Minus-end-directed motion of kinesin-coated microspheres driven by microtubule depolymerization

Centrosomal control of microtubule dynamics

In many animal cells, minus ends of microtubules (MTs) are thought to be capped by the centrosome whereas plus ends are free and display dynamic instability. We tested the role of the centrosome by examining MT behavior in cytoplasts from which the centrosome was removed. Cells were injected with Cy3-tubulin to fluorescently label MTs and were enucleated by using a centrifugation procedure. Enucleation resulted in a mixture of cytoplasts containing or lacking the centrosome. Fibroblast (CHO-K1) and epithelial (BSC-1) cells were investigated. In fibroblast cytoplasts containing the centrosome, MTs showed dynamic instability indistinguishable from that in intact cells. In contrast, in cytoplasts lacking the centrosome, MTs treadmilled-shortened at the minus end at about 12 micrometers/min while growing at the plus end at the same rate. The change in behavior of the plus end from dynamic instability to persistent growth correlated with an elevated level of free tubulin subunits (78% in centrosome-free cytoplasts vs. 44% in intact cells) generated by minus-end depolymerization. In contrast to fibroblast cells, in centrosome-free cytoplasts prepared from epithelial cells, MTs displayed dynamic instability at plus ends and relative stability at minus ends presumably because of specific minus-end stability factors distributed throughout the cytoplasm. We suggest that, in fibroblast cells, a minus-end depolymerization mechanism functions to eliminate errors in MT organization and that dynamic instability of MT plus ends is a result of capping of minus ends by the centrosome.

Dynamics of axonal microtubules regulate the topology of new membrane insertion into the growing neurites

Nerve growth depends on the delivery of cell body-synthesized material to the growing neuronal processes. The cellular mechanisms that determine the topology of new membrane addition to the axon are not known. Here we describe a technique to visualize the transport and sites of exocytosis of cell body- derived vesicles in growing axons. We found that in Xenopus embryo neurons in culture, cell body-derived vesicles were rapidly transported all the way down to the growth cone region, where they fused with the plasma membrane. Suppression of microtubule (MT) dynamic instability did not interfere with the delivery of new membrane material to the growth cone region; however, the insertion of vesicles into the plasma membrane was dramatically inhibited. Local disassembly of MTs by focal application of nocodazole to the middle axonal segment resulted in the addition of new membrane at the site of drug application. Our results suggest that the local destabilization of axonal MTs is necessary and sufficient for the delivery of membrane material to specific neuronal sites.

Overexpression of tau protein inhibits kinesin-dependent trafficking of vesicles, mitochondria, and endoplasmic reticulum: implications for Alzheimer's disease

The neuronal microtubule-associated protein tau plays an important role in establishing cell polarity by stabilizing axonal microtubules that serve as tracks for motor-protein-driven transport processes. To investigate the role of tau in intracellular transport, we studied the effects of tau expression in stably transfected CHO cells and differentiated neuroblastoma N2a cells. Tau causes a change in cell shape, retards cell growth, and dramatically alters the distribution of various organelles, known to be transported via microtubule-dependent motor proteins. Mitochondria fail to be transported to peripheral cell compartments and cluster in the vicinity of the microtubule-organizing center. The endoplasmic reticulum becomes less dense and no longer extends to the cell periphery. In differentiated N2a cells, the overexpression of tau leads to the disappearance of mitochondria from the neurites. These effects are caused by tau's binding to microtubules and slowing down intracellular transport by preferential impairment of plus-end-directed transport mediated by kinesin-like motor proteins. Since in Alzheimer's disease tau protein is elevated and mislocalized, these observations point to a possible cause for the gradual degeneration of neurons.

The dynamic behavior of individual microtubules associated with chromosomes in vitro

Mitotic movements of chromosomes are usually coupled to the elongation and shortening of the microtubules to which they are bound. The lengths of kinetochore-associated microtubules change by incorporation or loss of tubulin subunits, principally at their chromosome-bound ends. We have reproduced aspects of this phenomenon in vitro, using a real-time assay that displays directly the movements of individual chromosome-associated microtubules as they elongate and shorten. Chromosomes isolated from cultured Chinese hamster ovary cells were adhered to coverslips and then allowed to bind labeled microtubules. In the presence of tubulin and GTP, these microtubules could grow at their chromosome-bound ends, causing the labeled segments to move away from the chromosomes, even in the absence of ATP. Sometimes a microtubule would switch to shortening, causing the direction of movement to change abruptly. The link between a microtubule and a chromosome was mechanically strong; 15 pN of tension was generally insufficient to detach a microtubule, even though it could add subunits at the kinetochore-microtubule junction. The behavior of the microtubules in vitro was regulated by the chromosomes to which they were bound; the frequency of transitions from polymerization to depolymerization was decreased, and the speed of depolymerization-coupled movement toward chromosomes was only one-fifth the rate of shortening for microtubules free in solution. Our results are consistent with a model in which each microtubule interacts with an increasing number of chromosome-associated binding sites as it approaches the kinetochore.

Simulating the role of microtubules in depolymerization-driven transport: a Monte Carlo approach

In this paper we present a model that simulates the role of microtubules in depolymerization-driven transport. The model simulates a system that consists of a 13-protofilament microtubule with "five-start" helical structure and a motor protein-coated bead that moves along one of the protofilaments of the microtubule, as in in vitro experiments. The microtubule is simulated using the lateral cap model, with substantial generalizations. For the new terminal configurations in the presence of the bead, rate constants for association and dissociation events of tubulin molecules are calculated by exploring the geometric similarities between different patterns of terminal configurations and by decomposing complex patterns into simpler patterns whose corresponding rate constants are known. In comparison with a previous model, in which simplifications are made about the structure of the microtubule and in which the microtubule can only depolymerize, the detailed structure of the microtubule is taken into account in the present model. Furthermore, the microtubule can be either polymerizing or depolymerizing. Force-velocity curves are obtained for both zero and non-zero tubulin guanosine 5'-triphosphate (GTP) concentrations. By analyzing the trajectory of the bead under different parameters, the condition for "run and pause" is analyzed, and the time scale of "run" and "pause" is found to be different for different motor proteins. We also suggest experiments that can be used to examine the results predicted by the model.

Mechanisms of nuclear positioning

The mechanisms underlying two types of microtubule-dependent nuclear positioning are discussed. ‘MTOC-dependent nuclear positioning’ occurs when a nucleus is tightly associated with a microtubule organizing center (MTOC). ‘Nuclear tracking along microtubules’ is analogous to the motor-driven motility of other organelles and occurs when the nucleus lacks an associated MTOC. These two basic types of microtubule-dependent nuclear positioning may cooperate in many proliferating animal cells to achieve proper nuclear positioning. Microtubule polymerization and dynamics, motor proteins, MAPs and specialized sites such as cortical anchors function to control nuclear movements within cells.

The kinesin-related proteins, Kip2p and Kip3p, function differently in nuclear migration in yeast

The roles of two kinesin-related proteins, Kip2p and Kip3p, in microtubule function and nuclear migration were investigated. Deletion of either gene resulted in nuclear migration defects similar to those described for dynein and kar9 mutants. By indirect immunofluorescence, the cytoplasmic microtubules in kip2Delta were consistently short or absent throughout the cell cycle. In contrast, in kip3Delta strains, the cytoplasmic microtubules were significantly longer than wild type at telophase. Furthermore, in the kip3Delta cells with nuclear positioning defects, the cytoplasmic microtubules were misoriented and failed to extend into the bud. Localization studies found Kip2p exclusively on cytoplasmic microtubules throughout the cell cycle, whereas GFP-Kip3p localized to both spindle and cytoplasmic microtubules. Genetic analysis demonstrated that the kip2Delta kar9Delta double mutants were synthetically lethal, whereas kip3Delta kar9Delta double mutants were viable. Conversely, kip3Delta dhc1Delta double mutants were synthetically lethal, whereas kip2Delta dhc1Delta double mutants were viable. We suggest that the kinesin-related proteins, Kip2p and Kip3p, function in nuclear migration and that they do so by different mechanisms. We propose that Kip2p stabilizes microtubules and is required as part of the dynein-mediated pathway in nuclear migration. Furthermore, we propose that Kip3p functions, in part, by depolymerizing microtubules and is required for the Kar9p-dependent orientation of the cytoplasmic microtubules.

Endoplasmic reticulum membrane tubules are distributed by microtubules in living cells using three distinct mechanisms

BACKGROUND

The microtubule-dependent motility of endoplasmic reticulum (ER) tubules is fundamental to the structure and function of the ER. From in vitro assays, three mechanisms for ER tubule motility have arisen: the 'membrane sliding mechanism' in which ER tubules slide along microtubules using microtubule motor activity; the 'microtubule movement mechanism' in which ER attaches to moving microtubules; and the 'tip attachment complex (TAC) mechanism' in which ER tubules attach to growing plus ends of microtubules.

RESULTS

We have used multi-wavelength time-lapse epifluorescence microscopy to image the dynamic interactions between microtubules (by microinjection of X-rhodamine-labeled tubulin) and ER (by DiOC6(3) staining) in living cells to determine which mechanism contributes to the formation and motility of ER tubules in migrating cells in vivo. Newly forming ER tubules extended only in a microtubule plus-end direction towards the cell periphery: 31.4% by TACs and 68.6% by the membrane sliding mechanism. ER tubules, statically attached to microtubules, moved towards the cell center with microtubules through actomyosin-based retrograde flow. TACs did not change microtubule growth and shortening velocities, but reduced transitions between these states. Treatment of cells with 100 nM nocodazole to inhibit plus-end microtubule dynamics demonstrated that TAC motility required microtubule assembly dynamics, whereas membrane sliding and retrograde-flow-driven ER motility did not.

CONCLUSIONS

Both plus-end-directed membrane sliding and TAC mechanisms make significant contributions to the motility of ER towards the periphery of living cells, whereas ER removal from the lamella is powered by actomyosin-based retrograde flow of microtubules with ER attached as cargo. TACs in the ER modulate plus-end microtubule dynamics.

The yeast dynactin complex is involved in partitioning the mitotic spindle between mother and daughter cells during anaphase B

Although vertebrate cytoplasmic dynein can move to the minus ends of microtubules in vitro, its ability to translocate purified vesicles on microtubules depends on the presence of an accessory complex known as dynactin. We have cloned and characterized a novel gene, NIP100, which encodes the yeast homologue of the vertebrate dynactin complex protein p150(glued). Like strains lacking the cytoplasmic dynein heavy chain Dyn1p or the centractin homologue Act5p, nip100Delta strains are viable but undergo a significant number of failed mitoses in which the mitotic spindle does not properly partition into the daughter cell. Analysis of spindle dynamics by time-lapse digital microscopy indicates that the precise role of Nip100p during anaphase is to promote the translocation of the partially elongated mitotic spindle through the bud neck. Consistent with the presence of a true dynactin complex in yeast, Nip100p exists in a stable complex with Act5p as well as Jnm1p, another protein required for proper spindle partitioning during anaphase. Moreover, genetic depletion experiments indicate that the binding of Nip100p to Act5p is dependent on the presence of Jnm1p. Finally, we find that a fusion of Nip100p to the green fluorescent protein localizes to the spindle poles throughout the cell cycle. Taken together, these results suggest that the yeast dynactin complex and cytoplasmic dynein together define a physiological pathway that is responsible for spindle translocation late in anaphase.

Organelle transport and molecular motors in fungi

Polarized growth, secretion of exoenzymes, organelle inheritance, and organelle positioning require vectorial transport along cytoskeletal elements. The discovery of molecular motors and intensive studies on their biological function during the past 3 years confirmed a central role of these mechanoenzymes in morphogenesis and development of yeasts and filamentous fungi. Saccharomyces cerevisiae proved to be an excellent model system, in which the complete set of molecular motors is presumed to be known. Genetic studies combined with cell biological methods revealed unexpected functional relationships between these motors and has greatly improved our understanding of nuclear migration, exocytosis, and endocytosis in yeasts. Tip growth of elongated hyphae, compared to budding, however, does require vectorial transport over long distances. The identification of ubiquitous motors that are not present in yeast indicates that studies on filamentous fungi might be helpful to elucidate the role of motors in long-distance organelle transport within higher eukaryotic cells. Copyright 1998 Academic Press.

A role for microtubule dynamics in phagosome movement

We have shown previously that intracellular phagosome movement requires microtubules. Here we provide evidence that within cells phagosomes display two different kinds of microtubule-based movements in approximately equal proportions. The first type occurs predominantly in the cell periphery, often shortly after the phagosome is formed, and at speeds below 0.1 microm/second. The second is faster (0.2-1.5 micron/second) and occurs mainly after phagosomes have reached the cell interior. Treating cells with nanomolar concentrations of taxol or nocodazole alters microtubule dynamics without affecting either total polymer mass or microtubule organisation. Such treatments slow the accumulation of phagosomes in the perinuclear region and reduce the number of slow movements by up to 50% without affecting the frequency of fast movements. This suggests that a proportion of slow movements are mediated by microtubule dynamics while fast movements are powered by microtubule motors. In macrophages, interphase microtubules radiate from the microtubule organising centre with their plus-end towards the cell periphery. To understand the behaviour of ‘early’ phagosomes at the cell periphery we investigated their ability to bind microtubule plus-ends in vitro. We show that early phagosomes have a strong preference for microtubule plus-ends, whereas ‘late’ phagosomes do not, and that plus-end affinity requires the presence of microtubule-associated proteins within cytosol. We suggest that phagosomes can bind to the plus-ends of dynamic microtubules and move by following their shrinkage or growth.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

Bacterial cell division

Bacteria usually divide by building a central septum across the middle of the cell. This review focuses on recent results indicating that the tubulin-like FtsZ protein plays a central role in cytokinesis as a major component of a contractile cytoskeleton. Assembly of this cytoskeletal element abutting the membrane is a key point for regulation. The characterization of FtsZ homologues in Mycoplasmas, Archaea, and chloroplasts implies that the constriction mechanism is conserved and that FtsZ can constrict in the absence of peptidoglycan synthesis. In most Eubacteria, the internal cytoskeleton must also regulate synthesis of septal peptidoglycan. The Escherichia coli septum-specific penicillin-binding protein 3 (PBP3) forms a complex with other enzymes involved in murein metabolism, suggesting a centrally located transmembrane complex capable of splicing multiple new strands of peptidoglycan into the cell wall. Important questions remain about the spatial and temporal control of bacterial division.

Coupling cell division and cell death to microtubule dynamics

The mitotic spindle is a self-organizing structure that is constructed primarily from microtubules. Among the most important spindle microtubules are those that bind to kinetochores and form the fibers along which chromosomes move. Chemotherapeutics such as taxol and the vinca alkaloids perturb kinetochore-microtubule attachment and disrupt chromosome segregation. This activates a checkpoint pathway that delays cell cycle progression and induces programmed cell death. Recent work has identified at least four mammalian spindle assembly checkpoint proteins.

Measurement of the force-velocity relation for growing microtubules

Forces generated by protein polymerization are important for various forms of cellular motility. Assembling microtubules, for instance, are believed to exert pushing forces on chromosomes during mitosis. The force that a single microtubule can generate was measured by attaching microtubules to a substrate at one end and causing them to push against a microfabricated rigid barrier at the other end. The subsequent buckling of the microtubules was analyzed to determine both the force on each microtubule end and the growth velocity. The growth velocity decreased from 1.2 micrometers per minute at zero force to 0.2 micrometer per minute at forces of 3 to 4 piconewtons. The force-velocity relation fits well to a decaying exponential, in agreement with theoretical models, but the rate of decay is faster than predicted.

CENP-E is a plus end-directed kinetochore motor required for metaphase chromosome alignment

Mitosis requires dynamic attachment of chromosomes to spindle microtubules. This interaction is mediated largely by kinetochores. During prometaphase, forces exerted at kinetochores, in combination with polar ejection forces, drive congression of chromosomes to the metaphase plate. A major question has been whether kinetochore-associated microtubule motors play an important role in congression. Using immunodepletion from and antibody addition to Xenopus egg extracts, we show that the kinetochore-associated kinesin-like motor protein CENP-E is essential for positioning chromosomes at the metaphase plate. We further demonstrate that CENP-E powers movement toward microtubule plus ends in vitro. These findings support a model in which CENP-E functions in congression to tether kinetochores to dynamic microtubule plus ends.

The microtubule-dependent motor centromere-associated protein E (CENP-E) is an integral component of kinetochore corona fibers that link centromeres to spindle microtubules

Centromere-associated protein E (CENP-E) is a kinesin-related microtubule motor protein that is essential for chromosome congression during mitosis. Using immunoelectron microscopy, CENP-E is shown to be an integral component of the kinetochore corona fibers that tether centromeres to the spindle. Immediately upon nuclear envelope fragmentation, an associated plus end motor trafficks cytoplasmic CENP-E toward chromosomes along astral microtubules that enter the nuclear volume. Before or concurrently with initial lateral attachment of spindle microtubules, CENP-E targets to the outermost region of the developing kinetochores. After stable attachment, throughout chromosome congression, at metaphase, and throughout anaphase A, CENP-E is a constituent of the corona fibers, extending at least 50 nm away from the kinetochore outer plate and intertwining with spindle microtubules. In congressing chromosomes, CENP-E is preferentially associated with (or accessible at) the stretched, leading kinetochore known to provide the primary power for chromosome movement. Taken together, this evidence strongly supports a model in which CENP-E functions in congression to tether kinetochores to the disassembling microtubule plus ends.

Microtubules orient the mitotic spindle in yeast through dynein-dependent interactions with the cell cortex

Proper orientation of the mitotic spindle is critical for successful cell division in budding yeast. To investigate the mechanism of spindle orientation, we used a green fluorescent protein (GFP)-tubulin fusion protein to observe microtubules in living yeast cells. GFP-tubulin is incorporated into microtubules, allowing visualization of both cytoplasmic and spindle microtubules, and does not interfere with normal microtubule function. Microtubules in yeast cells exhibit dynamic instability, although they grow and shrink more slowly than microtubules in animal cells. The dynamic properties of yeast microtubules are modulated during the cell cycle. The behavior of cytoplasmic microtubules revealed distinct interactions with the cell cortex that result in associated spindle movement and orientation. Dynein-mutant cells had defects in these cortical interactions, resulting in misoriented spindles. In addition, microtubule dynamics were altered in the absence of dynein. These results indicate that microtubules and dynein interact to produce dynamic cortical interactions, and that these interactions result in the force driving spindle orientation.

The Saccharomyces cerevisiae kinesin-related motor Kar3p acts at preanaphase spindle poles to limit the number and length of cytoplasmic microtubules

The Saccharomyces cerevisiae kinesin-related motor Kar3p, though known to be required for karyogamy, plays a poorly defined, nonessential role during vegetative growth. We have found evidence suggesting that Kar3p functions to limit the number and length of cytoplasmic microtubules in a cell cycle-specific manner. Deletion of KAR3 leads to a dramatic increase in cytoplasmic microtubules, a phenotype which is most pronounced from START through the onset of anaphase but less so during late anaphase in synchronized cultures. We have immunolocalized HA-tagged Kar3p to the spindle pole body region, and fittingly, Kar3p was not detected by late anaphase. A microtubule depolymerizing activity may be the major vegetative role for Kar3p. Addition of the microtubule polymerization inhibitors nocodazol or benomyl to the medium or deletion of the nonessential alpha-tubulin TUB3 gene can mostly correct the abnormal microtubule arrays and other growth defects of kar3 mutants, suggesting that these phenotypes result from excessive microtubule polymerization. Microtubule depolymerization may also be the mechanism by which Kar3p acts in opposition to the anaphase B motors Cin8p and Kip1p. A preanaphase spindle collapse phenotype of cin8 kip1 mutants, previously shown to involve Kar3p, is markedly delayed when microtubule depolymerization is inhibited by the tub2-150 mutation. These results suggest that the Kar3p motor may act to regulate the length and number of microtubules in the preanaphase spindle.

The role of microtubule assembly dynamics in mitotic force generation and functional organization of living cells

This article summarizes the author's presentation at the Baylor Medical School Symposium on the Biophysics of Microtubules, held April 12 to 14, 1996, in Houston, Texas. It presents a brief historical sketch and discusses the role that assembly/disassembly of microtubules is likely to be playing in force generation for chromosome movement and related organellar positioning in living cells. The article starts out with how polarized light microscopy of living cells had laid the foundation for this concept in the 1950s and 1960s, but was then eclipsed for some 2 decades following the discovery of force generation by microtubule sliding powered by an ATP-hydrolyzing motor protein, dynein. The intriguing recent discoveries: that microtubules undergo dynamic instability; that they both assemble and disassemble right at the kinetochore where they are attached to the chromosome; and that assembling and disassembling microtubules can of themselves push and pull reasonable loads in model experiments, even in the absence of hydrolyzable nucleotides, have refocused serious attention on the probable role played by assembly/disassembly of microtubules. This mode of force generation may well be intricately coupled, and interact, with force-generating and/or dynamic attachment roles played by "motor" proteins, especially at the kinetochore.

How tubulin subunits are lost from the shortening ends of microtubules

Microtubules exhibit dynamic instability, switching between persistent states of growth and shortening at their ends. The switch between growth and shortening has been proposed to depend on end conformation where growing ends have "straight" tubulin protofilaments stabilized by a terminal cap of GTP-tubulin, while-shortening ends have lost their GTP-tubulin cap, allowing terminal GDP-tubulin dimers to curve inside-out and peel rapidly away from the microtubule lattice. This "conformational cap" model predicts that tubulin dissociation from shortening ends is a two-step process where the average lengths of curved GDP-tubulin protofilaments at a depolymerizing end will depend on the ratio of the rate of peeling to the rate of breakage of the longitudinal bonds between adjacent curved dimers. We have tested this model for the plus and minus ends of microtubules assembled with pure porcine tubulin off the ends of axoneme fragments in standard assembly buffer. Individual microtubule ends were imaged using video-enhanced differential interference contrast light microscopy. The rate of rapid shortening was systematically increased by isothermal dilution into assembly buffer containing various concentrations of Mg2+ or Ca2+ ions. At 1 mM Mg2+ and no Ca2+, shortening occurred at 20 (plus) and 45 (minus) microns/min. The ends appeared similar in contrast to growing ends and the core of the microtubule and the ends appeared blunt or slightly frayed by negative stain electron microscopy. Above 20 mM Mg2+ or above 5 mM Ca2+, microtubule shortening occurred at 60 (plus) and 115 (minus) microns/min or faster and "knobs" were distinctly visible at depolymerizing ends, particularly at the faster minus ends, and knob contrast remained constant during many micrometers of rapid shortening. Negative stain electron microscopy revealed that these knobs were "blossoms" of inside-out curved protofilaments, some extending for several helical turns (30 to 60 dimers in length) at constant curvature from the ends. At these high shortening velocities, the peeling of curved protofilaments was confined to within several dimers of the end of the microtubule cylinder, suggesting that dimer curling and protofilament peeling is constrained to the tip by interactions between adjacent straight protofilaments. Depolymerization is produced by conformational changes in GDP-tubulin since microtubules assembled with a slowly hydrolizable analog of GTP, GMPCPP, are stable even at 20 mM Mg2+ or 5 mM Ca2+. Monte Carlo simulations show that the ratio of the peeling to breakage rate constants can control the steady-state average length of curved GDP-tubulin protofilaments at the depolymerizing end.

How cells get the right chromosomes

When cells divide, the chromosomes must be delivered flawlessly to the daughter cells. Missing or extra chromosomes can result in birth defects and cancer. Chance events are the starting point for chromosome delivery, which makes the process prone to error. Errors are avoided by diverse uses of mechanical tension from mitotic forces. Tension stabilizes the proper chromosome configuration, controls a cell cycle checkpoint, and changes chromosome chemistry.

Dynamic elastic behavior of alpha-satellite DNA domains visualized in situ in living human cells

We have constructed a fluorescent alpha-satellite DNA-binding protein to explore the motile and mechanical properties of human centromeres. A fusion protein consisting of human CENP-B coupled to the green fluorescent protein (GFP) of A. victoria specifically targets to centromeres when expressed in human cells. Morphometric analysis revealed that the alpha-satellite DNA domain bound by CENPB-GFP becomes elongated in mitosis in a microtubule-dependent fashion. Time lapse confocal microscopy in live mitotic cells revealed apparent elastic deformations of the central domain of the centromere that occurred during metaphase chromosome oscillations. These observations demonstrate that the interior region of the centromere behaves as an elastic element that could play a role in the mechanoregulatory mechanisms recently identified at centromeres. Fluorescent labeling of centromeres revealed that they disperse throughout the nucleus in a nearly isometric expansion during chromosome decondensation in telophase and early G1. During interphase, centromeres were primarily stationary, although motility of individual or small groups of centromeres was occasionally observed at very slow rates of 7-10 microns/h.

Centrosome and spindle function of the Drosophila Ncd microtubule motor visualized in live embryos using Ncd-GFP fusion proteins

The Ncd microtubule motor protein is required for meiotic and early mitotic chromosome distribution in Drosophila. Null mutant females expressing the Ncd motor fused to the Aequorea victoria green fluorescent protein (GFP), regulated by the wild-type ncd promoter, are rescued for chromosome segregation and embryo viability. Analysis of mitosis in live embryos shows cell cycle-dependent localization of Ncd-GFP to centrosomes and spindles. The distribution of Ncd-GFP in spindles during metaphase differs strikingly from that of tubulin: the tubulin staining is excluded by the chromosomes at the metaphase plate; in contrast, Ncd-GFP forms filaments along the spindle microtubules that extend across the chromosomes. The existence of Ncd-GFP fibers that cross the metaphase plate suggests that Ncd interacts functionally with chromosomes in metaphase. Differences are no longer observed in anaphase when the chromosomes have moved off the metaphase plate. A mutant form of Ncd fused to GFP also localizes to spindles in live embryos. Mutant embryos show frequent centrosome and spindle abnormalities, including free centrosomes that dissociate from interphase nuclei, precociously split centrosomes, and spindles with microtubule spurs or bridges to nearby spindles. The precociously split and free centrosomes indicate that the Ncd motor acts in cleavage stage embryos to maintain centrosome integrity and attachment to nuclei. The frequent spindle spurs of mutant embryos are associated with mis-segregating chromosomes that partially detach from the spindle in metaphase, but can be recaptured in early anaphase. This implies that the Ncd motor functions to prevent chromosome loss by maintaining chromosome attachment to the spindle in metaphase, consistent with the Ncd-GFP fibers that across the metaphase plate.

Fission yeast pkl1 is a kinesin-related protein involved in mitotic spindle function

We have used anti-peptide antibodies raised against highly conserved regions of the kinesin motor domain to identify kinesin-related proteins in the fission yeast Schizosaccharomyces pombe. Here we report the identification of a new kinesin-related protein, which we have named pkl1. Sequence homology and domain organization place pkl1 in the Kar3/ncd subfamily of kinesin-related proteins. Bacterially expressed pkl1 fusion proteins display microtubule-stimulated ATPase activity, nucleotide-sensitive binding, and bundling of microtubules. Immunofluorescence studies with affinity-purified antibodies indicate that the pkl1 protein localizes to the nucleus and the mitotic spindle. Pkl1 null mutants are viable but have increased sensitivity to microtubule-disrupting drugs. Disruption of pkl1+ suppresses mutations in another kinesin-related protein, cut7, which is known to act in the spindle. Overexpression of pkl1 to very high levels causes a similar phenotype to that seen in cut7 mutants: V-shaped and star-shaped microtubule structures are observed, which we interpret to be spindles with unseparated spindle poles. These observations suggest that pkl1 and cut7 provide opposing forces in the spindle. We propose that pkl1 functions as a microtubule-dependent motor that is involved in microtubule organization in the mitotic spindle.

Kinetochore function: molecular motors, switches and gates

Kinetochores are essential for accurate chromosome segregation. Recent studies reveal that vertebrate kinetochores are sophisticated propulsion systems composed of not only force generators but also "navigation' and "fail-safe' mechanisms. Advances toward the understanding of the biochemical composition and activities of the components of the kinetochore have come from the molecular characterization of key proteins of the kinetochore complex.

Cytoskeleton: a catastrophic kinesin

The 'plus' ends of microtubules exhibit dynamic instability, switching stochastically from growth to shortening phases. The first endogenous regulator of such 'catastrophes' has been identified, and is a kinesin-related microtubule motor protein.

Kinesin proteins: a phylum of motors for microtubule-based motility

The cellular processes of transport, division and, possibly, early development all involve microtubule-based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin-related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule-motor proteins that share a 340-amino-acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles.

Motors involved in spindle assembly and chromosome segregation

During the past two years, major advances have been made in our understanding of the role of motor proteins in chromosome-microtubule interactions in the spindle. The discovery of kinesin-like proteins (KLPs) associated with chromosome arms has shed some light on the mechanism of chromosome congression and the establishment of spindle bipolarity. Recent results also indicate that kinetochore KLPs may tether the ends of growing and shrinking microtubules to kinetochores during chromosome movements. Finally, new data indicate that phosphorylation of KLPs may be one of the mechanisms by which they are targeted to specific spindle domains.

XKCM1: a Xenopus kinesin-related protein that regulates microtubule dynamics during mitotic spindle assembly

We isolated a cDNA clone encoding a kinesin-related protein, which we named XKCM1. Antibodies to XKCM1 stain mitotic centromeres and spindle poles. Immunodepletion and antibody addition experiments in an in vitro spindle assembly assay show that XKCM1 is required for both establishment and maintenance of mitotic spindles. The structures that form in the absence of XKCM1 contain abnormally long microtubules. This long microtubule defect can be rescued by the addition of purified XKCM1 protein. Analysis of microtubule dynamics in a clarified mitotic extract reveals that loss of XKCM1 function causes a 4-fold suppression in the catastrophe frequency. XKCM1 thus exhibits a novel activity for a kinesin-related protein by promoting microtubule depolymerization during mitotic spindle assembly.

Bacterial cell division protein FtsZ assembles into protofilament sheets and minirings, structural homologs of tubulin polymers

The bacterial cell division protein FtsZ is a homolog of tubulin, but it has not been determined whether FtsZ polymers are structurally related to the microtubule lattice. In the present study, we have obtained high-resolution electron micrographs of two FtsZ polymers that show remarkable similarity to tubulin polymers. The first is a two-dimensional sheet of protofilaments with a lattice very similar to that of the microtubule wall. The second is a miniring, consisting of a single protofilament in a sharply curved, planar conformation. FtsZ minirings are very similar to tubulin rings that are formed upon disassembly of microtubules but are about half the diameter. This suggests that the curved conformation occurs at every FtsZ subunit, but in tubulin rings the conformation occurs at either beta- or alpha-tubulin subunits but not both. We conclude that the functional polymer of FtsZ in bacterial cell division is a long thin sheet of protofilaments. There is sufficient FtsZ in Escherichia coli to form a protofilament that encircles the cell 20 times. The similarity of polymers formed by FtsZ and tubulin implies that the protofilament sheet is an ancient cytoskeletal system, originally functioning in bacterial cell division and later modified to make microtubules.

Phosphorylation by p34cdc2 regulates spindle association of human Eg5, a kinesin-related motor essential for bipolar spindle formation in vivo

We have isolated a human homolog of Xenopus Eg5, a kinesin-related motor protein implicated in the assembly and dynamics of the mitotic spindle. We report that microinjection of antibodies against human Eg5 (HsEg5) blocks centrosome migration and causes HeLa cells to arrest in mitosis with monoastral microtubule arrays. Furthermore, an evolutionarily conserved cdc2 phosphorylation site (Thr-927) in HsEg5 is phosphorylated specifically during mitosis in HeLa cells and by p34cdc2/cyclin B in vitro. Mutation of Thr-927 to nonphosphorylatable residues prevents HsEg5 from binding to centrosomes, indicating that phosphorylation controls the association of this motor with the spindle apparatus. These results indicate that HsEg5 is required for establishing a bipolar spindle and that p34cdc2 protein kinase directly regulates its localization.

Force production by depolymerizing microtubules: load-velocity curves and run-pause statistics

Experiments indicate that depolymerization of microtubules generates sufficient force to produce the minus-end-directed transport of chromosomes during mitosis (Koshland et al., 1988). In vitro, analogous transport of kinesin-coated microspheres exhibits a paradoxical effect. Minus-end-directed transport of the microspheres driven by depolymerization is enhanced by the presence of ATP, which fuels the motor action of kinesin driving the microspheres in the opposite direction, toward the plus end of the microtubule. Here we present a mathematical model to explain this behavior. We postulate that a microsphere at the plus end of the microtubule facilitates depolymerization and hence enhances minus-end-directed transport. The force-velocity curve of the model is derived; it has the peculiar feature that velocity is maximal at some positive load (opposing the motion) rather than at zero load. The model is used to simulate the stochastic process of microsphere-facilitated depolymerization-driven transport. Simulated trajectories at low load show distinctive runs and pauses, the statistics of which are calculated from the model. The statistics of the process provide sufficient information to determine all of the model's parameters.

Force generation by microtubule assembly/disassembly in mitosis and related movements

In this article, we review the dynamic nature of the filaments (microtubules) that make up the labile fibers of the mitotic spindle and asters, we discuss the roles that assembly and disassembly of microtubules play in mitosis, and we consider how such assembling and disassembling polymer filaments can generate forces that are utilized by the living cell in mitosis and related movements.

Cell polarity. The importance of being polar

Cell polarization is often accompanied by cytoskeletal rearrangements. Two signalling proteins, a GTPase and a kinase, are required for both actin and microtubule rearrangements. Are these two systems coupled?

Membrane/microtubule tip attachment complexes (TACs) allow the assembly dynamics of plus ends to push and pull membranes into tubulovesicular networks in interphase Xenopus egg extracts

We discovered by using high resolution video microscopy, that membranes become attached selectively to the growing plus ends of microtubules by membrane/microtubule tip attachment complexes (TACs) in interphase-arrested, undiluted, Xenopus egg extracts. Persistent plus end growth of stationary microtubules pushed the membranes into thin tubules and dragged them through the cytoplasm at the approximately 20 microns/min velocity typical of free plus ends. Membrane tubules also remained attached to plus ends when they switched to the shortening phase of dynamic instability at velocities typical of free ends, 50-60 microns/min. Over time, the membrane tubules contacted and fused with one another along their lengths, forming a polygonal network much like the distribution of ER in cells. Several components of the membrane networks formed by TACs were identified as ER by immunofluorescent staining using antibodies to ER-resident proteins. TAC motility was not inhibited by known inhibitors of microtubule motor activity, including 5 mM AMP-PNP, 250 microM orthovanadate, and ATP depletion. These results show that membrane/microtubule TACs enable polymerizing ends to push and depolymerizing ends to pull membranes into thin tubular extensions and networks at fast velocities.

Kinetic stabilization of microtubule dynamics at steady state in vitro by substoichiometric concentrations of tubulin-colchicine complex

We have analyzed the effects of tubulin-colchicine (TC)-complex on the dynamic instability behavior of bovine brain microtubules at steady state in vitro using video microscopy. Incorporation of low numbers of TC-complexes per microtubule strongly suppressed dynamics at the plus ends by reducing the rate and extent of growing and shortening and by increasing the time microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, TC-complex strongly suppressed the catastrophe frequency and increased the rescue frequency. At low concentrations (0.02-0.05 microM), TC-complex suppressed dynamics without reducing the polymer mass or the mean microtubule length. Such strong suppression of microtubule dynamics by low TC-complex concentrations in the absence of polymer mass changes demonstrates that microtubule dynamics are more sensitive to the actions of TC-complex than the polymer mass. Significant reduction of polymer mass occurred at relatively high TC-complex concentration (> 0.05 microM). However, the surviving microtubules were extremely stable. Thus, TC-complex stabilizes microtubules even though the microtubules can transiently depolymerize when TC-complex is added. The data also directly establish that kinetic suppression of dynamics by colchicine at low concentrations is effected by a low number of TC-complexes at the microtubule ends.

Microtubule dynamics. Kinetochores get a grip

Analysis of the interactions between purified motor proteins or isolated chromosomes and shrinking microtubules has shed light on the mechanism of chromosome segregation at mitosis.

Multi-dynein hypothesis

Axonemal dyneins and cytoplasmic dynein have evolved separate strategies to perform their tasks. The multi-dynein hypothesis accurately describes the highly specialized axonemal isoforms; each isoform is encoded by a separate gene, is located in a precise place, produces specific forces which contribute to the overall generation of propagated bending, and is not functionally interchangeable with other isoforms. In contrast, cytoplasmic dynein, although carrying many different cargoes, appears to be one isoform. An intriguing question is to determine whether there are additional cytoplasmic dyneins, heretofore uncharacterized, which, like their axonemal counterparts, are customized to perform specific tasks.