Three-dimensional ultrastructural analysis of the Saccharomyces cerevisiae mitotic spindle

Structure of the inner kinetochore CCAN complex assembled onto a centromeric nucleosome

In eukaryotes, accurate chromosome segregation in mitosis and meiosis maintains genome stability and prevents aneuploidy. Kinetochores are large protein complexes that, by assembling onto specialized Cenp-A nucleosomes1,2, function to connect centromeric chromatin to microtubules of the mitotic spindle3,4. Whereas the centromeres of vertebrate chromosomes comprise millions of DNA base pairs and attach to multiple microtubules, the simple point centromeres of budding yeast are connected to individual microtubules5,6. All 16 budding yeast chromosomes assemble complete kinetochores using a single Cenp-A nucleosome (Cenp-ANuc), each of which is perfectly centred on its cognate centromere7-9. The inner and outer kinetochore modules are responsible for interacting with centromeric chromatin and microtubules, respectively. Here we describe the cryo-electron microscopy structure of the Saccharomyces cerevisiae inner kinetochore module, the constitutive centromere associated network (CCAN) complex, assembled onto a Cenp-A nucleosome (CCAN-Cenp-ANuc). The structure explains the interdependency of the constituent subcomplexes of CCAN and shows how the Y-shaped opening of CCAN accommodates Cenp-ANuc to enable specific CCAN subunits to contact the nucleosomal DNA and histone subunits. Interactions with the unwrapped DNA duplex at the two termini of Cenp-ANuc are mediated predominantly by a DNA-binding groove in the Cenp-L-Cenp-N subcomplex. Disruption of these interactions impairs assembly of CCAN onto Cenp-ANuc. Our data indicate a mechanism of Cenp-A nucleosome recognition by CCAN and how CCAN acts as a platform for assembly of the outer kinetochore to link centromeres to the mitotic spindle for chromosome segregation.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Central-spindle microtubules are strongly coupled to chromosomes during both anaphase A and anaphase B

Spindle microtubules, whose dynamics vary over time and at different locations, cooperatively drive chromosome segregation. Measurements of microtubule dynamics and spindle ultrastructure can provide insight into the behaviors of microtubules, helping elucidate the mechanism of chromosome segregation. Much work has focused on the dynamics and organization of kinetochore microtubules, that is, on the region between chromosomes and poles. In comparison, microtubules in the central-spindle region, between segregating chromosomes, have been less thoroughly characterized. Here, we report measurements of the movement of central-spindle microtubules during chromosome segregation in human mitotic spindles and Caenorhabditis elegans mitotic and female meiotic spindles. We found that these central-spindle microtubules slide apart at the same speed as chromosomes, even as chromosomes move toward spindle poles. In these systems, damaging central-spindle microtubules by laser ablation caused an immediate and complete cessation of chromosome motion, suggesting a strong coupling between central-spindle microtubules and chromosomes. Electron tomographic reconstruction revealed that the analyzed anaphase spindles all contain microtubules with both ends between segregating chromosomes. Our results provide new dynamical, functional, and ultrastructural characterizations of central-spindle microtubules during chromosome segregation in diverse spindles and suggest that central-spindle microtubules and chromosomes are strongly coupled in anaphase.

Pivoting of microtubules driven by minus-end-directed motors leads to spindle assembly

BACKGROUND

At the beginning of mitosis, the cell forms a spindle made of microtubules and associated proteins to segregate chromosomes. An important part of spindle architecture is a set of antiparallel microtubule bundles connecting the spindle poles. A key question is how microtubules extending at arbitrary angles form an antiparallel interpolar bundle.

RESULTS

Here, we show in fission yeast that microtubules meet at an oblique angle and subsequently rotate into antiparallel alignment. Our live-cell imaging approach provides a direct observation of interpolar bundle formation. By combining experiments with theory, we show that microtubules from each pole search for those from the opposite pole by performing random angular movement. Upon contact, two microtubules slide sideways along each other in a directed manner towards the antiparallel configuration. We introduce the contour length of microtubules as a measure of activity of motors that drive microtubule sliding, which we used together with observation of Cut7/kinesin-5 motors and our theory to reveal the minus-end-directed motility of this motor in vivo.

CONCLUSION

Random rotational motion helps microtubules from the opposite poles to find each other and subsequent accumulation of motors allows them to generate forces that drive interpolar bundle formation.

The stoichiometry of the outer kinetochore is modulated by microtubule-proximal regulatory factors

Dhatchinamoorthy et al. suggest the stoichiometry of outer submodules of the budding yeast kinetochore is strongly influenced by factors at the kinetochore–microtubule interface such as Fin1 and Dam1. Outer kinetochore stoichiometry is remarkably plastic and responsive to microtubule-proximal regulation.

The kinetochore is a large molecular machine that attaches chromosomes to microtubules and facilitates chromosome segregation. The kinetochore includes submodules that associate with the centromeric DNA and submodules that attach to microtubules. Additional copies of several submodules of the kinetochore are added during anaphase, including the microtubule binding module Ndc80. While the factors governing plasticity are not known, they could include regulation based on microtubule–kinetochore interactions. We report that Fin1 localizes to the microtubule-proximal edge of the kinetochore cluster during anaphase based on single-particle averaging of super-resolution images. Fin1 is required for the assembly of normal levels of Dam1 and Ndc80 submodules. Levels of Ndc80 further depend on the Dam1 microtubule binding complex. Our results suggest the stoichiometry of outer kinetochore submodules is strongly influenced by factors at the kinetochore–microtubule interface such as Fin1 and Dam1, and phosphorylation by cyclin-dependent kinase. Outer kinetochore stoichiometry is remarkably plastic and responsive to microtubule-proximal regulation.

Astral microtubule forces alter nuclear organization and inhibit DNA repair in budding yeast

Dividing cells must balance the maintenance of genome integrity with the generation of cytoskeletal forces that control chromosome position. In this study, we investigate how forces on astral microtubules impact the genome during cell division by using live-cell imaging of the cytoskeleton, chromatin, and DNA damage repair in budding yeast. Our results demonstrate that dynein-dependent forces on astral microtubules are propagated through the spindle during nuclear migration and when in excess can increase the frequency of double-stranded breaks (DSBs). Under these conditions, we find that homology-directed repair of DSBs is delayed, indicating antagonism between nuclear migration and the mechanism of homology-directed repair. These effects are partially rescued by mutants that weaken pericentric cohesion or mutants that decrease constriction on the nucleus as it moves through the bud neck. We propose that minimizing nuclear movement aids in finding a donor strand for homologous recombination.

Theory of Cytoskeletal Reorganization during Cross-Linker-Mediated Mitotic Spindle Assembly

Cells grow, move, and respond to outside stimuli by large-scale cytoskeletal reorganization. A prototypical example of cytoskeletal remodeling is mitotic spindle assembly, during which microtubules nucleate, undergo dynamic instability, bundle, and organize into a bipolar spindle. Key mechanisms of this process include regulated filament polymerization, cross-linking, and motor-protein activity. Remarkably, using passive cross-linkers, fission yeast can assemble a bipolar spindle in the absence of motor proteins. We develop a torque-balance model that describes this reorganization because of dynamic microtubule bundles, spindle-pole bodies, the nuclear envelope, and passive cross-linkers to predict spindle-assembly dynamics. We compare these results to those obtained with kinetic Monte Carlo-Brownian dynamics simulations, which include cross-linker-binding kinetics and other stochastic effects. Our results show that rapid cross-linker reorganization to microtubule overlaps facilitates cross-linker-driven spindle assembly, a testable prediction for future experiments. Combining these two modeling techniques, we illustrate a general method for studying cytoskeletal network reorganization.

Mitotic Cell Division in Caenorhabditis elegans

Mitotic cell divisions increase cell number while faithfully distributing the replicated genome at each division. The Caenorhabditis elegans embryo is a powerful model for eukaryotic cell division. Nearly all of the genes that regulate cell division in C. elegans are conserved across metazoan species, including humans. The C. elegans pathways tend to be streamlined, facilitating dissection of the more redundant human pathways. Here, we summarize the virtues of C. elegans as a model system and review our current understanding of centriole duplication, the acquisition of pericentriolar material by centrioles to form centrosomes, the assembly of kinetochores and the mitotic spindle, chromosome segregation, and cytokinesis.

Electron cryotomography analysis of Dam1C/DASH at the kinetochore-spindle interface in situ

In dividing cells, depolymerizing spindle microtubules move chromosomes by pulling at their kinetochores. While kinetochore subcomplexes have been studied extensively in vitro, little is known about their in vivo structure and interactions with microtubules or their response to spindle damage. Here we combine electron cryotomography of serial cryosections with genetic and pharmacological perturbation to study the yeast chromosome segregation machinery in vivo. Each kinetochore microtubule has one (rarely, two) Dam1C/DASH outer kinetochore assemblies. Dam1C/DASH contacts the microtubule walls and does so with its flexible "bridges"; there are no contacts with the protofilaments' curved tips. In metaphase, ∼40% of the Dam1C/DASH assemblies are complete rings; the rest are partial rings. Ring completeness and binding position along the microtubule are sensitive to kinetochore attachment and tension, respectively. Our study and those of others support a model in which each kinetochore must undergo cycles of conformational change to couple microtubule depolymerization to chromosome movement.

The mitotic spindle is chiral due to torques within microtubule bundles

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins. Forces are required for the congression of chromosomes to the metaphase plate and their separation in anaphase. However, besides forces, torques may exist in the spindle, yet they have not been investigated. Here we show that the spindle is chiral. Chirality is evident from the finding that microtubule bundles in human spindles follow a left-handed helical path, which cannot be explained by forces but rather by torques. Kinesin-5 (Kif11/Eg5) inactivation abolishes spindle chirality. Our theoretical model predicts that bending and twisting moments may generate curved shapes of bundles. We found that bundles turn by about -2 deg µm-1 around the spindle axis, which we explain by a twisting moment of roughly -10 pNµm. We conclude that torques, in addition to forces, exist in the spindle and determine its chiral architecture.

An assay for de novo kinetochore assembly reveals a key role for the CENP-T pathway in budding yeast

Chromosome segregation depends on the kinetochore, the machine that establishes force-bearing attachments between DNA and spindle microtubules. Kinetochores are formed every cell cycle via a highly regulated process that requires coordinated assembly of multiple subcomplexes on specialized chromatin. To elucidate the underlying mechanisms, we developed an assay to assemble kinetochores de novo using centromeric DNA and budding yeast extracts. Assembly is enhanced by mitotic phosphorylation of the Dsn1 kinetochore protein and generates kinetochores capable of binding microtubules. We used this assay to investigate why kinetochores recruit the microtubule-binding Ndc80 complex via two receptors: the Mis12 complex and CENP-T. Although the CENP-T pathway is non-essential in yeast, we demonstrate that it becomes essential for viability and Ndc80c recruitment when the Mis12 pathway is crippled by defects in Dsn1 phosphorylation. Assembling kinetochores de novo in yeast extracts provides a powerful and genetically tractable method to elucidate critical regulatory events in the future.

Spindle Dynamics Model Explains Chromosome Loss Rates in Yeast Polyploid Cells

Faithful chromosome segregation, driven by the mitotic spindle, is essential for organismal survival. Neopolyploid cells from diverse species exhibit a significant increase in mitotic errors relative to their diploid progenitors, resulting in chromosome nondisjunction. In the model system Saccharomyces cerevisiae, the rate of chromosome loss in haploid and diploid cells is measured to be one thousand times lower than the rate of loss in isogenic tetraploid cells. Currently it is unknown what constrains the number of chromosomes that can be segregated with high fidelity in an organism. Here we developed a simple mathematical model to study how different rates of chromosome loss in cells with different ploidy can arise from changes in (1) spindle dynamics and (2) a maximum duration of mitotic arrest, after which cells enter anaphase. We apply this model to S. cerevisiae to show that this model can explain the observed rates of chromosome loss in S. cerevisiae cells of different ploidy. Our model describes how small increases in spindle assembly time can result in dramatic differences in the rate of chromosomes loss between cells of increasing ploidy and predicts the maximum duration of mitotic arrest.

Structure of the DASH/Dam1 complex shows its role at the yeast kinetochore-microtubule interface

Kinetochores connect mitotic-spindle microtubules with chromosomes, allowing microtubule depolymerization to pull chromosomes apart during anaphase while resisting detachment as the microtubule shortens. The heterodecameric DASH/Dam1 complex (DASH/Dam1c), an essential component of yeast kinetochores, assembles into a microtubule-encircling ring. The ring associates with rodlike Ndc80 complexes to organize the kinetochore-microtubule interface. We report the cryo-electron microscopy structure (at ~4.5-angstrom resolution) of a DASH/Dam1c ring and a molecular model of its ordered components, validated by evolutionary direct-coupling analysis. Integrating this structure with that of the Ndc80 complex and with published interaction data yields a molecular picture of kinetochore-microtubule attachment, including how flexible, C-terminal extensions of DASH/Dam1c subunits project and contact widely separated sites on the Ndc80 complex rod and how phosphorylation at previously identified sites might regulate kinetochore assembly.

How Kinetochore Architecture Shapes the Mechanisms of Its Function

The eukaryotic kinetochore is a sophisticated multi-protein machine that segregates chromosomes during cell division. To ensure accurate chromosome segregation, it performs three major functions using disparate molecular mechanisms. It operates a mechanosensitive signaling cascade known as the spindle assembly checkpoint (SAC) to detect and signal the lack of attachment to spindle microtubules, and delay anaphase onset in response. In addition, after attaching to spindle microtubules, the kinetochore generates the force necessary to move chromosomes. Finally, if the two sister kinetochores on a chromosome are both attached to microtubules emanating from the same spindle pole, they activate another mechanosensitive mechanism to correct the monopolar attachments. All three of these functions maintain genome stability during cell division. The outlines of the biochemical activities responsible for these functions are now available. How the kinetochore integrates the underlying molecular mechanisms is still being elucidated. In this Review, we discuss how the nanoscale protein organization in the kinetochore, which we refer to as kinetochore 'architecture', organizes its biochemical activities to facilitate the realization and integration of emergent mechanisms underlying its three major functions. For this discussion, we will use the relatively simple budding yeast kinetochore as a model, and extrapolate insights gained from this model to elucidate functional roles of the architecture of the much more complex human kinetochore.

Assessing the Contributions of Motor Enzymes and Microtubule Dynamics to Mitotic Chromosome Motions

During my graduate work with Keith Porter, I became fascinated by the mitotic spindle, an interest that has motivated much of my scientific work ever since. I began spindle studies by using electron microscopes, instruments that have made significant contributions to our understanding of spindle organization. Such instruments have helped to elucidate the distributions of spindle microtubules, the interactions among them, their molecular polarity, and their associations with both kinetochores and spindle poles. Our lab has also investigated some processes of spindle physiology: microtubule dynamics, the actions of microtubule-associated proteins (including motor enzymes), the character of forces generated by specific spindle components, and factors that control mitotic progression. Here, I give a personal perspective on some of this intellectual history and on what recent discoveries imply about the mechanisms of chromosome motion.

Complete microtubule-kinetochore occupancy favours the segregation of merotelic attachments

Kinetochores are multi-protein complexes that power chromosome movements by tracking microtubules plus-ends in the mitotic spindle. Human kinetochores bind up to 20 microtubules, even though single microtubules can generate sufficient force to move chromosomes. Here, we show that high microtubule occupancy at kinetochores ensures robust chromosome segregation by providing a strong mechanical force that favours segregation of merotelic attachments during anaphase. Using low doses of the microtubules-targeting agent BAL27862 we reduce microtubule occupancy and observe that spindle morphology is unaffected and bi-oriented kinetochores can still oscillate with normal intra-kinetochore distances. Inter-kinetochore stretching is, however, dramatically reduced. The reduction in microtubule occupancy and inter-kinetochore stretching does not delay satisfaction of the spindle assembly checkpoint or induce microtubule detachment via Aurora-B kinase, which was so far thought to release microtubules from kinetochores under low stretching. Rather, partial microtubule occupancy slows down anaphase A and increases incidences of lagging chromosomes due to merotelically attached kinetochores.

Single microtubules (MTs) can move chromosomes, but it is unclear why kinetochores bind up to 20 MTs. Here, the authors decrease the number of kinetochore MTs with BAL27862 and see lagging chromosomes, suggesting that numerous kinetochore MTs provide force ensuring robust chromosomal segregation.

Microtubules grow by the addition of bent guanosine triphosphate tubulin to the tips of curved protofilaments

How microtubules (MTs) grow during the addition of guanosine triphosphate (GTP) tubulin is not clear. McIntosh et al. now show that MTs elongating either in vivo or in vitro end in bent protofilaments that curve out from the microtubule axis, suggesting that GTP-tubulin is bent in solution and must straighten to join the MT wall.

We used electron tomography to examine microtubules (MTs) growing from pure tubulin in vitro as well as two classes of MTs growing in cells from six species. The tips of all these growing MTs display bent protofilaments (PFs) that curve away from the MT axis, in contrast with previously reported MTs growing in vitro whose tips are either blunt or sheetlike. Neither high pressure nor freezing is responsible for the PF curvatures we see. The curvatures of PFs on growing and shortening MTs are similar; all are most curved at their tips, suggesting that guanosine triphosphate–tubulin in solution is bent and must straighten to be incorporated into the MT wall. Variations in curvature suggest that PFs are flexible in their plane of bending but rigid to bending out of that plane. Modeling by Brownian dynamics suggests that PF straightening for MT growth can be achieved by thermal motions, providing a simple mechanism with which to understand tubulin polymerization.

High-quality ultrastructural preservation using cryofixation for 3D electron microscopy of genetically labeled tissues

Electron microscopy (EM) offers unparalleled power to study cell substructures at the nanoscale. Cryofixation by high-pressure freezing offers optimal morphological preservation, as it captures cellular structures instantaneously in their near-native state. However, the applicability of cryofixation is limited by its incompatibility with diaminobenzidine labeling using genetic EM tags and the high-contrast en bloc staining required for serial block-face scanning electron microscopy (SBEM). In addition, it is challenging to perform correlated light and electron microscopy (CLEM) with cryofixed samples. Consequently, these powerful methods cannot be applied to address questions requiring optimal morphological preservation. Here, we developed an approach that overcomes these limitations; it enables genetically labeled, cryofixed samples to be characterized with SBEM and 3D CLEM. Our approach is broadly applicable, as demonstrated in cultured cells, Drosophila olfactory organ and mouse brain. This optimization exploits the potential of cryofixation, allowing for quality ultrastructural preservation for diverse EM applications.

Purification of kinetochores from the budding yeast Saccharomyces cerevisiae

Chromosome segregation relies on forces generated by spindle microtubules that are translated into chromosome movement through interactions with kinetochores, highly conserved macromolecular machines that assemble on a specialized centromeric chromatin structure. Kinetochores not only have to stably attach to growing and shrinking microtubules, but they also need to recruit spindle assembly checkpoint proteins to halt cell cycle progression when there are attachment defects. Even the simplest kinetochore in budding yeast contains more than 50 unique components that are present in multiple copies, totaling more than 250 proteins in a single kinetochore. The complex nature of kinetochores makes it challenging to elucidate the contributions of individual components to its various functions. In addition, it is difficult to manipulate forces in vivo to understand how they regulate kinetochore-microtubule attachments and the checkpoint. To address these issues, we developed a technique to purify kinetochores from budding yeast that can be used to analyze kinetochore functions and composition as well as to reconstitute kinetochore-microtubule attachments in vitro.

Bidirectional motility of kinesin-5 motor proteins: structural determinants, cumulative functions and physiological roles

Mitotic kinesin-5 bipolar motor proteins perform essential functions in mitotic spindle dynamics by crosslinking and sliding antiparallel microtubules (MTs) apart within the mitotic spindle. Two recent studies have indicated that single molecules of Cin8, the Saccharomyces cerevisiae kinesin-5 homolog, are minus end-directed when moving on single MTs, yet switch directionality under certain experimental conditions (Gerson-Gurwitz et al., EMBO J 30:4942-4954, 2011; Roostalu et al., Science 332:94-99, 2011). This finding was unexpected since the Cin8 catalytic motor domain is located at the N-terminus of the protein, and such kinesins have been previously thought to be exclusively plus end-directed. In addition, the essential intracellular functions of kinesin-5 motors in separating spindle poles during mitosis can only be accomplished by plus end-directed motility during antiparallel sliding of the spindle MTs. Thus, the mechanism and possible physiological role of the minus end-directed motility of kinesin-5 motors remain unclear. Experimental and theoretical studies from several laboratories in recent years have identified additional kinesin-5 motors that are bidirectional, revealed structural determinants that regulate directionality, examined the possible mechanisms involved and have proposed physiological roles for the minus end-directed motility of kinesin-5 motors. Here, we summarize our current understanding of the remarkable ability of certain kinesin-5 motors to switch directionality when moving along MTs.

Microtubule architecture in vitro and in cells revealed by cryo-electron tomography

Electron microscopy is a key methodology for studying microtubule structure and organization. Here, the results of cryo-electron tomography experiments on in vitro-polymerized microtubules and comparisons with microtubule ultrastructure in cells are described.

The microtubule cytoskeleton is involved in many vital cellular processes. Microtubules act as tracks for molecular motors, and their polymerization and depolymerization can be harnessed to generate force. The structures of microtubules provide key information about the mechanisms by which their cellular roles are accomplished and the physiological context in which these roles are performed. Cryo-electron microscopy allows the visualization of in vitro-polymerized microtubules and has provided important insights into their overall morphology and the influence of a range of factors on their structure and dynamics. Cryo-electron tomography can be used to determine the unique three-dimensional structure of individual microtubules and their ends. Here, a previous cryo-electron tomography study of in vitro-polymerized GMPCPP-stabilized microtubules is revisited, the findings are compared with new tomograms of dynamic in vitro and cellular microtubules, and the information that can be extracted from such data is highlighted. The analysis shows the surprising structural heterogeneity of in vitro-polymerized microtubules. Lattice defects can be observed both in vitro and in cells. The shared ultrastructural properties in these different populations emphasize the relevance of three-dimensional structures of in vitro microtubules for understanding microtubule cellular functions.

Limited Resources Induce Bistability in Microtubule Length Regulation

The availability of protein is an important factor for the determination of the size of the mitotic spindle. Involved in spindle-size regulation is kinesin-8, a molecular motor and microtubule (MT) depolymerase, which is known to tightly control MT length. Here, we propose and analyze a theoretical model in which kinesin-induced MT depolymerization competes with spontaneous polymerization while supplies of both tubulin and kinesin are limited. In contrast to previous studies where resources were unconstrained, we find that, for a wide range of concentrations, MT length regulation is bistable. We test our predictions by conducting in vitro experiments and find that the bistable behavior manifests in a bimodal MT length distribution.

Mitotic spindle: kinetochore fibers hold on tight to interpolar bundles

When a cell starts to divide, it forms a spindle, a micro-machine made of microtubules, which separates the duplicated chromosomes. The attachment of microtubules to chromosomes is mediated by kinetochores, protein complexes on the chromosome. Spindle microtubules can be divided into three major classes: kinetochore microtubules, which form k-fibers ending at the kinetochore; interpolar microtubules, which extend from the opposite sides of the spindle and interact in the middle; and astral microtubules, which extend towards the cell cortex. Recent work in human cells has shown a close relationship between interpolar and kinetochore microtubules, where interpolar bundles are attached laterally to kinetochore fibers almost all along their length, acting as a bridge between sister k-fibers. Most of the interpolar bundles are attached to a pair of sister kinetochore fibers and vice versa. Thus, the spindle is made of modules consisting of a pair of sister kinetochore fibers and a bundle of interpolar microtubules that connects them. These interpolar bundles, termed bridging fibers, balance the forces acting at kinetochores and support the rounded shape of the spindle during metaphase. This review discusses the structure, function, and formation of kinetochore fibers and interpolar bundles, with an emphasis on how they interact. Their connections have an impact on the force balance in the spindle and on chromosome movement during mitosis because the forces in interpolar bundles are transmitted to kinetochore fibers and hence to kinetochores through these connections.

Screening of FOXD3 targets in lung cancer via bioinformatics analysis

The purpose of the present study was to explore the targets of forkhead box D3 (FOXD3) in lung cancer, and thus contribute to the diagnosis and therapy of the disease. The gene expression profile of GSE64513 was downloaded from the Gene Expression Omnibus database. The dataset contained 3 FOXD3 knockout A549 lung cancer cell samples and 3 normal A549 cell samples. The differentially expressed genes (DEGs) between the FOXD3-knockout and normal A549 cells were identified using the limma package in R. The alternative splicing genes (ASGs) in FOXD3-knockout samples were identified by Replicate Multivariate Analysis of Transcript Splicing software. The Database for Annotation, Visualization and Integrated Discovery was used to identify the enriched functions and pathways of DEGs and ASGs. A protein-protein interaction (PPI) network was constructed based on results from the Search Tool for the Retrieval of Interacting Genes database and visualized using Cytoscape software. A total of 1,853 DEGs and 2,249 ASGs were identified in FOXD3-knockout A549 cells compared with normal A549 cells. The DEGs were enriched in 338 Gene Ontology (GO) terms and 21 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and the ASGs were enriched in 470 GO terms and 22 KEGG pathways. A total of 199 overlaps between the DEGs and the ASGs were identified; a PPI network constructed based on the overlapping genes contained 97 nodes and 115 pairs. FOXD3 may serve an important role in regulating the growth, migration and proliferation of tumor cells in lung cancer. The present study indicates that a number of genes, including AURKA and NOS3, may be targets of FOXD3, mediating its effect in lung cancer.

Kinesin-5 mediated chromosome congression in insect spindles

INTRODUCTION

The microtubule motor protein kinesin-5 is well known to establish the bipolar spindle by outward sliding of antiparallel interpolar microtubules. In yeast, kinesin-5 also facilitates chromosome alignment "congression" at the spindle equator by preferentially depolymerizing long kinetochore microtubules (kMTs). The motor protein kinesin-8 has also been linked to chromosome congression. Therefore, we sought to determine whether kinesin-5 or kinesin-8 facilitates chromosome congression in insect spindles.

METHODS

RNAi of the kinesin-5 Klp61F and kinesin-8 Klp67A were performed separately in Drosophila melanogaster S2 cells to test for inhibited chromosome congression. Klp61F RNAi, Klp67A RNAi, and control metaphase mitotic spindles expressing fluorescent tubulin and fluorescent Cid were imaged, and their fluorescence distributions were compared.

RESULTS

RNAi of Klp61F with a weak Klp61F knockdown resulted in longer kMTs and less congressed kinetochores compared to control over a range of conditions, consistent with kinesin-5 length-dependent depolymerase activity. RNAi of the kinesin-8 Klp67A revealed that kMTs relative to the spindle lengths were not longer compared to control, but rather that the spindles were longer, indicating that Klp67A acts preferentially as a length-dependent depolymerase on interpolar microtubules without significantly affecting kMT length and chromosome congression.

CONCLUSIONS

This study demonstrates that in addition to establishing the bipolar spindle, kinesin-5 regulates kMT length to facilitate chromosome congression in insect spindles. It expands on previous yeast studies, and it expands the role of kinesin-5 to include kMT assembly regulation in eukaryotic mitosis.

Mitotic spindles revisited – new insights from 3D electron microscopy

The mitotic spindle is a complex three-dimensional (3D) apparatus that functions to ensure the faithful segregation of chromosomes during cell division. Our current understanding of spindle architecture is mainly based on a plethora of information derived from light microscopy with rather few insights about spindle ultrastructure obtained from electron microscopy. In this Review, we will provide insights into the history of imaging of mitotic spindles and highlight recent technological advances in electron tomography and data processing, which have delivered detailed 3D reconstructions of mitotic spindles in the early embryo of the nematode Caenorhabditis elegans. Tomographic reconstructions provide novel views on spindles and will enable us to revisit and address long-standing questions in the field of mitosis.

Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Cell division in mitosis and meiosis is governed by evolutionary highly conserved protein kinases and phosphatases, controlling the timely execution of key events such as nuclear envelope breakdown, spindle assembly, chromosome attachment to the spindle and chromosome segregation, and cell cycle exit. In mitosis, the spindle assembly checkpoint (SAC) controls the proper attachment to and alignment of chromosomes on the spindle. The SAC detects errors and induces a cell cycle arrest in metaphase, preventing chromatid separation. Once all chromosomes are properly attached, the SAC-dependent arrest is relieved and chromatids separate evenly into daughter cells. The signaling cascade leading to checkpoint arrest depends on several protein kinases that are conserved from yeast to man. In meiosis, haploid cells containing new genetic combinations are generated from a diploid cell through two specialized cell divisions. Though apparently less robust, SAC control also exists in meiosis. Recently, it has emerged that SAC kinases have additional roles in executing accurate chromosome segregation during the meiotic divisions. Here, we summarize the main differences between mitotic and meiotic cell divisions, and explain why meiotic divisions pose special challenges for correct chromosome segregation. The less-known meiotic roles of the SAC kinases are described, with a focus on two model systems: yeast and mouse oocytes. The meiotic roles of the canonical checkpoint kinases Bub1, Mps1, the pseudokinase BubR1 (Mad3), and Aurora B and C (Ipl1) will be discussed. Insights into the molecular signaling pathways that bring about the special chromosome segregation pattern during meiosis will help us understand why human oocytes are so frequently aneuploid.

Purification of Fluorescently Labeled Saccharomyces cerevisiae Spindle Pole Bodies

Centrosomes are components of the mitotic spindle responsible for organizing microtubules and establishing a bipolar spindle for accurate chromosome segregation. In budding yeast, Saccharomyces cerevisiae, the centrosome is called the spindle pole body, a highly organized trilaminar structure embedded in the nuclear envelope. Here we describe a detailed protocol for the purification of fluorescently labeled spindle pole bodes from S. cerevisiae. Spindle pole bodies are purified from yeast using a TAP-tag purification followed by velocity sedimentation.This highly reproducible TAP-tag purification method improves upon previous techniques and expands the scope of in vitro characterization of yeast spindle pole bodies. The genetic flexibility of this technique allows for the study of spindle pole body mutants as well as the study of spindle pole bodies during different stages of the cell cycle. The ease and reproducibility of the technique make it possible to study spindle pole bodies using a variety of biochemical, biophysical, and microscopic techniques.

Molecular Structures of Yeast Kinetochore Subcomplexes and Their Roles in Chromosome Segregation

Kinetochore molecular architecture exemplifies "form follows function." The simplifications that generated the one-chromosome:one-microtubule linkage in point-centromere yeast have enabled strategies for systematic structural analysis and high-resolution visualization of many kinetochore components, leading to specific proposals for molecular mechanisms. We describe here some structural features that allow a kinetochore to remain attached to the end of a depolymerizing microtubule (MT) and some characteristics of the connections between substructures that permit very sensitive regulation by differential kinase activities. We emphasize in particular the importance of flexible connections between rod-like structural members and the integration of these members into a compliant cage-like assembly anchored on the MT by a sliding molecular ring.

Spatial cues and not spindle pole maturation drive the asymmetry of astral microtubules between new and preexisting spindle poles

The distinct behavior of the spindle pole bodies (SPBs) during spindle orientation in yeast metaphase does not result from them being differently mature, but astral microtubule organization correlates with the subcellular position rather than the age of the SPBs.

In many asymmetrically dividing cells, the microtubule-organizing centers (MTOCs; mammalian centrosome and yeast spindle pole body [SPB]) nucleate more astral microtubules on one of the two spindle poles than the other. This differential activity generally correlates with the age of MTOCs and contributes to orienting the mitotic spindle within the cell. The asymmetry might result from the two MTOCs being in distinctive maturation states. We investigated this model in budding yeast. Using fluorophores with different maturation kinetics to label the outer plaque components of the SPB, we found that the Cnm67 protein is mobile, whereas Spc72 is not. However, these two proteins were rapidly as abundant on both SPBs, indicating that SPBs mature more rapidly than anticipated. Superresolution microscopy confirmed this finding for Spc72 and for the γ-tubulin complex. Moreover, astral microtubule number and length correlated with the subcellular localization of SPBs rather than their age. Kar9-dependent orientation of the spindle drove the differential activity of the SPBs in astral microtubule organization rather than intrinsic differences between the spindle poles. Together, our data establish that Kar9 and spatial cues, rather than the kinetics of SPB maturation, control the asymmetry of astral microtubule organization between the preexisting and new SPBs.

Chromosome segregation occurs by microtubule pushing in oocytes

During cell division, spindle microtubules ensure an equal repartition of chromosomes between the two daughter cells. While the kinetochore-dependent mechanisms that drive mitotic chromosome segregation are well understood, in oocytes of most species atypical spindles assembled in absence of centrosomes entail poorly understood mechanisms of chromosome segregation. In particular, the structure(s) responsible for force generation during meiotic chromosome separation in oocytes is unclear. Using quantitative light microscopy, electron tomography, laser-mediated ablation, and genetic perturbations in the Caenorhabditis elegans oocyte, we studied the mechanism of chromosome segregation in meiosis. We find spindle poles are largely dispensable, and in fact act as brakes for chromosome segregation. Instead, our results suggest that CLS-2-dependent microtubules of the meiotic central spindle, located between the segregating chromosomes and aligned along the axis of segregation, are essential. Our results support a model in which inter-chromosomal microtubules of the central spindle push chromosomes apart during meiotic anaphase in oocytes.

In oocytes of most species atypical spindles assembled in the absence of centrosomes drive chromosome segregation, however the forces driving this process are unclear. Here the authors found that spindle poles are largely dispensable and that inter-chromosomal microtubules of the central spindle control chromosomal segregation.

Molecular basis for inner kinetochore configuration through RWD domain-peptide interactions

Kinetochores are dynamic cellular structures that connect chromosomes to microtubules. They form from multi‐protein assemblies that are evolutionarily conserved between yeasts and humans. One of these assemblies—COMA—consists of subunits Ame1^CENP‐U, Ctf19^CENP‐P, Mcm21^CENP‐O and Okp1^CENP‐Q. A description of COMA molecular organization has so far been missing. We defined the subunit topology of COMA, bound with inner kinetochore proteins Nkp1 and Nkp2, from the yeast Kluyveromyces lactis, with nanoflow electrospray ionization mass spectrometry, and mapped intermolecular contacts with hydrogen‐deuterium exchange coupled to mass spectrometry. Our data suggest that the essential Okp1 subunit is a multi‐segmented nexus with distinct binding sites for Ame1, Nkp1‐Nkp2 and Ctf19‐Mcm21. Our crystal structure of the Ctf19‐Mcm21 RWD domains bound with Okp1 shows the molecular contacts of this important inner kinetochore joint. The Ctf19‐Mcm21 binding motif in Okp1 configures a branch of mitotic inner kinetochores, by tethering Ctf19‐Mcm21 and Chl4^CENP‐N‐Iml3^CENP‐L. Absence of this motif results in dependence on the mitotic checkpoint for viability.

Kinetochore Function from the Bottom Up

During a single human lifetime, nearly one quintillion chromosomes separate from their sisters and transit to their destinations in daughter cells. Unlike DNA replication, chromosome segregation has no template, and, unlike transcription, errors frequently lead to a total loss of cell viability. Rapid progress in recent years has shown how kinetochores enable faithful execution of this process by connecting chromosomal DNA to microtubules. These findings have transformed our idea of kinetochores from cytological features to immense molecular machines and now allow molecular interpretation of many long-appreciated kinetochore functions. In this review we trace kinetochore protein connectivity from chromosomal DNA to microtubules, relating new findings to important points of regulation and function.

Structural plasticity of the living kinetochore

Dhatchinamoorthy et al. use calibrated imaging, FRAP, and photoconversion to study the changes in kinetochore component copy numbers from G1 to anaphase and find that the Dam1 submodule is unchanged during anaphase, whereas MIND and Ndc80 submodules add copies, providing insight into the dynamics and plasticity of the kinetochore structure during chromosome segregation.

The kinetochore is a large, evolutionarily conserved protein structure that connects chromosomes with microtubules. During chromosome segregation, outer kinetochore components track depolymerizing ends of microtubules to facilitate the separation of chromosomes into two cells. In budding yeast, each chromosome has a point centromere upon which a single kinetochore is built, which attaches to a single microtubule. This defined architecture facilitates quantitative examination of kinetochores during the cell cycle. Using three independent measures—calibrated imaging, FRAP, and photoconversion—we find that the Dam1 submodule is unchanged during anaphase, whereas MIND and Ndc80 submodules add copies to form an “anaphase configuration” kinetochore. Microtubule depolymerization and kinesin-related motors contribute to copy addition. Mathematical simulations indicate that the addition of microtubule attachments could facilitate tracking during rapid microtubule depolymerization. We speculate that the minimal kinetochore configuration, which exists from G1 through metaphase, allows for correction of misattachments. Our study provides insight into dynamics and plasticity of the kinetochore structure during chromosome segregation in living cells.

Big Lessons from Little Yeast: Budding and Fission Yeast Centrosome Structure, Duplication, and Function

Centrosomes are a functionally conserved feature of eukaryotic cells that play an important role in cell division. The conserved γ-tubulin complex organizes spindle and astral microtubules, which, in turn, separate replicated chromosomes accurately into daughter cells. Like DNA, centrosomes are duplicated once each cell cycle. Although in some cell types it is possible for cell division to occur in the absence of centrosomes, these divisions typically result in defects in chromosome number and stability. In single-celled organisms such as fungi, centrosomes [known as spindle pole bodies (SPBs)] are essential for cell division. SPBs also must be inserted into the membrane because fungi undergo a closed mitosis in which the nuclear envelope (NE) remains intact. This poorly understood process involves events similar or identical to those needed for de novo nuclear pore complex assembly. Here, we review how analysis of fungal SPBs has advanced our understanding of centrosomes and NE events.

Transcriptome analysis reveals plant response to colchicine treatment during on chromosome doubling

Colchicine was commonly used to artificially double chromosomes while the transcriptome changes in colchicine treated plants has rarely been characterized. To understand the molecular mechanism of colchicine on chromosome doubling, we characterized transcriptome data of diploid orchardgrass root after colchicine treatment. Our results showed that 3381 of differentially expressed genes (DEGs) were mainly affected by water stress, 1258 DEGs that were expressed significantly in sample DacR5tr but not in DacR5ck were considered to be mainly affected by colchicine and combination of water and colchicine. These DEGs mainly regulated by colchicine were enriched to gene ontology (GO) accessions of cation binding, catalytic activity, membrane and transporter activity, and enriched to Kyoto Encyclopedia of Genes and Genome (KEGG) pathways of phenylpropanoid biosynthesis, phenylalanine metabolism, plant hormone signal transduction and starch and sucrose metabolism. Genes related to microtubule, spindle, chromosomal kinetochore, vesicle, cellulose and processes of cytoplasm movement, chromatid segregation, membrane and cell wall development were inhibited by colchicine. Our results revealed that colchicine restrained the microtubules and inhibited gene expression of cytokinesis, which might slow down the cell activity, delay the cell into anaerobic respiration, resulting in apoptosis at late stage, and relieving of waterlogging.

Structure of centromere chromatin: from nucleosome to chromosomal architecture

The centromere is essential for the segregation of chromosomes, as it serves as attachment site for microtubules to mediate chromosome segregation during mitosis and meiosis. In most organisms, the centromere is restricted to one chromosomal region that appears as primary constriction on the condensed chromosome and is partitioned into two chromatin domains: The centromere core is characterized by the centromere-specific histone H3 variant CENP-A (also called cenH3) and is required for specifying the centromere and for building the kinetochore complex during mitosis. This core region is generally flanked by pericentric heterochromatin, characterized by nucleosomes containing H3 methylated on lysine 9 (H3K9me) that are bound by heterochromatin proteins. During mitosis, these two domains together form a three-dimensional structure that exposes CENP-A-containing chromatin to the surface for interaction with the kinetochore and microtubules. At the same time, this structure supports the tension generated during the segregation of sister chromatids to opposite poles. In this review, we discuss recent insight into the characteristics of the centromere, from the specialized chromatin structures at the centromere core and the pericentromere to the three-dimensional organization of these regions that make up the functional centromere.

Intrinsic and Extrinsic Determinants Linking Spindle Pole Fate, Spindle Polarity, and Asymmetric Cell Division in the Budding Yeast S. cerevisiae

The budding yeast S. cerevisiae is a powerful model to understand the multiple layers of control driving an asymmetric cell division. In budding yeast, asymmetric targeting of the spindle poles to the mother and bud cell compartments respectively orients the mitotic spindle along the mother-bud axis. This program exploits an intrinsic functional asymmetry arising from the age distinction between the spindle poles-one inherited from the preceding division and the other newly assembled. Extrinsic mechanisms convert this age distinction into differential fate. Execution of this program couples spindle orientation with the segregation of the older spindle pole to the bud. Remarkably, similar stereotyped patterns of inheritance occur in self-renewing stem cell divisions underscoring the general importance of studying spindle polarity and differential fate in yeast. Here, we review the mechanisms accounting for this pivotal interplay between intrinsic and extrinsic asymmetries that translate spindle pole age into differential fate.

Mechanisms mitigating problems associated with multiple kinetochores on one microtubule in early mitosis

Proper chromosome segregation in mitosis relies on correct kinetochore interaction with spindle microtubules. In early mitosis, each kinetochore usually interacts with the lateral side of each microtubule and is subsequently tethered at the microtubule end. However, since eukaryotic cells carry multiple chromosomes, multiple kinetochores could occasionally interact with a single microtubule. The consequence of this is unknown. Here, we find that, although two kinetochores (two pairs of sister kinetochores) can interact with the lateral side of one microtubule, only one kinetochore can form a sustained attachment to the microtubule end in budding yeast (Saccharomyces cerevisiae). This leads to detachment of the other kinetochore from the microtubule end (or a location in its proximity). Intriguingly, in this context, kinetochore sliding along a microtubule towards a spindle pole delays and diminishes discernible kinetochore detachment. This effect expedites collection of the entire set of kinetochores to a spindle pole. We propose that cells are equipped with the kinetochore-sliding mechanism to mitigate problems associated with multiple kinetochores on one microtubule in early mitosis.

Summary: Given that eukaryotic cells carry multiple chromosomes, multiple kinetochores could occasionally interact with a single microtubule. We identify problems associated with this situation and find mechanisms mitigating these problems.

Stochastic Modeling Yields a Mechanistic Framework for Spindle Attachment Error Correction in Budding Yeast Mitosis

Proper segregation of the replicated genome requires that kinetochores form and maintain bioriented, amphitelic attachments to microtubules from opposite spindle poles and eliminate erroneous, syntelic attachments to microtubules from the same spindle pole. Phosphorylation of kinetochore proteins destabilizes low-tension kinetochore-microtubule attachments, yet tension stabilizes bioriented attachments. This conundrum for forming high-tension amphitelic attachments is recognized as the "initiation problem of biorientation (IPBO)." A delay before kinetochore-microtubule detachment solves the IPBO, but it lacks a mechanistic framework. We developed a stochastic mathematical model for kinetochore-microtubule error correction in yeast that reveals: (1) under low chromatin tension, requiring a large number of phosphorylation events at multiple sites to achieve detachment provides the necessary delay; and (2) kinetochore-induced microtubule depolymerization generates tension in amphitelic, but not syntelic, attachments. With these requirements, the model provides a mechanistic framework for the delay before detachment to solve the IPBO and demonstrates the high degree of amphitely observed experimentally for wild-type spindles under optimal conditions.

C. elegans chromosomes connect to centrosomes by anchoring into the spindle network

The mitotic spindle ensures the faithful segregation of chromosomes. Here we combine the first large-scale serial electron tomography of whole mitotic spindles in early C. elegans embryos with live-cell imaging to reconstruct all microtubules in 3D and identify their plus- and minus-ends. We classify them as kinetochore (KMTs), spindle (SMTs) or astral microtubules (AMTs) according to their positions, and quantify distinct properties of each class. While our light microscopy and mutant studies show that microtubules are nucleated from the centrosomes, we find only a few KMTs directly connected to the centrosomes. Indeed, by quantitatively analysing several models of microtubule growth, we conclude that minus-ends of KMTs have selectively detached and depolymerized from the centrosome. In toto, our results show that the connection between centrosomes and chromosomes is mediated by an anchoring into the entire spindle network and that any direct connections through KMTs are few and likely very transient.

A connection between centrosomes and chromosomes is a key feature of mitotic spindles. Here the authors generate 3D reconstructions of whole mitotic spindles in early C. elegans embryos and show that chromosomes are anchored by the entire spindle network and that connections through kinetochore microtubules are few and likely very transient.

Direct measurement of the strength of microtubule attachment to yeast centrosomes

Laser trapping is used to manipulate single attached microtubules in vitro. Direct mechanical measurement shows that attachment of microtubule minus ends to yeast spindle pole bodies is extraordinarily strong.

Centrosomes, or spindle pole bodies (SPBs) in yeast, are vital mechanical hubs that maintain load-bearing attachments to microtubules during mitotic spindle assembly, spindle positioning, and chromosome segregation. However, the strength of microtubule-centrosome attachments is unknown, and the possibility that mechanical force might regulate centrosome function has scarcely been explored. To uncover how centrosomes sustain and regulate force, we purified SPBs from budding yeast and used laser trapping to manipulate single attached microtubules in vitro. Our experiments reveal that SPB–microtubule attachments are extraordinarily strong, rupturing at forces approximately fourfold higher than kinetochore attachments under identical loading conditions. Furthermore, removal of the calmodulin-binding site from the SPB component Spc110 weakens SPB–microtubule attachment in vitro and sensitizes cells to increased SPB stress in vivo. These observations show that calmodulin binding contributes to SPB mechanical integrity and suggest that its removal may cause pole delamination and mitotic failure when spindle forces are elevated. We propose that the very high strength of SPB–microtubule attachments may be important for spindle integrity in mitotic cells so that tensile forces generated at kinetochores do not cause microtubule detachment and delamination at SPBs.

Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles

The separation of sister chromatids during anaphase is the culmination of mitosis and one of the most strikingly beautiful examples of cellular movement. It consists of two distinct processes: Anaphase A, the movement of chromosomes toward spindle poles via shortening of the connecting fibers, and anaphase B, separation of the two poles from one another via spindle elongation. I focus here on anaphase A chromosome-to-pole movement. The chapter begins by summarizing classical observations of chromosome movements, which support the current understanding of anaphase mechanisms. Live cell fluorescence microscopy studies showed that poleward chromosome movement is associated with disassembly of the kinetochore-attached microtubule fibers that link chromosomes to poles. Microtubule-marking techniques established that kinetochore-fiber disassembly often occurs through loss of tubulin subunits from the kinetochore-attached plus ends. In addition, kinetochore-fiber disassembly in many cells occurs partly through 'flux', where the microtubules flow continuously toward the poles and tubulin subunits are lost from minus ends. Molecular mechanistic models for how load-bearing attachments are maintained to disassembling microtubule ends, and how the forces are generated to drive these disassembly-coupled movements, are discussed.

Centrochromatin of Fungi

The centromere is an essential chromosomal locus that dictates the nucleation point for assembly of the kinetochore and subsequent attachment of spindle microtubules during chromosome segregation. Research over the last decades demonstrated that centromeres are defined by a combination of genetic and epigenetic factors. Recent work showed that centromeres are quite diverse and flexible and that many types of centromere sequences and centromeric chromatin ("centrochromatin") have evolved. The kingdom of the fungi serves as an outstanding example of centromere plasticity, including organisms with centromeres as diverse as 0.15-300 kb in length, and with different types of chromatin states for most species examined thus far. Some of the species in the less familiar taxa provide excellent opportunities to help us better understand centromere biology in all eukaryotes, which may improve treatment options against fungal infection, and biotechnologies based on fungi. This review summarizes the current knowledge of fungal centromeres and centrochromatin, including an outlook for future research.

A Brief History of Research on Mitotic Mechanisms

This chapter describes in summary form some of the most important research on chromosome segregation, from the discovery and naming of mitosis in the nineteenth century until around 1990. It gives both historical and scientific background for the nine chapters that follow, each of which provides an up-to-date review of a specific aspect of mitotic mechanism. Here, we trace the fruits of each new technology that allowed a deeper understanding of mitosis and its underlying mechanisms. We describe how light microscopy, including phase, polarization, and fluorescence optics, provided descriptive information about mitotic events and also enabled important experimentation on mitotic functions, such as the dynamics of spindle fibers and the forces generated for chromosome movement. We describe studies by electron microscopy, including quantitative work with serial section reconstructions. We review early results from spindle biochemistry and genetics, coupled to molecular biology, as these methods allowed scholars to identify key molecular components of mitotic mechanisms. We also review hypotheses about mitotic mechanisms whose testing led to a deeper understanding of this fundamental biological event. Our goal is to provide modern scientists with an appreciation of the work that has laid the foundations for their current work and interests.

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.

MOZART1 and γ-tubulin complex receptors are both required to turn γ-TuSC into an active microtubule nucleation template

Cells use γ-tubulin complex to nucleate microtubules. The assembly of active microtubule nucleator is spatially and temporally regulated through the cell cycle. Lin et al. show that the protein Mzt1/MOZART1 and γ-tubulin complex receptors directly interact and act together to assemble the γ-tubulin small complex into an active microtubule nucleation template and that such interaction is conserved between Candida albicans and human cells.

MOZART1/Mzt1 is required for the localization of γ-tubulin complexes to microtubule (MT)–organizing centers from yeast to human cells. Nevertheless, the molecular function of MOZART1/Mzt1 is largely unknown. Taking advantage of the minimal MT nucleation system of Candida albicans, we reconstituted the interactions of Mzt1, γ-tubulin small complex (γ-TuSC), and γ-tubulin complex receptors (γ-TuCRs) Spc72 and Spc110 in vitro. With affinity measurements, domain deletion, and swapping, we show that Spc110 and Mzt1 bind to distinct regions of the γ-TuSC. In contrast, both Mzt1 and γ-TuSC interact with the conserved CM1 motif of Spc110/Spc72. Spc110/Spc72 and Mzt1 constitute “oligomerization chaperones,” cooperatively promoting and directing γ-TuSC oligomerization into MT nucleation-competent rings. Consistent with the functions of Mzt1, human MOZART1 directly interacts with the CM1-containing region of the γ-TuCR CEP215. MOZART1 depletion in human cells destabilizes the large γ-tubulin ring complex and abolishes CEP215^CM1-induced ectopic MT nucleation. Together, we reveal conserved functions of MOZART1/Mzt1 through interactions with γ-tubulin complex subunits and γ-TuCRs.

Analysis of Cell Cycle Progression in Saccharomyces cerevisiae Using the Budding Index and Tubulin Staining

The budding index and the morphology of the spindle and the nucleus are excellent markers for the analysis of the progression through the different stages of the cell cycle in Saccharomyces cerevisiae. Here, we describe a protocol to evaluate the budding index in this model organism using phase contrast microscopy. We also describe an indirect immunofluorescence method designed for the visualization of microtubules and the nucleus in S. cerevisiae. Finally, we explain how both methodologies can be used in order to analyze cell cycle progression in the budding yeast.