlap generates a length-regulated mechanical link between neighboring microtubule asters. Enabled by a novel experimental approach, our study illuminates an essential process of embryonic multicellularity.

Fission yeast Ase1PRC1 is required for the G2-microtubule damage response

Schizosaccharomyces pombe delays entry into mitosis following G₂ microtubule damage. This pathway is dependent on Rad26^ATRIP, the regulatory subunit of the Rad26^ATRIP/Rad3^ATR DNA damage response (DDR) complex. However, this G₂ microtubule damage response pathway acts independently of the G₂ DNA damage checkpoint pathway. To identify other proteins in this G₂ microtubule damage pathway, we previously screened a cDNA overexpression library for genes that rescued the sensitivity of rad26Δ cells to the microtubule poison thiabendazole. A partial cDNA fragment encoding only the C-terminal regulatory region of the microtubule bundling protein Ase1 ^PRC1 was isolated. This fragment lacks the Ase1^PRC1 dimerization and microtubule binding domains and retains the conserved C-terminal unstructured regulatory region. Here, we report that ase1Δ cells fail to delay entry into mitosis following G₂ microtubule damage. Microscopy revealed that Rad26^ATRIP foci localized alongside Ase1^PRC1 filaments, although we suggest that this is related to microtubule-dependent double strand break mobility that facilitates homologous recombination events. Indeed, we report that the DNA repair protein Rad52 co-localizes with Rad26^ATRIP at these foci, and that localization of Rad26^ATRIP to these foci depends on a Rad26^ATRIP N-terminal region containing a checkpoint recruitment domain. To our knowledge, this is the first report implicating Ase1^PRC1 in regulation of the G₂/M transition.

Slk19 enhances cross-linking of microtubules by Ase1 and Stu1

The Saccharomyces cerevisiae protein Slk19 has been shown to localize to kinetochores throughout mitosis and to the spindle midzone in anaphase. However, Slk19 clearly also has an important role for spindle formation and stabilization in prometaphase and metaphase, albeit this role is unresolved. Here we show that Slk19’s localization to metaphase spindles in vivo and to microtubules (MTs) in vitro depends on the MT cross-linking protein Ase1 and the MT cross-linking and stabilizing protein Stu1. By analyzing a slk19 mutant that specifically fails to localize to spindles and MTs, we surprisingly found that the presence of Slk19 amplified the amount of Ase1 strongly and that of Stu1 moderately at the metaphase spindle in vivo and at MTs in vitro. Furthermore, Slk19 markedly enhanced the cross-linking of MTs in vitro when added together with Ase1 or Stu1. We therefore suggest that Slk19 recruits additional Ase1 and Stu1 to the interpolar MTs (ipMTs) of metaphase spindles and thus increases their cross-linking and stabilization. This is in agreement with our observation that cells with defective Slk19 localization exhibit shorter metaphase spindles, an increased number of unaligned nuclear MTs, and most likely reduced ipMT overlaps.

Gradual compaction of the central spindle decreases its dynamicity in PRC1 and EB1 gene-edited cells

Although different anaphase proteins bind with characteristically different strength to the central spindle, the overall central spindle dynamicity slows down as mitosis proceeds.

During mitosis, the spindle undergoes morphological and dynamic changes. It reorganizes at the onset of the anaphase when the antiparallel bundler PRC1 accumulates and recruits central spindle proteins to the midzone. Little is known about how the dynamic properties of the central spindle change during its morphological changes in human cells. Using gene editing, we generated human cells that express from their endogenous locus fluorescent PRC1 and EB1 to quantify their native spindle distribution and binding/unbinding turnover. EB1 plus end tracking revealed a general slowdown of microtubule growth, whereas PRC1, similar to its yeast orthologue Ase1, binds increasingly strongly to compacting antiparallel microtubule overlaps. KIF4A and CLASP1 bind more dynamically to the central spindle, but also show slowing down turnover. These results show that the central spindle gradually becomes more stable during mitosis, in agreement with a recent “bundling, sliding, and compaction” model of antiparallel midzone bundle formation in the central spindle during late mitosis.

Differential regulation of single microtubules and bundles by a three-protein module

A remarkable feature of the microtubule cytoskeleton is co-existence of sub-populations having different dynamic properties. A prominent example is the anaphase spindle, where stable antiparallel bundles exist alongside dynamic microtubules and provide spatial cues for cytokinesis. How are dynamics of spatially proximal arrays differentially regulated? We reconstitute a minimal system of three midzone proteins: microtubule-crosslinker PRC1, and its interactors CLASP1 and Kif4A, proteins that promote and suppress microtubule elongation, respectively. We find their collective activity promotes elongation of single microtubules, while simultaneously stalling polymerization of crosslinked bundles. This differentiation arises from (i) Strong rescue activity of CLASP1, which overcomes weaker effects of Kif4A on single microtubules, (ii) Lower microtubule and PRC1-binding affinity of CLASP1, which permit dominance of Kif4A at overlaps. In addition to canonical mechanisms where antagonistic regulators set microtubule lengths, our findings illuminate design principles by which collective regulator activity creates microenvironments of arrays with distinct dynamic properties.

Protein regulator of cytokinesis 1 regulates chromosome dynamics and cytoplasmic division during mouse oocyte meiotic maturation and early embryonic development

In contrast to the homeokinesis of mitosis, asymmetric division of cytoplasm is the conspicuous feature of meiosis in mammalian oocytes. Protein regulator of cytokinesis 1 (PRC1) is an important regulator during mitotic spindle assembly and cytoplasmic division, but its functions in oocyte meiosis and early embryo development have not been fully elucidated. In this study, we detected PRC1 expression and localization and revealed a nuclear, spindle midzone-related dynamic pattern throughout meiotic and mitotic progressions. Treatment of oocytes with the reagents taxol or nocodazole disturbed the distribution of PRC1 in metaphase II oocytes. Further, PRC1 depletion led to failure of first polar body (PB1) extrusion and spindle migration, aneuploidy and defective kinetochore-microtubule attachment and spindle assembly. Overexpression of PRC1 resulted in PB1 extrusion failure, aneuploidy and serious defects of spindle assembly. To investigate PRC1 function in early embryos, we injected Prc1 morpholino into zygotes and 2-cell stage embryos. Depletion of PRC1 in zygotes impaired 4-cell, morula and blastocyst formation. Loss of PRC1 in single or double blastomeres in 2-cell stage embryos significantly impaired cell division, indicating its indispensable role in early embryo development. Co-immunoprecipitation showed that PRC1 interacts with polo-like kinase 1 (PLK1), and functional knockdown and rescue experiments demonstrated that PRC1 recruits PLK1 to the spindle midzone to regulate cytoplasmic division during meiosis. Finally, kinesin family member 4 knockdown downregulates PRC1 expression and leads to PRC1 localization failure. Taken together, our data suggest PRC1 plays an important role during oocyte maturation and early embryonic development by regulating chromosome dynamics and cytoplasmic division.

Ase1 domains dynamically slow anaphase spindle elongation and recruit Bim1 to the midzone

How cells regulate microtubule cross-linking activity to control the rate and duration of spindle elongation during anaphase is poorly understood. In this study, we test the hypothesis that PRC1/Ase1 proteins use distinct microtubule-binding domains to control the spindle elongation rate. Using the budding yeast Ase1, we identify unique contributions for the spectrin and carboxy-terminal domains during different phases of spindle elongation. We show that the spectrin domain uses conserved basic residues to promote the recruitment of Ase1 to the midzone before anaphase onset and slow spindle elongation during early anaphase. In contrast, a partial Ase1 carboxy-terminal truncation fails to form a stable midzone in late anaphase, produces higher elongation rates after early anaphase, and exhibits frequent spindle collapses. We find that the carboxy-terminal domain interacts with the plus-end tracking protein EB1/Bim1 and recruits Bim1 to the midzone to maintain midzone length. Overall, our results suggest that the Ase1 domains provide cells with a modular system to tune midzone activity and control elongation rates.

Inclusion Formation and Toxicity of the ALS Protein RGNEF and Its Association with the Microtubule Network

The Rho guanine nucleotide exchange factor (RGNEF) protein encoded by the ARHGEF28 gene has been implicated in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Biochemical and pathological studies have shown that RGNEF is a component of the hallmark neuronal cytoplasmic inclusions in ALS-affected neurons. Additionally, a heterozygous mutation in ARHGEF28 has been identified in a number of familial ALS (fALS) cases that may give rise to one of two truncated variants of the protein. Little is known about the normal biological function of RGNEF or how it contributes to ALS pathogenesis. To further explore RGNEF biology we have established and characterized a yeast model and characterized RGNEF expression in several mammalian cell lines. We demonstrate that RGNEF is toxic when overexpressed and forms inclusions. We also found that the fALS-associated mutation in ARGHEF28 gives rise to an inclusion-forming and toxic protein. Additionally, through unbiased screening using the split-ubiquitin system, we have identified RGNEF-interacting proteins, including two ALS-associated proteins. Functional characterization of other RGNEF interactors identified in our screen suggest that RGNEF functions as a microtubule regulator. Our findings indicate that RGNEF misfolding and toxicity may cause impairment of the microtubule network and contribute to ALS pathogenesis.

Saccharomyces cerevisiae polo-like kinase, Cdc5 exhibits ATP-dependent Mg2+-enhanced kinase activity in vitro

Phosphorylation of proteins on serine/threonine residues represents an important biochemical mechanism to regulate several cellular processes. Polo-like kinases (PLKs) are a family of serine-threonine kinases that play an imminent role in cell cycle regulation in yeast to humans, and thus an important therapeutic target for cancers. The present study provides insights into the enzymatic features of Saccharomyces cerevisiae PLK, Cdc5 using in vitro casein phosphorylation assays. The recombinant yeast PLK, GST-Cdc5 showed maximum casein phosphorylation activity at 30 °C, pH 9 and 45 min of incubation period. GST-Cdc5 exhibited a K_M of 1.35 μM for casein, and high affinity for ATP, since addition of non-radioactive ATP chased out casein phosphorylation by radiolabeled ATP. The recombinant enzyme showed maximum kinase activity at 2.7 μM of GST-Cdc5. Casein was found to be the best in vitro substrate of GST-Cdc5 followed by BSA (Bovine Serum Albumin) and MBP (Myelin Basic Protein). Of the metal ions tested, Mg²⁺ (at 20 mM) was found to enhance GST-Cdc5 kinase activity, while Ca²⁺ (at 5 mM) and Mn²⁺ (at 10 mM) inhibited the same. The presence of EDTA, SDS and PMSF inhibited phosphorylation by GST-Cdc5, while DTT had no effect. The recombinant GST-Cdc5 can be used as a tool for deciphering PLKs’ structure and functions, which are still at infancy.

How Essential Kinesin-5 Becomes Non-Essential in Fission Yeast: Force Balance and Microtubule Dynamics Matter

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.

Inhibition of spindle extension through the yeast S phase checkpoint is coupled to replication fork stability and the integrity of centromeric DNA

Budding yeast treated with hydroxyurea (HU) activate the S phase checkpoint kinase Rad53, which prevents DNA replication forks from undergoing aberrant structural transitions and nuclease processing. Rad53 is also required to prevent premature extension of the mitotic spindle that assembles during a HU-extended S phase. Here we present evidence that checkpoint restraint of spindle extension is directly coupled to Rad53 control of replication fork stability. In budding yeast, centromeres are flanked by replication origins that fire in early S phase. Mutations affecting the Zn²⁺-finger of Dbf4, an origin activator, preferentially reduce centromere-proximal origin firing in HU, corresponding with suppression of rad53 spindle extension. Inactivating Exo1 nuclease or displacing centromeres from origins provides a similar suppression. Conversely, short-circuiting Rad53 targeting of Dbf4, Sld3, and Dun1, substrates contributing to fork stability, induces spindle extension. These results reveal spindle extension in HU-treated rad53 mutants is a consequence of replication fork catastrophes at centromeres. When such catastrophes occur, centromeres become susceptible to nucleases, disrupting kinetochore function and spindle force balancing mechanisms. At the same time, our data indicate centromere duplication is not required to stabilize S phase spindle structure, leading us to propose a model for how monopolar kinetochore-spindle attachments may contribute to spindle force balance in HU.

Mechanisms of the Ase1/PRC1/MAP65 family in central spindle assembly

During cytokinesis, the organization of the spindle midzone and chromosome segregation is controlled by the central spindle, a microtubule cytoskeleton containing kinesin motors and non-motor microtubule-associated proteins. The anaphase spindle elongation 1/protein regulator of cytokinesis 1/microtubule associated protein 65 (Ase1/PRC1/MAP65) family of microtubule-bundling proteins are key regulators of central spindle assembly, mediating microtubule crosslinking and spindle elongation in the midzone. Ase1/PRC1/MAP65 serves as a complex regulatory platform for the recruitment of other midzone proteins at the spindle midzone. Herein, we summarize recent advances in understanding of the structural domains and molecular kinetics of the Ase1/PRC1/MAP65 family. We summarize the regulatory network involved in post-translational modifications of Ase1/PRC1 by cyclin-dependent kinase 1 (Cdk1), cell division cycle 14 (Cdc14) and Polo-like kinase 1 (Plk1) and also highlight multiple functions of Ase1/PRC1 in central spindle organization, spindle elongation and cytokinesis during cell division.

Self-Organization of Minimal Anaphase Spindle Midzone Bundles

In anaphase spindles, antiparallel microtubules associate to form tight midzone bundles, as required for functional spindle architecture and correct chromosome segregation. Several proteins selectively bind to these overlaps to control cytokinesis. How midzone bundles assemble is poorly understood. Here, using an in vitro reconstitution approach, we demonstrate that minimal midzone bundles can reliably self-organize in solution from dynamic microtubules, the microtubule crosslinker PRC1, and the motor protein KIF4A. The length of the central antiparallel overlaps in these microtubule bundles is similar to that observed in cells and is controlled by the PRC1/KIF4A ratio. Experiments and computer simulations demonstrate that minimal midzone bundle formation results from promoting antiparallel microtubule crosslinking, stopping microtubule plus-end dynamicity, and motor-driven midzone compaction and alignment. The robustness of this process suggests that a similar self-organization mechanism may contribute to the reorganization of the spindle architecture during the metaphase to anaphase transition in cells.

High-resolution imaging reveals how the spindle midzone impacts chromosome movement

Microtubule bundles in the spindle midzone have been reported to either promote or hinder chromosome movement. Pamula et al. examine the assembly dynamics of midzone microtubule bundles during anaphase and how chromosome segregation is impacted by aberrant bundle assembly.

In the spindle midzone, microtubules from opposite half-spindles form bundles between segregating chromosomes. Microtubule bundles can either push or restrict chromosome movement during anaphase in different cellular contexts, but how these activities are achieved remains poorly understood. Here, we use high-resolution live-cell imaging to analyze individual microtubule bundles, growing filaments, and chromosome movement in dividing human cells. Within bundles, filament overlap length marked by the cross-linking protein PRC1 decreases during anaphase as chromosome segregation slows. Filament ends within microtubule bundles appear capped despite dynamic PRC1 turnover and submicrometer proximity to growing microtubules. Chromosome segregation distance and rate are increased in two human cell lines when microtubule bundle assembly is prevented via PRC1 knockdown. Upon expressing a mutant PRC1 with reduced microtubule affinity, bundles assemble but chromosome hypersegregation is still observed. We propose that microtubule overlap length reduction, typically linked to pushing forces generated within filament bundles, is needed to properly restrict spindle elongation and position chromosomes within daughter cells.

A Gradient in Metaphase Tension Leads to a Scaled Cellular Response in Mitosis

During mitosis, motor proteins associate with microtubules to exert pushing forces that establish a mitotic spindle. These pushing forces generate opposing tension in the chromatin that connects oppositely attached sister chromatids, which may then act as a mechanical signal to ensure the fidelity of chromosome segregation during mitosis. However, the role of tension in mitotic cellular signaling remains controversial. In this study, we generated a gradient in tension over multiple isogenic budding yeast cell lines by genetically altering the magnitude of motor-based spindle forces. We found that a decreasing gradient in tension led to an increasing gradient in the rates of kinetochore detachment and anaphase chromosome mis-segregration, and in metaphase time. Simulations and experiments indicated that these tension responses originate from a tension-dependent kinetochore phosphorylation gradient. We conclude that the cell is exquisitely tuned to the magnitude of tension as a signal to detect potential chromosome segregation errors during mitosis.

Kinesin-6 regulates cell-size-dependent spindle elongation velocity to keep mitosis duration constant in fission yeast

The length of the mitotic spindle scales with cell size in a wide range of organisms during embryonic development. Interestingly, in C. elegans embryos, this goes along with temporal regulation: larger cells speed up spindle assembly and elongation. We demonstrate that, similarly in fission yeast, spindle length and spindle dynamics adjust to cell size, which allows to keep mitosis duration constant. Since prolongation of mitosis was shown to affect cell viability, this may resemble a mechanism to regulate mitosis duration. We further reveal how the velocity of spindle elongation is regulated: coupled to cell size, the amount of kinesin-6 Klp9 molecules increases, resulting in an acceleration of spindle elongation in anaphase B. In addition, the number of Klp9 binding sites to microtubules increases overproportionally to Klp9 molecules, suggesting that molecular crowding inversely correlates to cell size and might have an impact on spindle elongation velocity control.

Discrete regions of the kinesin-8 Kip3 tail differentially mediate astral microtubule stability and spindle disassembly

To function in diverse cellular processes, the dynamic properties of microtubules must be tightly regulated. Cellular microtubules are influenced by a multitude of regulatory proteins, but how their activities are spatiotemporally coordinated within the cell, or on specific microtubules, remains mostly obscure. The conserved kinesin-8 motor proteins are important microtubule regulators, and family members from diverse species combine directed motility with the ability to modify microtubule dynamics. Yet how kinesin-8 activities are appropriately deployed in the cellular context is largely unknown. Here we reveal the importance of the nonmotor tail in differentially controlling the physiological functions of the budding yeast kinesin-8, Kip3. We demonstrate that the tailless Kip3 motor domain adequately governs microtubule dynamics at the bud tip to allow spindle positioning in early mitosis. Notably, discrete regions of the tail mediate specific functions of Kip3 on astral and spindle microtubules. The region proximal to the motor domain operates to spatially regulate astral microtubule stability, while the distal tail serves a previously unrecognized role to control the timing of mitotic spindle disassembly. These findings provide insights into how nonmotor tail domains differentially control kinesin functions in cells and the mechanisms that spatiotemporally control the stability of cellular microtubules.

Changes in microtubule overlap length regulate kinesin-14-driven microtubule sliding

Microtubule-crosslinking motor proteins, which slide antiparallel microtubules, are required for remodeling of microtubule networks. Hitherto, all microtubule-crosslinking motors have been shown to slide microtubules at constant velocity until no overlap between the microtubules remains, leading to breakdown of the initial microtubule geometry. Here, we show in vitro that the sliding velocity of microtubules, driven by human kinesin-14, HSET, decreases when microtubules start to slide apart, resulting in the maintenance of finite-length microtubule overlaps. We quantitatively explain this feedback by the local interaction kinetics of HSET with overlapping microtubules, causing retention of HSET in shortening overlaps. Consequently, the increased HSET density in the overlaps leads to a density-dependent decrease in sliding velocity and the generation of an entropic force antagonizing the force exerted by the motors. Our results demonstrate that a spatial arrangement of microtubules can regulate the collective action of molecular motors through local alteration of their individual interaction kinetics.

Interrogation of γ-tubulin alleles using high-resolution fitness measurements reveals a distinct cytoplasmic function in spindle alignment

γ-Tubulin has a well-established role in nucleating the assembly of microtubules, yet how phosphorylation regulates its activity remains unclear. Here, we use a time-resolved, fitness-based SGA approach to compare two γ-tubulin alleles, and find that the genetic interaction profile of γtub-Y362E is enriched in spindle positioning and cell polarity genes relative to that of γtub-Y445D, which is enriched in genes involved in spindle assembly and stability. In γtub-Y362E cells, we find a defect in spindle alignment and an increase in the number of astral microtubules at both spindle poles. Our results suggest that the γtub-Y362E allele is a separation-of-function mutation that reveals a role for γ-tubulin phospho-regulation in spindle alignment. We propose that phosphorylation of the evolutionarily conserved Y362 residue of budding yeast γ-tubulin contributes to regulating the number of astral microtubules associated with spindle poles, and promoting efficient pre-anaphase spindle alignment.

Three Cdk1 sites in the kinesin-5 Cin8 catalytic domain coordinate motor localization and activity during anaphase

The bipolar kinesin-5 motors perform essential functions in mitotic spindle dynamics. We previously demonstrated that phosphorylation of at least one of the Cdk1 sites in the catalytic domain of the Saccharomyces cerevisiae kinesin-5 Cin8 (S277, T285, S493) regulates its localization to the anaphase spindle. The contribution of these three sites to phospho-regulation of Cin8, as well as the timing of such contributions, remains unknown. Here, we examined the function and spindle localization of phospho-deficient (serine/threonine to alanine) and phospho-mimic (serine/threonine to aspartic acid) Cin8 mutants. In vitro, the three Cdk1 sites undergo phosphorylation by Clb2-Cdk1. In cells, phosphorylation of Cin8 affects two aspects of its localization to the anaphase spindle, translocation from the spindle-pole bodies (SPBs) region to spindle microtubules (MTs) and the midzone, and detachment from the mitotic spindle. We found that phosphorylation of S277 is essential for the translocation of Cin8 from SPBs to spindle MTs and the subsequent detachment from the spindle. Phosphorylation of T285 mainly affects the detachment of Cin8 from spindle MTs during anaphase, while phosphorylation at S493 affects both the translocation of Cin8 from SPBs to the spindle and detachment from the spindle. Only S493 phosphorylation affected the anaphase spindle elongation rate. We conclude that each phosphorylation site plays a unique role in regulating Cin8 functions and postulate a model in which the timing and extent of phosphorylation of the three sites orchestrates the anaphase function of Cin8.

Elevated PRC1 in gastric carcinoma exerts oncogenic function and is targeted by piperlongumine in a p53-dependent manner

Gastric carcinoma is one of the most common malignancies worldwide and the second most frequent cause of cancer-related death in China. Protein regulator of cytokinesis 1 (PRC1) is involved in cytokinesis and plays key roles in microtubule organization in eukaryotes. This study was aimed to analyse the expression and to investigate the functional role of PRC1 in gastric tumorigenesis. The expression of PRC1 was evaluated by qRT-PCR, Western blot and immunohistochemistry. The biological function of PRC1 was determined by CCK-8 proliferation assays, monolayer colony formation, xenografted nude mice and cell invasion assays by shRNA-mediated knockdown in AGS and HGC27 cells. The regulation of PRC1 expression by piperlongumine was also investigated using dual-luciferase reporter assay and ChIP-qPCR analysis. PRC1 was up-regulated in primary gastric cancers. Overexpression of PRC1 in gastric cancers was associated with poor disease-specific survival and overall survival. PRC1 knockdown in AGS and HGC27 cell lines suppressed proliferation, reduced monolayer colony formation, inhibited cell invasion and migration ability and induced cell-cycle arrest and apoptosis. Inhibition of PRC1 also suppressed tumour growth in vivo. We finally confirmed that PRC1 is a novel downstream target of piperlongumine in gastric cancer. Our findings supported the oncogenic role of PRC1 in gastric carcinogenesis. PRC1 might serve as a prognostic biomarker and potential therapeutic target for gastric carcinoma.

Kinesin-5-independent mitotic spindle assembly requires the antiparallel microtubule crosslinker Ase1 in fission yeast

Bipolar spindle assembly requires a balance of forces where kinesin-5 produces outward pushing forces to antagonize the inward pulling forces from kinesin-14 or dynein. Accordingly, Kinesin-5 inactivation results in force imbalance leading to monopolar spindle and chromosome segregation failure. In fission yeast, force balance is restored when both kinesin-5 Cut7 and kinesin-14 Pkl1 are deleted, restoring spindle bipolarity. Here we show that the cut7Δpkl1Δ spindle is fully competent for chromosome segregation independently of motor activity, except for kinesin-6 Klp9, which is required for anaphase spindle elongation. We demonstrate that cut7Δpkl1Δ spindle bipolarity requires the microtubule antiparallel bundler PRC1/Ase1 to recruit CLASP/Cls1 to stabilize microtubules. Brownian dynamics-kinetic Monte Carlo simulations show that Ase1 and Cls1 activity are sufficient for initial bipolar spindle formation. We conclude that pushing forces generated by microtubule polymerization are sufficient to promote spindle pole separation and the assembly of bipolar spindle in the absence of molecular motors.

Bipolar spindle assembly requires a balance of kinesin 14 pulling and kinesin 5 pushing forces. Here, the authors show that in fission yeast, spindle formation can occur in the absence of kinesin 5 (Cut7) and 14 (Pkl1) but requires the microtubule-associated protein Ase1 for spindle bipolarity.

Microtubule-associated proteins, Bik1 and Bim1, are required for faithful partitioning of the endogenous 2 micron plasmids in budding yeast

The 2 μ plasmid of budding yeast shows high mitotic stability similar to that of chromosomes by using its self-encoded systems, namely partitioning and amplification. The partitioning system consists of the plasmid-borne proteins Rep1, Rep2 and a cis-acting locus STB that, along with several host factors, ensures efficient segregation of the plasmid. The plasmids show high stability as they presumably co-segregate with chromosomes through utilization of various host factors. To acquire these host factors, the plasmids are thought to localize to a certain sub-nuclear locale probably assisted by the motor protein, Kip1 and microtubules. Here, we show that the microtubule-associated proteins Bik1 and Bim1 are also important host factors in this process, perhaps by acting as an adapter between the plasmid and the motor and thus helping to anchor the plasmid to microtubules. Abrogation of Kip1 recruitment at STB in the absence of Bik1 argues for its function at STB upstream of Kip1. Consistent with this, both Bik1 and Bim1 associate with plasmids without any assistance from the Rep proteins. As observed earlier with other host factors, lack of Bik1 or Bim1 also causes a cohesion defect between sister plasmids leading to plasmid missegregation.

Phosphorylation of MAP65-1 by Arabidopsis Aurora Kinases Is Required for Efficient Cell Cycle Progression

Aurora kinases are key effectors of mitosis. Plant Auroras are functionally divided into two clades. The alpha Auroras (Aurora1 and Aurora2) associate with the spindle and the cell plate and are implicated in controlling formative divisions throughout plant development. The beta Aurora (Aurora3) localizes to centromeres and likely functions in chromosome separation. In contrast to the wealth of data available on the role of Aurora in other kingdoms, knowledge on their function in plants is merely emerging. This is exemplified by the fact that only histone H3 and the plant homolog of TPX2 have been identified as Aurora substrates in plants. Here we provide biochemical, genetic, and cell biological evidence that the microtubule-bundling protein MAP65-1-a member of the MAP65/Ase1/PRC1 protein family, implicated in central spindle formation and cytokinesis in animals, yeasts, and plants-is a genuine substrate of alpha Aurora kinases. MAP65-1 interacts with Aurora1 in vivo and is phosphorylated on two residues at its unfolded tail domain. Its overexpression and down-regulation antagonistically affect the alpha Aurora double mutant phenotypes. Phospho-mutant analysis shows that Aurora contributes to the microtubule bundling capacity of MAP65-1 in concert with other mitotic kinases.

Anaphase B

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.

Reconstitution of Basic Mitotic Spindles in Spherical Emulsion Droplets

Mitotic spindle assembly, positioning and orientation depend on the combined forces generated by microtubule dynamics, microtubule motor proteins and cross-linkers. Growing microtubules can generate pushing forces, while depolymerizing microtubules can convert the energy from microtubule shrinkage into pulling forces, when attached, for example, to cortical dynein or chromosomes. In addition, motor proteins and diffusible cross-linkers within the spindle contribute to spindle architecture by connecting and sliding anti-parallel microtubules. In vivo, it has proven difficult to unravel the relative contribution of individual players to the overall balance of forces. Here we present the methods that we recently developed in our efforts to reconstitute basic mitotic spindles bottom-up in vitro. Using microfluidic techniques, centrosomes and tubulin are encapsulated in water-in-oil emulsion droplets, leading to the formation of geometrically confined (double) microtubule asters. By additionally introducing cortically anchored dynein, plus-end directed microtubule motors and diffusible cross-linkers, this system is used to reconstitute spindle-like structures. The methods presented here provide a starting point for reconstitution of more complete mitotic spindles, allowing for a detailed study of the contribution of each individual component, and for obtaining an integrated quantitative view of the force-balance within the mitotic spindle.

Organization of microtubule assemblies in Dictyostelium syncytia depends on the microtubule crosslinker, Ase1

It has long been known that the interphase microtubule (MT) array is a key cellular scaffold that provides structural support and directs organelle trafficking in eukaryotic cells. Although in animal cells, a combination of centrosome nucleating properties and polymer dynamics at the distal microtubule ends is generally sufficient to establish a radial, polar array of MTs, little is known about how effector proteins (motors and crosslinkers) are coordinated to produce the diversity of interphase MT array morphologies found in nature. This diversity is particularly important in multinucleated environments where multiple MT arrays must coexist and function. We initiate here a study to address the higher ordered coordination of multiple, independent MT arrays in a common cytoplasm. Deletion of a MT crosslinker of the MAP65/Ase1/PRC1 family disrupts the spatial integrity of multiple arrays in Dictyostelium discoideum, reducing the distance between centrosomes and increasing the intermingling of MTs with opposite polarity. This result, coupled with previous dynein disruptions suggest a robust mechanism by which interphase MT arrays can utilize motors and crosslinkers to sense their position and minimize overlap in a common cytoplasm.

Factors that Control Mitotic Spindle Dynamics

Mitosis is the last phase of the cell cycle and it leads to the formation of two daughter cells with the same genetic information. This process must occurr in a very precise way and this task is essential to preserve genetic stability and to maintain cell viability. Accurate chromosome segregation during mitosis is brought about by an important cellular organelle: the mitotic spindle. This structure is made of microtubules, polymers of alpha and beta tubulin, and it is highly dynamic during the cell cycle: it emanates from two microtubules organizing centers (Spindle Pole Bodies, SPBs, in yeast) that are essential to build a short bipolar spindle, and it undergoes two steps of elongation during anaphase A and anaphase B in order to separate sister chromatids. Several proteins are involved in the control of mitotic spindle dynamics and their activity is tightly coordinated with other cell cycle events and with cell cycle progression.

MAPanalyzer: a novel online tool for analyzing microtubule-associated proteins

The wide functional impacts of microtubules are unleashed and controlled by a battery of microtubule-associated proteins (MAPs). Specialists in the field appreciate the diversity of known MAPs and propel the identifications of novel MAPs. By contrast, there is neither specific database to record known MAPs, nor MAP predictor that can facilitate the discovery of potential MAPs. We here report the establishment of a MAP-centered online analysis tool MAPanalyzer, which consists of a MAP database and a MAP predictor. In the database, a core MAP dataset, which is fully manually curated from the literature, is further enriched by MAP information collected via automated pipeline. The core dataset, on the other hand, enables the building of a novel MAP predictor which combines specialized machine learning classifiers and the BLAST homology searching tool. Benchmarks on the curated testing dataset and the Arabidopsis thaliana whole genome dataset have shown that the proposed predictor outperforms not only its own components (i.e. the machine learning classifiers and BLAST), but also another popular homology searching tool, PSI-BLAST. Therefore, MAPanalyzer will serve as a promising computational resource for the investigations of MAPs.

Database URL:http://systbio.cau.edu.cn/mappred/.

Kinetochore components are required for central spindle assembly

A critical structure poised to coordinate chromosome segregation with division plane specification is the central spindle that forms between separating chromosomes after anaphase onset. The central spindle acts as a signalling centre that concentrates proteins essential for division plane specification and contractile ring constriction. However, the molecular mechanisms that control the initial stages of central spindle assembly remain elusive. Using Caenorhabditis elegans zygotes, we found that the microtubule-bundling protein SPD-1(PRC1) and the motor ZEN-4(MKLP-1) are required for proper central spindle structure during its elongation. In contrast, we found that the kinetochore controls the initiation of central spindle assembly. Specifically, central spindle microtubule assembly is dependent on kinetochore recruitment of the scaffold protein KNL-1, as well as downstream partners BUB-1, HCP-1/2(CENP-F) and CLS-2(CLASP); and is negatively regulated by kinetochore-associated protein phosphatase 1 activity. This in turn promotes central spindle localization of CLS-2(CLASP) and initial central spindle microtubule assembly through its microtubule polymerase activity. Together, our results reveal an unexpected role for a conserved kinetochore protein network in coupling two critical events of cell division: chromosome segregation and cytokinesis.

CYK4 promotes antiparallel microtubule bundling by optimizing MKLP1 neck conformation

Centralspindlin, a constitutive 2:2 heterotetramer of MKLP1 (a kinesin-6) and the non-motor subunit CYK4, plays important roles in cytokinesis. It is crucial for the formation of central spindle microtubule bundle structure. Its accumulation at the central antiparallel overlap zone is key for recruitment and regulation of downstream cytokinesis factors and for stable anchoring of the plasma membrane at the midbody. Both MKLP1 and CYK4 are required for efficient microtubule bundling. However, the mechanism by which CYK4 contributes to this is unclear. Here we performed structural and functional analyses of centralspindlin using high-speed atomic force microscopy, Fӧrster resonance energy transfer analysis, and in vitro reconstitution. Our data reveal that CYK4 binds to a globular mass in the atypically long MKLP1 neck domain between the catalytic core and the coiled coil and thereby reconfigures the two motor domains in the MKLP1 dimer to be suitable for antiparallel microtubule bundling. Our work provides insights into the microtubule bundling during cytokinesis and into the working mechanisms of the kinesins with non-canonical neck structures.

Cell division depends on the antiparallel bundling of microtubules by a motor complex called centralpindlin. This study reveals how the centralspindlin non-motor subunit CYK4 reconfigures the motor domains of the kinesin subunit MKLP1 to help it carry out this role.

Cell division requires coordination of many different cellular components. Cytokinesis is the process by which the cytoplasm divides between the two forming daughter cells. During cytokinesis, centralspindlin is truly central, as it organizes microtubule bundle structures, recruits other factors to the site of division, and anchors the plasma membrane at the inter-cellular bridge while the two daughter cells are waiting for the final separation. Centralspindlin is a heterotetramer composed of two molecules of a kinesin-6 motor subunit, MKLP1, and two molecules of the second subunit, CYK4. For efficient microtubule bundling, both the microtubule motor subunit MKLP1 and the non-motor CYK4 subunit are required. However, it has remained unclear how CYK4 contributes to this activity. Here, we took a combinatorial approach to investigate this process, using in vitro reconstitution and structural analyses by atomic force microscopy and Förster resonance energy transfer. We revealed that the CYK4 dimer binds to a hitherto unknown globular domain at the neck of the MKLP1 dimer and optimizes the configuration of two motor domains, making them suitable for antiparallel microtubule bundling. This provides novel insight into how other kinesin superfamily molecules with non-canonical neck structures may work.

Fission yeast Scp3 potentially maintains microtubule orientation through bundling

Microtubules play important roles in organelle transport, the maintenance of cell polarity and chromosome segregation and generally form bundles during these processes. The fission yeast gene scp3+ was identified as a multicopy suppressor of the cps3-81 mutant, which is hypersensitive to isopropyl N-3-chlorophenylcarbamate (CIPC), a poison that induces abnormal multipolar spindle formation in higher eukaryotes. In this study, we investigated the function of Scp3 along with the effect of CIPC in the fission yeast Schizosaccharomyces pombe. Microscopic observation revealed that treatment with CIPC, cps3-81 mutation and scp3+ gene deletion disturbed the orientation of microtubules in interphase cells. Overexpression of scp3+ suppressed the abnormal orientation of microtubules by promoting bundling. Functional analysis suggested that Scp3 functions independently from Ase1, a protein largely required for the bundling of the mitotic spindle. A strain lacking the ase1+ gene was more sensitive to CIPC, with the drug affecting the integrity of the mitotic spindle, indicating that CIPC has a mitotic target that has a role redundant with Ase1. These results suggested that multiple systems are independently involved to ensure microtubule orientation by bundling in fission yeast.

The microtubule cross-linker Feo controls the midzone stability, motor composition, and elongation of the anaphase B spindle in Drosophila embryos

The antiparallel MT-MT cross-linking nonmotor MAP, Feo, controls the organization, stability, and motor composition of the Drosophila embryo anaphase B spindle midzone, thereby facilitating the kinesin-5-driven sliding filament mechanism underlying proper anaphase B spindle elongation and chromosome segregation.

Chromosome segregation during anaphase depends on chromosome-to-pole motility and pole-to-pole separation. We propose that in Drosophila embryos, the latter process (anaphase B) depends on a persistent kinesin-5–generated interpolar (ip) microtubule (MT) sliding filament mechanism that “engages” to push apart the spindle poles when poleward flux is turned off. Here we investigated the contribution of the midzonal, antiparallel MT-cross-linking nonmotor MAP, Feo, to this “slide-and-flux-or-elongate” mechanism. Whereas Feo homologues in other systems enhance the midzone localization of the MT-MT cross-linking motors kinesin-4, -5 and -6, the midzone localization of these motors is respectively enhanced, reduced, and unaffected by Feo. Strikingly, kinesin-5 localizes all along ipMTs of the anaphase B spindle in the presence of Feo, including at the midzone, but the antibody-induced dissociation of Feo increases kinesin-5 association with the midzone, which becomes abnormally narrow, leading to impaired anaphase B and incomplete chromosome segregation. Thus, although Feo and kinesin-5 both preferentially cross-link MTs into antiparallel polarity patterns, kinesin-5 cannot substitute for loss of Feo function. We propose that Feo controls the organization, stability, and motor composition of antiparallel ipMTs at the midzone, thereby facilitating the kinesin-5–driven sliding filament mechanism underlying proper anaphase B spindle elongation and chromosome segregation.

Determination of phosphorylation sites in microtubule associated protein MAP65-1

Reorganization of microtubules during cell cycle depends on the modulation of activity of microtubule-associated proteins. MAP65 is one of the main microtubule structural proteins in plants responsible for the formation of bundles of parallel and antiparallel microtubules. A member of MAP65 protein family, MAP65-1, binds to microtubules of preprophase band during early stages of cell division and later to the midzone of anaphase spindle and the phragmoplast, but exhibits no or reduced microtubule binding during metaphase. Artificially induced interaction of MAP65-1 with microtubules during metaphase promotes excessive formation of pole-to-pole microtubule bundles and causes delay of anaphase onset. The exact mechanism of this delay is not known, but it was suggested that microtubule bundles induced by MAP65 impose spatial constraints on the chromosome movement obstructing their alignment in the metaphase plate. Interaction of MAP65-1 with microtubules is controlled by phosphorylation. This chapter describes a strategy for the identification of phosphorylation residues responsible for the cell-cycle control of MAP65-1 activity.

Minus-end-directed Kinesin-14 motors align antiparallel microtubules to control metaphase spindle length

During cell division, a microtubule-based mitotic spindle mediates the faithful segregation of duplicated chromosomes into daughter cells. Proper length control of the metaphase mitotic spindle is critical to this process and is thought to be achieved through a mechanism in which spindle pole separation forces from plus-end-directed motors are balanced by forces from minus-end-directed motors that pull spindle poles together. However, in contrast to this model, metaphase mitotic spindles with inactive kinesin-14 minus-end-directed motors often have shorter spindle lengths, along with poorly aligned spindle microtubules. A mechanistic explanation for this paradox is unknown. Using computational modeling, in vitro reconstitution, live-cell fluorescence microscopy, and electron microscopy, we now find that the budding yeast kinesin-14 molecular motor Kar3-Cik1 can efficiently align spindle microtubules along the spindle axis. This then allows plus-end-directed kinesin-5 motors to efficiently exert the outward microtubule sliding forces needed for proper spindle bipolarity.

Mechanical design principles of a mitotic spindle

An organised spindle is crucial to the fidelity of chromosome segregation, but the relationship between spindle structure and function is not well understood in any cell type. The anaphase B spindle in fission yeast has a slender morphology and must elongate against compressive forces. This 'pushing' mode of chromosome transport renders the spindle susceptible to breakage, as observed in cells with a variety of defects. Here we perform electron tomographic analyses of the spindle, which suggest that it organises a limited supply of structural components to increase its compressive strength. Structural integrity is maintained throughout the spindle's fourfold elongation by organising microtubules into a rigid transverse array, preserving correct microtubule number and dynamically rescaling microtubule length.

Chromosome length and perinuclear attachment constrain resolution of DNA intertwines

Independent of the presence of rDNA repeats, topological constraints imposed by chromosome length and perinuclear attachment determine the efficiency with which sister chromatid intertwines are resolved by topoisomerase II and dynamic microtubules during anaphase.

To allow chromosome segregation, topoisomerase II (topo II) must resolve sister chromatid intertwines (SCI) formed during deoxynucleic acid (DNA) replication. How this process extends to the full genome is not well understood. In budding yeast, the unique structure of the ribosomal DNA (rDNA) array is thought to cause late SCI resolution of this genomic region during anaphase. In this paper, we show that chromosome length, and not the presence of rDNA repeats, is the critical feature determining the time of topo II–dependent segregation. Segregation of chromosomes lacking rDNA also requires the function of topo II in anaphase, and increasing chromosome length aggravates missegregation in topo II mutant cells. Furthermore, anaphase Stu2-dependent microtubule dynamics are critical for separation of long chromosomes. Finally, defects caused by topo II or Stu2 impairment depend on attachment of telomeres to the nuclear envelope. We propose that topological constraints imposed by chromosome length and perinuclear attachment determine the amount of SCI that topo II and dynamic microtubules resolve during anaphase.

A TOGL domain specifically targets yeast CLASP to kinetochores to stabilize kinetochore microtubules

The two different TOGL domains of the budding yeast CLASP Stu1 are responsible for its distinct mitotic activities, and these activities are only partially mediated by tight microtubule binding.

Cytoplasmic linker–associated proteins (CLASPs) are proposed to function in cell division based on their ability to bind tubulin via arrayed tumor overexpressed gene (TOG)–like (TOGL) domains. Structure predictions suggest that CLASPs have at least two TOGL domains. We show that only TOGL2 of Saccharomyces cerevisiae CLASP Stu1 binds to tubulin and is required for polymerization of spindle microtubules (MTs) in vivo. In contrast, TOGL1 recruits Stu1 to kinetochores (KTs), where it is essential for the stability and tension-dependent regulation of KT MTs. Stu1 is also recruited to spindle MTs by different mechanisms depending on the mitotic phase: in metaphase, Stu1 binds directly to the MT lattice, whereas in anaphase, it is localized indirectly to the spindle midzone. In both phases, the activity of TOGL2 is essential for interpolar MT stability, whereas TOGL1 is not involved. Thus, the two TOGL domains of yeast CLASP have different activities and execute distinct mitotic functions.

Increased Aurora B activity causes continuous disruption of kinetochore-microtubule attachments and spindle instability

Aurora B kinase regulates the proper biorientation of sister chromatids during mitosis. Lack of Aurora B kinase function results in the inability to correct erroneous kinetochore-microtubule attachments and gives rise to aneuploidy. Interestingly, increased Aurora B activity also leads to problems with chromosome segregation, and overexpression of this kinase has been observed in various types of cancer. However, little is known about the mechanisms by which an increase in Aurora B kinase activity can impair mitotic progression and cell viability. Here, using a yeast model, we demonstrate that increased Aurora B activity as a result of the overexpression of the Aurora B and inner centromere protein homologs triggers defects in chromosome segregation by promoting the continuous disruption of chromosome-microtubule attachments even when sister chromatids are correctly bioriented. This disruption leads to a constitutive activation of the spindle-assembly checkpoint, which therefore causes a lack of cytokinesis even though spindle elongation and chromosome segregation take place. Finally, we demonstrate that this increase in Aurora B activity causes premature collapse of the mitotic spindle by promoting instability of the spindle midzone.

Replicative stress induces intragenic transcription of the ASE1 gene that negatively regulates Ase1 activity

Intragenic transcripts initiate within the coding region of a gene, thereby producing shorter mRNAs and proteins. Although intragenic transcripts are widely expressed [1], their role in the functional regulation of genes remains largely unknown. In budding yeast, DNA replication stress activates the S phase checkpoint that stabilizes replication forks and arrests cells in S phase with a short spindle [2-4]. When yeast cells were treated with hydroxyurea (HU) to block DNA synthesis and induce replication stress, we found that Ase1, a conserved spindle midzone protein [5], appeared as two short protein isoforms in addition to the full-length protein. We further demonstrated that the short isoforms result from intragenic transcription of ASE1, which depends on the S phase checkpoint. Blocking generation of the short isoforms leads to a destabilized S phase spindle, characterized by increased spindle dynamics and frequent spindle collapse. Because the short Ase1 isoforms localize at the spindle in HU-treated cells and overexpression of the short Ase1 isoforms impairs the spindle midzone localization of full-length Ase1, it is likely that the presence of short Ase1 isoforms stabilizes the spindle by antagonizing full-length Ase1. Together, our results reveal intragenic transcription as a unique mechanism to downregulate gene functions in response to DNA replication stress.

Segregation of diffusible and directionally moving particles on a polar filament

Directed transport in living cells relies on the action of motor proteins. These enzymes can transform chemical energy into mechanical work and move directionally along filamentous tracks. At the same time, these filaments serve as a substrate for the binding of proteins performing other functions, but that also obstruct the motors' motion. Motivated by the mobile cross-linker Ase1, we theoretically study a system of molecular motors in the presence of diffusible particles. Both the motors and the obstacles shuttle between the filament and its surrounding. Numerical simulations of this system show a segregation between motors and obstacles if the filament ends act as diffusion barriers for the obstacles. A phenomenological mean-field theory captures the essential effects observed in the simulations.