In vitro reactivation of anaphase spindle elongation using isolated diatom spindles

Lateral and longitudinal compaction of PRC1 overlap zones drives stabilization of interzonal microtubules

During anaphase, antiparallel-overlapping midzone microtubules elongate and form bundles, contributing to chromosome segregation and the location of contractile ring formation. Midzone microtubules are dynamic in early but not late anaphase; however, the kinetics and mechanisms of stabilization are incompletely understood. Using photoactivation of cells expressing PA-EGFP-α-tubulin we find that immediately after anaphase onset, a single highly dynamic population of midzone microtubules is present; as anaphase progresses, both dynamic and stable populations of midzone microtubules coexist. By mid-cytokinesis, only static, non-dynamic microtubules are detected. The velocity of microtubule sliding also decreases as anaphase progresses, becoming undetectable by late anaphase. Following depletion of PRC1, midzone microtubules remain highly dynamic in anaphase and fail to form static arrays in telophase despite furrowing. Cells depleted of Kif4a contain elongated PRC1 overlap zones and fail to form static arrays in telophase. Cells blocked in cytokinesis form short PRC1 overlap zones that do not coalesce laterally; these cells also fail to form static arrays in telophase. Together, our results demonstrate that dynamic turnover and sliding of midzone microtubules is gradually reduced during anaphase and that the final transition to a static array in telophase requires both lateral and longitudinal compaction of PRC1 containing overlap zones.

Anaphase B: Long-standing models meet new concepts

Mitotic cell divisions ensure stable transmission of genetic information from a mother to daughter cells in a series of generations. To ensure this crucial task is accomplished, the cell forms a bipolar structure called the mitotic spindle that divides sister chromatids to the opposite sides of the dividing mother cell. After successful establishment of stable attachments of microtubules to chromosomes and inspection of connections between them, at the heart of mitosis, the cell starts the process of segregation. This spectacular moment in the life of a cell is termed anaphase, and it involves two distinct processes: depolymerization of microtubules bound to chromosomes, which is also known as anaphase A, and elongation of the spindle or anaphase B. Both processes ensure physical separation of disjointed sister chromatids. In this chapter, we review the mechanisms of anaphase B spindle elongation primarily in mammalian systems, combining different pioneering ideas and concepts with more recent findings that shed new light on the force generation and regulation of biochemical modules operating during spindle elongation. Finally, we present a comprehensive model of spindle elongation that includes structural, biophysical, and molecular aspects of anaphase B.

Kinesin-6 Klp9 orchestrates spindle elongation by regulating microtubule sliding and growth

Mitotic spindle function depends on the precise regulation of microtubule dynamics and microtubule sliding. Throughout mitosis, both processes have to be orchestrated to establish and maintain spindle stability. We show that during anaphase B spindle elongation in Schizosaccharomyces pombe, the sliding motor Klp9 (kinesin-6) also promotes microtubule growth in vivo. In vitro, Klp9 can enhance and dampen microtubule growth, depending on the tubulin concentration. This indicates that the motor is able to promote and block tubulin subunit incorporation into the microtubule lattice in order to set a well-defined microtubule growth velocity. Moreover, Klp9 recruitment to spindle microtubules is dependent on its dephosphorylation mediated by XMAP215/Dis1, a microtubule polymerase, creating a link between the regulation of spindle length and spindle elongation velocity. Collectively, we unravel the mechanism of anaphase B, from Klp9 recruitment to the motors dual-function in regulating microtubule sliding and microtubule growth, allowing an inherent coordination of both processes.

Spindle scaling mechanisms

The mitotic spindle robustly scales with cell size in a plethora of different organisms. During development and throughout evolution, the spindle adjusts to cell size in metazoans and yeast in order to ensure faithful chromosome separation. Spindle adjustment to cell size occurs by the scaling of spindle length, spindle shape and the velocity of spindle assembly and elongation. Different mechanisms, depending on spindle structure and organism, account for these scaling relationships. The limited availability of critical spindle components, protein gradients, sequestration of spindle components, or post-translational modification and differential expression levels have been implicated in the regulation of spindle length and the spindle assembly/elongation velocity in a cell size-dependent manner. In this review, we will discuss the phenomenon and mechanisms of spindle length, spindle shape and spindle elongation velocity scaling with cell size.

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.

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

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

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.

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.

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.

Mitochondria, the Cell Cycle, and the Origin of Sex via a Syncytial Eukaryote Common Ancestor

Theories for the origin of sex traditionally start with an asexual mitosing cell and add recombination, thereby deriving meiosis from mitosis. Though sex was clearly present in the eukaryote common ancestor, the order of events linking the origin of sex and the origin of mitosis is unknown. Here, we present an evolutionary inference for the origin of sex starting with a bacterial ancestor of mitochondria in the cytosol of its archaeal host. We posit that symbiotic association led to the origin of mitochondria and gene transfer to host's genome, generating a nucleus and a dedicated translational compartment, the eukaryotic cytosol, in which-by virtue of mitochondria-metabolic energy was not limiting. Spontaneous protein aggregation (monomer polymerization) and Adenosine Tri-phosphate (ATP)-dependent macromolecular movement in the cytosol thereby became selectable, giving rise to continuous microtubule-dependent chromosome separation (reduction division). We propose that eukaryotic chromosome division arose in a filamentous, syncytial, multinucleated ancestor, in which nuclei with insufficient chromosome numbers could complement each other through mRNA in the cytosol and generate new chromosome combinations through karyogamy. A syncytial (or coenocytic, a synonym) eukaryote ancestor, or Coeca, would account for the observation that the process of eukaryotic chromosome separation is more conserved than the process of eukaryotic cell division. The first progeny of such a syncytial ancestor were likely equivalent to meiospores, released into the environment by the host's vesicle secretion machinery. The natural ability of archaea (the host) to fuse and recombine brought forth reciprocal recombination among fusing (syngamy and karyogamy) progeny-sex-in an ancestrally meiotic cell cycle, from which the simpler haploid and diploid mitotic cell cycles arose. The origin of eukaryotes was the origin of vertical lineage inheritance, and sex was required to keep vertically evolving lineages viable by rescuing the incipient eukaryotic lineage from Muller's ratchet. The origin of mitochondria was, in this view, the decisive incident that precipitated symbiosis-specific cell biological problems, the solutions to which were the salient features that distinguish eukaryotes from prokaryotes: A nuclear membrane, energetically affordable ATP-dependent protein-protein interactions in the cytosol, and a cell cycle involving reduction division and reciprocal recombination (sex).

Imaging and analysis of the microtubule cytoskeleton in giardia

Giardia intestinalis, a common parasitic protist, possesses a complex microtubule cytoskeleton critical for cellular function and transitioning between the cyst and trophozoite life cycle stages. The giardial microtubule cytoskeleton is comprised of highly dynamic and stable structures. Novel microtubule structures include the ventral disc that is essential for the parasite's attachment to the intestinal villi to avoid peristalsis. The completed Giardia genome combined with new molecular genetic tools and live imaging will aid in the characterization and analysis of cytoskeletal dynamics in Giardia. Fundamental areas of giardial cytoskeletal biology remain to be explored and knowledge of the molecular mechanisms of cytoskeletal functioning is needed to better understand Giardia's unique biology and pathogenesis.

Cell division screens and dynamin

Cell division is the most fundamental process in the development of all living organisms. The generation of cell diversity throughout development, the multiplication of cells during wound repair and the maintenance of stem cells in several tissues and organs all rely on proper progress through cell division. Historically, biochemical studies of cell division proved to be difficult, since mitosis is a moving target. The rapid and dynamic nature of mitosis means necessary proteins often exist in multiple isoforms and some for only brief moments during a particular stage in the cell cycle. The advent of proteomics and the introduction of stage-specific inhibitors have enabled the field to identify numerous factors required at distinct steps in the cell cycle. One such factor identified in many of these screens was the highly conserved protein dynamin. Dynamin, long known for its role in endocytosis, is also necessary for co-ordinating actin assembly at membranes. Our knowledge of its precise cell cycle function and upstream/downstream targets, however, is unclear. Our review will describe current knowledge regarding the impacts of several cell division screens and the multiple roles that dynamin may play during mitosis.

W. Zacheus Cande: Evolutionary biologist in cell biologist's clothing. Interview by Ruth Williams

Zac Cande wants to get to the roots of cell division and cytoskeletal mechanics.

Schizosaccharomyces pombehomolog of Survivin, Bir1p, exhibits a novel dynamic behavior at the spindle mid-zone

Members of the BIR-domain containing Survivin family of proteins have been identified in a variety of eukaryotes and are known to play important roles in the regulation of mitosis. The Schizosaccharomyces pombe homolog of Survivin, Bir1p, is essential for chromosome condensation and spindle elongation and integrity. Bir1p, a nuclear protein, resides at the kinetochores in metaphase and anaphase A and spreads to the spindle mid-zone in anaphase B. Here we show that this relocation requires Cdk (Cyclin dependent kinase) inactivation and intact microtubules. With the aid of a kinesin mutant, klp5Delta, we also show that completion of anaphase A is vital for effecting Bir1p re-location to the spindle mid-zone. Although minimal exchange of Bir1p sub-units occurs between the spindle and the nucleoplasm, the protein redistributes laterally within the mid-zone region. Bir1p dynamics therefore significantly differs from that of tubulin on an anaphase B spindle, which is loaded at the plus ends of growing microtubules and shows no lateral redistribution within the spindle. Thus, Bir1p, and possibly its associated proteins, might organize a dynamic mid-zone region that helps spindle elongation and maintenance.

Live analysis of lagging chromosomes during anaphase and their effect on spindle elongation rate in fission yeast

The fission yeast Schizosaccharomyces pombe is widely used as a model system for studies of the cell cycle and chromosome biology. To enhance these studies we have fused GFP to the chromodomain protein Swi6p, thus allowing nuclear and chromosome behaviour to be followed in living cells using time-lapse fluorescence microscopy. Like endogenous Swi6p, GFP-Swi6p localises to the nucleus and is concentrated at the heterochromatic centromeres and telomeres. The nucleus is highly dynamic during interphase: the clustered centromeres, in particular, are highly mobile. By expressing GFP-(α)2-tubulin and GFP-Swi6p in the same cells we observe that the clustered centromeres move in concert with the cytoplasmic microtubules, which is likely to reflect their association with the spindle pole body. Drug treatment indicates that this movement is dependent on intact cytoplasmic microtubules. We have also used GFP-Swi6p to investigate the properties of lagging chromosomes observed in mutants with defects in chromosome segregation. Lagging chromosomes display a variety of behaviours on anaphase spindles, most surprisingly, chromosomes appear to initiate microtubule interactions and move to the poles late in anaphase B. Interestingly, in cells displaying lagging chromosomes, the rate of spindle elongation is slowed by a factor of two. This suggests that cells are able to sense the presence of a lagging chromosome and slow anaphase B in order to allow it extra time to reach the pole. However, this mechanism is not dependent on the spindle checkpoint proteins Bub1p or Dma1p, raising the possibility that a novel checkpoint mechanism operates to retard spindle elongation if lagging chromosomes are detected. An alternative model is also discussed in which single defective kinetochores on lagging chromatids are able to interact simultaneously with microtubules emanating from both poles and affect spindle dynamics by counteracting the spindle elongation force.

A switch in microtubule dynamics at the onset of anaphase B in the mitotic spindle of Schizosaccharomyces pombe

Microtubule dynamics have key roles in mitotic spindle assembly and chromosome movement [1]. Fast turnover of spindle microtubules at metaphase and polewards flux of microtubules (polewards movement of the microtubule lattice with depolymerization at the poles) at both metaphase and anaphase have been observed in mammalian cells [2]. Imaging spindle dynamics in genetically tractable yeasts is now possible using green fluorescent protein (GFP)-tagging of tubulin and sites on chromosomes [3] [4] [5] [6] [7] [8]. We used photobleaching of GFP-labeled tubulin to observe microtubule dynamics in the fission yeast Schizosaccharomyces pombe. Photobleaching did not perturb progress through mitosis. Bleached marks made on the spindle during metaphase recovered their fluorescence rapidly, indicating fast microtubule turnover. Recovery was spatially non-uniform, but we found no evidence for polewards flux. Marks made during anaphase B did not recover fluorescence, and were observed to slide away from each other at the same rate as spindle elongation. Fast microtubule turnover at metaphase and a switch to stable microtubules at anaphase suggest the existence of a cell-cycle-regulated molecular switch that controls microtubule dynamics and that may be conserved in evolution. Unlike the situation for vertebrate spindles, microtubule depolymerization at poles and polewards flux may not occur in S. pombe mitosis. We conclude that GFP-tubulin photobleaching in conjunction with mutant cells should aid research on molecular mechanisms causing and regulating dynamics.

CDK1 inactivation regulates anaphase spindle dynamics and cytokinesis in vivo

Through association with CDK1, cyclin B accumulation and destruction govern the G2/M/G1 transitions in eukaryotic cells. To identify CDK1 inactivation-dependent events during late mitosis, we expressed a nondestructible form of cyclin B (cyclin BDelta90) by microinjecting its mRNA into prometaphase normal rat kidney cells. The injection inhibited chromosome decondensation and nuclear envelope formation. Chromosome disjunction occurred normally, but anaphase-like movement persisted until the chromosomes reached the cell periphery, whereupon they often somersaulted and returned to the cell center. Injection of rhodamine-tubulin showed that this movement occurred in the absence of a central anaphase spindle. In 82% of cells cytokinesis was inhibited; the remainder split themselves into two parts in a process reminiscent of Dictyostelium cytofission. In all cells injected, F-actin and myosin II were diffusely localized with no detectable organization at the equator. Our results suggest that a primary effect of CDK1 inactivation is on spindle dynamics that regulate chromosome movement and cytokinesis. Prolonged CDK1 activity may prevent cytokinesis through inhibiting midzone microtubule formation, the behavior of proteins such as TD60, or through the phosphorylation of myosin II regulatory light chain.

Cell model systems in plant cytoskeleton studies

Cytoskeletons play an essential role in cellular functions in both animal and plant cells. In studies of the molecular mechanisms of their functions, a variety of cell model systems, mainly of animal cells, have yielded much information. With plant cells, cell model systems have mostly been restricted to studies on the mechanism of cytoplasmic streaming. Recently, however, there have been several reports of studies employing plant cell model systems to investigate plant cytoskeletons that have revealed new concepts about their structure and functions. To promote and support a general understanding of cell model systems, this review attempts to categorize them, present currently known information on the structure and function of plant cytoskeletons, and offer a possible role of cell model systems in future studies of plant cytoskeletons.

Anaphase in vitro

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DSK1, a novel kinesin-related protein from the diatom Cylindrotheca fusiformis that is involved in anaphase spindle elongation

We have identified an 80-kD protein that is involved in mitotic spindle elongation in the diatom Cylindrotheca fusiformis. DSK1 (Diatom Spindle Kinesin 1) was isolated using a peptide antibody raised against a conserved region in the motor domain of the kinesin superfamily. By sequence homology, DSK1 belongs to the central motor family of kinesin-related proteins. Immunoblots using an antibody raised against a non-conserved region of DSK1 show that DSK1 is greatly enriched in mitotic spindle preparations. Anti-DSK1 stains in diatom central spindle with a bias toward the midzone, and staining is retained in the spindle midzone during spindle elongation in vitro. Furthermore, preincubation with anti-DSK1 blocks function in an in vitro spindle elongation assay. This inhibition of spindle elongation can be rescued by preincubating concurrently with the fusion protein against which anti-DSK1 was raised. We conclude that DSK1 is involved in spindle elongation and is likely to be responsible for pushing hal-spindles apart in the spindle midzone.

Two microtubule-associated proteins required for anaphase spindle movement in Saccharomyces cerevisiae

In many eucaryotic cells, the midzone of the mitotic spindle forms a distinct structure containing a specific set of proteins. We have isolated ASE1, a gene encoding a component of the Saccharomyces cerevisiae spindle midzone. Strains lacking both ASE1 and BIK1, which encodes an S. cerevisiae microtubule-associated protein, are inviable. The analysis of the phenotype of a bik1 ase1 conditional double mutant suggests that BIK1 and ASE1 are not required for the assembly of a bipolar spindle, but are essential for anaphase spindle elongation. The steady-state levels of Ase1p are regulated in a manner that is consistent with a function during anaphase: they are low in G1, accumulate to maximal levels after S phase and then drop as cells exit mitosis. Components of the spindle midzone may therefore be required in vivo for anaphase spindle movement. Additionally, anaphase spindle movement may depend on a dedicated set of genes whose expression is induced at G2/M.

The Drosophila kinesin-like protein KLP3A is a midbody component required for central spindle assembly and initiation of cytokinesis

We describe here a new member of the kinesin superfamily in Drosophila, KLP3A (Kinesin-Like-Protein-at-3A). The KLP3A protein localizes to the equator of the central spindle during late anaphase and telophase of male meiosis. Mutations in the KLP3A gene disrupt the interdigitation of microtubules in spermatocyte central spindles. Despite this defect, anaphase B spindle elongation is not obviously aberrant. However, cytokinesis frequently fails after both meiotic divisions in mutant testes. Together, these findings strongly suggest that the KLP3A presumptive motor protein is a critical component in the establishment or stabilization of the central spindle. Furthermore, these results imply that the central spindle is the source of signals that initiate the cleavage furrow in higher cells.

Saccharomyces cerevisiae kinesin- and dynein-related proteins required for anaphase chromosome segregation

The Saccharomyces cerevisiae kinesin-related gene products Cin8p and Kip1p function to assemble the bipolar mitotic spindle. The cytoplasmic dynein heavy chain homologue Dyn1p (also known as Dhc1p) participates in proper cellular positioning of the spindle. In this study, the roles of these motor proteins in anaphase chromosome segregation were examined. While no single motor was essential, loss of function of all three completely halted anaphase chromatin separation. As combined motor activity was diminished by mutation, both the velocity and extent of chromatin movement were reduced, suggesting a direct role for all three motors in generating a chromosome-separating force. Redundancy for function between different types of microtubule-based motor proteins was also indicated by the observation that cin8 dyn1 double-deletion mutants are inviable. Our findings indicate that the bulk of anaphase chromosome segregation in S. cerevisiae is accomplished by the combined actions of these three motors.

Mitotic spindle pole separation

Eukaryotic cells utilize a microtubular spindle to segregate chromosomes during mitosis. Chromosome segregation requires the timely separation of the mitotic spindle poles to which the chromosomes are attached. Recent studies at the molecular and cellular levels have provided new insights into the mechanism and regulation of this process. On the one hand, the process now seems more complex, as redundant mechanisms apparently overlap in function during cell division. On the other hand, some of these processes may be acting continuously during the various stages of spindle pole separation, suggesting an underlying simplicity.

Inhibition of anaphase spindle elongation in vitro by a peptide antibody that recognizes kinesin motor domain

Isolated central spindles or spindles in detergent-permeabilized cells from the diatom Cylindrotheca fusiformis can undergo ATP-dependent reactivation of spindle elongation in vitro. We have used a peptide antibody raised against a 10-amino acid portion common to the kinesin superfamily motor domain to look for kinesin-like motor activity during anaphase B of mitosis. The peptide antibody localizes to central spindles. Upon ATP reactivation of spindle elongation, antigens recognized by the antibody are associated exclusively with the central spindle midzone where antiparallel microtubules of each half-spindle overlap. The antibody recognizes several polypeptides by immunoblot using isolated spindle extracts. One of these polypeptides behaves like kinesin with respect to nucleotide-specific binding to and release from taxol-stabilized microtubules. Preincubation of the spindle model with the peptide antibody inhibits subsequent ATP reactivation of spindle elongation. Coincubation of the peptide antibody with peptide antigen rescues spindle function. These results support a role for kinesin-related protein(s) in spindle elongation (anaphase B) of mitosis and suggest that one or several polypeptides that we have identified in spindle extracts may fulfill this function.

Physiological evidence for involvement of a kinesin-related protein during anaphase spindle elongation in diatom central spindles

We have developed a new model system for studying spindle elongation in vitro using the pennate, marine diatom Cylindrotheca fusiformis. C. fusiformis can be grown in bulk to high densities while in log phase growth and synchronized by a simple light/dark regime. Isolated spindles can be attained in quantities sufficient for biochemical analysis and spindle tubulin is approximately 5% of the total protein present. The spindle isolation procedure results in a 10-fold enrichment of diatom tubulin and a calculated 40-fold increase in spindle protein. Isolated spindles or spindles in permeabilized cells can elongate in vitro by the same mechanism and with the same pharmacological sensitivities as described for other anaphase B models (Cande and McDonald, 1986; Masuda et al., 1990). Using this model, in vitro spindle elongation rate profiles were developed for a battery of nucleotide triphosphates and ATP analogs. The relative rates of spindle elongation produced by various nucleotide triphosphates parallel relative rates seen for kinesin-based motility in microtubule gliding assays. Likewise ATP analogs that allow discrimination between myosin-, dynein-, and kinesin-mediated motility produce relative spindle elongation rates characteristic of kinesin motility. Also, isolated spindle fractions are enriched for a kinesin related protein as identified by a peptide antibody against a conserved region of the kinesin superfamily. These data suggest that kinesin-like motility contributes to spindle elongation during anaphase B of mitosis.

Sister chromatid separation in frog egg extracts requires DNA topoisomerase II activity during anaphase

We have produced metaphase spindles and induced them to enter anaphase in vitro. Sperm nuclei were added to frog egg extracts, allowed to replicate their DNA, and driven into metaphase by the addition of cytoplasm containing active maturation promoting factor (MPF) and cytostatic factor (CSF), an activity that stabilizes MPF. Addition of calcium induces the inactivation of MPF, sister chromatid separation and anaphase chromosome movement. DNA topoisomerase II inhibitors prevent chromosome segregation at anaphase, demonstrating that the chromatids are catenated at metaphase and that decatenation occurs at the start of anaphase. Topoisomerase II activity towards exogenous substrates does not increase at the metaphase to anaphase transition, showing that chromosome separation at anaphase is not triggered by a bulk activation of topoisomerase II.

Nucleotide specificity of the enzymatic and motile activities of dynein, kinesin, and heavy meromyosin

The substrate specificities of dynein, kinesin, and myosin substrate turnover activity and cytoskeletal filament-driven translocation were examined using 15 ATP analogues. The dyneins were more selective in their substrate utilization than bovine brain kinesin or muscle heavy meromyosin, and even different types of dyneins, such as 14S and 22S dynein from Tetrahymena cilia and the beta-heavy chain-containing particle from the outer-arm dynein of sea urchin flagella, could be distinguished by their substrate specificities. Although bovine brain kinesin and muscle heavy meromyosin both exhibited broad substrate specificities, kinesin-induced microtubule translocation varied over a 50-fold range in speed among the various substrates, whereas heavy meromyosin-induced actin translocation varied only by fourfold. With both kinesin and heavy meromyosin, the relative velocities of filament translocation did not correlate well with the relative filament-activated substrate turnover rates. Furthermore, some ATP analogues that did not support the filament translocation exhibited filament-activated substrate turnover rates. Filament-activated substrate turnover and power production, therefore, appear to become uncoupled with certain substrates. In conclusion, the substrate specificities and coupling to motility are distinct for different types of molecular motor proteins. Such nucleotide "fingerprints" of enzymatic activities of motor proteins may prove useful as a tool for identifying what type of motor is involved in powering a motility-related event that can be reconstituted in vitro.

Chromosomal passengers: toward an integrated view of mitosis

The major events of mitosis have traditionally been considered to represent two distinct pathways and have been studied by two separate groups of workers. The chromosomal events (chromosome condensation and sister chromatid disjunction) have been the principal focus for one group, while the cytoskeletal events (nuclear envelope breakdown, chromosomal movements, cytokinesis) have been the focus for the other. This historical division is epitomized by the view of many cell biologists, which was aptly caught by Mazia's comparison of the role of the chromosome arms in mitosis to that of "the corpse at the funeral" which "provide a reason for the proceedings but do not take an active part in them" (Mazia 1961). More recent studies have demonstrated that the role of the chromosomes in mitotic movements is somewhat more active than this. That the kinetochore may play an important role in chromosome movements has long been suspected (see early references in Mazia 1961) but was only proven rather recently (Brinkley et al. 1988; Gorbsky et al. 1987; Nicklas 1989). This has led to a burst of recent interest in all aspects of kinetochore structure and function. Our studies have led us to ask whether chromosomes may play an even more extensive role in the events of mitosis. We suggest here that in addition to their active role in movements, the chromosome may make important structural contributions to the anaphase spindle and cleavage furrow, which are normally thought of as "cytoskeletal" functions. These structural contributions may be made by members of a new class of "chromosomal passenger" proteins that use the chromosomes as a means of conveyance so that they are correctly positioned at the metaphase plate to carry out their nonchromosomal functions during anaphase and the subsequent mitotic events.

Conserved beta-tubulin binding domain for the microtubule-associated motors underlying sperm motility and fast axonal transport

An antiserum against tubulin, NS20, has been previously shown to inhibit anterograde and retrograde axonal transport by 50% in vivo and in vitro. We report here that Protein A purified NS20 antibodies also attenuate sperm motility by 50% in demembranated sea urchin sperm. This inhibition is absorbed out by preincubating the NS20 antibodies with a biochemically purified porcine microtubule preparation, with recombinant Trypanosoma beta- (but not alpha-) tubulin and most specifically, with a 37 amino acid (a.a.) synthetic peptide corresponding to a domain near (but not including) the porcine beta-tubulin C terminus. Furthermore, addition of this beta-tubulin peptide alone is sufficient to attenuate motility by 50% in demembranated sperm, indicating that this critical 37a.a. NS20 antigen is a motor binding domain. Together, the results suggest that at least two phenotypically distinct forms of microtubule-based motility, axonal transport and flagellar beating, are homologous at the fundamental level of the microtubule domains (the beta-tubulin peptide and we suggest a distinct but similarly located alpha-tubulin domain) mediating the attachment of tubulin-associated motors.

Reactivation of isolated mitotic apparatus: metaphase versus anaphase spindles

Mitotic spindles isolated from sea urchin eggs can be reactivated to undergo mitotic processes in vitro. Spindles incubated in reactivation media containing sea urchin tubulin and nucleotides undergo pole-pole elongation similar to that observed in living cells during anaphase-B. The in vitro behavior of spindles isolated during metaphase and anaphase are compared. Both metaphase and anaphase spindles undergo pole-pole elongation with similar rates, but only in the presence of added tubulin. In contrast, metaphase but not anaphase spindles increase chromosome-pole distance in the presence of exogenous tubulin, suggesting that in vitro, tubulin can be incorporated at the kinetochores of metaphase but not anaphase chromosomes. The rate of spindle elongation, ultimate length achieved, and the increase in chromosome-pole distance for isolated metaphase spindles is related to the concentration of available tubulin. Pole-pole elongation and chromosome-pole elongation does not require added adenosine triphosphate (ATP). Guanosine triphosphate (GTP) will support all activities observed. Thus, the force generation mechanism for anaphase-B in isolated sea urchin spindles is independent of added ATP, but dependent on the availability of tubulin. These results support the hypothesis that the mechanism of force generation for anaphase-B is linked to the incorporation of tubulin into the mitotic apparatus. (If, in addition, a microtubule-dependent motor-protein(s) is acting to generate force, it does not appear to be dependent on ATP as the exclusive energy source.

Inhibition of spindle elongation by taxol

During anaphase B spindle elongation, interzonal microtubules lengthen to accomplish pole-pole separation, while at the same time remaining highly dynamic [Shelden and Wadsworth, J. Cell Sci. 97:273-281, 1990]. To further examine the role of microtubule polymerization and dynamics during spindle elongation, cells have been treated with taxol, which induces microtubule polymerization and stabilizes microtubules. Taxol was added to PtK1 cells 3 minutes after initial chromatid separation, so that the effect on anaphase B could be observed with minimal disruption to anaphase A movement. In 20 microM taxol, the rate and extent of pole-pole separation, measured from time-lapse video records, are reduced to 4% and 9.5% of controls, respectively. The organization of microtubules in taxol treated cells was examined using tubulin immunofluorescence and confocal fluorescence microscopy. Taxol induces a dramatic reorganization of interzonal microtubules resulting in a narrow gap, which is nearly completely lacking in MTs, across the center of the interzone. Furthermore, microtubules in taxol treated cells are resistant to nocodazole induced microtubule disassembly. Our results reveal that taxol rapidly inhibits anaphase B spindle elongation; inhibition is accompanied by a depletion of interdigitated interzonal microtubules and a reduction in microtubule dynamic behavior.

Analysis of inducible contractile rings suggests a role for protein kinase C in embryonic cytokinesis and wound healing

A semi-in vitro system derived from Xenopus oocytes which allows induction of contractile ring (CR) formation and closure is described and exploited to elucidate regulatory and structural features of cytokinesis. The inducible CRs (ICRs) are composed of actin filaments and closure is actin filament-dependent as is cytokinesis in vivo. ICR closure in this system is calcium-dependent and pH-sensitive, as is cytokinesis in permeabilized cells (Cande: Journal of Cell Biology 87:326, 1980). Closure of ICRs proceeds at a rate and with a kinetic pattern similar to embryonic cytokinesis. Collectively, these data demonstrate that this system is a faithful mimic of cytokinesis in vivo. ICR formation and closure is protein kinase C (PKC)-dependent and neomycin-sensitive, indicating that the PKC branch of the polyphosphoinositide pathway regulates formation of the actomyosin ring which is the effector of cytokinesis. Kinetic measurements show that the rate of ICR closure reaches a peak of 4-8 microns/sec. Since the maximum measured velocity of actin filament translocation by vertebrate, non-muscle myosins is 0.04 micron/sec, the later observations support a model in which the CR is segmented, containing multiple sites where filaments overlap in a "sliding filament" fashion. Because the rate decreases after reaching a peak, the results also suggest that the number of overlap sites decrease with time.

In vitro reactivation of spindle elongation in fission yeast nuc2 mutant cells

To investigate the mechanisms of spindle elongation and chromosome separation in the fission yeast Schizosaccharomyces pombe, we have developed an in vitro assay using a temperature-sensitive mutant strain, nuc2. At the restrictive temperature, nuc2 cells are arrested at a metaphase-like stage with short spindles and condensed chromosomes. After permeabilization of spheroplasts of the arrested cells, spindle elongation was reactivated by addition of ATP and neurotubulin both at the restrictive and the permissive temperatures, but chromosome separation was not. This suggests that the nuc2 cells are impaired in function at a stage before sister chromatid disjunction. Spindle elongation required both ATP and exogenous tubulin and was inhibited by adenylyl imidodiphosphate (AMPPNP) or vanadate. The ends of yeast half-spindle microtubules pulse-labeled with biotinylated tubulin moved past each other during spindle elongation and a gap formed between the original half-spindles. These results suggest that the primary mechanochemical event responsible for spindle elongation is the sliding apart of antiparallel microtubules of the two half-spindles.

Identification of dynamin, a novel mechanochemical enzyme that mediates interactions between microtubules

We report that calf brain microtubules prepared without nucleotide contain, in addition to kinesin and dynein, a polypeptide of 100 kd that could be dissociated by nucleotide. The protein was selectively extracted from microtubules using a combination of GTP and AMP-PNP. The extract contained microtubule-stimulated (6-fold) MgATPase activity that partitioned into two components upon further purification: the 100 kd polypeptide and a soluble activating fraction. The 100 kd protein induced microtubules to form hexagonally packed bundles containing periodic cross bridges spaced 13 nm apart. In the presence of ATP and the activating fraction, bundles fragmented, elongated, and exhibited other behavior indicative of sliding between microtubules. These findings indicate that the 100 kd protein is part of a novel mechanochemical enzyme, which we term "dynamin", that may mediate microtubule sliding in vivo.

The motor for poleward chromosome movement in anaphase is in or near the kinetochore

I have tested two contending views of chromosome-to-pole movement in anaphase. Chromosomes might be pulled poleward by a traction fiber consisting of the kinetochore microtubules and associated motors, or they might propel themselves by a motor in the kinetochore. I cut through the spindle of demembranated grasshopper spermatocytes between the chromosomes and one pole and swept the polar region away, removing a portion of the would-be traction fiber. Chromosome movement continued, and in the best examples, chromosomes moved to within 1 micron of the cut edge. There is nothing beyond the edge to support movement, and a push from the rear is unlikely because cuts in the interzone behind the separating chromosomes did not stop movement. Therefore, I conclude that the motor must be in the kinetochore or within 1 micron of it. Less conclusive evidence points to the kinetochore itself as the motor. The alternative is an external motor pulling on the kinetochore microtubules or directly on the kinetochore. A pulling motor would move kinetochore microtubules along with the chromosome, so that in a cut half-spindle, the microtubules should protrude from the cut edge as chromosomes move toward it. No protrusion was seen; however, the possibility that microtubules depolymerize as they are extruded, though unlikely, is not ruled out. What is certain is that the motor for poleward chromosome movement in anaphase must be in the kinetochore or very close to it.

Preparation and characterization of mitotic cytoskeletons from embryos of the sea urchin Strongylocentrotus franciscanus

We present here a preparation protocol and molecular characterization of mitotic cytoskeletons isolated from embryos of the spiny red sea urchin, Strongylocentrotus franciscanus. The mitotic cytoskeletons are produced by detergent lysis of S. franciscanus embryos during cell division and consist of a mitotic apparatus enclosed in a thin shell of cortical filaments. Mitotic cytoskeletons produced in this way are stable for a year at liquid nitrogen temperature. S. franciscanus is of interest because it is commercially available in large quantities and each animal provides prodigious amounts of gametes. Furthermore, embryos from this sea urchin will develop synchronously at high densities and are an excellent source of quantities of cytoskeletal proteins appropriate for biochemical studies.

Selective reduction of anaphase B in quinacrine-treated PtK1 cells

Quinacrine, an acridine derivative which competitively binds to ATP binding sites, has previously been shown to cause the reorganization of metaphase spindle microtubules (MTs) due to changes in interactions of non-kinetochore microtubules (nkMTs) of opposite polarity (Armstrong and Snyder: Cell Motil. Cytoskeleton 7:10-19, 1987). In the study presented here, mitotic PtK1 cells were treated in early anaphase with concentrations of quinacrine ranging from 2 to 12 microM to determine energy requirements for chromosome motion. The rate and extent of chromosome-to-pole movements (anaphase A) were not affected by these quinacrine treatments. The extent of anaphase B (kinetochore-kinetochore separation) was reduced with increasing concentrations of quinacrine. Five micromolar quinacrine reduced the extent of kinetochore-kinetochore separation by 20%, and addition of 12 microM quinacrine reduced the kinetochore-kinetochore separation by 40%. To determine the role of nkMTs in anaphase spindle elongation, quinacrine-treated metaphase cells were treated with hyperosmotic sucrose concentrations, and spindle elongation was measured (Snyder et al.: Eur J. Cell Biol. 39:373-379, 1985). Metaphase cells treated with 2-10 microM concentrations of quinacrine for 2-5 min reduced spindle lengths by 10-50% prior to 0.5 M sucrose treatment for 5 min. This treatment showed a significant reduction in the ability of sucrose to induce spindle elongation in cells pretreated with quinacrine. As spindle length and birefringence was reduced by quinacrine treatment, sucrose-induced elongation was concomitantly diminished. These data suggest that quinacrine-sensitive linkages are necessary for anaphase B motions. Reduction in these linkages and/or MT length in the nkMT continuum may reduce the ability of the nkMTs to hold compression at metaphase. This form of energy is thought to drive a significant proportion of normal anaphase B in PtK1 cells and sucrose-induced metaphase spindle elongation.

Distribution of phosphorylated spindle-associated proteins in the diatom Stephanopyxis turris

Mitotic spindles isolated from the diatom Stephanopyxis turris become thiophosphorylated in the presence of ATP gamma S at specific locations within the mitotic apparatus, resulting in a stimulation of ATP-dependent spindle elongation in vitro. Here, using indirect immunofluorescence, we compare the staining pattern of an antibody against thiophosphorylated proteins to that of MPM-2, an antibody against mitosis-specific phosphoproteins, in isolated spindles. Both antibodies label spindle poles, kinetochores, and the midzone. Neither antibody exhibits reduced labeling in salt-extracted spindles, although prior salt extraction inhibits thiophosphorylation in ATP gamma S. Furthermore, both antibodies recognize a 205 kd band on immunoblots of spindle extracts. Microtubule-organizing centers and mitotic spindles label brightly with the MPM-2 antibody in intact cells. These results show that functional mitotic spindles isolated from S. turris are phosphorylated both in vivo and in vitro. We discuss the possible role of phosphorylated cytoskeletal proteins in the control of mitotic spindle function.

The mechanism of anaphase spindle elongation: uncoupling of tubulin incorporation and microtubule sliding during in vitro spindle reactivation

To study tubulin polymerization and microtubule sliding during spindle elongation in vitro, we developed a method of uncoupling the two processes. When isolated diatom spindles were incubated with biotinylated tubulin (biot-tb) without ATP, biot-tb was incorporated into two regions flanking the zone of microtubule overlap, but the spindles did not elongate. After biot-tb was removed, spindle elongation was initiated by addition of ATP. The incorporated biot-tb was found in the midzone between the original half-spindles. The extent and rate of elongation were increased by preincubation in biot-tb. Serial section reconstruction of spindles elongating in tubulin and ATP showed that the average length of half-spindle microtubules increased due to growth of microtubules from the ends of native microtubules. The characteristic packing pattern between antiparallel microtubules was retained even in the "new" overlap region. Our results suggest that the forces required for spindle elongation are generated by enzymes in the overlap zone that mediate the sliding apart of antiparallel microtubules, and that tubulin polymerization does not contribute to force generation. Changes in the extent of microtubule overlap during spindle elongation were affected by tubulin and ATP concentration in the incubation medium. Spindles continued to elongate even after the overlap zone was composed entirely of newly polymerized microtubules, suggesting that the enzyme responsible for microtubule translocation either is bound to a matrix in the spindle midzone, or else can move on one microtubule toward the spindle midzone and push another microtubule of opposite polarity toward the pole.

Direct observation of mitotic spindle elongation in vitro

Successful reactivation in vitro of anaphase B has recently been achieved with mitotic spindles isolated from the diatom Stephanopyxis turris. When a population of isolated spindles was studied indirectly by using immunofluorescence, nearly all of them were found to have elongated; however, when studied directly by using video microscopy, only a small proportion of spindles elongated. We report here conditions that allow nearly all of the spindles to elongate when observed directly with video microscopy. These direct observations validate previous ones made using indirect immunofluorescence. In addition, we find that the isolated spindles elongate with a linear rate, that the elongation is unchanged after the chromatin surrounding the spindles is digested with DNase I, and that during elongation a phase-dense matrix may accumulate in the spindle midzone.