Reconstitution of physiological microtubule dynamics using purified components

More is different: Reconstituting complexity in microtubule regulation

Microtubules are dynamic cytoskeletal filaments that undergo stochastic switching between phases of polymerization and depolymerization-a behavior known as dynamic instability. Many important cellular processes, including cell motility, chromosome segregation, and intracellular transport, require complex spatiotemporal regulation of microtubule dynamics. This coordinated regulation is achieved through the interactions of numerous microtubule-associated proteins (MAPs) with microtubule ends and lattices. Here, we review the recent advances in our understanding of microtubule regulation, focusing on results arising from biochemical in vitro reconstitution approaches using purified multiprotein ensembles. We discuss how the combinatory effects of MAPs affect both the dynamics of individual microtubule ends, as well as the stability and turnover of the microtubule lattice. In addition, we highlight new results demonstrating the roles of protein condensates in microtubule regulation. Our overall intent is to showcase how lessons learned from reconstitution approaches help unravel the regulatory mechanisms at play in complex cellular environments.

Integrated regulation of tubulin tyrosination and microtubule stability by human α-tubulin isotypes

Tubulin isotypes are critical for the functions of cellular microtubules, which exhibit different stability and harbor various post-translational modifications. However, how tubulin isotypes determine the activities of regulators for microtubule stability and modifications remains unknown. Here, we show that human α4A-tubulin, a conserved genetically detyrosinated α-tubulin isotype, is a poor substrate for enzymatic tyrosination. To examine the stability of microtubules reconstituted with defined tubulin compositions, we develop a strategy to site-specifically label recombinant human tubulin for single-molecule TIRF microscopy-based in vitro assays. The incorporation of α4A-tubulin into the microtubule lattice stabilizes the polymers from passive and MCAK-stimulated depolymerization. Further characterization reveals that the compositions of α-tubulin isotypes and tyrosination/detyrosination states allow graded control for the microtubule binding and the depolymerization activities of MCAK. Together, our results uncover the tubulin isotype-dependent enzyme activity for an integrated regulation of α-tubulin tyrosination/detyrosination states and microtubule stability, two well-correlated features of cellular microtubules.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

A mathematical model of kinetochore-microtubule attachment regulated by Aurora A activity gradient describes chromosome oscillation and correction of erroneous attachments

Chromosome oscillation during metaphase is attenuated in cancer cell lines, concomitant with the reduction of Aurora A activity on kinetochores, which results in reduced mitotic fidelity. To verify the correlation between Aurora A activity, chromosome oscillation, and error correction efficiency, we developed a mathematical model of kinetochore-microtubule dynamics, based on stochastic attachment/detachment events regulated by Aurora A activity gradient centered at spindle poles. The model accurately reproduced the oscillatory movements of chromosomes, which were suppressed not only when Aurora A activity was inhibited, but also when it was upregulated, mimicking the situation in cancer cells. Our simulation also predicted efficient correction of erroneous attachments through chromosome oscillation, which was hampered by both inhibition and upregulation of Aurora A activity. Our model provides a framework to understand the physiological role of chromosome oscillation in the correction of erroneous attachments that is intrinsically related to Aurora A activity.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Growth rate-dependent flexural rigidity of microtubules influences pattern formation in collective motion

Background

Microtubules (MTs) are highly dynamic tubular cytoskeleton filaments that are essential for cellular morphology and intracellular transport. In vivo, the flexural rigidity of MTs can be dynamically regulated depending on their intracellular function. In the in vitro reconstructed MT-motor system, flexural rigidity affects MT gliding behaviors and trajectories. Despite the importance of flexural rigidity for both biological functions and in vitro applications, there is no clear interpretation of the regulation of MT flexural rigidity, and the results of many studies are contradictory. These discrepancies impede our understanding of the regulation of MT flexural rigidity, thereby challenging its precise manipulation.

Results

Here, plausible explanations for these discrepancies are provided and a new method to evaluate the MT rigidity is developed. Moreover, a new relationship of the dynamic and mechanic of MTs is revealed that MT flexural rigidity decreases through three phases with the growth rate increases, which offers a method of designing MT flexural rigidity by regulating its growth rate. To test the validity of this method, the gliding performances of MTs with different flexural rigidities polymerized at different growth rates are examined. The growth rate-dependent flexural rigidity of MTs is experimentally found to influence the pattern formation in collective motion using gliding motility assay, which is further validated using machine learning.

Conclusion

Our study establishes a robust quantitative method for measurement and design of MT flexural rigidity to study its influences on MT gliding assays, collective motion, and other biological activities in vitro. The new relationship about the growth rate and rigidity of MTs updates current concepts on the dynamics and mechanics of MTs and provides comparable data for investigating the regulation mechanism of MT rigidity in vivo in the future.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00960-y.

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.

Measuring collagen fibril diameter with differential interference contrast microscopy

Collagen fibrils, linear arrangements of collagen monomers, 20-500 nm in diameter, comprising hundreds of molecules in their cross-section, are the fundamental structural unit in a variety of load-bearing tissues such as tendons, ligaments, skin, cornea, and bone. These fibrils often assemble into more complex structures, providing mechanical stability, strength, or toughness to the host tissue. Unfortunately, there is little information available on individual fibril dynamics, mechanics, growth, aggregation and remodeling because they are difficult to image using visible light as a probe. The principle quantity of interest is the fibril diameter, which is difficult to extract accurately, dynamically, in situ and non-destructively. An optical method, differential interference contrast (DIC) microscopy has been used to visualize dynamic structures that are as small as microtubules (25 nm diameter) and has been shown to be sensitive to the size of objects smaller than the wavelength of light. In this investigation, we take advantage of DIC microscopy's ability to report dimensions of nanometer scale objects to generate a curve that relates collagen diameter to DIC edge intensity shift (DIC-EIS). We further calibrate the curve using electron microscopy and demonstrate a linear correlation between fibril diameter and the DIC-EIS. Using a non-oil immersion, 40x objective (NA 0.6), collagen fibril diameters between ~100 nm to ~ 300 nm could be obtained with ±11 and ±4 nm accuracy for dehydrated and hydrated fibrils, respectively. This simple, nondestructive, label free method should advance our ability to directly examine fibril dynamics under experimental conditions that are physiologically relevant.

Spatial variation of microtubule depolymerization in large asters

Microtubule plus-end depolymerization rate is a potentially important target of physiological regulation, but it has been challenging to measure, so its role in spatial organization is poorly understood. Here we apply a method for tracking plus ends based on time difference imaging to measure depolymerization rates in large interphase asters growing in Xenopus egg extract. We observed strong spatial regulation of depolymerization rates, which were higher in the aster interior compared with the periphery, and much less regulation of polymerization or catastrophe rates. We interpret these data in terms of a limiting component model, where aster growth results in lower levels of soluble tubulin and microtubule-associated proteins (MAPs) in the interior cytosol compared with that at the periphery. The steady-state polymer fraction of tubulin was ∼30%, so tubulin is not strongly depleted in the aster interior. We propose that the limiting component for microtubule assembly is a MAP that inhibits depolymerization, and that egg asters are tuned to low microtubule density.

Microtubule catastrophe under force: mathematical and computational results from a Brownian ratchet model

In the intracellular environment, the intrinsic dynamics of microtubule filaments is often hindered by the presence of barriers of various kind, such as kinetochore complexes and cell cortex, which impact their polymerisation force and dynamical properties such as catastrophe frequency. We present a theoretical study of the effect of a forced barrier, also subjected to thermal noise, on the statistics of catastrophe events in a single microtubule as well as a 'bundle' of two parallel microtubules. For microtubule dynamics, which includes growth, detachment, hydrolysis and the consequent dynamic instability, we employ a one-dimensional discrete stochastic model. The dynamics of the barrier is captured by over-damped Langevin equation, while its interaction with a growing filament is assumed to be hard-core repulsion. A unified treatment of the continuum dynamics of the barrier and the discrete dynamics of the filament is realized using a hybrid Fokker-Planck equation. An explicit mathematical formula for the force-dependent catastrophe frequency of a single microtubule is obtained by solving the above equation, under some assumptions. The prediction agrees well with results of numerical simulations in the appropriate parameter regime. More general situations are studied via numerical simulations. To investigate the extent of 'load-sharing' in a microtubule bundle, and its impact on the frequency of catastrophes, the dynamics of a two-filament bundle is also studied. Here, two parallel, non-interacting microtubules interact with a common, forced barrier. The equations for the two-filament model, when solved using a mean-field assumption, predicts equal sharing of load between the filaments. However, numerical results indicate the existence of a wide spectrum of load-sharing behaviour, which is characterized using a dimensionless parameter.

Clathrin's adaptor interaction sites are repurposed to stabilize microtubules during mitosis

Clathrin ensures mitotic spindle stability and efficient chromosome alignment, independently of its vesicle trafficking function. Although clathrin localizes to the mitotic spindle and kinetochore fiber microtubule bundles, the mechanisms by which clathrin stabilizes microtubules are unclear. We show that clathrin adaptor interaction sites on clathrin heavy chain (CHC) are repurposed during mitosis to directly recruit the microtubule-stabilizing protein GTSE1 to the spindle. Structural analyses reveal that these sites interact directly with clathrin-box motifs on GTSE1. Disruption of this interaction releases GTSE1 from spindles, causing defects in chromosome alignment. Surprisingly, this disruption destabilizes astral microtubules, but not kinetochore-microtubule attachments, and chromosome alignment defects are due to a failure of chromosome congression independent of kinetochore-microtubule attachment stability. GTSE1 recruited to the spindle by clathrin stabilizes microtubules by inhibiting the microtubule depolymerase MCAK. This work uncovers a novel role of clathrin adaptor-type interactions to stabilize nonkinetochore fiber microtubules to support chromosome congression, defining for the first time a repurposing of this endocytic interaction mechanism during mitosis.

The Tubulin Code in Microtubule Dynamics and Information Encoding

Microtubules are non-covalent mesoscale polymers central to the eukaryotic cytoskeleton. Microtubule structure, dynamics, and mechanics are modulated by a cell's choice of tubulin isoforms and post-translational modifications, a "tubulin code," which is thought to support the diverse morphology and dynamics of microtubule arrays across various cell types, cell cycle, and developmental stages. We give a brief historical overview of research into tubulin diversity and highlight recent progress toward uncovering the mechanistic underpinnings of the tubulin code. As a large number of essential pathways converge upon the microtubule cytoskeleton, understanding how cells utilize tubulin diversity is crucial to understanding cellular physiology and disease.

The New Age of Cell-Free Biology

The cell-free molecular synthesis of biochemical systems is a rapidly growing field of research. Advances in the Human Genome Project, DNA synthesis, and other technologies have allowed the in vitro construction of biochemical systems, termed cell-free biology, to emerge as an exciting domain of bioengineering. Cell-free biology ranges from the molecular to the cell-population scales, using an ever-expanding variety of experimental platforms and toolboxes. In this review, we discuss the ongoing efforts undertaken in the three major classes of cell-free biology methodologies, namely protein-based, nucleic acids–based, and cell-free transcription–translation systems, and provide our perspectives on the current challenges as well as the major goals in each of the subfields.

Collective effects of XMAP215, EB1, CLASP2, and MCAK lead to robust microtubule treadmilling

Treadmilling is a complex behavior of active polymers characterized by polymerization at one polymer end and simultaneous depolymerization at the other end. Treadmilling is an essential feature of cytoskeletal filaments driving actin-based cell motility, bacterial cell division and transport, and reorganization of microtubule arrays in plants. Although microtubule treadmilling occurs in many cellular contexts, how cells coordinate growth at microtubule plus ends and shrinkage at microtubule minus ends to achieve treadmilling is not understood. Here, we employ predictive computational modeling and a multiprotein in vitro assay to reconstitute cellular-like microtubule treadmilling. Our work provides a deeper understanding of how active polymer systems can be tuned to give rise to robust yet dynamic cytoskeletal architectures.

Microtubule network remodeling is essential for fundamental cellular processes including cell division, differentiation, and motility. Microtubules are active biological polymers whose ends stochastically and independently switch between phases of growth and shrinkage. Microtubule treadmilling, in which the microtubule plus end grows while the minus end shrinks, is observed in cells; however, the underlying mechanisms are not known. Here, we use a combination of computational and in vitro reconstitution approaches to determine the conditions leading to robust microtubule treadmilling. We find that microtubules polymerized from tubulin alone can treadmill, albeit with opposite directionality and order-of-magnitude slower rates than observed in cells. We then employ computational simulations to predict that the combinatory effects of four microtubule-associated proteins (MAPs), namely EB1, XMAP215, CLASP2, and MCAK, can promote fast and sustained plus-end-leading treadmilling. Finally, we experimentally confirm the predictions of our computational model using a multi-MAP, in vitro microtubule dynamics assay to reconstitute robust plus-end-leading treadmilling, consistent with observations in cells. Our results demonstrate how microtubule dynamics can be modulated to achieve a dynamic balance between assembly and disassembly at opposite polymer ends, resulting in treadmilling over long periods of time. Overall, we show how the collective effects of multiple components give rise to complex microtubule behavior that may be used for global network remodeling in cells.

XMAP215 promotes microtubule-F-actin interactions to regulate growth cone microtubules during axon guidance in Xenopus laevis

It has long been established that neuronal growth cone navigation depends on changes in microtubule (MT) and F-actin architecture downstream of guidance cues. However, the mechanisms by which MTs and F-actin are dually coordinated remain a fundamentally unresolved question. Here, we report that the well-characterized MT polymerase, XMAP215 (also known as CKAP5), plays an important role in mediating MT-F-actin interaction within the growth cone. We demonstrate that XMAP215 regulates MT-F-actin alignment through its N-terminal TOG 1-5 domains. Additionally, we show that XMAP215 directly binds to F-actin in vitro and co-localizes with F-actin in the growth cone periphery. We also find that XMAP215 is required for regulation of growth cone morphology and response to the guidance cue, Ephrin A5. Our findings provide the first strong evidence that XMAP215 coordinates MT and F-actin interaction in vivo We suggest a model in which XMAP215 regulates MT extension along F-actin bundles into the growth cone periphery and that these interactions may be important to control cytoskeletal dynamics downstream of guidance cues. This article has an associated First Person interview with the first author of the paper.

The role of Patronin in Drosophila mitosis

Background

The calmodulin-regulated spectrin-associated proteins (CAMSAPs) belong to a conserved protein family, which includes members that bind the polymerizing mcrotubule (MT) minus ends and remain associated with the MT lattice formed by minus end polymerization. Only one of the three mammalian CAMSAPs, CAMSAP1, localizes to the mitotic spindle but its function is unclear. In Drosophila, there is only one CAMSAP, named Patronin. Previous work has shown that Patronin stabilizes the minus ends of non-mitotic MTs and is required for proper spindle elongation. However, the precise role of Patronin in mitotic spindle assembly is poorly understood.

Results

Here we have explored the role of Patronin in Drosophila mitosis using S2 tissue culture cells as a model system. We show that Patronin associates with different types of MT bundles within the Drosophila mitotic spindle, and that it is required for their stability. Imaging of living cells expressing Patronin-GFP showed that Patronin displays a dynamic behavior. In prometaphase cells, Patronin accumulates on short segments of MT bundles located near the chromosomes. These Patronin “seeds” extend towards the cell poles and stop growing just before reaching the poles. Our data also suggest that Patronin localization is largely independent of proteins acting at the MT minus ends such as Asp and Klp10A.

Conclusion

Our results suggest a working hypothesis about the mitotic role of Patronin. We propose that Patronin binds the minus ends within MT bundles, including those generated from the walls of preexisting MTs via the augmin-mediated pathway. This would help maintaining MT association within the mitotic bundles, thereby stabilizing the spindle structure. Our data also raise the intriguing possibility that the minus ends of bundled MTs can undergo a limited polymerization.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0189-0) contains supplementary material, which is available to authorized users.

Minimal in vitro systems shed light on cell polarity

Cell polarity - the morphological and functional differentiation of cellular compartments in a directional manner - is required for processes such as orientation of cell division, directed cellular growth and motility. How the interplay of components within the complexity of a cell leads to cell polarity is still heavily debated. In this Review, we focus on one specific aspect of cell polarity: the non-uniform accumulation of proteins on the cell membrane. In cells, this is achieved through reaction-diffusion and/or cytoskeleton-based mechanisms. In reaction-diffusion systems, components are transformed into each other by chemical reactions and are moving through space by diffusion. In cytoskeleton-based processes, cellular components (i.e. proteins) are actively transported by microtubules (MTs) and actin filaments to specific locations in the cell. We examine how minimal systems - in vitro reconstitutions of a particular cellular function with a minimal number of components - are designed, how they contribute to our understanding of cell polarity (i.e. protein accumulation), and how they complement in vivo investigations. We start by discussing the Min protein system from Escherichia coli, which represents a reaction-diffusion system with a well-established minimal system. This is followed by a discussion of MT-based directed transport for cell polarity markers as an example of a cytoskeleton-based mechanism. To conclude, we discuss, as an example, the interplay of reaction-diffusion and cytoskeleton-based mechanisms during polarity establishment in budding yeast.

Analysis of Microtubule Dynamics Heterogeneity in Cell Culture

Microtubules (MTs) are dynamic components of the cytoskeleton playing an important role in a large number of cell functions. Individual MTs in living cells undergo stochastic switching between alternate states of growth, shortening and attenuated phase, a phenomenon known as tempered dynamic instability. Dynamic instability of MTs is usually analyzed by labeling MTs with +TIPs, namely, EB proteins. Tracking of +TIP trajectories allows analyzing MT growth in cells with a different density of MTs. Numerous labs now use +TIP to track growing MTs in a variety of cell cultures. However, heterogeneity of MT dynamics is usually underestimated, and rather small sampling for the description of dynamic instability parameters is often used. The strategy described in this chapter is the method for repetitive quantitative analysis of MT growth rate within the same cell that allows minimization of the variation in MT dynamics measurement. We show that variability in MT dynamics within a cell when using repeated measurements is significantly less than between different cells in the same chamber. This approach allows better estimation of the heterogeneity of cells' responses to different treatments. To compare the effects of different MT inhibitors, the protocol using normalized values for MT dynamics and repetitive measurements for each cell is employed. This chapter provides detailed methods for analysis of MT dynamics in tissue cultures. We describe protocols for imaging MT dynamics by fluorescent microscopy, contrast enhancement technique, and MT dynamics analysis using triple color-coded display based on sequential subtraction analysis.

Human β-Tubulin Isotypes Can Regulate Microtubule Protofilament Number and Stability

Cell biological studies have shown that protofilament number, a fundamental feature of microtubules, can correlate with the expression of different tubulin isotypes. However, it is not known if tubulin isotypes directly control this basic microtubule property. Here, we report high-resolution cryo-EM reconstructions (3.5-3.65 Å) of purified human α1B/β3 and α1B/β2B microtubules and find that the β-tubulin isotype can determine protofilament number. Comparisons of atomic models of 13- and 14-protofilament microtubules reveal how tubulin subunit plasticity, manifested in "accordion-like" distributed structural changes, can accommodate distinct lattice organizations. Furthermore, compared to α1B/β3 microtubules, α1B/β2B filaments are more stable to passive disassembly and against depolymerization by MCAK or chTOG, microtubule-associated proteins with distinct mechanisms of action. Mixing tubulin isotypes in different proportions results in microtubules with protofilament numbers and stabilities intermediate to those of isotypically pure filaments. Together, our findings indicate that microtubule protofilament number and stability can be controlled through β-tubulin isotype composition.

Proteomic Profiling of Microtubule Self-organization in M-phase

Microtubules (MTs) and associated proteins can self-organize into complex structures such as the bipolar spindle, a process in which RanGTP plays a major role. Addition of RanGTP to M-phase Xenopus egg extracts promotes the nucleation and self-organization of MTs into asters and bipolar-like structures in the absence of centrosomes or chromosomes. We show here that the complex proteome of these RanGTP-induced MT assemblies is similar to that of mitotic spindles. Using proteomic profiling we show that MT self-organization in the M-phase cytoplasm involves the non-linear and non-stoichiometric recruitment of proteins from specific functional groups. Our study provides for the first time a temporal understanding of the protein dynamics driving MT self-organization in M-phase.

The ABCs of Centriole Architecture: The Form and Function of Triplet Microtubules

The centriole is a defining feature of many eukaryotic cells. It nucleates a cilium, organizes microtubules as part of the centrosome, and is duplicated in coordination with the cell cycle. Centrioles have a remarkable structure, consisting of microtubules arranged in a barrel with ninefold radial symmetry. At their base, or proximal end, centrioles have unique triplet microtubules, formed from three microtubules linked to each other. This microtubule organization is not found anywhere else in the cell, is conserved in all major branches of the eukaryotic tree, and likely was present in the last eukaryotic common ancestor. At their tip, or distal end, centrioles have doublet microtubules, which template the cilium. Here, we consider the structures of the compound microtubules in centrioles and discuss potential mechanisms for their formation and their function. We propose that triplet microtubules are required for the structural integrity of centrioles, allowing the centriole to serve as the essential nucleator of the cilium.

Cep120 promotes microtubule formation through a unique tubulin binding C2 domain

Centrioles are microtubule-based structures that play essential roles in cell division and cilia biogenesis. Cep120 is an important protein for correct centriole formation and mutations in the Cep120 gene cause severe human diseases like Joubert syndrome and complex ciliopathies. Here, we show that Cep120 contains three consecutive C2 domains that are followed by a coiled-coil dimerization domain. Surprisingly, unlike the classical C2 domains, all three Cep120 C2 domains lack calcium- and phospholipid-binding activities. However, biophysical and biochemical assays revealed that the N-terminal Cep120 C2 domain (C2A) binds to both tubulin and microtubules, and promotes microtubule formation. Structural analyses coupled with mutagenesis identified a highly conserved, positively charged residue patch on the surface of Cep120 C2A, which mediates the interaction with tubulin and microtubules. Together, our results establish Cep120 C2A as a unique microtubule-binding domain. They further provide insights into the molecular mechanism of Cep120 during centriole biogenesis.

The family-specific α4-helix of the kinesin-13, MCAK, is critical to microtubule end recognition

Kinesins that influence the dynamics of microtubule growth and shrinkage require the ability to distinguish between the microtubule end and the microtubule lattice. The microtubule depolymerizing kinesin MCAK has been shown to specifically recognize the microtubule end. This ability is key to the action of MCAK in regulating microtubule dynamics. We show that the α4-helix of the motor domain is crucial to microtubule end recognition. Mutation of the residues K524, E525 and R528, which are located in the C-terminal half of the α4-helix, specifically disrupts the ability of MCAK to recognize the microtubule end. Mutation of these residues, which are conserved in the kinesin-13 family and discriminate members of this family from translocating kinesins, impairs the ability of MCAK to discriminate between the microtubule lattice and the microtubule end.

Control of molecular shuttles by designing electrical and mechanical properties of microtubules

Kinesin-driven microtubules have been focused on to serve as molecular transporters, called "molecular shuttles," to replace micro/nanoscale molecular manipulations necessitated in micro total analysis systems. Although transport, concentration, and detection of target molecules have been demonstrated, controllability of the transport directions is still a major challenge. Toward broad applications of molecular shuttles by defining multiple moving directions for selective molecular transport, we integrated a bottom-up molecular design of microtubules and a top-down design of a microfluidic device. The surface charge density and stiffness of microtubules were controlled, allowing us to create three different types of microtubules, each with different gliding directions corresponding to their electrical and mechanical properties. The measured curvature of the gliding microtubules enabled us to optimize the size and design of the device for molecular sorting in a top-down approach. The integrated bottom-up and top-down design achieved separation of stiff microtubules from negatively charged, soft microtubules under an electric field. Our method guides multiple microtubules by integrating molecular control and microfluidic device design; it is not only limited to molecular sorters but is also applicable to various molecular shuttles with the high controllability in their movement directions.

The role of TACC3 in mitotic spindle organization

TACC3 regulates spindle organization during mitosis and also regulates centrosome-mediated microtubule nucleation by affecting γ-Tubulin ring complexes. In addition, it interacts with different proteins (such as ch-TOG, clathrin and Aurora-A) to function in mitotic spindle assembly and stability. By forming the TACC3/ch-TOG complex, TACC3 acts as a plus end-tracking protein to promote microtubule elongation. The TACC3/ch-TOG/clathrin complex is formed to stabilize kinetochore fibers by crosslinking adjacent microtubules. Furthermore, the phosphorylation of TACC3 by Aurora-A is important for the formation of TACC3/ch-TOG/clathrin and its recruitment to kinetochore fibers. Recently, the aberrant expression of TACC3 in a variety of human cancers has been linked with mitotic defects. Thus, in this review, we will discuss our current understanding of the biological roles of TACC3 in mitotic spindle organization.

Centriolar CPAP/SAS-4 Imparts Slow Processive Microtubule Growth

Centrioles are fundamental and evolutionarily conserved microtubule-based organelles whose assembly is characterized by microtubule growth rates that are orders of magnitude slower than those of cytoplasmic microtubules. Several centriolar proteins can interact with tubulin or microtubules, but how they ensure the exceptionally slow growth of centriolar microtubules has remained mysterious. Here, we bring together crystallographic, biophysical, and reconstitution assays to demonstrate that the human centriolar protein CPAP (SAS-4 in worms and flies) binds and “caps” microtubule plus ends by associating with a site of β-tubulin engaged in longitudinal tubulin-tubulin interactions. Strikingly, we uncover that CPAP activity dampens microtubule growth and stabilizes microtubules by inhibiting catastrophes and promoting rescues. We further establish that the capping function of CPAP is important to limit growth of centriolar microtubules in cells. Our results suggest that CPAP acts as a molecular lid that ensures slow assembly of centriolar microtubules and, thereby, contributes to organelle length control.

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CPAP's PN2-3 domain binds to an exposed site on β-tubulin at microtubule plus ends

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CPAP tracks and caps microtubule plus ends in vitro

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CPAP dampens microtubule growth in vitro

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The capping function of CPAP limits centriolar microtubule growth in human cells

A microtubule dynamics reconstitutional convention

In vitro reconstitution is the fundamental test for identification of the core components of a biological process. In this issue, Moriwaki and Goshima (2016. J. Cell Biol. https://doi.org/10.1083/jcb.201604118) reconstitute all phases of microtubule dynamics through the inclusion of five key regulators and demonstrate that Polo kinase activity shifts the system from an interphase mode into an enhanced mitotic mode.

Mechanisms of kinetic stabilization by the drugs paclitaxel and vinblastine

Chemotherapeutic agents that target microtubule dynamics promote a universal phenotype of kinetic stabilization. Integrated computational modeling and fluorescence microscopy identify the fundamental kinetic and thermodynamic mechanisms that result in kinetic stabilization, specifically by the drugs paclitaxel and vinblastine.

Microtubule-targeting agents (MTAs), widely used as biological probes and chemotherapeutic drugs, bind directly to tubulin subunits and “kinetically stabilize” microtubules, suppressing the characteristic self-assembly process of dynamic instability. However, the molecular-level mechanisms of kinetic stabilization are unclear, and the fundamental thermodynamic and kinetic requirements for dynamic instability and its elimination by MTAs have yet to be defined. Here we integrate a computational model for microtubule assembly with nanometer-scale fluorescence microscopy measurements to identify the kinetic and thermodynamic basis of kinetic stabilization by the MTAs paclitaxel, an assembly promoter, and vinblastine, a disassembly promoter. We identify two distinct modes of kinetic stabilization in live cells, one that truly suppresses on-off kinetics, characteristic of vinblastine, and the other a “pseudo” kinetic stabilization, characteristic of paclitaxel, that nearly eliminates the energy difference between the GTP- and GDP-tubulin thermodynamic states. By either mechanism, the main effect of both MTAs is to effectively stabilize the microtubule against disassembly in the absence of a robust GTP cap.

Mitotic spindle assembly in animal cells: a fine balancing act

The mitotic spindle has a crucial role in ensuring the accurate segregation of chromosomes into the two daughter cells during cell division, which is paramount for maintaining genome integrity. It is a self-organized and dynamic macromolecular structure that is constructed from microtubules, microtubule-associated proteins and motor proteins. Thirty years of research have led to the identification of centrosome-, chromatin- and microtubule-mediated microtubule nucleation pathways that each contribute to mitotic spindle assembly. Far from being redundant pathways, data are now emerging regarding how they function together to ensure the timely completion of mitosis. We are also beginning to comprehend the multiple mechanisms by which cells regulate spindle scaling. Together, this research has increased our understanding of how cells coordinate hundreds of proteins to assemble the dynamic, precise and robust structure that is the mitotic spindle.

Physical basis of large microtubule aster growth

Microtubule asters - radial arrays of microtubules organized by centrosomes - play a fundamental role in the spatial coordination of animal cells. The standard model of aster growth assumes a fixed number of microtubules originating from the centrosomes. However, aster morphology in this model does not scale with cell size, and we recently found evidence for non-centrosomal microtubule nucleation. Here, we combine autocatalytic nucleation and polymerization dynamics to develop a biophysical model of aster growth. Our model predicts that asters expand as traveling waves and recapitulates all major aspects of aster growth. With increasing nucleation rate, the model predicts an explosive transition from stationary to growing asters with a discontinuous jump of the aster velocity to a nonzero value. Experiments in frog egg extract confirm the main theoretical predictions. Our results suggest that asters observed in large fish and amphibian eggs are a meshwork of short, unstable microtubules maintained by autocatalytic nucleation and provide a paradigm for the assembly of robust and evolvable polymer networks.

DOI:http://dx.doi.org/10.7554/eLife.19145.001

Cells must carefully organize their contents in order to work effectively. Protein filaments called microtubules often play important roles in this organization, as well as giving structure to the cell. Many cells contain structures called asters that are formed of microtubules that radiate out from a central point (much like a star shape). Textbooks generally state that all microtubules in the aster grow outward from its center. If this was the case, the microtubules at the edge of large asters – such as those found in frog egg cells and other extremely large cells – would be spread relatively far apart from each other. However, even at the edges of large asters, the microtubules are quite densely packed.

In 2014, a group of researchers proposed that new microtubules could form throughout the aster instead of all originating from the center. This model had not been tested; it was also unclear under what conditions an aster would be able to grow to fill a large cell.

Ishihara et al. – including some of the researchers involved in the 2014 work – have now developed a mathematical theory of aster growth that is based on the assumption that microtubules stimulate the generation of new microtubules. The theory reproduces the key features seen during the growth of asters in large cells, and predicts that the asters may stay at a constant size or grow continuously. The condition required for the aster to grow is simple: each microtubule in it has to trigger the generation of at least one new microtubule during its lifetime. Ishihara et al. have named this process “collective growth”.

Experiments performed using microtubules taken from crushed frog eggs and assembled under a cover slip provided further evidence that asters grow via a collective growth process. Future studies could now investigate whether collective growth also underlies the formation of other cellular structures.

DOI:http://dx.doi.org/10.7554/eLife.19145.002

Five factors can reconstitute all three phases of microtubule polymerization dynamics

Cytoplasmic microtubules (MTs) undergo growth, shrinkage, and pausing. However, how MT polymerization cycles are produced and spatiotemporally regulated at a molecular level is unclear, as the entire cycle has not been recapitulated in vitro with defined components. In this study, we reconstituted dynamic MT plus end behavior involving all three phases by mixing tubulin with five Drosophila melanogaster proteins (EB1, XMAP215Msps, Sentin, kinesin-13Klp10A, and CLASPMast/Orbit). When singly mixed with tubulin, CLASPMast/Orbit strongly inhibited MT catastrophe and reduced the growth rate. However, in the presence of the other four factors, CLASPMast/Orbit acted as an inducer of pausing. The mitotic kinase Plk1Polo modulated the activity of CLASPMast/Orbit and kinesin-13Klp10A and increased the dynamic instability of MTs, reminiscent of mitotic cells. These results suggest that five conserved proteins constitute the core factors for creating dynamic MTs in cells and that Plk1-dependent phosphorylation is a crucial event for switching from the interphase to mitotic mode.

Nonequilibrium Diffusion and Capture Mechanism Ensures Tip Localization of Regulating Proteins on Dynamic Filaments

Diffusive motion of regulatory enzymes on biopolymers with eventual capture at a reaction site is a common feature in cell biology. Using a lattice gas model we study the impact of diffusion and capture for a microtubule polymerase and a depolymerase. Our results show that the capture mechanism localizes the proteins and creates large-scale spatial correlations. We develop an analytic approximation that globally accounts for relevant correlations and yields results that are in excellent agreement with experimental data. Our results show that diffusion and capture operates most efficiently at cellular enzyme concentrations which points to in vivo relevance.

Tracking of plus-ends reveals microtubule functional diversity in different cell types

Many cellular processes are tightly connected to the dynamics of microtubules (MTs). While in neuronal axons MTs mainly regulate intracellular trafficking, they participate in cytoskeleton reorganization in many other eukaryotic cells, enabling the cell to efficiently adapt to changes in the environment. We show that the functional differences of MTs in different cell types and regions is reflected in the dynamic properties of MT tips. Using plus-end tracking proteins EB1 to monitor growing MT plus-ends, we show that MT dynamics and life cycle in axons of human neurons significantly differ from that of fibroblast cells. The density of plus-ends, as well as the rescue and catastrophe frequencies increase while the growth rate decreases toward the fibroblast cell margin. This results in a rather stable filamentous network structure and maintains the connection between nucleus and membrane. In contrast, plus-ends are uniformly distributed along the axons and exhibit diverse polymerization run times and spatially homogeneous rescue and catastrophe frequencies, leading to MT segments of various lengths. The probability distributions of the excursion length of polymerization and the MT length both follow nearly exponential tails, in agreement with the analytical predictions of a two-state model of MT dynamics.

Computational reconstitution of spine calcium transients from individual proteins

We have built a stochastic model in the program MCell that simulates Ca(2+) transients in spines from the principal molecular components believed to control Ca(2+) entry and exit. Proteins, with their kinetic models, are located within two segments of dendrites containing 88 intact spines, centered in a fully reconstructed 6 × 6 × 5 μm(3) cube of hippocampal neuropil. Protein components include AMPA- and NMDA-type glutamate receptors, L- and R-type voltage-dependent Ca(2+) channels, Na(+)/Ca(2+) exchangers, plasma membrane Ca(2+) ATPases, smooth endoplasmic reticulum Ca(2+) ATPases, immobile Ca(2+) buffers, and calbindin. Kinetic models for each protein were taken from published studies of the isolated proteins in vitro. For simulation of electrical stimuli, the time course of voltage changes in the dendritic spine was generated with the desired stimulus in the program NEURON. Voltage-dependent parameters were then continuously re-adjusted during simulations in MCell to reproduce the effects of the stimulus. Nine parameters of the model were optimized within realistic experimental limits by a process that compared results of simulations to published data. We find that simulations in the optimized model reproduce the timing and amplitude of Ca(2+) transients measured experimentally in intact neurons. Thus, we demonstrate that the characteristics of individual isolated proteins determined in vitro can accurately reproduce the dynamics of experimentally measured Ca(2+) transients in spines. The model will provide a test bed for exploring the roles of additional proteins that regulate Ca(2+) influx into spines and for studying the behavior of protein targets in the spine that are regulated by Ca(2+) influx.

Axon injury triggers EFA-6 mediated destabilization of axonal microtubules via TACC and doublecortin like kinase

Axon injury triggers a series of changes in the axonal cytoskeleton that are prerequisites for effective axon regeneration. In Caenorhabditis elegans the signaling protein Exchange Factor for ARF-6 (EFA-6) is a potent intrinsic inhibitor of axon regrowth. Here we show that axon injury triggers rapid EFA-6-dependent inhibition of axonal microtubule (MT) dynamics, concomitant with relocalization of EFA-6. EFA-6 relocalization and axon regrowth inhibition require a conserved 18-aa motif in its otherwise intrinsically disordered N-terminal domain. The EFA-6 N-terminus binds the MT-associated proteins TAC-1/Transforming-Acidic-Coiled-Coil, and ZYG-8/Doublecortin-Like-Kinase, both of which are required for regenerative growth cone formation, and which act downstream of EFA-6. After injury TAC-1 and EFA-6 transiently relocalize to sites marked by the MT minus end binding protein PTRN-1/Patronin. We propose that EFA-6 acts as a bifunctional injury-responsive regulator of axonal MT dynamics, acting at the cell cortex in the steady state and at MT minus ends after injury.

DOI:http://dx.doi.org/10.7554/eLife.08695.001

In the nervous system, cells called neurons carry information around the body. These cells have long thin projections called axons that allow the information to pass very quickly along the cell to junctions with other neurons. Neurons in adult mammals are limited in their ability to regenerate, so any damage to axons, for example, due to a stroke or a brain injury, tends to be permanent. Therefore, an important goal in neuroscience research is to discover the genes and proteins that are involved in regenerating axons as this may make it possible to develop new therapies.

An internal scaffold called the cytoskeleton supports the three-dimensional shape of the axons. Changes in the cytoskeleton are required to allow neurons to regenerate axons after injury, and drugs that stabilize filaments called microtubules in the cytoskeleton can promote these changes. Chen et al. used a technique called laser microsurgery to sever individual axons in a roundworm known as C. elegans and then observed whether these axons could regenerate. The experiments reveal that a protein called EFA-6 blocks the regeneration of neurons by preventing rearrangements in the cytoskeleton.

EFA-6 is normally found at the membrane that surrounds the neuron. However, Chen et al. show that when the axon is damaged, this protein rapidly moves to areas near the ends of microtubule filaments. EFA-6 interacts with two other proteins that are associated with microtubules and are required for axons to be able to regenerate. Chen et al.'s findings demonstrate that several proteins that regulate microtubule filaments play a key role in regenerating axons. All three of these proteins are found in humans and other animals so they have the potential to be targeted by drug therapies in future. The next challenge is to understand the details of how EFA-6 activity is affected by axon injury, and how this alters the cytoskeleton.

DOI:http://dx.doi.org/10.7554/eLife.08695.002

Emergent Properties of the Metaphase Spindle

A metaphase spindle is a complex structure consisting of microtubules and a myriad of different proteins that modulate microtubule dynamics together with chromatin and kinetochores. A decade ago, a full description of spindle formation and function seemed a lofty goal. Here, we describe how work in the last 10 years combining cataloging of spindle components, the characterization of their biochemical activities using single-molecule techniques, and theory have advanced our knowledge. Taken together, these advances suggest that a full understanding of spindle assembly and function may soon be possible.

Reconstitution of mitotic chromatids with a minimum set of purified factors

The assembly of mitotic chromosomes, each composed of a pair of rod-shaped chromatids, is an essential prerequisite for accurate transmission of the genome during cell division. It remains poorly understood, however, how this fundamental process might be achieved and regulated in the cell. Here we report an in vitro system in which mitotic chromatids can be reconstituted by mixing a simple substrate with only six purified factors: core histones, three histone chaperones (nucleoplasmin, Nap1 and FACT), topoisomerase II (topo II) and condensin I. We find that octameric nucleosomes containing the embryonic variant H2A.X-F are highly susceptible to FACT and function as the most productive substrate for subsequent actions of topo II and condensin I. Cdk1 phosphorylation of condensin I is the sole mitosis-specific modification required for chromatid reconstitution. This experimental system will enhance our understanding of the mechanisms of action of individual factors and their cooperation during this process.

Building the Microtubule Cytoskeleton Piece by Piece

The microtubule (MT) cytoskeleton gives cells their shape, organizes the cellular interior, and segregates chromosomes. These functions rely on the precise arrangement of MTs, which is achieved by the coordinated action of MT-associated proteins (MAPs). We highlight the first and most important examples of how different MAP activities are combined in vitro to create an ensemble function that exceeds the simple addition of their individual activities, and how the Xenopus laevis egg extract system has been utilized as a powerful intermediate between cellular and purified systems to uncover the design principles of self-organized MT networks in the cell.

The contribution of αβ-tubulin curvature to microtubule dynamics

Microtubules are dynamic polymers of αβ-tubulin that form diverse cellular structures, such as the mitotic spindle for cell division, the backbone of neurons, and axonemes. To control the architecture of microtubule networks, microtubule-associated proteins (MAPs) and motor proteins regulate microtubule growth, shrinkage, and the transitions between these states. Recent evidence shows that many MAPs exert their effects by selectively binding to distinct conformations of polymerized or unpolymerized αβ-tubulin. The ability of αβ-tubulin to adopt distinct conformations contributes to the intrinsic polymerization dynamics of microtubules. αβ-Tubulin conformation is a fundamental property that MAPs monitor and control to build proper microtubule networks.

Molecular mechanisms for microtubule length regulation by kinesin-8 and XMAP215 proteins

The cytoskeleton is regulated by a plethora of enzymes that influence the stability and dynamics of cytoskeletal filaments. How microtubules (MTs) are controlled is of particular importance for mitosis, during which dynamic MTs are responsible for proper segregation of chromosomes. Molecular motors of the kinesin-8 protein family have been shown to depolymerize MTs in a length-dependent manner, and recent experimental and theoretical evidence suggests a possible role for kinesin-8 in the dynamic regulation of MTs. However, so far the detailed molecular mechanisms of how these molecular motors interact with the growing MT tip remain elusive. Here we show that two distinct scenarios for the interactions of kinesin-8 with the MT tip lead to qualitatively different MT dynamics, including accurate length control as well as intermittent dynamics. We give a comprehensive analysis of the regimes where length regulation is possible and characterize how the stationary length depends on the biochemical rates and the bulk concentrations of the various proteins. For a neutral scenario, where MTs grow irrespective of whether the MT tip is occupied by a molecular motor, length regulation is possible only for a narrow range of biochemical rates, and, in particular, limited to small polymerization rates. By contrast, for an inhibition scenario, where the presence of a motor at the MT tip inhibits MT growth, the regime where length regulation is possible is extremely broad and includes high growth rates. These results also apply to situations where a polymerizing enzyme like XMAP215 and kinesin-8 mutually exclude each other from the MT tip. Moreover, we characterize the differences in the stochastic length dynamics between the two scenarios. While for the neutral scenario length is tightly controlled, length dynamics is intermittent for the inhibition scenario and exhibits extended periods of MT growth and shrinkage. On a broader perspective, the set of models established in this work quite generally suggest that mutual exclusion of molecules at the ends of cytoskeletal filaments is an important factor for filament dynamics and regulation.

Regulation of localization and activity of the microtubule depolymerase MCAK

Mitotic Centromere Associated Kinesin (MCAK) is a potent microtubule depolymerizing and catastrophe-inducing factor, which uses the energy of ATP hydrolysis to destabilize microtubule ends. MCAK is localized to inner centromeres, kinetochores and spindle poles of mitotic cells, and is also present in the cytoplasm. Both in interphase and in mitosis, MCAK can specifically accumulate at the growing microtubule ends. Here we discuss the mechanisms, which modulate subcellular localization and activity of MCAK through the interaction with the End Binding (EB) proteins and phosphorylation.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

Microtubule plus end-tracking proteins play critical roles in directional growth of hyphae by regulating the dynamics of cytoplasmic microtubules in Aspergillus nidulans

Cytoplasmic microtubules (MTs) serve as a rate-limiting factor for hyphal tip growth in the filamentous fungus Aspergillus nidulans. We hypothesized that this function depended on the MT plus end-tracking proteins (+TIPs) including the EB1 family protein EBA that decorated the MT plus ends undergoing polymerization. The ebAΔ mutation reduced colony growth and the mutant hyphae appeared in an undulating pattern instead of exhibiting unidirectional growth in the control. These phenotypes were enhanced by a mutation in another +TIP gene clipA. EBA was required for plus end-tracking of CLIPA, the Kinesin-7 motor KipA, and the XMAP215 homologue AlpA. In addition, cytoplasmic dynein also depended on EBA to track on most polymerizing MT plus ends, but not for its conspicuous appearance at the MT ends near the hyphal apex. The loss of EBA reduced the number of cytoplasmic MTs and prolonged dwelling times for MTs after reaching the hyphal apex. Finally, we found that colonies were formed in the absence of EBA, CLIPA, and NUDA together, suggesting that they were dispensable for fundamental functions of MTs. This study provided a comprehensive delineation of the relationship among different +TIPs and their contributions to MT dynamics and unidirectional hyphal expansion in filamentous fungi.

Experiments inside a box lead to out-of-the-box ideas on cellular organization

Microtubules are biopolymers that assemble from tubulin dimers into hollow tubes and play an important role in cellular organization. Their fascinating properties and variety of functions, like for example chromosome segregation, sperm propagation and polarity establishment, have made them a popular subject of study. In this perspective I focus on the contribution of minimal in vitro systems to our understanding of microtubule organization within the physical confinement of a cell.

γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning

γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examine a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, we propose a role for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.

Lessons from in vitro reconstitution analyses of plant microtubule-associated proteins

Plant microtubules, composed of tubulin GTPase, are irreplaceable cellular components that regulate the directions of cell expansion and cell division, chromosome segregation and cell plate formation. To accomplish these functions, plant cells organize microtubule structures by regulating microtubule dynamics. Each microtubule localizes to the proper position with repeated growth and shortening. Although it is possible to reconstitute microtubule dynamics with pure tubulin solution in vitro, many microtubule-associated proteins (MAPs) govern microtubule dynamics in cells. In plants, major MAPs are identified as microtubule stabilizers (CLASP and MAP65 etc.), microtubule destabilizers (kinesin-13, katanin, MAP18 and MDP25), and microtubule dynamics promoters (EB1, MAP215, MOR1, MAP200, SPR2). Mutant analyses with forward and reverse genetics have shown the importance of microtubules and individual MAPs in plants. However, it is difficult to understand how each MAP regulates microtubule dynamics, such as growth and shortening, through mutant analyses. In vitro reconstitution analyses with individual purified MAPs and tubulin are powerful tools to reveal how each MAP regulates microtubule dynamics at the molecular level. In this review, I summarize the results of in vitro reconstitution analyses and introduce current models of how each MAP regulates microtubule dynamic instability.

Aurora B and Kif2A control microtubule length for assembly of a functional central spindle during anaphase

A gradient of Aurora B activity determines the distribution of the microtubule depolymerase Kif2A at the central spindle and specifies the subsequent spindle structure necessary for proper cytokinesis.

The central spindle is built during anaphase by coupling antiparallel microtubules (MTs) at a central overlap zone, which provides a signaling scaffold for the regulation of cytokinesis. The mechanisms underlying central spindle morphogenesis are still poorly understood. In this paper, we show that the MT depolymerase Kif2A controls the length and alignment of central spindle MTs through depolymerization at their minus ends. The distribution of Kif2A was limited to the distal ends of the central spindle through Aurora B–dependent phosphorylation and exclusion from the spindle midzone. Overactivation or inhibition of Kif2A affected interchromosomal MT length and disorganized the central spindle, resulting in uncoordinated cell division. Experimental data and model simulations suggest that the steady-state length of the central spindle and its symmetric position between segregating chromosomes are predominantly determined by the Aurora B activity gradient. On the basis of these results, we propose a robust self-organization mechanism for central spindle formation.

Chromatin-bound Xenopus Dppa2 shapes the nucleus by locally inhibiting microtubule assembly

Nuclear shape and size vary between species, during development, and in many tissue pathologies, but the causes and effects of these differences remain poorly understood. During fertilization, sperm nuclei undergo a dramatic conversion from a heavily compacted form into decondensed, spherical pronuclei, accompanied by rapid nucleation of microtubules from centrosomes. Here we report that the assembly of the spherical nucleus depends on a critical balance of microtubule dynamics, which is regulated by the chromatin-binding protein Developmental pluripotency-associated 2 (Dppa2). Whereas microtubules normally promote sperm pronuclear expansion, in Dppa2-depleted Xenopus egg extracts excess microtubules cause pronuclear assembly defects, leading to abnormal morphology and disorganized DNA replication. Dppa2 inhibits microtubule polymerization in vitro, and Dppa2 activity is needed at a precise time and location during nascent pronuclear formation. This demonstrates a strict spatiotemporal requirement for local suppression of microtubules during nuclear formation, fulfilled by chromatin-bound microtubule regulators.

XMAP215 activity sets spindle length by controlling the total mass of spindle microtubules

Metaphase spindles are microtubule-based structures that use a multitude of proteins to modulate their morphology and function. Today, we understand many details of microtubule assembly, the role of microtubule-associated proteins, and the action of molecular motors. Ultimately, the challenge remains to understand how the collective behaviour of these nanometre-scale processes gives rise to a properly sized spindle on the micrometre scale. By systematically engineering the enzymatic activity of XMAP215, a processive microtubule polymerase, we show that Xenopus laevis spindle length increases linearly with microtubule growth velocity, whereas other parameters of spindle organization, such as microtubule density, lifetime and spindle shape, remain constant. We further show that mass balance can be used to link the global property of spindle size to individual microtubule dynamic parameters. We propose that spindle length is set by a balance of non-uniform nucleation and global microtubule disassembly in a liquid-crystal-like arrangement of microtubules.