Opposing motor activities are required for the organization of the mammalian mitotic spindle pole

From equator to pole: splitting chromosomes in mitosis and meiosis

During eukaryotic cell division, chromosomes must be precisely partitioned to daughter cells. This relies on a mechanism to move chromosomes in defined directions within the parental cell. While sister chromatids are segregated from one another in mitosis and meiosis II, specific adaptations enable the segregation of homologous chromosomes during meiosis I to reduce ploidy for gamete production. Many of the factors that drive these directed chromosome movements are known, and their molecular mechanism has started to be uncovered. Here we review the mechanisms of eukaryotic chromosome segregation, with a particular emphasis on the modifications that ensure the segregation of homologous chromosomes during meiosis I.

Asymmetric friction of nonmotor MAPs can lead to their directional motion in active microtubule networks

Diverse cellular processes require microtubules to be organized into distinct structures, such as asters or bundles. Within these dynamic motifs, microtubule-associated proteins (MAPs) are frequently under load, but how force modulates these proteins' function is poorly understood. Here, we combine optical trapping with TIRF-based microscopy to measure the force dependence of microtubule interaction for three nonmotor MAPs (NuMA, PRC1, and EB1) required for cell division. We find that frictional forces increase nonlinearly with MAP velocity across microtubules and depend on filament polarity, with NuMA's friction being lower when moving toward minus ends, EB1's lower toward plus ends, and PRC1's exhibiting no directional preference. Mathematical models predict, and experiments confirm, that MAPs with asymmetric friction can move directionally within actively moving microtubule pairs they crosslink. Our findings reveal how nonmotor MAPs can generate frictional resistance in dynamic cytoskeletal networks via micromechanical adaptations whose anisotropy may be optimized for MAP localization and function within cellular structures.

Function and regulation of dynein in mitotic chromosome segregation

Cytoplasmic dynein is a large minus-end-directed microtubule motor complex, involved in many different cellular processes including intracellular trafficking, organelle positioning, and microtubule organization. Furthermore, dynein plays essential roles during cell division where it is implicated in multiple processes including centrosome separation, chromosome movements, spindle organization, spindle positioning, and mitotic checkpoint silencing. How is a single motor able to fulfill this large array of functions and how are these activities temporally and spatially regulated? The answer lies in the unique composition of the dynein motor and in the interactions it makes with multiple regulatory proteins that define the time and place where dynein becomes active. Here, we will focus on the different mitotic processes that dynein is involved in, and how its regulatory proteins act to support dynein. Although dynein is highly conserved amongst eukaryotes (with the exception of plants), there is significant variability in the cellular processes that depend on dynein in different species. In this review, we concentrate on the functions of cytoplasmic dynein in mammals but will also refer to data obtained in other model organisms that have contributed to our understanding of dynein function in higher eukaryotes.

An improved optical tweezers assay for measuring the force generation of single kinesin molecules

Numerous microtubule-associated molecular motors, including several kinesins and cytoplasmic dynein, produce opposing forces that regulate spindle and chromosome positioning during mitosis. The motility and force generation of these motors are therefore critical to normal cell division, and dysfunction of these processes may contribute to human disease. Optical tweezers provide a powerful method for studying the nanometer motility and piconewton force generation of single motor proteins in vitro. Using kinesin-1 as a prototype, we present a set of step-by-step, optimized protocols for expressing a kinesin construct (K560-GFP) in Escherichia coli, purifying it, and studying its force generation in an optical tweezers microscope. We also provide detailed instructions on proper alignment and calibration of an optical trapping microscope. These methods provide a foundation for a variety of similar experiments.

Cell cycle-regulated membrane binding of NuMA contributes to efficient anaphase chromosome separation

The mitotic apparatus protein NuMA has an intrinsic membrane-targeting mechanism that is regulated by CDK1-mediated phosphorylation, underlies anaphase-specific cortical accumulation of dynein, and contributes to chromosome separation.

Accurate and efficient separation of sister chromatids during anaphase is critical for faithful cell division. It has been proposed that cortical dynein–generated pulling forces on astral microtubules contribute to anaphase spindle elongation and chromosome separation. In mammalian cells, however, definitive evidence for the involvement of cortical dynein in chromosome separation is missing. It is believed that dynein is recruited and anchored at the cell cortex during mitosis by the α subunit of heterotrimeric G protein (Gα)/mammalian homologue of Drosophila Partner of Inscuteable/nuclear mitotic apparatus (NuMA) ternary complex. Here we uncover a Gα/LGN-independent lipid- and membrane-binding domain at the C-terminus of NuMA. We show that the membrane binding of NuMA is cell cycle regulated—it is inhibited during prophase and metaphase by cyclin-dependent kinase 1 (CDK1)–mediated phosphorylation and only occurs after anaphase onset when CDK1 activity is down-regulated. Further studies indicate that cell cycle–regulated membrane association of NuMA underlies anaphase-specific enhancement of cortical NuMA and dynein. By replacing endogenous NuMA with membrane-binding-deficient NuMA, we can specifically reduce the cortical accumulation of NuMA and dynein during anaphase and demonstrate that cortical NuMA and dynein contribute to efficient chromosome separation in mammalian cells.

Two appendages homologous between basal bodies and centrioles are formed using distinct Odf2 domains

Analysis of Odf2 deletion mutants reveals regions important for the formation of basal body transition fibers and centriole distal appendages and distinct regions required for basal feet and subdistal appendages.

Ciliogenesis is regulated by context-dependent cellular cues, including some transduced through appendage-like structures on ciliary basal bodies called transition fibers and basal feet. However, the molecular basis for this regulation is not fully understood. The Odf2 gene product, ODF2/cenexin, is essential for both ciliogenesis and the formation of the distal and subdistal appendages on centrioles, which become basal bodies. We examined the effects of Odf2 deletion constructs on ciliogenesis in Odf2-knockout F9 cells. Electron microscopy revealed that ciliogenesis and transition fiber formation required the ODF2/cenexin fragment containing amino acids (aa) 188–806, whereas basal foot formation required aa 1–59 and 188–806. These sequences also formed distal and subdistal appendages, respectively, indicating that the centriole appendages are molecularly analogous to those on basal bodies. We used the differential formation of appendages by Odf2 deletion constructs to study the incorporation and function of molecules associated with each appendage type. We found that transition fibers and distal appendages were required for ciliogenesis and subdistal appendages stabilized the centrosomal microtubules.

Nudel/NudE and Lis1 promote dynein and dynactin interaction in the context of spindle morphogenesis

Nudel/NudE facilitates the binding of Lis1 to dynein, which subsequently enhances the recruitment of dynactin to dynein, and dynactin antagonizes Lis1 to relieve Lis1-induced dynein stall on microtubules.

Lis1, Nudel/NudE, and dynactin are regulators of cytoplasmic dynein, a minus end–directed, microtubule (MT)-based motor required for proper spindle assembly and orientation. In vitro studies have shown that dynactin promotes processive movement of dynein on MTs, whereas Lis1 causes dynein to enter a persistent force-generating state (referred to here as dynein stall). Yet how the activities of Lis1, Nudel/NudE, and dynactin are coordinated to regulate dynein remains poorly understood in vivo. Working in Xenopus egg extracts, we show that Nudel/NudE facilitates the binding of Lis1 to dynein, which enhances the recruitment of dynactin to dynein. We further report a novel Lis1-dependent dynein–dynactin interaction that is essential for the organization of mitotic spindle poles. Finally, using assays for MT gliding and spindle assembly, we demonstrate an antagonistic relationship between Lis1 and dynactin that allows dynactin to relieve Lis1-induced dynein stall on MTs. Our findings suggest the interesting possibility that Lis1 and dynactin could alternately engage with dynein to allow the motor to promote spindle assembly.

Cytoplasmic dynein crosslinks and slides anti-parallel microtubules using its two motor domains

Cytoplasmic dynein is the predominant minus-end-directed microtubule (MT) motor in most eukaryotic cells. In addition to transporting vesicular cargos, dynein helps to organize MTs within MT networks such as mitotic spindles. How dynein performs such non-canonical functions is unknown. Here we demonstrate that dynein crosslinks and slides anti-parallel MTs in vitro. Surprisingly, a minimal dimeric motor lacking a tail domain and associated subunits can cause MT sliding. Single molecule imaging reveals that motors pause and frequently reverse direction when encountering an anti-parallel MT overlap, suggesting that the two motor domains can bind both MTs simultaneously. In the mitotic spindle, inward microtubule sliding by dynein counteracts outward sliding generated by kinesin-5, and we show that a tailless, dimeric motor is sufficient to drive this activity in mammalian cells. Our results identify an unexpected mechanism for dynein-driven microtubule sliding, which differs from filament sliding mechanisms described for other motor proteins.

DOI:http://dx.doi.org/10.7554/eLife.00943.001

When cells divide, they must also divide their contents. In particular, both ‘mother’ and ‘daughter’ cells require full sets of chromosomes, which must first be duplicated, and then evenly distributed between the cells. Protein filaments called microtubules form a network that helps to accurately segregate the chromosomes. Microtubules emanate from structures at each end of the dividing cell known as spindle poles; after the chromosomes have duplicated, the microtubules latch onto them and align the pairs in the middle of the cell. As the two cells separate, microtubules at opposite spindle poles reel in one chromosome from each pair.

Microtubules are composed of alternating copies of two different types of a protein called tubulin, and have ends with distinct properties. The ‘minus’ ends are directed outwards, away from the chromosomes; the ‘plus’ ends—which can actively add tubulin—grow toward the middle of the cell, and can also bind to chromosomes. Microtubules can be manipulated by motor proteins that ‘walk’ along them carrying cargoes, which can include other microtubules. The combined actions of many motor proteins rearrange the microtubule network into a configuration that enables the chromosomes, and other cellular structures, to partition equally between the mother and daughter cells.

Motor proteins such as dynein and kinesin transport cargoes along microtubules; each motor is composed of two identical copies of the protein bound to each other. Kinesin walks toward the plus end of a microtubule, propelling itself using ‘feet’ that are called motor domains; it binds cargoes (including other microtubules) through additional regions located at the opposite end of the protein. In contrast, dynein walks toward the minus end of a microtubule. Although dynein is known to carry certain cargoes through regions outside its motor domain, how it transports other microtubules is not well understood.

Tanenbaum et al. now show that regions outside the motor domain of dynein are unnecessary to transport microtubule cargoes. When two dynein motor domains are isolated and linked to each other in vitro, each can bind to a separate microtubule. By walking toward the minus ends of their respective microtubules, the motor domains drive the microtubules in opposite directions, sliding them apart. These studies thus provide insight into the mechanism through which dynein works with additional motor proteins (such as kinesin) to rearrange microtubules during cell division—and also to ensure that chromosomes segregate evenly between mother and daughter cells.

DOI:http://dx.doi.org/10.7554/eLife.00943.002

Development of a Drosophila cell-based error correction assay

Accurate transmission of the genome through cell division requires microtubules from opposing spindle poles to interact with protein super-structures called kinetochores that assemble on each sister chromatid. Most kinetochores establish erroneous attachments that are destabilized through a process called error correction. Failure to correct improper kinetochore-microtubule (kt-MT) interactions before anaphase onset results in chromosomal instability (CIN), which has been implicated in tumorigenesis and tumor adaptation. Thus, it is important to characterize the molecular basis of error correction to better comprehend how CIN occurs and how it can be modulated. An error correction assay has been previously developed in cultured mammalian cells in which incorrect kt-MT attachments are created through the induction of monopolar spindle assembly via chemical inhibition of kinesin-5. Error correction is then monitored following inhibitor wash out. Implementing the error correction assay in Drosophila melanogaster S2 cells would be valuable because kt-MT attachments are easily visualized and the cells are highly amenable to RNAi and high-throughput screening. However, Drosophila kinesin-5 (Klp61F) is unaffected by available small molecule inhibitors. To overcome this limitation, we have rendered S2 cells susceptible to kinesin-5 inhibitors by functionally replacing Klp61F with human kinesin-5 (Eg5). Eg5 expression rescued the assembly of monopolar spindles typically caused by Klp61F depletion. Eg5-mediated bipoles collapsed into monopoles due, in part, to kinesin-14 (Ncd) activity when treated with the kinesin-5 inhibitor S-trityl-L-cysteine (STLC). Furthermore, bipolar spindles reassembled and error correction was observed after STLC wash out. Importantly, error correction in Eg5-expressing S2 cells was dependent on the well-established error correction kinase Aurora B. This system provides a powerful new cell-based platform for studying error correction and CIN.

Self-organization of stabilized microtubules by both spindle and midzone mechanisms in Xenopus egg cytosol

Pineapples, or self-organized, Taxol-stabilized microtubule assemblies, reveal the richness of self-organizing mechanisms that operate on assembled microtubules during cell division and provide a biochemically tractable system for investigating these mechanisms during meiosis and cytokinesis.

Previous study of self-organization of Taxol-stabilized microtubules into asters in Xenopus meiotic extracts revealed motor-dependent organizational mechanisms in the spindle. We revisit this approach using clarified cytosol with glycogen added back to supply energy and reducing equivalents. We added probes for NUMA and Aurora B to reveal microtubule polarity. Taxol and dimethyl sulfoxide promote rapid polymerization of microtubules that slowly self-organize into assemblies with a characteristic morphology consisting of paired lines or open circles of parallel bundles. Minus ends align in NUMA-containing foci on the outside, and plus ends in Aurora B–containing foci on the inside. Assemblies have a well-defined width that depends on initial assembly conditions, but microtubules within them have a broad length distribution. Electron microscopy shows that plus-end foci are coated with electron-dense material and resemble similar foci in monopolar midzones in cells. Functional tests show that two key spindle assembly factors, dynein and kinesin-5, act during assembly as they do in spindles, whereas two key midzone assembly factors, Aurora B and Kif4, act as they do in midzones. These data reveal the richness of self-organizing mechanisms that operate on microtubules after they polymerize in meiotic cytoplasm and provide a biochemically tractable system for investigating plus-end organization in midzones.

Building complexity: insights into self-organized assembly of microtubule-based architectures

Successful completion of diverse cellular functions, such as mitosis, positioning organelles, and assembling cilia, depends on the proper assembly of microtubule-based structures. While essentially all of the proteins needed to assemble these structures are now known, we cannot explain how even simple features such as size and shape are determined. As steps toward filling this knowledge gap, there have been several recent efforts toward reconstituting, with purified proteins, the basic structural motifs that recur in diverse cytoskeletal arrays. We discuss these studies and highlight how they shed light on the self-organized assembly of complex and dynamic cytoskeleton-based cellular structures.

New insights into the mechanism of force generation by kinesin-5 molecular motors

Kinesin-5 motors are members of a superfamily of microtubule-dependent ATPases and are widely conserved among eukaryotes. Kinesin-5s typically form homotetramers with pairs of motor domains located at either end of a dumbbell-shaped molecule. This quaternary structure enables cross-linking and ATP-driven sliding of pairs of microtubules, although the exact molecular mechanism of this activity is still unclear. Kinesin-5 function has been characterized in greatest detail in cell division, although a number of interphase roles have also been defined. The kinesin-5 ATPase is tuned for slow microtubule sliding rather than cellular transport and-in vertebrates-can be inhibited specifically by allosteric small molecules currently in cancer clinical trials. The biophysical and structural basis of kinesin-5 mechanochemistry is being elucidated and has provided further insight into kinesin-5 activities. However, it is likely that the precise mechanism of these important motors has evolved according to functional context and regulation in individual organisms.

Kinesin spindle protein SiRNA slows tumor progression

The kinesin spindle protein (KSP), a member of the kinesin superfamily of microtubule-based motors, plays a critical role in mitosis as it mediates centrosome separation and bipolar spindle assembly and maintenance. Inhibition of KSP function leads to cell cycle arrest at mitosis with the formation of monoastral microtubule arrays, and ultimately, to cell death. Several KSP inhibitors are currently being studied in clinical trials and provide new opportunities for the development of novel anticancer therapeutics. RNA interference (RNAi) may represent a powerful strategy to interfere with key molecular pathways involved in cancer. In this study, we have established an efficient method for intratumoral delivery of siRNA. We evaluated short interfering RNA (siRNA) duplexes targeting luciferase as surrogate marker or KSP sequence. To examine the potential feasibility of RNAi therapy, the siRNA was transfected into pre-established lesions by means of intratumor electro-transfer of RNA therapeutics (IERT). This technology allowed cell permeation of the nucleic acids and to efficiently knock down gene expression, albeit transiently. The KSP-specific siRNA drastically reduced outgrowth of subcutaneous melanoma and ovarian cancer lesions. Our results show that intratumoral electro-transfer of siRNA is feasible and KSP-specific siRNA may provide a novel strategy for therapeutic intervention.

Dynactin's pointed-end complex is a cargo-targeting module

Dynactin serves as an adaptor that allows the dynein motor to bind cargoes, but how dynactin associates with its diverse complement of subcellular binding partners remains mysterious. We show that the “pointed-end complex” of dynactin is a bipartite structural domain that stabilizes dynactin and supports its binding to different subcellular structures.

Dynactin is an essential part of the cytoplasmic dynein motor that enhances motor processivity and serves as an adaptor that allows dynein to bind cargoes. Much is known about dynactin's interaction with dynein and microtubules, but how it associates with its diverse complement of subcellular binding partners remains mysterious. It has been suggested that cargo specification involves a group of subunits referred to as the “pointed-end complex.” We used chemical cross-linking, RNA interference, and protein overexpression to characterize interactions within the pointed-end complex and explore how it contributes to dynactin's interactions with endomembranes. The Arp11 subunit, which caps one end of dynactin's Arp1 filament, and p62, which binds Arp11 and Arp1, are necessary for dynactin stability. These subunits also allow dynactin to bind the nuclear envelope prior to mitosis. p27 and p25, by contrast, are peripheral components that can be removed without any obvious impact on dynactin integrity. Dynactin lacking these subunits shows reduced membrane binding. Depletion of p27 and p25 results in impaired early and recycling endosome movement, but late endosome movement is unaffected, and mitotic spindles appear normal. We conclude that the pointed-end complex is a bipartite structural domain that stabilizes dynactin and supports its binding to different subcellular structures.

Microtubule redistribution in growth cones elicited by focal inactivation of kinesin-5

In order for growth cones to turn, microtubules from the central domain must preferentially invade the peripheral domain in the direction of the turn. Recent studies suggest that kinesin-5 (also called Eg5 or kif11) suppresses the invasion of microtubules into the peripheral domain on the side of the growth cone opposite the direction of turning. In theory, kinesin-5 could elicit these effects by acting on the microtubules within the peripheral domain itself, by acting on microtubules in the central domain, or in the transition zone between these two domains. In rat neurons expressing kinesin-5, we documented the presence of kinesin-5 in both domains of the growth cone and especially enriched in the transition zone. We then focally inactivated kinesin-5 in various regions of the growth cone, using micro-chromophore-assisted laser inactivation. We found that a greater invasion of microtubules into the peripheral domain occurred when kinesin-5 was inactivated specifically in the transition zone. However, there was no effect on microtubule invasion into the peripheral domain when kinesin-5 was inactivated in the peripheral domain itself or in the central domain. In other experiments, frog growth cones were observed to turn toward a gradient of a drug that inhibits kinesin-5, confirming that asymmetric inactivation of kinesin-5 can cause the growth cone to turn. Finally, expression of a phospho-mutant of kinesin-5 resulted in greater microtubule invasion throughout the peripheral domain and an inhibition of growth cone turning, implicating phosphorylation as a means by which kinesin-5 is regulated in the growth cone.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Biophysics of mitosis

Mitosis is the process by which eukaryotic cells organize and segregate their chromosomes in preparation for cell division. It is accomplished by a cellular machine composed largely of microtubules (MTs) and their associated proteins. This article reviews literature on mitosis from a biophysical point of view, drawing attention to the assembly and motility processes required to do this complex job with precision. Work from both the recent and the older literature is integrated into a description of relevant biological events and the experiments that probe their mechanisms. Theoretical work on specific subprocesses is also reviewed. Our goal is to provide a document that will expose biophysicists to the fascination of this quite amazing process and provide them with a good background from which they can pursue their own research interests in the subject.

PRC1 controls spindle polarization and recruitment of cytokinetic factors during monopolar cytokinesis

PRC1 and KIF4A are believed to play a critical role in organizing antiparallel microtubules of the central spindle. Separable and non­redundant roles for these proteins were uncovered using cells with compromised spindle bipolarity, in which cytokinesis can be induced by bypassing the spindle assembly checkpoint.

The central spindle is a postanaphase array of microtubules that plays an essential role in organizing the signaling machinery for cytokinesis. The model by which the central spindle organizes the cytokinetic apparatus is premised on an antiparallel arrangement of microtubules, yet cells lacking spindle bipolarity are capable of generating a distal domain of ectopic furrowing when forced into mitotic exit. Because protein regulator of cytokinesis (PRC1) and kinesin family member 4A (KIF4A) are believed to play a principal role in organizing the antiparallel midzone array, we sought to clarify their roles in monopolar cytokinesis. Although both factors localized to the distal ends of microtubules during monopolar cytokinesis, depletion of PRC1 and KIF4A displayed different phenotypes. Cells depleted of PRC1 failed to form a polarized microtubule array or ectopic furrows following mitotic exit, and recruitment of Aurora B kinase, male germ cell Rac GTPase-activating protein, and RhoA to the cortex was impaired. In contrast, KIF4A depletion impaired neither polarization nor ectopic furrowing, but it did result in elongated spindles with a diffuse distribution of cytokinetic factors. Thus, even in the absence of spindle bipolarity, PRC1 appears to be essential for polarizing parallel microtubules and concentrating the factors responsible for contractile ring assembly, whereas KIF4A is required for limiting the length of anaphase microtubules.

Centrioles resist forces applied on centrosomes during G2/M transition

BACKGROUND INFORMATION

Centrosome movements at the onset of mitosis result from a balance between the pulling and pushing forces mediated by microtubules. The structural stability of the centrosome core structure, the centriole pair, is correlated with a heavy polyglutamylation of centriole tubulin.

RESULTS

Using HeLa cells stably expressing centrin-green fluorescent protein as a centriole marker, we monitored the effect of microinjecting an anti-(polyglutamylated tubulin) monoclonal antibody, GT335, in G1/S or G2 cells. In contrast with the slow effect of the monoclonal antibody GT335 during interphase, a dramatic and rapid centrosome fragmentation occurred in cells microinjected in G2 that was both Eg5- and dynein-dependent. Inhibition of either one of these two motors significantly decreased the scattering of centrosome fragments, and inhibition of centrosome segregation by impairing microtubule dynamics abolished centrosome fragmentation.

CONCLUSIONS

Our results demonstrate that the compact structure of the mitotic centrosome is capable of absorbing most of the pulling and pushing forces during G2/M transition and suggest that centrosomes could act as mechanosensors integrating tensions during cell division.

Loop formation of microtubules during gliding at high density

The microtubule cytoskeleton, including the associated proteins, forms a complex network essential to multiple cellular processes. Microtubule-associated motor proteins, such as kinesin-1, travel on microtubules to transport membrane bound vesicles across the crowded cell. Other motors, such as cytoplasmic dynein and kinesin-5, are used to organize the cytoskeleton during mitosis. In order to understand the self-organization processes of motors on microtubules, we performed filament-gliding assays with kinesin-1 motors bound to the cover glass with a high density of microtubules on the surface. To observe microtubule organization, 3% of the microtubules were fluorescently labeled to serve as tracers. We find that microtubules in these assays are not confined to two dimensions and can cross one other. This causes microtubules to align locally with a relatively short correlation length. At high density, this local alignment is enough to create 'intersections' of perpendicularly oriented groups of microtubules. These intersections create vortices that cause microtubules to form loops. We characterize the radius of curvature and time duration of the loops. These different behaviors give insight into how crowded conditions, such as those in the cell, might affect motor behavior and cytoskeleton organization.

Spindle pole mechanics studied in mitotic asters: dynamic distribution of spindle forces through compliant linkages

During cell division, chromosomes must faithfully segregate to maintain genome integrity, and this dynamic mechanical process is driven by the macromolecular machinery of the mitotic spindle. However, little is known about spindle mechanics. For example, spindle microtubules are organized by numerous cross-linking proteins yet the mechanical properties of those cross-links remain unexplored. To examine the mechanical properties of microtubule cross-links we applied optical trapping to mitotic asters that form in mammalian mitotic extracts. These asters are foci of microtubules, motors, and microtubule-associated proteins that reflect many of the functional properties of spindle poles and represent centrosome-independent spindle-pole analogs. We observed bidirectional motor-driven microtubule movements, showing that microtubule linkages within asters are remarkably compliant (mean stiffness 0.025 pN/nm) and mediated by only a handful of cross-links. Depleting the motor Eg5 reduced this stiffness, indicating that Eg5 contributes to the mechanical properties of microtubule asters in a manner consistent with its localization to spindle poles in cells. We propose that compliant linkages among microtubules provide a mechanical architecture capable of accommodating microtubule movements and distributing force among microtubules without loss of pole integrity-a mechanical paradigm that may be important throughout the spindle.

Kinesin-5, a mitotic microtubule-associated motor protein, modulates neuronal migration

Kinesin-5 (also called Eg5 or kif11) is a homotetrameric motor protein that functions by modulating microtubule (MT)-MT interactions. In the case of mitosis, kinesin-5 slows the rate of separation of the half-spindles. In the case of the axon, kinesin-5 limits the frequency of transport of short MTs, and also limits the rate of axonal growth. Here we show that experimental inhibition of kinesin-5 in cultured migratory neurons results in a faster but more randomly moving neuron with a shorter leading process. As is the case with axons of stationary neurons, short MT transport frequency is notably enhanced in the leading process of the migratory neuron when kinesin-5 is inhibited. Conversely, overexpression of kinesin-5, both in culture and in developing cerebral cortex, causes migration to slow and even cease. Regions of anti-parallel MT organization behind the centrosome were shown to be especially rich in kinesin-5, implicating these regions as potential sites where kinesin-5 forces may be especially relevant. We posit that kinesin-5 acts as a "brake" on MT-MT interactions that modulates the advance of the entire MT apparatus. In so doing, kinesin-5 regulates the rate and directionality of neuronal migration and possibly the cessation of migration when the neuron reaches its destination.

Coupling between microtubule sliding, plus-end growth and spindle length revealed by kinesin-8 depletion

Mitotic spindle length control requires coordination between microtubule (MT) dynamics and motor-generated forces. To investigate how MT plus-end polymerization contributes to spindle length in Drosophila embryos, we studied the dynamics of the MT plus-end depolymerase, kinesin-8, and the effects of kinesin-8 inhibition using mutants and antibody microinjection. As expected, kinesin-8 was found to contribute to anaphase A. Furthermore, kinesin-8 depletion caused: (i) excessive polymerization of interpolar (ip) MT plus ends, which "overgrow" to penetrate distal half spindles; (ii) an increase in the poleward ipMT sliding rate that is coupled to MT plus-end polymerization; (iii) premature spindle elongation during metaphase/anaphase A; and (iv) an increase in the anaphase B spindle elongation rate which correlates linearly with the MT sliding rate. This is best explained by a revised "ipMT sliding/minus-end depolymerization" model for spindle length control which incorporates a coupling between ipMT plus end dynamics and the outward ipMT sliding that drives poleward flux and spindle elongation.

Cytoplasmic microtubule sliding: An unconventional function of conventional kinesin

There are well known examples in nature of microtubules dramatically changing their function by re-organizing their structure. Most interphase animal cells rely on the radial organization of the microtubule network for precise cargo delivery. Dividing cells re-organize microtubules with the help of motor proteins to form the spindle and drive the segregation of chromosomes into daughter cells. These examples present a kind of dichotomy: microtubules can be utilized as stationary tracks along which motor proteins move, or they can perform work themselves by utilizing the power of motor proteins. While both occur during mitosis, our recent findings demonstrate that both functions may occur simultaneously in interphase cells as well. We find that kinesin-1 (a motor known for its role in transporting cargo along microtubule tracks) powers microtubule sliding in non-dividing cells and this mechanism is used to form cellular protrusions.

Microtubule organization by the antagonistic mitotic motors kinesin-5 and kinesin-14

During cell division, different molecular motors act synergistically to rearrange microtubules. Minus end-directed motors are thought to have a dual role: focusing microtubule ends to poles and establishing together with plus end-directed motors a balance of force between antiparallel microtubules in the spindle. We study here the competing action of Xenopus laevis kinesin-14 and -5 in vitro in situations in which these motors with opposite directionality cross-link and slide microtubules. We find that full-length kinesin-14 can form microtubule asters without additional factors, whereas kinesin-5 does not, likely reflecting an adaptation to mitotic function. A stable balance of force is not established between two antiparallel microtubules with these motors. Instead, directional instability is generated, promoting efficient motor and microtubule sorting. A nonmotor microtubule cross-linker can suppress directional instability but also impedes microtubule sorting, illustrating a conflict between stability and dynamicity of organization. These results establish the basic organizational properties of these antagonistic mitotic motors and a microtubule bundler.

Spindly/CCDC99 is required for efficient chromosome congression and mitotic checkpoint regulation

Spindly recruits a fraction of cytoplasmic dynein to kinetochores for poleward movement of chromosomes and control of mitotic checkpoint signaling. Here we show that human Spindly is a cell cycle-regulated mitotic phosphoprotein that interacts with the Rod/ZW10/Zwilch (RZZ) complex. The kinetochore levels of Spindly are regulated by microtubule attachment and biorientation induced tension. Deletion mutants lacking the N-terminal half of the protein (NDelta253), or the conserved Spindly box (DeltaSB), strongly localized to kinetochores and failed to respond to attachment or tension. In addition, these mutants prevented the removal of the RZZ complex and that of MAD2 from bioriented chromosomes and caused cells to arrest at metaphase, showing that RZZ-Spindly has to be removed from kinetochores to terminate mitotic checkpoint signaling. Depletion of Spindly by RNAi, however, caused cells to arrest in prometaphase because of a delay in microtubule attachment. Surprisingly, this defect was alleviated by codepletion of ZW10. Thus, Spindly is not only required for kinetochore localization of dynein but is a functional component of a mechanism that couples dynein-dependent poleward movement of chromosomes to their efficient attachment to microtubules.

Proper organization of microtubule minus ends is needed for midzone stability and cytokinesis

Successful cytokinesis is critical for maintaining genome stability and requires the assembly of a robust central spindle to specify the cleavage furrow position, to prevent separated chromosomes from coming back together, and to contribute to midbody abscission. A proper central spindle is assembled and maintained by a number of microtubule-associated and molecular motor proteins that sort microtubules into bundles with their plus ends overlapping at the center. The mechanisms by which different factors organize the central spindle microtubules remain unclear. We found that perturbation of the minus-end-directed Kinesin-14 HSET increased the frequency of polyploid cells, which resulted from a failure in cytokinesis. In addition, HSET knockdown resulted in severe midzone microtubule organization, most notably at microtubule minus ends, as well as mislocalization of several midbody-associated proteins. Biochemical analysis showed that both human HSET and Xenopus XCTK2 cofractionated with the gamma-tubulin ring complexes on sucrose gradients and that XCTK2 associated with gamma-tubulin and Xgrip109 by immunoprecipitation. Our data reveal the novel finding that a minus-end-directed motor contributes to the organization and stability of the central spindle, which is needed for proper cytokinesis.

Patterns of kinesin evolution reveal a complex ancestral eukaryote with a multifunctional cytoskeleton

BACKGROUND

The genesis of the eukaryotes was a pivotal event in evolution and was accompanied by the acquisition of numerous new cellular features including compartmentalization by cytoplasmic organelles, mitosis and meiosis, and ciliary motility. Essential for the development of these features was the tubulin cytoskeleton and associated motors. It is therefore possible to map ancient cell evolution by reconstructing the evolutionary history of motor proteins. Here, we have used the kinesin motor repertoire of 45 extant eukaryotes to infer the ancestral state of this superfamily in the last common eukaryotic ancestor (LCEA).

RESULTS

We bioinformatically identified 1624 putative kinesin proteins, determined their protein domain architectures and calculated a comprehensive Bayesian phylogeny for the kinesin superfamily with statistical support. These data enabled us to define 51 anciently-derived kinesin paralogs (including three new kinesin families) and 105 domain architectures. We then mapped these characters across eukaryotes, accounting for secondary loss within established eukaryotic groupings, and alternative tree topologies.

CONCLUSIONS

We show that a minimum of 11 kinesin families and 3 protein domain architectures were present in the LCEA. This demonstrates that the microtubule-based cytoskeleton of the LCEA was surprisingly highly developed in terms of kinesin motor types, but that domain architectures have been extensively modified during the diversification of the eukaryotes. Our analysis provides molecular evidence for the existence of several key cellular functions in the LCEA, and shows that a large proportion of motor family diversity and cellular complexity had already arisen in this ancient cell.

Prometaphase spindle maintenance by an antagonistic motor-dependent force balance made robust by a disassembling lamin-B envelope

We tested the classical hypothesis that astral, prometaphase bipolar mitotic spindles are maintained by balanced outward and inward forces exerted on spindle poles by kinesin-5 and -14 using modeling of in vitro and in vivo data from Drosophila melanogaster embryos. Throughout prometaphase, puncta of both motors aligned on interpolar microtubules (MTs [ipMTs]), and motor perturbation changed spindle length, as predicted. Competitive motility of purified kinesin-5 and -14 was well described by a stochastic, opposing power stroke model incorporating motor kinetics and load-dependent detachment. Motor parameters from this model were applied to a new stochastic force-balance model for prometaphase spindles, providing a good fit to data from embryos. Maintenance of virtual spindles required dynamic ipMTs and a narrow range of kinesin-5 to kinesin-14 ratios matching that found in embryos. Functional perturbation and modeling suggest that this range can be extended significantly by a disassembling lamin-B envelope that surrounds the prometaphase spindle and augments the finely tuned, antagonistic kinesin force balance to maintain robust prometaphase spindles as MTs assemble and chromosomes are pushed to the equator.

Glyfoline induces mitotic catastrophe and apoptosis in cancer cells

Glyfoline exhibits cytotoxic activity in vitro and antitumor activity in mice bearing murine or human solid tumors, but the underlying mechanisms are unknown. In our study, we found that glyfoline inhibited cell growth and induced accumulation of mitotic cells in human cancer cell lines. Glyfoline induced the appearance of spindle abnormalities, chromosome mis-segregation, multipolar cell division and multiple nuclei, all of which are indicative of mitotic catastrophe. However, glyfoline did not bind to DNA and did not inhibit or stabilize tubulin polymerization, but slightly increased the resistance of mitotic spindles to nocodazole-induced disassembly. In addition, microtubule aster formation was significantly enhanced in the extract prepared from glyfoline-arrested mitotic cells compared to that from synchronized mitotic cells. When Eg5, a mitotic kinesin that plays an essential role in establishing mitotic spindle bipolarity, was inhibited using S-trityl-cysteine in glyfoline-treated cells, formation of spindle multipolarity, multipolar cell division, and multinuclei was significantly reduced. After glyfoline-mediated arrest of cells at mitosis, considerable poly(ADP-ribose) polymerase degradation was induced and the number of annexin V-positive cells significantly increased, indicating that glyfoline ultimately induces apoptosis. Small interfering RNA-mediated silencing of the spindle checkpoint proteins BUBR1 and MAD2 markedly reduced induction of mitotic cell accumulation, but did not affect glyfoline-induced mitotic catastrophe and apoptosis. Thus, glyfoline induces mitotic catastrophe probably by enhancing microtubule aster formation and subsequent apoptosis in cancer cells independently of spindle checkpoint function.

Mitotic functions of kinesin-5

In all eukaryotic cells, molecular motor proteins play essential roles in spindle assembly and function. The homotetrameric kinesin-5 motors in particular generate outward forces that establish and maintain spindle bipolarity and contribute to microtubule flux. Cell-cycle dependent phosphorylation of kinesin-5 motors regulates their localization to the mitotic spindle. Analysis of live cells further shows that kinesin-5 motors are highly dynamic in the spindle. Understanding the interactions of kinesin-5 motors with microtubules and other spindle proteins is likely to broaden the documented roles of kinesin-5 motors during cell division.

Pins homolog LGN regulates meiotic spindle organization in mouse oocytes

Mouse oocytes undergo polarization during meiotic maturation, and this polarization is essential for asymmetric cell divisions that maximize retention of maternal components required for early development. Without conventional centrosomes, the meiotic spindle has less focused poles and is barrel-shaped. The migration of meiotic spindles to the cortex is accompanied by a local reorganization and polarization of the cortex. LGN is a conserved protein involved in cell polarity and regulation of spindle organization. In the present study, we characterized the localization dynamics of LGN during mouse oocyte maturation and analyzed the effects of LGN upregulation and downregulation on meiotic spindle organization. At the germinal vesicle stage, LGN is distributed both cytoplasmically and at the cortex. During maturation, LGN localizes to the meiotic spindle apparatus and cortical LGN becomes less concentrated at the actin cap region. Excessive LGN induces meiotic spindle organization defects by elongating the spindle and enhancing pole focusing, whereas depletion of LGN by RNA interference results in meiotic spindle deformation and chromosome misalignment. Furthermore, the N-terminus of LGN has the ability of full-length LGN to regulate spindle organization, whereas the C-terminus of LGN controls cortical localization and polarization. Our results reveal that LGN is cortically polarized in mouse oocytes and is critical for meiotic spindle organization.

Compression regulates mitotic spindle length by a mechanochemical switch at the poles

BACKGROUND

Although the molecules involved in mitosis are becoming better characterized, we still lack an understanding of the emergent mechanical properties of the mitotic spindle. For example, we cannot explain how spindle length is determined. To gain insight into how forces are generated and responded to in the spindle, we developed a method to apply controlled mechanical compression to metaphase mitotic spindles in living mammalian cells while monitoring microtubules and kinetochores by fluorescence microscopy.

RESULTS

Compression caused reversible spindle widening and lengthening to a new steady state. Widening was a passive mechanical response, and lengthening was an active mechanochemical process requiring microtubule polymerization but not kinesin-5 activity. Spindle morphology during lengthening and drug perturbations suggested that kinetochore fibers are pushed outward by pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurred by inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochore stretching, or kinetochore dynamics.

CONCLUSIONS

We propose that spindle length is controlled by a mechanochemical switch at the poles that regulates the depolymerization rate of kinetochore fibers in response to compression and discuss models for how this switch is controlled. Poleward force appears to be exerted along kinetochore fibers by some mechanism other than kinesin-5 activity, and we speculate that it may arise from polymerization pressure from growing plus ends of interpolar microtubules whose minus ends are anchored in the fiber. These insights provide a framework for conceptualizing mechanical integration within the spindle.

A shift from kinesin 5-dependent metaphase spindle function during preimplantation development in mouse

Microtubules within meiotic and mitotic spindles continually move towards spindle poles in a process termed poleward flux, which is essential for spindle integrity and faithful chromosome segregation. Kinesin 5 is a longstanding candidate for a molecular motor that might drive poleward flux, and has been shown to drive flux and to be necessary for spindle bipolarity in Xenopus egg extracts. However, kinesin 5 is not necessary for poleward flux or for maintaining metaphase spindle bipolarity in intact mammalian cells, and the reason for the different results in these systems is unknown. The experiments presented here test the hypothesis that these results might reflect developmental differences in spindle function by examining the role of kinesin 5 in mouse eggs and preimplantation embryos. In contrast to cultured somatic cells, poleward flux in mouse eggs is critically dependent upon kinesin 5. Inhibition of poleward flux leads to spindle shortening as a result of continued microtubule depolymerisation at the pole, and eventual loss of spindle bipolarity. Spindle bipolarity is also dependent upon kinesin 5 during the first three embryonic cleavages, but becomes kinesin 5-independent in the majority of spindles by the blastocyst stage. This switch occurs asynchronously in different blastomeres but is independent of clonal cell heritage and of whether the blastomere is within the inner cell mass or the trophoectoderm. These experiments reveal a novel developmental switch in the requirements for spindle function and chromosome segregation during preimplantation development.

NuMA is required for proper spindle assembly and chromosome alignment in prometaphase

BACKGROUND

NuMA is a protein that has been previously shown to play a role in focusing microtubules at the mitotic spindle poles. However, most previous work relies on experimental methods that might cause dominant side effects on spindle formation, such as microinjection of antibodies, overexpression of mutant protein, or immunodepletion of NuMA-containing protein complexes.

FINDINGS

To circumvent these technical problems, we performed siRNA experiments in which we depleted the majority of NuMA in human cultured cells. Depleted mitotic cells show a prolonged duration of prometaphase, with spindle pole defects and with unattached, unaligned chromosomes.

CONCLUSION

Our data confirm that NuMA is important for spindle pole formation, and for cohesion of centrosome-derived microtubules with the bulk of spindle microtubules. Our findings of NuMA-dependent defects in chromosome alignment suggest that NuMA is involved in stabilizing kinetochore fibres.

Cohesin associates with spindle poles in a mitosis-specific manner and functions in spindle assembly in vertebrate cells

Cohesin is an essential protein complex required for sister chromatid cohesion. Cohesin associates with chromosomes and establishes sister chromatid cohesion during interphase. During metaphase, a small amount of cohesin remains at the chromosome-pairing domain, mainly at the centromeres, whereas the majority of cohesin resides in the cytoplasm, where its functions remain unclear. We describe the mitosis-specific recruitment of cohesin to the spindle poles through its association with centrosomes and interaction with nuclear mitotic apparatus protein (NuMA). Overexpression of NuMA enhances cohesin accumulation at spindle poles. Although transient cohesin depletion does not lead to visible impairment of normal spindle formation, recovery from nocodazole-induced spindle disruption was significantly impaired. Importantly, selective blocking of cohesin localization to centromeres, which disrupts centromeric sister chromatid cohesion, had no effect on this spindle reassembly process, clearly separating the roles of cohesin at kinetochores and spindle poles. In vitro, chromosome-independent spindle assembly using mitotic extracts was compromised by cohesin depletion, and it was rescued by addition of cohesin that was isolated from mitotic, but not S phase, cells. The combined results identify a novel spindle-associated role for human cohesin during mitosis, in addition to its function at the centromere/kinetochore regions.

Kinesin-5-dependent poleward flux and spindle length control in Drosophila embryo mitosis

We used antibody microinjection and genetic manipulations to dissect the various roles of the homotetrameric kinesin-5, KLP61F, in astral, centrosome-controlled Drosophila embryo spindles and to test the hypothesis that it slides apart interpolar (ip) microtubules (MT), thereby controlling poleward flux and spindle length. In wild-type and Ncd null mutant embryos, anti-KLP61F dissociated the motor from spindles, producing a spatial gradient in the KLP61F content of different spindles, which was visible in KLP61F-GFP transgenic embryos. The resulting mitotic defects, supported by gene dosage experiments and time-lapse microscopy of living klp61f mutants, reveal that, after NEB, KLP61F drives persistent MT bundling and the outward sliding of antiparallel MTs, thereby contributing to several processes that all appear insensitive to cortical disruption. KLP61F activity contributes to the poleward flux of both ipMTs and kinetochore MTs and to the length of the metaphase spindle. KLP61F activity maintains the prometaphase spindle by antagonizing Ncd and another unknown force-generator and drives anaphase B, although the rate of spindle elongation is relatively insensitive to the motor's concentration. Finally, KLP61F activity contributes to normal chromosome congression, kinetochore spacing, and anaphase A rates. Thus, a KLP61F-driven sliding filament mechanism contributes to multiple aspects of mitosis in this system.

Kinesin-5 is essential for growth-cone turning

Inhibition of kinesin-5, a mitotic motor protein also expressed in neurons, causes axons to grow faster as a result of alterations in the forces on microtubules (MTs) in the axonal shaft. Here, we investigate whether kinesin-5 plays a role in growth-cone guidance. Growth-cone turning requires that MTs in the central (C-) domain enter the peripheral (P-) domain in the direction of the turn. We found that inhibition of kinesin-5 in cultured neurons prevents MTs from polarizing within growth cones and causes them to grow past cues that would normally cause them to turn. We found that kinesin-5 is enriched in the transition (T-) zone of the growth cone and that kinesin-5 is preferentially phosphorylated on the side opposite the invasion of MTs. Moreover, when a growth cone encounters a turning cue, phospho-kinesin-5 polarizes even before the growth cone turns. Additional studies indicate that kinesin-5 works in part by antagonizing cytoplasmic dynein and that these motor-driven forces function together with the dynamic properties of the MTs to determine whether MTs can enter the P-domain. We propose that kinesin-5 permits MTs to selectively invade one side of the growth cone by opposing their entry into the other side.

Human papillomavirus E7 protein deregulates mitosis via an association with nuclear mitotic apparatus protein 1

We previously observed that high-risk human papillomavirus type 16 (HPV16) E7 expression leads to the delocalization of dynein from mitotic spindles (C. L. Nguyen, M. E. McLaughlin-Drubin, and K. Munger, Cancer Res. 68:8715-8722, 2008). Here, we show that HPV16 E7 associates with nuclear mitotic apparatus protein 1 (NuMA) and that NuMA binding and the ability to induce dynein delocalization map to similar carboxyl-terminal sequences of E7. Additionally, we show that the delocalization of dynein from mitotic spindles by HPV16 E7 and the interaction between HPV16 E7 and NuMA correlate with the induction of defects in chromosome alignment during prometaphase even in cells with normal centrosome numbers. Furthermore, low-risk HPV6b and HPV11 E7s also associate with NuMA and also induce a similar mitotic defect. It is possible that the disruption of mitotic events by HPV E7, via targeting of the NuMA/dynein complex and potentially other NuMA-containing complexes, contributes to viral maintenance and propagation potentially through abrogating the differentiation program of the infected epithelium. Furthermore, in concert with activities specific to high-risk HPV E6 and E7, such as the inactivation of the p53 and pRB tumor suppressors, respectively, the disruption of the NuMA/dynein network may result in mitotic errors that would make an infected cell more prone to the accumulation of aneuploidy even in the absence of supernumerary centrosomes.

Phosphorylation regulates targeting of cytoplasmic dynein to kinetochores during mitosis

Cytoplasmic dynein functions at several sites during mitosis; however, the basis of targeting to each site remains unclear. Tandem mass spectrometry analysis of mitotic dynein revealed a phosphorylation site in the dynein intermediate chains (ICs) that mediates binding to kinetochores. IC phosphorylation directs binding to zw10 rather than dynactin, and this interaction is needed for kinetochore dynein localization. Phosphodynein associates with kinetochores from nuclear envelope breakdown to metaphase, but bioriented microtubule (MT) attachment and chromosome alignment induce IC dephosphorylation. IC dephosphorylation stimulates binding to dynactin and poleward streaming. MT depolymerization, release of kinetochore tension, and a PP1-gamma mutant each inhibited IC dephosphorylation, leading to the retention of phosphodynein at kinetochores and reduced poleward streaming. The depletion of kinetochore dynactin by moderate levels of p50(dynamitin) expression disrupted the ability of dynein to remove checkpoint proteins by streaming at metaphase but not other aspects of kinetochore dynein activity. Together, these results suggest a new model for localization of kinetochore dynein and the contribution of kinetochore dynactin.

Kinesin-5 is not essential for mitotic spindle elongation in Dictyostelium

The proper assembly and operation of the mitotic spindle is essential to ensure the accurate segregation of chromosomes and to position the cytokinetic furrow during cell division in eukaryotes. Not only are dynamic microtubules required but also the concerted actions of multiple motor proteins are necessary to effect spindle pole separation, chromosome alignment, chromatid segregation, and spindle elongation. Although a number of motor proteins are known to play a role in mitosis, there remains a limited understanding of their full range of functions and the details by which they interact with other spindle components. The kinesin-5 (BimC/Eg5) family of motors is largely considered essential to drive spindle pole separation during the initial and latter stages of mitosis. We have deleted the gene encoding the kinesin-5 member in Dictyostelium, (kif13), and find that, in sharp contrast with results found in vertebrate, fly, and yeast organisms, kif13(-) cells continue to grow at rates indistinguishable from wild type. Phenotype analysis reveals a slight increase in spindle elongation rates in the absence of Kif13. More importantly, there is a dramatic, premature separation of spindle halves in kif13(-) cells, suggesting a novel role of this motor in maintaining spindle integrity at the terminal stages of division.

Diffusion and directed movement: in vitro motile properties of fission yeast kinesin-14 Pkl1

Fission yeast Pkl1 is a kinesin-14A family member that is known to be localized at the cellular spindle and is capable of hydrolyzing ATP. However, its motility has not been detected. Here, we show that Pkl1 is a slow, minus end-directed microtubule motor with a maximum velocity of 33+/-9 nm/s. The Km,MT value of steady-state ATPase activity of Pkl1 was as low as 6.4+/-1.1 nM, which is 20-30 times smaller than that of kinesin-1 and another kinesin-14A family member, Ncd, indicating a high affinity of Pkl1 for microtubules. However, the duty ratio of 0.05 indicates that Pkl1 spends only a small fraction of the ATPase cycle strongly associated with a microtubule. By using total internal reflection fluorescence microscopy, we demonstrated that single molecules of Pkl1 were not highly processive but only exhibited biased one-dimensional diffusion along microtubules, whereas several molecules of Pkl1, probably fewer than 10 molecules, cooperatively moved along microtubules and substantially reduced the diffusive component in the movement. Our results suggest that Pkl1 molecules work in groups to move and generate forces in a cooperative manner for their mitotic functions.

Cohesin subunit SMC1 associates with mitotic microtubules at the spindle pole

Accurate mitotic chromosome segregation depends on the formation of a microtubule-based bipolar spindle apparatus. We report that the cohesin subunit structural maintenance of chromosomes subunit 1 (SMC1) is recruited to microtubule-bound RNA export factor 1 (Rae1) at the mitotic spindle pole. We locate the Rae1-binding site to a 21-residue-long region, SMC1(947-967) and provide several lines of evidence that phosphorylation of Ser(957) and Ser(966) of SMC1 stimulates binding to Rae1. Imbalances in these assembly pathways caused formation of multipolar spindles. Our data suggest that cohesin's known bundling function for chromatids in mitotic and interphase cells extends to microtubules at the spindle pole.

Poleward transport of Eg5 by dynein-dynactin in Xenopus laevis egg extract spindles

Molecular motors are required for spindle assembly and maintenance during cell division. How motors move and interact inside spindles is unknown. Using photoactivation and photobleaching, we measure mitotic motor movement inside a dynamic spindle. We find that dynein-dynactin transports the essential motor Eg5 toward the spindle poles in Xenopus laevis egg extract spindles, revealing a direct interplay between two motors of opposite directionality. This transport occurs throughout the spindle except at the very spindle center and at the spindle poles, where Eg5 remains stationary. The variation of Eg5 dynamics with its position in the spindle is indicative of position-dependent functions of this motor protein. Our results suggest that Eg5 drives microtubule flux by antiparallel microtubule sliding in the spindle center, whereas the dynein-dependent concentration of Eg5 outside the spindle center could contribute to parallel microtubule cross-linking. These results emphasize the importance of spatially differentiated functions of motor proteins and contribute to our understanding of spindle organization.

Live-cell analysis of mitotic spindle formation in taxol-treated cells

Taxol functions to suppress the dynamic behavior of individual microtubules, and induces multipolar mitotic spindles. However, little is known about the mechanisms by which taxol disrupts normal bipolar spindle assembly in vivo. Using live imaging of GFP-alpha tubulin expressing cells, we examined spindle assembly after taxol treatment. We find that as taxol-treated cells enter mitosis, there is a dramatic re-distribution of the microtubule network from the centrosomes to the cell cortex. As they align there, the cortical microtubules recruit NuMA to their embedded ends, followed by the kinesin motor HSET. These cortical microtubules then bud off to form cytasters, which fuse into multipolar spindles. Cytoplasmic dynein and dynactin do not re-localize to cortical microtubules, and disruption of dynein/dynactin interactions by over-expression of p50 "dynamitin" does not prevent cytaster formation. Taxol added well before spindle poles begin to form induces multipolarity, but taxol added after nascent spindle poles are visible-but before NEB is complete-results in bipolar spindles. Our results suggest that taxol prevents rapid transport of key components, such as NuMA, to the nascent spindle poles. The net result is loss of mitotic spindle pole cohesion, microtubule re-distribution, and cytaster formation.