In vitro reconstitution of the functional interplay between MCAK and EB3 at microtubule plus ends

Direct Functional Interaction of the Kinesin-13 Family Member Kinesin-like Protein 2A (Kif2A) and Arf GAP with GTP-binding Protein-like, Ankyrin Repeats and PH Domains1 (AGAP1)

The molecular basis for control of the cytoskeleton by the Arf GTPase-activating protein AGAP1 has not been characterized. AGAP1 is composed of G-protein-like (GLD), pleckstrin homology (PH), Arf GAP, and ankyrin repeat domains. Kif2A was identified in screens for proteins that bind to AGAP1. The GLD and PH domains of AGAP1 bound the motor domain of Kif2A. Kif2A increased GAP activity of AGAP1, and a protein composed of the GLD and PH domains of AGAP1 increased ATPase activity of Kif2A. Knockdown (KD) of Kif2A or AGAP1 slowed cell migration and accelerated cell spreading. The effect of Kif2A KD on spreading could be rescued by expression of Kif2A-GFP or FLAG-AGAP1, but not by Kif2C-GFP. The effect of AGAP1 KD could be rescued by FLAG-AGAP1, but not by an AGAP1 mutant that did not bind Kif2A efficiently, ArfGAP1-HA or Kif2A-GFP. Taken together, the results support the hypothesis that the Kif2A·AGAP1 complex contributes to control of cytoskeleton remodeling involved in cell movement.

High-resolution Time-lapse Imaging and Automated Analysis of Microtubule Dynamics in Living Human Umbilical Vein Endothelial Cells

The physiological process by which new vasculature forms from existing vasculature requires specific signaling events that trigger morphological changes within individual endothelial cells (ECs). These processes are critical for homeostatic maintenance such as wound healing, and are also crucial in promoting tumor growth and metastasis. EC morphology is defined by the organization of the cytoskeleton, a tightly regulated system of actin and microtubule (MT) dynamics that is known to control EC branching, polarity and directional migration, essential components of angiogenesis. To study MT dynamics, we used high-resolution fluorescence microscopy coupled with computational image analysis of fluorescently-labeled MT plus-ends to investigate MT growth dynamics and the regulation of EC branching morphology and directional migration. Time-lapse imaging of living Human Umbilical Vein Endothelial Cells (HUVECs) was performed following transfection with fluorescently-labeled MT End Binding protein 3 (EB3) and Mitotic Centromere Associated Kinesin (MCAK)-specific cDNA constructs to evaluate effects on MT dynamics. PlusTipTracker software was used to track EB3-labeled MT plus ends in order to measure MT growth speeds and MT growth lifetimes in time-lapse images. This methodology allows for the study of MT dynamics and the identification of how localized regulation of MT dynamics within sub-cellular regions contributes to the angiogenic processes of EC branching and migration.

Coordinating neuronal actin-microtubule dynamics

The growth and migration of neurons require continuous remodelling of the neuronal cytoskeleton, providing a versatile cellular framework for force generation and guided movement, in addition to structural support. Actin filaments and microtubules are central to the dynamic action of the cytoskeleton and rapid advances in imaging technologies are enabling ever more detailed visualisation of the dynamic intracellular networks that they form. However, these filaments do not act individually and an expanding body of evidence emphasises the importance of actin-microtubule crosstalk in orchestrating cytoskeletal dynamics. Here, we summarise our current understanding of the structure and dynamics of actin and microtubules in isolation, before reviewing both the mechanisms and the molecular players involved in mediating actin-microtubule crosstalk in neurons.

Oxidative stress decreases microtubule growth and stability in ventricular myocytes

Microtubules (MTs) have many roles in ventricular myocytes, including structural stability, morphological integrity, and protein trafficking. However, despite their functional importance, dynamic MTs had never been visualized in living adult myocytes. Using adeno-associated viral vectors expressing the MT-associated protein plus end binding protein 3 (EB3) tagged with EGFP, we were able to perform live imaging and thus capture and quantify MT dynamics in ventricular myocytes in real time under physiological conditions. Super-resolution nanoscopy revealed that EB1 associated in puncta along the length of MTs in ventricular myocytes. The vast (~80%) majority of MTs grew perpendicular to T-tubules at a rate of 0.06μm∗s(-1) and growth was preferentially (82%) confined to a single sarcomere. Microtubule catastrophe rate was lower near the Z-line than M-line. Hydrogen peroxide increased the rate of catastrophe of MTs ~7-fold, suggesting that oxidative stress destabilizes these structures in ventricular myocytes. We also quantified MT dynamics after myocardial infarction (MI), a pathological condition associated with increased production of reactive oxygen species (ROS). Our data indicate that the catastrophe rate of MTs increases following MI. This contributed to decreased transient outward K(+) currents by decreasing the surface expression of Kv4.2 and Kv4.3 channels after MI. On the basis of these data, we conclude that, under physiological conditions, MT growth is directionally biased and that increased ROS production during MI disrupts MT dynamics, decreasing K(+) channel trafficking.

The far C-terminus of MCAK regulates its conformation and spindle pole focusing

Spatial regulation of microtubule dynamics is critical for proper spindle assembly. The far C-terminus of the microtubule-depolymerizing kinesin-13 MCAK regulates MCAK localization at spindle poles, which is needed for proper pole focusing.

To ensure proper spindle assembly, microtubule (MT) dynamics needs to be spatially regulated within the cell. The kinesin-13 MCAK is a potent MT depolymerase with a complex subcellular localization, yet how MCAK spatial regulation contributes to spindle assembly is not understood. Here we show that the far C-terminus of MCAK plays a critical role in regulating MCAK conformation, subspindle localization, and spindle assembly in Xenopus egg extracts. Alteration of MCAK conformation by the point mutation E715A/E716A in the far C-terminus increased MCAK targeting to the poles and reduced MT lifetimes, which induced spindles with unfocused poles. These effects were phenocopied by the Aurora A phosphomimetic mutation, S719E. Furthermore, addition of the kinesin-14 XCTK2 to spindle assembly reactions rescued the unfocused-pole phenotype. Collectively our work shows how the regional targeting of MCAK regulates MT dynamics, highlighting the idea that multiple phosphorylation pathways of MCAK cooperate to spatially control MT dynamics to maintain spindle architecture.

Divergent microtubule assembly rates after short- versus long-term loss of end-modulating kinesins

Depletion of microtubule (MT) regulators can initiate stable alterations in MT assembly rates that affect chromosome instability and mitotic spindle function, but the manner by which cellular MT assembly rates can stably increase or decrease is not understood. To investigate this phenomenon, we measured the response of microtubule assembly to both rapid and long-term loss of MT regulators MCAK/Kif2C and Kif18A. Depletion of MCAK/Kif2C by siRNA stably decreases MT assembly rates in mitotic spindles, whereas depletion of Kif18A stably increases rates of assembly. Surprisingly, this is not phenocopied by rapid rapamycin-dependent relocalization of MCAK/Kif2C and Kif18A to the plasma membrane. Instead, this treatment yields opposite affects on MT assembly. Rapidly increased MT assembly rates are balanced by a decrease in nucleated microtubules, whereas nucleation appears to be maximal and limiting for decreased MT assembly rates and also for long-term treatments. We measured amplified tubulin synthesis during long-term depletion of MT regulators and hypothesize that this is the basis for different phenotypes arising from long-term versus rapid depletion of MT regulators.

Encoding the microtubule structure: Allosteric interactions between the microtubule +TIP complex master regulators and TOG-domain proteins

Since their initial discovery, the intriguing proteins of the +TIP network have been the focus of intense investigation. Although many of the individual +TIP functions have been revealed, the capacity for +TIP proteins to regulate each other has not been widely addressed. Importantly, recent studies involving EBs, the master regulators of the +TIP complex, and several TOG-domain proteins have uncovered a novel mechanism of mutual +TIP regulation: allosteric interactions through changes in microtubule structure. These findings have added another level of complexity to the existing evidence on +TIP regulation and highlight the cooperative nature of the +TIP protein network.

TTBK2 with EB1/3 regulates microtubule dynamics in migrating cells through KIF2A phosphorylation

The microtubule (MT) plus end–tracking protein TTBK2 phosphorylates kinesin-13 family MT depolymerase KIF2A and removes it from MTs, thereby antagonizing KIF2A-induced depolymerization at MT plus ends during cell migration.

Microtubules (MTs) play critical roles in various cellular events, including cell migration. End-binding proteins (EBs) accumulate at the ends of growing MTs and regulate MT end dynamics by recruiting other plus end–tracking proteins (+TIPs). However, how EBs contribute to MT dynamics through +TIPs remains elusive. We focused on tau-tubulin kinase 2 (TTBK2) as an EB1/3-binding kinase and confirmed that TTBK2 acted as a +TIP. We identified MT-depolymerizing kinesin KIF2A as a novel substrate of TTBK2. TTBK2 phosphorylated KIF2A at S135 in intact cells in an EB1/3-dependent fashion and inactivated its MT-depolymerizing activity in vitro. TTBK2 depletion reduced MT lifetime (facilitated shrinkage and suppressed rescue) and impaired HeLa cell migration, and these phenotypes were partially restored by KIF2A co-depletion. Expression of nonphosphorylatable KIF2A, but not wild-type KIF2A, reduced MT lifetime and slowed down the cell migration. These findings indicate that TTBK2 with EB1/3 phosphorylates KIF2A and antagonizes KIF2A-induced depolymerization at MT plus ends for cell migration.

Spatiotemporal dynamics of Aurora B-PLK1-MCAK signaling axis orchestrates kinetochore bi-orientation and faithful chromosome segregation

Chromosome segregation in mitosis is orchestrated by the dynamic interactions between the kinetochore and spindle microtubules. The microtubule depolymerase mitotic centromere-associated kinesin (MCAK) is a key regulator for an accurate kinetochore-microtubule attachment. However, the regulatory mechanism underlying precise MCAK depolymerase activity control during mitosis remains elusive. Here, we describe a novel pathway involving an Aurora B-PLK1 axis for regulation of MCAK activity in mitosis. Aurora B phosphorylates PLK1 on Thr210 to activate its kinase activity at the kinetochores during mitosis. Aurora B-orchestrated PLK1 kinase activity was examined in real-time mitosis using a fluorescence resonance energy transfer-based reporter and quantitative analysis of native PLK1 substrate phosphorylation. Active PLK1, in turn, phosphorylates MCAK at Ser715 which promotes its microtubule depolymerase activity essential for faithful chromosome segregation. Importantly, inhibition of PLK1 kinase activity or expression of a non-phosphorylatable MCAK mutant prevents correct kinetochore-microtubule attachment, resulting in abnormal anaphase with chromosome bridges. We reason that the Aurora B-PLK1 signaling at the kinetochore orchestrates MCAK activity, which is essential for timely correction of aberrant kinetochore attachment to ensure accurate chromosome segregation during mitosis.

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.

BRCA2 diffuses as oligomeric clusters with RAD51 and changes mobility after DNA damage in live cells

Nuclear BRCA2 is oligomeric and associated with RAD51, possibly sequestering it until it is delivered to DNA damage sites.

Genome maintenance by homologous recombination depends on coordinating many proteins in time and space to assemble at DNA break sites. To understand this process, we followed the mobility of BRCA2, a critical recombination mediator, in live cells at the single-molecule level using both single-particle tracking and fluorescence correlation spectroscopy. BRCA2-GFP and -YFP were compared to distinguish diffusion from fluorophore behavior. Diffusive behavior of fluorescent RAD51 and RAD54 was determined for comparison. All fluorescent proteins were expressed from endogenous loci. We found that nuclear BRCA2 existed in oligomeric clusters, and exhibited heterogeneous mobility. DNA damage increased BRCA2 transient binding, presumably including binding to damaged sites. Despite its very different size, RAD51 displayed mobility similar to BRCA2, which indicates physical interaction between these proteins both before and after induction of DNA damage. We propose that BRCA2-mediated sequestration of nuclear RAD51 serves to prevent inappropriate DNA interactions and that all RAD51 is delivered to DNA damage sites in association with BRCA2.

Regulatory mechanisms that control mitotic kinesins

During mitosis, the mitotic spindle is assembled to align chromosomes at the spindle equator in metaphase, and to separate the genetic material equally to daughter cells in anaphase. The spindle itself is a macromolecular machine composed of an array of dynamic microtubules and associated proteins that coordinate the diverse events of mitosis. Among the microtubule associated proteins are a plethora of molecular motor proteins that couple the energy of ATP hydrolysis to force production. These motors, including members of the kinesin superfamily, must function at the right time and in the right place to insure the fidelity of mitosis. Misregulation of mitotic motors in disease states, such as cancer, underlies their potential utility as targets for antitumor drug development and highlights the importance of understanding the molecular mechanisms for regulating their function. Here, we focus on recent progress about regulatory mechanisms that control the proper function of mitotic kinesins and highlight new findings that lay the path for future studies.

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.

Actin-microtubule coordination at growing microtubule ends

To power dynamic processes in cells, the actin and microtubule cytoskeletons organize into complex structures. Although it is known that cytoskeletal coordination is vital for cell function, the mechanisms by which cross-linking proteins coordinate actin and microtubule activities remain poorly understood. In particular, it is unknown how the distinct mechanical properties of different actin architectures modulate the outcome of actin-microtubule interactions. To address this question, we engineered the protein TipAct, which links growing microtubule ends via end-binding proteins to actin filaments. We show that growing microtubules can be captured and guided by stiff actin bundles, leading to global actin-microtubule alignment. Conversely, growing microtubule ends can transport, stretch and bundle individual actin filaments, thereby globally defining actin filament organization. Our results provide a physical basis to understand actin-microtubule cross-talk, and reveal that a simple cross-linker can enable a mechanical feedback between actin and microtubule organization that is relevant to diverse biological contexts.

Reconstitution of a hierarchical +TIP interaction network controlling microtubule end tracking of dynein

Growing microtubule end regions recruit a variety of proteins collectively termed +TIPs, which confer local functions to the microtubule cytoskeleton. +TIPs form dynamic interaction networks whose behaviour depends on a number of potentially competitive and hierarchical interaction modes. The rules that determine which of the various +TIPs are recruited to the limited number of available binding sites at microtubule ends remain poorly understood. Here we examined how the human dynein complex, the main minus-end-directed motor and an important +TIP (refs , , ), is targeted to growing microtubule ends in the presence of different +TIP competitors. Using a total internal reflection fluorescence microscopy-based reconstitution assay, we found that a hierarchical recruitment mode targets the large dynactin subunit p150Glued to growing microtubule ends via EB1 and CLIP-170 in the presence of competing SxIP-motif-containing peptides. We further show that the human dynein complex is targeted to growing microtubule ends through an interaction of the tail domain of dynein with p150Glued. Our results highlight how the connectivity and hierarchy within dynamic +TIP networks are orchestrated.

EB1 accelerates two conformational transitions important for microtubule maturation and dynamics

BACKGROUND

The dynamic properties of microtubules depend on complex nanoscale structural rearrangements in their end regions. Members of the EB1 and XMAP215 protein families interact autonomously with microtubule ends. EB1 recruits several other proteins to growing microtubule ends and has seemingly antagonistic effects on microtubule dynamics: it induces catastrophes, and it increases growth velocity, as does the polymerase XMAP215.

RESULTS

Using a combination of in vitro reconstitution, time-lapse fluorescence microscopy, and subpixel-precision image analysis and convolved model fitting, we have studied the effects of EB1 on conformational transitions in growing microtubule ends and on the time course of catastrophes. EB1 density distributions at growing microtubule ends reveal two consecutive conformational transitions in the microtubule end region, which have growth-velocity-independent kinetics. EB1 binds to the microtubule after the first and before the second conformational transition has occurred, positioning it several tens of nanometers behind XMAP215, which binds to the extreme microtubule end. EB1 binding accelerates conformational maturation in the microtubule, most likely by promoting lateral protofilament interactions and by accelerating reactions of the guanosine triphosphate (GTP) hydrolysis cycle. The microtubule maturation time is directly linked to the duration of a growth pause just before microtubule depolymerization, indicating an important role of the maturation time for the control of dynamic instability.

CONCLUSIONS

These activities establish EB1 as a microtubule maturation factor and provide a mechanistic explanation for its effects on microtubule growth and catastrophe frequency, which cause microtubules to be more dynamic.

Mechanical and geometrical constraints control kinesin-based microtubule guidance

Proper organization of microtubule networks depends on microtubule-associated proteins and motors that use different spatial cues to guide microtubule growth [1-3]. For example, it has been proposed that the uniform minus-end-out microtubule organization in dendrites of Drosophila neurons is maintained by steering of polymerizing microtubules along the stable ones by kinesin-2 motors bound to growing microtubule plus ends [4]. To explore the mechanics of kinesin-guided microtubule growth, we reconstituted this process in vitro. In the presence of microtubule plus-end tracking EB proteins, a constitutively active kinesin linked to the EB-interacting motif SxIP effectively guided polymerizing microtubules along other microtubules both in cells and in vitro. Experiments combined with modeling revealed that at angles larger than 90°, guidance efficiency is determined by the force needed for microtubule bending. At angles smaller than 90°, guidance requires microtubule growth, and guidance efficiency depends on the ability of kinesins to maintain contact between the two microtubules despite the geometrical constraints imposed by microtubule length and growth rate. Our findings provide a conceptual framework for understanding microtubule guidance during the generation of different types of microtubule arrays.

In vitro reconstitution of dynamic microtubules interacting with actin filament networks

Interactions between microtubules and actin filaments (F-actin) are essential for eukaryotic cell migration, polarization, growth, and division. Although the importance of these interactions has been long recognized, the inherent complexity of the cell interior hampers a detailed mechanistic study of how these two cytoskeletal systems influence each other. In this chapter, we show how in vitro reconstitution can be employed to study how actin filaments and dynamic microtubules affect each other's organization. While we focus here on the effect of steric interactions, these assays provide an ideal starting point to develop more complex studies through the addition of known F-actin-microtubule cross-linkers, or myosin II motors.

The molecular basis for kinesin functional specificity during mitosis

Microtubule-based motor proteins play key roles during mitosis to assemble the bipolar spindle, define the cell division axis, and align and segregate the chromosomes. The majority of mitotic motors are members of the kinesin superfamily. Despite sharing a conserved catalytic core, each kinesin has distinct functions and localization, and is uniquely regulated in time and space. These distinct behaviors and functional specificity are generated by variations in the enzymatic domain as well as the non-conserved regions outside of the kinesin motor domain and the stalk. These flanking regions can directly modulate the properties of the kinesin motor through dimerization or self-interactions, and can associate with extrinsic factors, such as microtubule or DNA binding proteins, to provide additional functional properties. This review discusses the recently identified molecular mechanisms that explain how the control and functional specification of mitotic kinesins is achieved. © 2013 Wiley Periodicals, Inc.

The human kinesin-14 HSET tracks the tips of growing microtubules in vitro

Tip-tracking of kinesin-14 motor proteins is believed to be crucial for the assembly and maintenance of dynamic microtubule arrays. However, in contrast to other members of the kinesin-14 family, H. sapiens kinesin-14 HSET has so far never been observed to be prominently located at microtubule plus ends. Here, using an in vitro microtubule dynamics reconstitution assay we observe tip-tracking of GFP-HSET in the presence of H. sapiens EB1 (hsEB1). Tip-tracking depended on the SxIP-like motif in HSET as well as on the EB homology domain in hsEB1. D. melanogaster Ncd and S. pombe Klp2 tip-tracking reconstitution assays accompanied by kinesin-14 amino acid sequence comparisons suggest that SxIP-like motif mediated tip-tracking dependent on EB family proteins is conserved in the kinesin-14 family of molecular motors.

Modern methods to interrogate microtubule dynamics

Microtubules are essential protein filaments required to organize and rearrange the interior of the cell. They must be stiff with mechanical integrity to support the structure of the cell. Yet, they must also be dynamic to enable rearrangements of the cell during cell division and development. This dynamic nature is inherent to microtubules and comes about through the hydrolysis of chemical energy stored in guanosine triphosphate (GTP). Dynamic instability has been studied with a number of microscopy techniques both in cells and in reconstituted systems. In this article, we review the techniques used to examine microtubule dynamic instability and highlight future avenues and still open questions about this vital and fascinating activity.

End binding proteins are obligatory dimers

End binding (EB) proteins are responsible for the recruitment of an array of microtubule plus-end tracking proteins (+TIPs) to growing microtubules ends. EBs encompass an N-terminal calponin homology domain that confers microtubule tip tracking activity to the protein. The C-terminal domain of EBs contains a coiled coil that mediates the parallel dimerization of EB monomers. This part of the protein is also responsible for partner binding. While dimerization is not essential for microtubule tip tracking by EBs it is a prerequisite for +TIP partner binding. The concentration of EBs in cells has been estimated to be in the range of hundreds of nanomoles. In contrast, in in vitro single molecule experiments EB concentrations of subnanomoles are employed. From a mechanistic point of view it is important to assess the oligomerization state of EBs at physiologically and experimentally relevant protein concentrations, in particular if the goal of a study is to model the behavior of EB-dependent dynamic +TIP networks. Here we have determined the stability of the EB1 and EB3 dimers using multi-angle light scattering and fluorescence analytical ultracentrifugation. We show that these EBs form stable dimers and do not dissociate even at very low nanomolar concentrations. The dimers remained stable at both room temperature as well as at the physiologically relevant temperature of 37°C. Together, our results reveal that EBs are obligatory dimers, a conclusion that has implications for the mechanistic understanding of these key proteins involved in the orchestration of dynamic protein networks at growing microtubule ends.

Lateral to end-on conversion of chromosome-microtubule attachment requires kinesins CENP-E and MCAK

BACKGROUND

Proper attachment of chromosomes to microtubules is crucial for the accurate segregation of chromosomes. Human chromosomes attach initially to lateral walls of microtubules. Subsequently, attachments to lateral walls disappear and attachments to microtubule ends (end-on attachments) predominate. While it is known in yeasts that lateral to end-on conversion of attachments occurs through a multistep process, equivalent conversion steps in humans remain unknown.

RESULTS

By developing a high-resolution imaging assay to visualize intermediary steps of the lateral to end-on conversion process, we show that the mechanisms that bring a laterally bound chromosome and its microtubule end closer to each other are indispensable for proper end-on attachment because laterally attached chromosomes seldom detach. We show that end-on conversion requires (1) the plus-end-directed motor CENP-E to tether the lateral kinetochore onto microtubule walls and (2) the microtubule depolymerizer MCAK to release laterally attached microtubules after a partial end-on attachment is formed.

CONCLUSIONS

By uncovering a CENP-E mediated wall-tethering event and a MCAK-mediated wall-removing event, we establish that human chromosome-microtubule attachment is achieved through a set of deterministic sequential events rather than stochastic direct capture of microtubule ends.

Aurora B spatially regulates EB3 phosphorylation to coordinate daughter cell adhesion with cytokinesis

During mitosis, human cells round up, decreasing their adhesion to extracellular substrates. This must be quickly reestablished by poorly understood cytoskeleton remodeling mechanisms that prevent detachment from epithelia, while ensuring the successful completion of cytokinesis. Here we show that the microtubule end-binding (EB) proteins EB1 and EB3 play temporally distinct roles throughout cell division. Whereas EB1 was involved in spindle orientation before anaphase, EB3 was required for stabilization of focal adhesions and coordinated daughter cell spreading during mitotic exit. Additionally, EB3 promoted midbody microtubule stability and, consequently, midbody stabilization necessary for efficient cytokinesis. Importantly, daughter cell adhesion and cytokinesis completion were spatially regulated by distinct states of EB3 phosphorylation on serine 176 by Aurora B. This EB3 phosphorylation was enriched at the midbody and shown to control cortical microtubule growth. These findings uncover differential roles of EB proteins and explain the importance of an Aurora B phosphorylation gradient for the spatiotemporal regulation of microtubule function during mitotic exit and cytokinesis.

Evolving tip structures can explain age-dependent microtubule catastrophe

Microtubules are key structural and transport elements in cells. The dynamics at microtubule ends are characterized by periods of slow growth, followed by stochastic switching events termed "catastrophes," in which microtubules suddenly undergo rapid shortening. Growing microtubules are thought to be protected from catastrophe by a GTP-tubulin "cap": GTP-tubulin subunits add to the tips of growing microtubules but are subsequently hydrolyzed to GDP-tubulin subunits once they are incorporated into the microtubule lattice. Loss of the GTP-tubulin cap exposes GDP-tubulin subunits at the microtubule tip, resulting in a catastrophe event. However, the mechanistic basis for sudden loss of the GTP cap, leading to catastrophe, is not known. To investigate microtubule catastrophe events, we performed 3D mechanochemical simulations that account for interactions between neighboring protofilaments. We found that there are two separate factors that contribute to catastrophe events in the 3D simulation: the GTP-tubulin cap size, which settles into a steady-state value that depends on the free tubulin concentration during microtubule growth, and the structure of the microtubule tip. Importantly, 3D simulations predict, and both fluorescence and electron microscopy experiments confirm, that microtubule tips become more tapered as the microtubule grows. This effect destabilizes the tip and ultimately contributes to microtubule catastrophe. Thus, the likelihood of a catastrophe event may be intimately linked to the aging physical structure of the growing microtubule tip. These results have important consequences for catastrophe regulation in cells, as microtubule-associated proteins could promote catastrophe events in part by modifying microtubule tip structures.

End-binding proteins sensitize microtubules to the action of microtubule-targeting agents

Microtubule-targeting agents (MTAs) are widely used for treatment of cancer and other diseases, and a detailed understanding of the mechanism of their action is important for the development of improved microtubule-directed therapies. Although there is a large body of data on the interactions of different MTAs with purified tubulin and microtubules, much less is known about how the effects of MTAs are modulated by microtubule-associated proteins. Among the regulatory factors with a potential to have a strong impact on MTA activity are the microtubule plus end-tracking proteins, which control multiple aspects of microtubule dynamic instability. Here, we reconstituted microtubule dynamics in vitro to investigate the influence of end-binding proteins (EBs), the core components of the microtubule plus end-tracking protein machinery, on the effects that MTAs exert on microtubule plus-end growth. We found that EBs promote microtubule catastrophe induction in the presence of all MTAs tested. Analysis of microtubule growth times supported the view that catastrophes are microtubule age dependent. This analysis indicated that MTAs affect microtubule aging in multiple ways: destabilizing MTAs, such as colchicine and vinblastine, accelerate aging in an EB-dependent manner, whereas stabilizing MTAs, such as paclitaxel and peloruside A, induce not only catastrophes but also rescues and can reverse the aging process.

Reconstitution of dynamic microtubules with Drosophila XMAP215, EB1, and Sentin

XMAP215^msps and the EB1–Sentin duo act individually and cooperatively to accelerate microtubule growth and increase rescue events but also to promote frequent catastrophes.

Dynamic microtubules (MTs) are essential for various intracellular events, such as mitosis. In Drosophila melanogaster S2 cells, three MT tip-localizing proteins, Msps/XMAP215, EB1, and Sentin (an EB1 cargo protein), have been identified as being critical for accelerating MT growth and promoting catastrophe events, thus resulting in the formation of dynamic MTs. However, the molecular activity of each protein and the basis of the modulation of MT dynamics by these three factors are unknown. In this paper, we showed in vitro that XMAP215^msps had a potent growth-promoting activity at a wide range of tubulin concentrations, whereas Sentin, when recruited by EB1 to the growing MT tip, accelerated growth and also increased catastrophe frequency. When all three factors were combined, the growth rate was synergistically enhanced, and rescue events were observed most frequently, but frequent catastrophes restrained the lengthening of the MTs. We propose that MT dynamics are promoted by the independent as well as the cooperative action of XMAP215^msps polymerase and the EB1–Sentin duo.

End-binding proteins and Ase1/PRC1 define local functionality of structurally distinct parts of the microtubule cytoskeleton

The microtubule cytoskeleton is crucial for the intracellular organization of eukaryotic cells. It is a dynamic scaffold that has to perform a variety of very different functions. This multitasking is achieved through the activity of numerous microtubule-associated proteins. Two prominent classes of proteins are central to the selective recognition of distinct transiently existing structural features of the microtubule cytoskeleton. They define local functionality through tightly regulated protein recruitment. Here we summarize the recent developments in elucidating the molecular mechanism underlying the action of microtubule end-binding proteins (EBs) and antiparallel microtubule crosslinkers of the Ase1/PRC1 family that represent the core of these two recruitment modules. Despite their fundamentally different activities, these conserved families share several common features.

New insights into the mechanism for chromosome alignment in metaphase

During mitosis, duplicated sister chromatids are properly aligned at the metaphase plate of the mitotic spindle before being segregated into two daughter cells. This requires a complex process to ensure proper interactions between chromosomes and spindle microtubules. The kinetochore, the proteinaceous complex assembled at the centromere region on each chromosome, serves as the microtubule attachment site and powers chromosome movement in mitosis. Numerous proteins/protein complexes have been implicated in the connection between kinetochores and dynamic microtubules. Recent studies have advanced our understanding on the nature of the interface between kinetochores and microtubule plus ends in promoting and maintaining their stable attachment. These efforts have demonstrated the importance of this process to ensure accurate chromosome segregation, an issue which has great significance for understanding and controlling abnormal chromosome segregation (aneuploidy) in human genetic diseases and in cancer progression.

Dissecting the nanoscale distributions and functions of microtubule-end-binding proteins EB1 and ch-TOG in interphase HeLa cells

Recently, the EB1 and XMAP215/TOG families of microtubule binding proteins have been demonstrated to bind autonomously to the growing plus ends of microtubules and regulate their behaviour in in vitro systems. However, their functional redundancy or difference in cells remains obscure. Here, we compared the nanoscale distributions of EB1 and ch-TOG along microtubules using high-resolution microscopy techniques, and also their roles in microtubule organisation in interphase HeLa cells. The ch-TOG accumulation sites protruded ∼100 nm from the EB1 comets. Overexpression experiments showed that ch-TOG and EB1 did not interfere with each other’s localisation, confirming that they recognise distinct regions at the ends of microtubules. While both EB1 and ch-TOG showed similar effects on microtubule plus end dynamics and additively increased microtubule dynamicity, only EB1 exhibited microtubule-cell cortex attachment activity. These observations indicate that EB1 and ch-TOG regulate microtubule organisation differently via distinct regions in the plus ends of microtubules.

VE-cadherin signaling induces EB3 phosphorylation to suppress microtubule growth and assemble adherens junctions

Vascular endothelial (VE)-cadherin homophilic adhesion controls endothelial barrier permeability through assembly of adherens junctions (AJs). We observed that loss of VE-cadherin-mediated adhesion induced the activation of Src and phospholipase C (PLC)γ2, which mediated Ca(2+) release from endoplasmic reticulum (ER) stores, resulting in activation of calcineurin (CaN), a Ca(2+)-dependent phosphatase. Downregulation of CaN activity induced phosphorylation of serine 162 in end binding (EB) protein 3. This phospho-switch was required to destabilize the EB3 dimer, suppress microtubule (MT) growth, and assemble AJs. The phospho-defective S162A EB3 mutant, in contrast, induced MT growth in confluent endothelial monolayers and disassembled AJs. Thus, VE-cadherin outside-in signaling regulates cytosolic Ca(2+) homeostasis and EB3 phosphorylation, which are required for assembly of AJs. These results identify a pivotal function of VE-cadherin homophilic interaction in modulating endothelial barrier through the tuning of MT dynamics.

Microtubule catastrophe and rescue

Microtubules are long cylindrical polymers composed of tubulin subunits. In cells, microtubules play an essential role in architecture and motility. For example, microtubules give shape to cells, serve as intracellular transport tracks, and act as key elements in important cellular structures such as axonemes and mitotic spindles. To accomplish these varied functions, networks of microtubules in cells are very dynamic, continuously remodeling through stochastic length fluctuations at the ends of individual microtubules. The dynamic behavior at the end of an individual microtubule is termed 'dynamic instability'. This behavior manifests itself by periods of persistent microtubule growth interrupted by occasional switching to rapid shrinkage (called microtubule 'catastrophe'), and then by switching back from shrinkage to growth (called microtubule 'rescue'). In this review, we summarize recent findings which provide new insights into the mechanisms of microtubule catastrophe and rescue, and discuss the impact of these findings in regards to the role of microtubule dynamics inside of cells.

Kinesin-13 and tubulin posttranslational modifications regulate microtubule growth in axon regeneration

The microtubule (MT) cytoskeleton of a mature axon is maintained in a stabilized steady state, yet after axonal injury it can be transformed into a dynamic structure capable of supporting axon regrowth. Using Caenorhabditis elegans mechanosensory axons and in vivo imaging, we find that, in mature axons, the growth of MTs is restricted in the steady state by the depolymerizing kinesin-13 family member KLP-7. After axon injury, we observe a two-phase process of MT growth upregulation. First, the number of growing MTs increases at the injury site, concomitant with local downregulation of KLP-7. A second phase of persistent MT growth requires the cytosolic carboxypeptidase CCPP-6, which promotes Δ2 modification of α-tubulin. Both phases of MT growth are coordinated by the DLK-1 MAP kinase cascade. Our results define how the stable MT cytoskeleton of a mature neuron is converted into the dynamically growing MT cytoskeleton of a regrowing axon.

Move in for the kill: motile microtubule regulators

The stereotypical function of kinesin superfamily motors is to transport cargo along microtubules. However, some kinesins also shape the microtubule track by regulating microtubule assembly and disassembly. Recent work has shown that the kinesin-8 family of motors emerge as key regulators of cellular microtubule length. The studied kinesin-8s are highly processive motors that walk towards the microtubule plus-end. Once at plus-ends, they have complex effects on polymer dynamics; kinesin-8s either destabilize or stabilize microtubules, depending on the context. This review focuses on the mechanisms underlying kinesin-8-microtubule interactions and microtubule length control. We compare and contrast kinesin-8s with the other major microtubule-regulating kinesins (kinesin-4 and kinesin-13), to survey the current understanding of the diverse ways that kinesins control microtubule dynamics.

A Proteome-wide screen for mammalian SxIP motif-containing microtubule plus-end tracking proteins

Microtubule plus-end tracking proteins (+TIPs) are structurally and functionally diverse factors that accumulate at the growing microtubule plus-ends, connect them to various cellular structures, and control microtubule dynamics [1, 2]. EB1 and its homologs are +TIPs that can autonomously recognize growing microtubule ends and recruit to them a variety of other proteins. Numerous +TIPs bind to end binding (EB) proteins through natively unstructured basic and serine-rich polypeptide regions containing a core SxIP motif (serine-any amino acid-isoleucine-proline) [3]. The SxIP consensus sequence is short, and the surrounding sequences show high variability, raising the possibility that undiscovered SxIP containing +TIPs are encoded in mammalian genomes. Here, we performed a proteome-wide search for mammalian SxIP-containing +TIPs by combining biochemical and bioinformatics approaches. We have identified a set of previously uncharacterized EB partners that have the capacity to accumulate at the growing microtubule ends, including protein kinases, a small GTPase, centriole-, membrane-, and actin-associated proteins. We show that one of the newly identified +TIPs, CEP104, interacts with CP110 and CEP97 at the centriole and is required for ciliogenesis. Our study reveals the complexity of the mammalian +TIP interactome and provides a basis for investigating the molecular crosstalk between microtubule ends and other cellular structures.

+TIPs: SxIPping along microtubule ends

+TIPs are a heterogeneous class of proteins that specifically bind to growing microtubule ends. Because dynamic microtubules are essential for many intracellular processes, +TIPs play important roles in regulating microtubule dynamics and microtubule interactions with other intracellular structures. End-binding proteins (EBs) recognize a structural cap at growing microtubule ends, and have emerged as central adaptors that mediate microtubule plus-end tracking of potentially all other +TIPs. The majority of these +TIPs bind to EBs through a short hydrophobic (S/T)x(I/L)P sequence motif (SxIP) and surrounding electrostatic interactions. These recent discoveries have resulted in a rapid expansion of the number of possible +TIPs. In this review, we outline our current understanding of the molecular mechanism of plus-end tracking and provide an overview of SxIP-recruited +TIPs.

Sequence determinants of a microtubule tip localization signal (MtLS)

Microtubule plus-end-tracking proteins (+TIPs) specifically localize to the growing plus-ends of microtubules to regulate microtubule dynamics and functions. A large group of +TIPs contain a short linear motif, SXIP, which is essential for them to bind to end-binding proteins (EBs) and target microtubule ends. The SXIP sequence site thus acts as a widespread microtubule tip localization signal (MtLS). Here we have analyzed the sequence-function relationship of a canonical MtLS. Using synthetic peptide arrays on membrane supports, we identified the residue preferences at each amino acid position of the SXIP motif and its surrounding sequence with respect to EB binding. We further developed an assay based on fluorescence polarization to assess the mechanism of the EB-SXIP interaction and to correlate EB binding and microtubule tip tracking of MtLS sequences from different +TIPs. Finally, we investigated the role of phosphorylation in regulating the EB-SXIP interaction. Together, our results define the sequence determinants of a canonical MtLS and provide the experimental data for bioinformatics approaches to carry out genome-wide predictions of novel +TIPs in multiple organisms.

Epothilone B inhibits migration of glioblastoma cells by inducing microtubule catastrophes and affecting EB1 accumulation at microtubule plus ends

Invasion of normal brain tissue by tumor cells is a major contributing factor to the recurrence of glioblastoma and its resistance to therapy. Here, we have assessed the efficacy of the microtubule (MT) targeting agent Epothilone B (patupilone) on glioblastoma cell migration, a prerequisite for invasive tumor cell behavior. At non-cytotoxic concentrations, patupilone inhibited glioblastoma cell movement, as shown by transwell cell migration, random motility and spheroid assays. This anti-migratory effect was associated with a reduced accumulation of EB1 and other MT plus end tracking proteins at MT ends and with the induction of MT catastrophes, while the MT growth rate and other MT dynamic instability parameters remained unaltered. An increase in MT catastrophes led to the reduction of the number of MTs reaching the leading edge. Analysis of the effect of patupilone on MT dynamics in a reconstituted in vitro system demonstrated that the induction of MT catastrophes and an alteration of EB1 accumulation at MT plus end are intrinsic properties of patupilone activity. We have thus demonstrated that patupilone antagonizes glioblastoma cell migration by a novel mechanism, which is distinct from suppression of MT dynamic instability. Taken together, our results suggest that EB proteins may represent a new potential target for anti-cancer therapy in highly invasive tumors.

Microtubule-severing enzymes at the cutting edge

ATP-dependent severing of microtubules was first reported in Xenopus laevis egg extracts in 1991. Two years later this observation led to the purification of the first known microtubule-severing enzyme, katanin. Katanin homologs have now been identified throughout the animal kingdom and in plants. Moreover, members of two closely related enzyme subfamilies, spastin and fidgetin, have been found to sever microtubules and might act alongside katanins in some contexts (Roll-Mecak and McNally, 2010; Yu et al., 2008; Zhang et al., 2007). Over the past few years, it has become clear that microtubule-severing enzymes contribute to a wide range of cellular activities including mitosis and meiosis, morphogenesis, cilia biogenesis and disassembly, and migration. Thus, this group of enzymes is revealing itself to be among the most important of the microtubule regulators. This Commentary focuses on our growing understanding of how microtubule-severing enzymes contribute to the organization and dynamics of diverse microtubule arrays, as well as the structural and biophysical characteristics that afford them the unique capacity to catalyze the removal of tubulin from the interior microtubule lattice. Our goal is to provide a broader perspective, focusing on a limited number of particularly informative, representative and/or timely findings.

EBs recognize a nucleotide-dependent structural cap at growing microtubule ends

Growing microtubule ends serve as transient binding platforms for essential proteins that regulate microtubule dynamics and their interactions with cellular substructures. End-binding proteins (EBs) autonomously recognize an extended region at growing microtubule ends with unknown structural characteristics and then recruit other factors to the dynamic end structure. Using cryo-electron microscopy, subnanometer single-particle reconstruction, and fluorescence imaging, we present a pseudoatomic model of how the calponin homology (CH) domain of the fission yeast EB Mal3 binds to the end regions of growing microtubules. The Mal3 CH domain bridges protofilaments except at the microtubule seam. By binding close to the exchangeable GTP-binding site, the CH domain is ideally positioned to sense the microtubule's nucleotide state. The same microtubule-end region is also a stabilizing structural cap protecting the microtubule from depolymerization. This insight supports a common structural link between two important biological phenomena, microtubule dynamic instability and end tracking.

MCAK activity at microtubule tips regulates spindle microtubule length to promote robust kinetochore attachment

The kinesin MCAK binds to end-binding proteins and antagonizes centrosome separation and promotes robust kinetochore attachments to spindle microtubules.

Mitotic centromere-associated kinesin (MCAK) is a microtubule-depolymerizing kinesin-13 member that can track with polymerizing microtubule tips (hereafter referred to as tip tracking) during both interphase and mitosis. MCAK tracks with microtubule tips by binding to end-binding proteins (EBs) through the microtubule tip localization signal SKIP, which lies N terminal to MCAK’s neck and motor domain. The functional significance of MCAK’s tip-tracking behavior during mitosis has never been explained. In this paper, we identify and define a mitotic function specific to the microtubule tip–associated population of MCAK: negative regulation of microtubule length within the assembling bipolar spindle. This function depends on MCAK’s ability to bind EBs and track with polymerizing nonkinetochore microtubule tips. Although this activity antagonizes centrosome separation during bipolarization, it ultimately benefits the dividing cell by promoting robust kinetochore attachments to the spindle microtubules.

Cooperative stabilization of microtubule dynamics by EB1 and CLIP-170 involves displacement of stably bound P(i) at microtubule ends

End binding protein 1 (EB1) and cytoplasmic linker protein of 170 kDa (CLIP-170) are two well-studied microtubule plus-end-tracking proteins (+TIPs) that target growing microtubule plus ends in the form of comet tails and regulate microtubule dynamics. However, the mechanism by which they regulate microtubule dynamics is not well understood. Using full-length EB1 and a minimal functional fragment of CLIP-170 (ClipCG12), we found that EB1 and CLIP-170 cooperatively regulate microtubule dynamic instability at concentrations below which neither protein is effective. By use of small-angle X-ray scattering and analytical ultracentrifugation, we found that ClipCG12 adopts a largely extended conformation with two noninteracting CAP-Gly domains and that it formed a complex in solution with EB1. Using a reconstituted steady-state mammalian microtubule system, we found that at a low concentration of 250 nM, neither EB1 nor ClipCG12 individually modulated plus-end dynamic instability. Higher concentrations (up to 2 μM) of the two proteins individually did modulate dynamic instability, perhaps by a combination of effects at the tips and along the microtubule lengths. However, when low concentrations (250 nM) of EB1 and ClipCG12 were present together, the mixture modulated dynamic instability considerably. Using a pulsing strategy with [γ(32)P]GTP, we further found that unlike EB1 or ClipCG12 alone, the EB1-ClipCG12 mixture partially depleted the microtubule ends of stably bound (32)P(i). Together, our results suggest that EB1 and ClipCG12 act cooperatively to regulate microtubule dynamics. They further indicate that stabilization of microtubule plus ends by the EB1-ClipCG12 mixture may involve modification of an aspect of the stabilizing cap.

Kif2C minimal functional domain has unusual nucleotide binding properties that are adapted to microtubule depolymerization

The kinesin-13 Kif2C hydrolyzes ATP and uses the energy released to disassemble microtubules. The mechanism by which this is achieved remains elusive. Here we show that Kif2C-(sN+M), a monomeric construct consisting of the motor domain with the proximal part of the N-terminal Neck extension but devoid of its more distal, unstructured, and highly basic part, has a robust depolymerase activity. When detached from microtubules, the Kif2C-(sN+M) nucleotide-binding site is occupied by ATP at physiological concentrations of adenine nucleotides. As a consequence, Kif2C-(sN+M) starts its interaction with microtubules in that state, which differentiates kinesin-13s from motile kinesins. Moreover, in this ATP-bound conformational state, Kif2C-(sN+M) has a higher affinity for soluble tubulin compared with microtubules. We propose a mechanism in which, in the first step, the specificity of ATP-bound Kif2C for soluble tubulin causes it to stabilize a curved conformation of tubulin heterodimers at the ends of microtubules. Data from an ATPase-deficient Kif2C mutant suggest that, then, ATP hydrolysis precedes and is required for tubulin release to take place. Finally, comparison with Kif2C-Motor indicates that the binding specificity for curved tubulin and, accordingly, the microtubule depolymerase activity are conferred to the motor domain by its N-terminal Neck extension.

Microtubules cut loose at the cell cortex

The ability of the microtubule cytoskeleton to rapidly and locally reorganize itself in response to intra- and extracellular signals is essential to its wide range of functions. A site of tightly regulated microtubule dynamics--and the major interface between the microtubule cytoskeleton and the extracellular environment--is the cell cortex, where the selective stabilization and destabilization of microtubule plus-ends is required for normal cell division, morphogenesis and migration. In a recent study, we found that the cortex of Drosophila S2 and D17 cells is coated with the microtubule severing enzyme and plus-end depolymerase, Kat-60, which actively suppresses microtubule growth and stability along the cell edge. We have proposed that cortical Kat-60 functions by uncapping plus-ends, thereby activating another microtubule depolymerase, KLP10A, preloaded onto the end. The localized destruction of microtubule plus-ends at a specific cortical could feed into larger regulatory pathways, such as those in control of the actin cytoskeleton, to influence cell polarization and motility.