Suppression of microtubule dynamic instability by the +TIP protein EB1 and its modulation by the CAP-Gly domain of p150glued

TRIM Proteins and Antiviral Microtubule Reorganization: A Novel Component in Innate Immune Responses?

TRIM proteins are a family of innate immune factors that play diverse roles in innate immunity and protect the cell against viral and bacterial aggression. As part of this special issue on TRIM proteins, we will take advantage of our findings on TRIM69, which acts by reorganizing the microtubules (MTs) in a manner that is fundamentally antiviral, to more generally discuss how host-pathogen interactions that take place for the control of the MT network represent a crucial facet of the struggle that opposes viruses to their cell environment. In this context, we will present several other TRIM proteins that are known to interact with microtubules in situations other than viral infection, and we will discuss evidence that may suggest a possible contribution to viral control. Overall, the present review will highlight the importance that the control of the microtubule network bears in host-pathogen interactions.

EB1 Increases the Dynamics of Tau Droplets and Inhibits Tau Aggregation: Implications in Tauopathies

EB1, a microtubule plus end-tracking protein (+TIP), regulates microtubule dynamics. Recent evidence indicates cross-talk between EB proteins and tau, a microtubule-associated neuronal protein that is important for the growth and stability of microtubules. We investigated the interaction between tau and EB1 and the effect of binding of EB1 on tau function and aggregation. EB1 colocalized with tau in SH-SY5Y cells and coimmunoprecipitated with tau. Further, purified EB1 impaired the ability of adult tau to induce tubulin polymerization in vitro. EB1 bound to tau with a dissociation constant of 2.5 ± 0.7 μM. EB1 reduced heparin-induced tau aggregation with a half-maximal inhibitory concentration of 4.3 ± 0.2 μM, and increased the dynamics of tau in phase-separated droplets. The fluorescence recovery rate in tau droplets increased from 0.02 ± 0.01 to 0.07 ± 0.03 s-1, while the half-time of recovery decreased from 44.5 ± 14 to 13.5 ± 6 s in the presence of 8 μM EB1, suggesting a delay in the transition of tau from the soluble to aggregated form in tau liquid-liquid phase separation. EB1 decreased the rate of aggregation and increased the critical concentration of tau aggregation. Dynamic light scattering, atomic force microscopy, dot blot assays, and SDS-PAGE analysis showed that EB1 inhibited the formation of oligomers and higher-order aggregates of tau. The data suggest a novel role for EB1 as a regulator of tau function and aggregation, and the findings indicated the role of the EB family proteins in neuronal function and neurodegeneration.

NudC regulated Lis1 stability is essential for the maintenance of dynamic microtubule ends in axon terminals

In the axon terminal, microtubule stability is decreased relative to the axon shaft. The dynamic microtubule plus ends found in the axon terminal have many functions, including serving as a docking site for the Cytoplasmic dynein motor. Here, we report an unexplored function of dynein in microtubule regulation in axon terminals: regulation of microtubule stability. Using a forward genetic screen, we identified a mutant with an abnormal axon terminal structure owing to a loss of function mutation in NudC. We show that, in the axon terminal, NudC is a chaperone for the protein Lis1. Decreased Lis1 in nudc axon terminals causes dynein/dynactin accumulation and increased microtubule stability. Microtubule dynamics can be restored by pharmacologically inhibiting dynein, implicating excess dynein motor function in microtubule stabilization. Together, our data support a model in which local NudC-Lis1 modulation of the dynein motor is critical for the regulation of microtubule stability in the axon terminal.

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NudC, a dynein regulator, is crucial for axon terminal structure

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NudC mutation leads to a near complete loss of Lis1 protein in axon terminals

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Lis1 deficits cause accumulation of dynein and cargo in axon terminals

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Local elevation of dynein increases axon terminal microtubule stability

Manipulation of the Tubulin Code Alters Directional Cell Migration and Ciliogenesis

Conjunction of epithelial cells into monolayer sheets implies the ability to migrate and to undergo apicobasal polarization. Both processes comprise reorganization of cytoskeletal elements and rearrangements of structural protein interactions. We modulated expression of tubulin tyrosin ligase (TTL), the enzyme that adds tyrosine to the carboxy terminus of detyrosinated α-tubulin, to study the role of tubulin detyrosination/-tyrosination in the orientation of cell motility and in epithelial morphogenesis. Oriented cell migration and the organization of focal adhesions significantly lose directionality with diminishing amounts of microtubules enriched in detyrosinated tubulin. On the other hand, increasing quantities of detyrosinated tubulin results in faster plus end elongation of microtubules in migrating and in polarized epithelial cells. These plus ends are decorated by the plus end binding protein 1 (EB1), which mediates interaction between microtubules enriched in detyrosinated tubulin and the integrin-ILK complex at focal adhesions. EB1 accumulates at the apical cell pole at the base of the primary cilium following apicobasal polarization. Polarized cells almost devoid of detyrosinated tubulin form stunted primary cilia and multiluminal cysts in 3D-matrices. We conclude that the balance between detyrosinated and tyrosinated tubulin alters microtubule dynamics, affects the orientation of focal adhesions and determines the organization of primary cilia on epithelial cells.

AKAP350 enables p150glued /EB1 interaction at the spindle poles

A-kinase anchoring protein 350 (AKAP350) is a centrosomal/Golgi scaffold protein, critical for the regulation of microtubule dynamics. AKAP350 recruits end-binding protein 1 (EB1) to the centrosome in mitotic cells, ensuring proper spindle orientation in epithelial cells. AKAP350 also interacts with p150^glued, the main component of the dynactin complex. In the present work, we found that AKAP350 localized p150^glued to the spindle poles, facilitating p150^glued/EB1 interaction at these structures. Our results further showed that the decrease in AKAP350 expression reduced p150^glued localization at astral microtubules and impaired the elongation of astral microtubules during anaphase. Overall, this study provides mechanistic data on how microtubule regulatory proteins gather to define microtubule dynamics in mitotic cells.

PTPN22 interacts with EB1 to regulate T-cell receptor signaling

The PTPN22 gene encoding the Lyp/Pep protein tyrosine phosphatase is a negative regulator of T-cell receptor (TCR) signaling. Recent studies have shown that phosphorylation of end-binding protein 1 (EB1) is associated with the TCR activation. In this study, using 2-hybrid and mass spectrometry analyses, we identified EB1 as a protein associated with PTPN22. Furthermore, we discovered that EB1 specifically bound to the P1 domain of PTPN22 by competing with CSK, and the variant PTPN22-R620W does not affect the association with EB1, which is instrumental with respect to the regulation of TCR signaling. In addition, PTPN22 dephosphorylates EB1 at tyrosine-247 (Y247), which decreases the expression of the T-cell activation markers CD25 and CD69 and the phosphorylation levels of the TCR molecules ZAP-70, LAT, and Erk, leading to the eventual downregulation of the transcription factor NFAT and reduced the levels of secreted IL-2. The findings of this study provide new insights into the TCR signaling and the T-cell immune response, which are important for clarifying the mechanism of PTPN22-related autoimmune diseases.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Tubulin in Platelets: When the Shape Matters

Platelets are anuclear cells with a short lifespan that play an essential role in many pathophysiological processes, including haemostasis, inflammation, infection, vascular integrity, and metastasis. Billions of platelets are produced daily from megakaryocytes (platelet precursors). Despite this high production, the number of circulating platelets is stable and, under resting conditions, they maintain their typical discoid shape thanks to cytoskeleton proteins. The activation of platelets is associated with dynamic and rapid changes in the cytoskeleton. Two cytoskeletal polymer systems exist in megakaryocytes and platelets: actin filaments and microtubules, based on actin, and α- and β-tubulin heterodimers, respectively. Herein, we will focus on platelet-specific tubulins and their alterations and role of the microtubules skeleton in platelet formation (thrombopoiesis). During this process, microtubules mediate elongation of the megakaryocyte extensions (proplatelet) and granule trafficking from megakaryocytes to nascent platelets. In platelets, microtubules form a subcortical ring, the so-called marginal band, which confers the typical platelet discoid shape and is also responsible for changes in platelet morphology upon activation. Molecular alterations in the gene encoding β1 tubulin and microtubules post-translational modifications may result in quantitative or qualitative changes in tubulin, leading to altered cytoskeleton reorganization that may induce changes in the platelet number (thrombocytopenia), morphology or function. Consequently, β1-tubulin modifications may participate in pathological and physiological processes, such as development.

GTP-binding facilitates EB1 recruitment onto microtubules by relieving its auto-inhibition

Microtubule plus end-binding protein, EB1 is a key regulator of microtubule dynamics. Auto-inhibitory interaction in EB1 has previously been shown to inhibit its ability to bind to microtubules and regulate microtubule dynamics. However, the factors that promote its microtubule regulatory activity by over-coming the auto-inhibition are less known. Here, we show that GTP plays a critical role in promoting the microtubule-targeting activity of EB1 by suppressing its auto-inhibition. Our biophysical data demonstrate that GTP binds to EB1 at a distinct site in its conserved N-terminal domain. Detailed analyses reveal that GTP-binding suppresses the intra-molecular inhibitory interaction between the globular N-terminus and the C-terminal coiled-coil domain. We further show that mutation of the GTP-binding site residues in N-terminus weakens the affinity for GTP, but also for the C-terminus, indicating overlapping binding sites. Confocal imaging and biochemical analysis reveal that EB1 localization on the microtubules is significantly increased upon mutations of the GTP-binding site residues. The results demonstrate a unique role of GTP in facilitating EB1 interaction with the microtubules by relieving its intra-molecular inhibition. They also implicate that GTP-binding may regulate the functions of EB1 on the cellular microtubules.

Kinetochore-microtubule interactions in chromosome segregation: lessons from yeast and mammalian cells

Chromosome congression and segregation require robust yet dynamic attachment of the kinetochore with the spindle microtubules. Force generated at the kinetochore-microtubule interface plays a vital role to drive the attachment, as it is required to move chromosomes and to provide signal to sense correct attachments. To understand the mechanisms underlying these processes, it is critical to describe how the force is generated and how the molecules at the kinetochore-microtubule interface are organized and assembled to withstand the force and respond to it. Research in the past few years or so has revealed interesting insights into the structural organization and architecture of kinetochore proteins that couple kinetochore attachment to the spindle microtubules. Interestingly, despite diversities in the molecular players and their modes of action, there appears to be architectural similarity of the kinetochore-coupling machines in lower to higher eukaryotes. The present review focuses on the most recent advances in understanding of the molecular and structural aspects of kinetochore-microtubule interaction based on the studies in yeast and vertebrate cells.

Regulation of paclitaxel activity by microtubule-associated proteins in cancer chemotherapy

Microtubules, highly dynamic components of the cytoskeleton, participate in diverse cellular activities such as mitosis, cell migration, and intracellular trafficking. Dysregulation of microtubule dynamics contributes to the development of serious diseases, including cancer. The dynamic properties and functions of microtubule network are regulated by microtubule-associated proteins. Paclitaxel, an anti-microtubule agent of the taxane family, has shown a success in clinical treatment of many cancer patients. However, the variable response activity of patients and acquired resistance to paclitaxel limit the clinical use of the drug. Accumulating studies show that microtubule-associated proteins can regulate paclitaxel sensitivity in a wide range of cancer types. In this review, we will describe the roles of various microtubule-associated proteins in the regulation of paclitaxel in cancers. Particularly, we will focus on the modulation of centrosomal proteins in paclitaxel resistance. Improved understandings of how these proteins act might predict treatment responses and provide insights into more rational chemotherapeutic regimens in clinical practice.

Interactions between Tau and Different Conformations of Tubulin: Implications for Tau Function and Mechanism

Tau is a multifaceted neuronal protein that stabilizes microtubules (MTs), but the mechanism of this activity remains poorly understood. Questions include whether Tau binds MTs laterally or longitudinally and whether Tau's binding affinity depends on the nucleotide state of tubulin. We observed that Tau binds tightly to Dolastatin-10 tubulin rings and promotes the formation of Dolastatin-10 ring stacks, implying that Tau can crosslink MT protofilaments laterally. In addition, we found that Tau prefers GDP-like tubulin conformations, which implies that Tau binding to the MT surface is biased away from the dynamic GTP-rich MT tip. To investigate the potential impact of these Tau activities on MT stabilization, we incorporated them into our previously developed dimer-scale computational model of MT dynamics. We found that lateral crosslinking activities have a much greater effect on MT stability than do longitudinal crosslinking activities, and that introducing a bias toward GDP tubulin has little impact on the observed MT stabilization. To address the question of why Tau is GDP-tubulin-biased, we tested whether Tau might affect MT binding of the +TIP EB1. We confirmed recent reports that Tau binds directly to EB1 and that Tau competes with EB1 for MT binding. Our results lead to a conceptual model where Tau stabilizes the MT lattice by strengthening lateral interactions between protofilaments. We propose that Tau's GDP preference allows the cell to independently regulate the dynamics of the MT tip and the stability of the lattice.

EB1 regulates attachment of Ska1 with microtubules by forming extended structures on the microtubule lattice

Kinetochore couples chromosome movement to dynamic microtubules, a process that is fundamental to mitosis in all eukaryotes but poorly understood. In vertebrates, spindle-kinetochore-associated (Ska1–3) protein complex plays an important role in this process. However, the proteins that stabilize Ska-mediated kinetochore-microtubule attachment remain unknown. Here we show that microtubule plus-end tracking protein EB1 facilitates Ska localization on microtubules in vertebrate cells. EB1 depletion results in a significant reduction of Ska1 recruitment onto microtubules and defects in mitotic chromosome alignment, which is also reflected in computational modelling. Biochemical experiments reveal that EB1 interacts with Ska1, facilitates Ska1-microtubule attachment and together stabilizes microtubules. Structural studies reveal that EB1 either with Ska1 or Ska complex forms extended structures on microtubule lattice. Results indicate that EB1 promotes Ska association with K-fibres and facilitates kinetochore-microtubule attachment. They also implicate that in vertebrates, chromosome coupling to dynamic microtubules could be mediated through EB1-Ska extended structures.

Ska1 is a kinetochore-localised protein that couples kinetochore movement to microtubule (MT) depolymerisation. Here Thomas et al. show that the MT +TIP binding protein EB1 recruits Ska1 to the MT-kinetochore interface and stabilises the interaction between Ska1 and MTs.

Aurora A's Functions During Mitotic Exit: The Guess Who Game

Until recently, the knowledge of Aurora A kinase functions during mitosis was limited to pre-metaphase events, particularly centrosome maturation, G2/M transition, and mitotic spindle assembly. However, an involvement of Aurora A in post-metaphase events was also suspected, but not clearly demonstrated due to the technical difficulty to perform the appropriate experiments. Recent developments of both an analog-specific version of Aurora A and small molecule inhibitors have led to the first demonstration that Aurora A is required for the early steps of cytokinesis. As in pre-metaphase, Aurora A plays diverse functions during anaphase, essentially participating in astral microtubules dynamics and central spindle assembly and functioning. The present review describes the experimental systems used to decipher new functions of Aurora A during late mitosis and situate these functions into the context of cytokinesis mechanisms.

+TIP EB1 downregulates paclitaxel‑induced proliferation inhibition and apoptosis in breast cancer cells through inhibition of paclitaxel binding on microtubules

Microtubule plus‑end‑binding protein (+TIP) EB1 has been shown to be upregulated in breast cancer cells and promote breast tumor growth in vivo. However, its effect on the cellular actions of microtubule‑targeted drugs in breast cancer cells has remained poorly understood. By using cellular and biochemical assays, we demonstrate that EB1 plays a critical role in regulating the sensitivity of breast cancer cells to anti‑microtubule drug, paclitaxel (PTX). Cell viability assays revealed that EB1 expression in the breast cancer cell lines correlated with the reduction of their sensitivity to PTX. Knockdown of EB1 by enzymatically‑prepared siRNA pools (esiRNAs) increased PTX‑induced cytotoxicity and sensitized cells to PTX‑induced apoptosis in three breast cancer cell lines, MCF‑7, MDA MB‑231 and T47D. Apoptosis was associated with activation of caspase‑9 and an increase in the cleavage of poly(ADP‑ribose) polymerase (PARP). p53 and Bax were upregulated and Bcl2 was downregulated in the EB1‑depleted PTX‑treated MCF‑7 cells, indicating that the apoptosis occurs via a p53‑dependent pathway. Following its upregulation, the nuclear accumulation of p53 and its association with cellular microtubules were increased. EB1 depletion increased PTX‑induced microtubule bundling in the interphase cells and induced formation of multiple spindle foci with abnormally elongated spindles in the mitotic MCF‑7 cells, indicating that loss of EB1 promotes PTX‑induced stabilization of microtubules. EB1 inhibited PTX‑induced microtubule polymerization and diminished PTX binding to microtubules in vitro, suggesting that it modulates the binding sites of PTX at the growing microtubule ends. Results demonstrate that EB1 downregulates inhibition of PTX‑induced proliferation and apoptosis in breast cancer cells through a mechanism in which it impairs PTX‑mediated stabilization of microtubule polymerization and inhibits PTX binding on microtubules.

Mechanical effects of EB1 on microtubules depend on GTP hydrolysis state and presence of paclitaxel

Using the nonhydrolyzable GTP analog GMPCPP and the slowly hydrolyzable GTPγS, we polymerize microtubules that recapitulate the end binding behavior of the plus end interacting protein (+TIP) EB1 along their entire length, and use these to investigate the impact of EB1 binding on microtubule mechanics. To measure the stiffness of single filaments, we use a spectral analysis method to determine the ensemble of shapes adopted by a freely diffusing, fluorescently labeled microtubule. We find that the presence of EB1 can stiffen microtubules in a manner that depends on the hydrolysis state of the tubulin-bound nucleotide, as well as the presence of the small-molecule stabilizer paclitaxel. We find that the magnitude of the EB1-induced stiffening is not proportional to the EB1-microtubule binding affinity, suggesting that the stiffening effect does not arise purely from an increase in the total amount of bound EB1. Additionally, we find that EB1 binds cooperatively to microtubules in manner that depends on tubulin-bound nucleotide state.

Microtubule +TIP protein EB1 binds to GTP and undergoes dissociation from dimer to monomer on binding GTP

The +TIP protein EB1 autonomously tracks the growing plus end of microtubules and regulates plus-end dynamics. Previous studies have indicated that EB1 can recognize GTP-bound tubulin structures at the plus end, and it localizes on the microtubule surface at a site close to the exchangeable GTP-binding site of tubulin. Although the GTP-dependent structural change in tubulin has been demonstrated to be a critical determinant for recognition of plus ends by EB1, the effect of GTP on the structure of EB1 has remained unclear. Here, we have used spectroscopic, calorimetric, and biochemical methods to analyze the effect of GTP on EB1 in vitro. Isothermal titration calorimetry and tryptophan fluorescence quenching experiments demonstrated that EB1 binds to GTP with a dissociation constant ~30 μM. Circular dichroism measurements showed that EB1 undergoes changes in its secondary structure on binding GTP. Size-exclusion chromatography and urea-induced unfolding analyses revealed that GTP binding induces dissociation of the EB1 dimer to monomers. Size-exclusion chromatography followed by biochemical analysis further determined that EB1-GTP binding involves association of approximately one molecule of GTP per EB1 monomer. The results reveal a hitherto unknown GTP-dependent mechanism of dimer-to-monomer transition in EB1 and further implicate its possible role in regulating the stability of the EB1 dimer vs monomer as well as plus-end regulation in cells.

Molecular basis for age-dependent microtubule acetylation by tubulin acetyltransferase

Acetylation of α-tubulin Lys40 by tubulin acetyltransferase (TAT) is the only known posttranslational modification in the microtubule lumen. It marks stable microtubules and is required for polarity establishment and directional migration. Here, we elucidate the mechanistic underpinnings for TAT activity and its preference for microtubules with slow turnover. 1.35 Å TAT cocrystal structures with bisubstrate analogs constrain TAT action to the microtubule lumen and reveal Lys40 engaged in a suboptimal active site. Assays with diverse tubulin polymers show that TAT is stimulated by microtubule interprotofilament contacts. Unexpectedly, despite the confined intraluminal location of Lys40, TAT efficiently scans the microtubule bidirectionally and acetylates stochastically without preference for ends. First-principles modeling and single-molecule measurements demonstrate that TAT catalytic activity, not constrained luminal diffusion, is rate limiting for acetylation. Thus, because of its preference for microtubules over free tubulin and its modest catalytic rate, TAT can function as a slow clock for microtubule lifetimes.

An EB1-kinesin complex is sufficient to steer microtubule growth in vitro

Proper microtubule polarity underlies overall neuronal polarity, but mechanisms for maintaining microtubule polarity are not well understood. Previous live imaging in Drosophila dendritic arborization neurons showed that while microtubules are uniformly plus-end out in axons, dendrites possess uniformly minus-end-out microtubules [1]. Thus, maintaining uniform microtubule polarity in dendrites requires that growing microtubule plus ends entering branch points be actively directed toward the cell body. A model was proposed in which EB1 tracks the plus ends of microtubules growing into a branch and an associated kinesin-2 motor walks along a static microtubule to steer the plus end toward the cell body. However, the fast plus-end binding dynamics of EB1 [2-5] appear to be at odds with this proposed mechanical function. To test this model in vitro, we reconstituted the system by artificially dimerizing EB1 to kinesin, growing microtubules from immobilized seeds, and imaging encounters between growing microtubule plus ends and static microtubules. Consistent with in vivo observations, the EB1-kinesin complex actively steered growing microtubules. Thus, EB1 kinetics and mechanics are sufficient to bend microtubules for several seconds. Other kinesins also demonstrated this activity, suggesting this is a general mechanism for organizing and maintaining proper microtubule polarity in cells.

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.

Dynactin subunit p150(Glued) is a neuron-specific anti-catastrophe factor

The dynein partner dynactin not only binds to microtubules, but is found to potently influence microtubule dynamics in neurons.

Regulation of microtubule dynamics in neurons is critical, as defects in the microtubule-based transport of axonal organelles lead to neurodegenerative disease. The microtubule motor cytoplasmic dynein and its partner complex dynactin drive retrograde transport from the distal axon. We have recently shown that the p150^Glued subunit of dynactin promotes the initiation of dynein-driven cargo motility from the microtubule plus-end. Because plus end-localized microtubule-associated proteins like p150^Glued may also modulate the dynamics of microtubules, we hypothesized that p150^Glued might promote cargo initiation by stabilizing the microtubule track. Here, we demonstrate in vitro using assembly assays and TIRF microscopy, and in primary neurons using live-cell imaging, that p150^Glued is a potent anti-catastrophe factor for microtubules. p150^Glued alters microtubule dynamics by binding both to microtubules and to tubulin dimers; both the N-terminal CAP-Gly and basic domains of p150^Glued are required in tandem for this activity. p150^Glued is alternatively spliced in vivo, with the full-length isoform including these two domains expressed primarily in neurons. Accordingly, we find that RNAi of p150^Glued in nonpolarized cells does not alter microtubule dynamics, while depletion of p150^Glued in neurons leads to a dramatic increase in microtubule catastrophe. Strikingly, a mutation in p150^Glued causal for the lethal neurodegenerative disorder Perry syndrome abrogates this anti-catastrophe activity. Thus, we find that dynactin has multiple functions in neurons, both activating dynein-mediated retrograde axonal transport and enhancing microtubule stability through a novel anti-catastrophe mechanism regulated by tissue-specific isoform expression; disruption of either or both of these functions may contribute to neurodegenerative disease.

Microtubules are polymers of tubulin that undergo successive cycles of growth and shrinkage so that the cell can maintain a stable yet adaptable cytoskeleton. In neurons, the microtubule motor protein dynein and its partner complex dynactin drive retrograde transport along microtubules from the distal axon towards the cell body. In addition to binding to dynein, the p150^Glued subunit of dynactin independently binds directly to microtubules. We hypothesized that by binding to microtubules, p150^Glued might also alter microtubule dynamics. We demonstrate using biochemistry and microscopy in vitro and in cells that p150^Glued stabilizes microtubules by inhibiting the transition from growth to shrinkage. We show that specific domains of p150^Glued encoded by neuronally enriched splice-forms are necessary for this activity. Although depletion of p150^Glued in nonpolarized cells does not alter microtubule dynamics, depletion of endogenous p150^Glued in neurons leads to dramatic microtubule instability. Strikingly, a mutation in p150^Glued known to cause the neurodegenerative disorder Perry syndrome abolishes this activity. In summary, we identified a previously unappreciated function of dynactin in direct regulation of the microtubule cytoskeleton. This activity may enhance generic microtubule stability in the cell, but could be especially important in specific areas of the cell where dynactin and dynein are loaded onto microtubules.

Anti-migratory effect of vinflunine in endothelial and glioblastoma cells is associated with changes in EB1 C-terminal detyrosinated/tyrosinated status

We previously showed that vinflunine, a microtubule-targeting drug of the Vinca-alkaloid family exerted its anti-angiogenic/anti-migratory activities through an increase in microtubule dynamics and an inhibition of microtubule targeting to adhesion sites. Such effect was associated with a reduction of EB1 comet length at microtubule (+) ends. In this work we first showed that the pro-angiogenic vascular endothelial growth factor VEGF suppressed microtubule dynamics in living Human Umbilical Vein Endothelial Cells (HUVECs), increased EB1 comet length by 40%, and induced EB1 to bind all along the microtubules, without modifying its expression level. Such microtubule (+) end stabilization occurred close to the plasma membrane in the vicinity of focal adhesion as shown by TIRF microscopy experiments. Vinflunine completely abolished the effect of VEGF on EB1 comets. Interestingly, we found a correlation between the reduction of EB1 comet length by vinflunine and the inhibition of cell migration. By using 2D gel electrophoresis we demonstrated for the first time that EB1 underwent several post-translational modifications in endothelial and tumor cells. Particularly, the C-terminal EEY sequence was poorly detectable in control and VEGF-treated HUVECs suggesting the existence of a non-tyrosinated form of EB1. By using specific antibodies that specifically recognized and discriminated the native tyrosinated form of EB1 and a putative C-terminal detyrosinated form, we showed that a detyrosinated form of EB1 exists in HUVECs and tumor cells. Interestingly, vinflunine decreased the level of the detyrosinated form and increased the native tyrosinated form of EB1. Using 3-L-Nitrotyrosine incorporation experiments, we concluded that the EB1 C-terminal modifications result from a detyrosination/retyrosination cycle as described for tubulin.

Altogether, our results show that vinflunine inhibits endothelial cell migration through an alteration of EB1 comet length and EB1 detyrosination/retyrosination cycle.

Arabidopsis AUGMIN subunit8 is a microtubule plus-end binding protein that promotes microtubule reorientation in hypocotyls

In plant cells, cortical microtubules provide tracks for cellulose-synthesizing enzymes and regulate cell division, growth, and morphogenesis. The role of microtubules in these essential cellular processes depends on the spatial arrangement of the microtubules. Cortical microtubules are reoriented in response to changes in cell growth status and cell shape. Therefore, an understanding of the mechanism that underlies the change in microtubule orientation will provide insight into plant cell growth and morphogenesis. This study demonstrated that AUGMIN subunit8 (AUG8) in Arabidopsis thaliana is a novel microtubule plus-end binding protein that participates in the reorientation of microtubules in hypocotyls when cell elongation slows down. AUG8 bound to the plus ends of microtubules and promoted tubulin polymerization in vitro. In vivo, AUG8 was recruited to the microtubule branch site immediately before nascent microtubules branched out. It specifically associated with the plus ends of growing cortical microtubules and regulated microtubule dynamics, which facilitated microtubule reorientation when microtubules changed their growth trajectory or encountered obstacle microtubules during microtubule reorientation. This study thus reveals a novel mechanism underlying microtubule reorientation that is critical for modulating cell elongation in Arabidopsis.

Aurora A is involved in central spindle assembly through phosphorylation of Ser 19 in P150Glued

A human Aurora A kinase engineered to be specifically inhibited by the ATP analog 1-Na-PP1 allows dissection of a novel role for this protein in central spindle assembly.

Knowledge of Aurora A kinase functions is limited to premetaphase events, particularly centrosome maturation, G2/M transition, and mitotic spindle assembly. The involvement of Aurora A in events after metaphase has only been suggested because appropriate experiments are technically difficult. We report here the design of the first human Aurora A kinase (as-AurA) engineered by chemical genetics techniques. This kinase is fully functional biochemically and in cells, and is rapidly and specifically inhibited by the ATP analogue 1-Naphthyl-PP1 (1-Na-PP1). By treating cells exclusively expressing the as-AurA with 1-Na-PP1, we discovered that Aurora A is required for central spindle assembly in anaphase through phosphorylation of Ser 19 of P150Glued. This paper thus describes a new Aurora A function that takes place after the metaphase-to-anaphase transition and a new powerful tool to search for and study new Aurora A functions.

Synergy between XMAP215 and EB1 increases microtubule growth rates to physiological levels

In cells, a complex network of proteins regulates the dynamic growth of microtubules that is essential for division and migration. In vitro approaches with purified components have so far been unable to reconstitute fast microtubule growth observed in vivo . Here we show that two well-studied plus-end-binding proteins-end-tracking protein EB1 and microtubule polymerase XMAP215-act together to strongly promote microtubule growth to cellular rates. Unexpectedly, the combined effects of XMAP215 and EB1 are highly synergistic, with acceleration of growth well beyond the product of the individual effects of either protein. The synergistic growth promotion does not rely on any of the canonical EB1 interactions, suggesting an allosteric interaction through the microtubule end. This hypothesis is supported by the finding that taxol and XMAP215, which have non-overlapping binding sites on tubulin, also act synergistically on growth. The increase in growth rates is accompanied by a strong enhancement of microtubule catastrophe by EB1, thereby rendering the fast and dynamic microtubule behaviour typically observed in cells.

Three-dimensional structure of CAP-gly domain of mammalian dynactin determined by magic angle spinning NMR spectroscopy: conformational plasticity and interactions with end-binding protein EB1

Microtubules and their associated proteins play important roles in vesicle and organelle transport, cell motility and cell division. Perturbation of these processes by mutation typically gives rise to severe pathological conditions. In our efforts to obtain atomic information on microtubule-associated protein/microtubule interactions with the goal to understand mechanisms that might potentially assist in the development of treatments for these diseases, we have determined the three-dimensional structure of CAP-Gly (cytoskeleton-associated protein, glycine-rich) domain of mammalian dynactin by magic angle spinning NMR spectroscopy. We observe two conformations in the β2 strand encompassing residues T43-V44-A45, residues that are adjacent to the disease-associated mutation, G59S. Upon binding of CAP-Gly to microtubule plus-end tracking protein EB1, the CAP-Gly shifts to a single conformer. We find extensive chemical shift perturbations in several stretches of residues of CAP-Gly upon binding to EB1, from which we define accurately the CAP-Gly/EB1 binding interface. We also observe that the loop regions may exhibit unique flexibility, especially in the GKNDG motif, which participates in the microtubule binding. This study in conjunction with our previous reports suggests that conformational plasticity is an intrinsic property of CAP-Gly likely due to its unusually high loop content and may be required for its biological functions.

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.

Multiparametric analysis of CLASP-interacting protein functions during interphase microtubule dynamics

The microtubule (MT) plus-end tracking protein (+TIP) CLASP mediates dynamic cellular behaviors and interacts with numerous cytoplasmic proteins. While the influence of some CLASP interactors on MT behavior is known, a comprehensive survey of the proteins in the CLASP interactome as MT regulators is missing. Ultimately, we are interested in understanding how CLASP collaborates with functionally linked proteins to regulate MT dynamics. Here, we utilize multiparametric analysis of time-lapse MT +TIP imaging data acquired in Drosophila melanogaster S2R+ cells to assess the effects on individual microtubule dynamics for RNA interference-mediated depletion of 48 gene products previously identified to be in vivo genetic CLASP interactors. While our analysis corroborates previously described functions of several known CLASP interactors, its multiparametric resolution reveals more detailed functional profiles (fingerprints) that allow us to precisely classify the roles that CLASP-interacting genes play in MT regulation. Using these data, we identify subnetworks of proteins with novel yet overlapping MT-regulatory roles and also uncover subtle distinctions between the functions of proteins previously thought to act via similar mechanisms.

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-binding sites of the CH domain of EB1 and its autoinhibition revealed by NMR

End-binding protein 1 (EB1) is one of the best studied plus-end tracking proteins. It is known that EB1 specifically binds the plus ends of microtubules (MTs) and promotes MT growth. EB1 activity is thought to be autoinhibited by an intramolecular interaction. Recent cryo-EM analyses showed that the CH domain of Mal3p (Schizosaccharomyces pombe EB1 homolog) binds to GMPCPP-MT (Sandblad, L. Cell 127 (2006) 1415-24), and strongly binds GTPγS-MT which is proposed to mimic MT plus ends better than GMPCPP-MT (Maurer S.P. et al. Cell 149 (2012) 371-82). Here, we report on the MT binding sites of the CH domain of EB1 as revealed by NMR using the transferred cross-saturation method. In this study, we used GMPCPP-MT and found that the MT binding sites are very similar to the binding site for GTPγS-MT as suggested by cryo-EM (Maurer S.P. et al. Cell 149 (2012) 371-82). Notably, the N-terminal tip of helix α6 of the CH domain did not make contact with GMPCPP-MT, in contrast to the cryo-EM study which showed that it is closely located to a putative switch region of β-tubulin in GTPγS-MT (Maurer S.P. et al. Cell 149 (2012) 371-82). Further, we found that the intramolecular interaction site of EB1 overlaps the MT binding sites, indicating that the MT binding sites are masked by interaction with the C-terminal domain. We propose a structural view of autoinhibition and its release mechanism through competition binding with binding partners such as adenomatous polyposis coli protein.

Microtubule plus-ends within a mitotic cell are 'moving platforms' with anchoring, signalling and force-coupling roles

The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.

End-binding protein 1 controls signal propagation from the T cell receptor

The role of microtubules (MTs) in the control and dynamics of the immune synapse (IS) remains unresolved. Here, we show that T cell activation requires the growth of MTs mediated by the plus-end specific protein end-binding 1 (EB1). A direct interaction of the T cell receptor (TCR) complex with EB1 provides the molecular basis for EB1 activity promoting TCR encounter with signalling vesicles at the IS. EB1 knockdown alters TCR dynamics at the IS and prevents propagation of the TCR activation signal to LAT, thus inhibiting activation of PLCγ1 and its localization to the IS. These results identify a role for EB1 interaction with the TCR in controlling TCR sorting and its connection with the LAT/PLCγ1 signalosome.

The Ndc80 kinetochore complex directly modulates microtubule dynamics

The conserved Ndc80 complex is an essential microtubule-binding component of the kinetochore. Recent findings suggest that the Ndc80 complex influences microtubule dynamics at kinetochores in vivo. However, it was unclear if the Ndc80 complex mediates these effects directly, or by affecting other factors localized at the kinetochore. Using a reconstituted system in vitro, we show that the human Ndc80 complex directly stabilizes the tips of disassembling microtubules and promotes rescue (the transition from microtubule shortening to growth). In vivo, an N-terminal domain in the Ndc80 complex is phosphorylated by the Aurora B kinase. Mutations that mimic phosphorylation of the Ndc80 complex prevent stable kinetochore-microtubule attachment, and mutations that block phosphorylation damp kinetochore oscillations. We find that the Ndc80 complex with Aurora B phosphomimetic mutations is defective at promoting microtubule rescue, even when robustly coupled to disassembling microtubule tips. This impaired ability to affect dynamics is not simply because of weakened microtubule binding, as an N-terminally truncated complex with similar binding affinity is able to promote rescue. Taken together, these results suggest that in addition to regulating attachment stability, Aurora B controls microtubule dynamics through phosphorylation of the Ndc80 complex.

Vinflunine: a new vision that may translate into antiangiogenic and antimetastatic activity

Microtubules and tubulin are major dynamic and structural cellular components that play a key role in several cell functions, including division, signalling and intracellular trafficking. Normal epithelial cells have a highly structured, rigid cytoskeletal network that is compatible with cell motility. Thus, tubulin and microtubules are compelling cellular targets for chemotherapy. In fact, among anticancer agents, those that target microtubules constitute one of the most effective classes of chemotherapeutics in cancer. The list of compounds that target either tubulin or microtubules is extensive and consists of chemically unique compounds that bind to the tubulin dimers and destabilize microtubules (Vinca alkaloids) and those that bind to the microtubule polymer and stabilize microtubules (taxanes). Tumour-induced angiogenesis, the formation of new capillaries from existing blood vessels, and epithelial-mesenchymal transition are two steps that are critical for both tumour growth and metastatic spread. Three possible mechanisms of action are described with vinflunine, the new-generation Vinca alkaloid to arrive in clinical practice are as follows: it acts against tubulin and microtubules, disrupts newly formed blood vessels and seems to be able to reduce the metastatic process as shown in preclinical studies. These findings support the hypothesis that vinflunine, by blocking microtubule functions that contribute to cell shape, polarization, migration and other processes, might be responsible not only for tumour-cytostatic but also for specific antiangiogenic or antiepithelial-mesenchymal transition effects.

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.

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.

Insights into EB1 structure and the role of its C-terminal domain for discriminating microtubule tips from the lattice

End-binding proteins (EBs) comprise a conserved family of microtubule plus end-tracking proteins. The concerted action of calponin homology (CH), linker, and C-terminal domains of EBs is important for their autonomous microtubule tip tracking, regulation of microtubule dynamics, and recruitment of numerous partners to microtubule ends. Here we report the detailed structural and biochemical analysis of mammalian EBs. Small-angle X-ray scattering, electron microscopy, and chemical cross-linking in combination with mass spectrometry indicate that EBs are elongated molecules with two interacting CH domains, an arrangement reminiscent of that seen in other microtubule- and actin-binding proteins. Removal of the negatively charged C-terminal tail did not affect the overall conformation of EBs; however, it increased the dwell times of EBs on the microtubule lattice in microtubule tip-tracking reconstitution experiments. An even more stable association with the microtubule lattice was observed when the entire negatively charged C-terminal domain of EBs was replaced by a neutral coiled-coil motif. In contrast, the interaction of EBs with growing microtubule tips was not significantly affected by these C-terminal domain mutations. Our data indicate that long-range electrostatic repulsive interactions between the C-terminus and the microtubule lattice drive the specificity of EBs for growing microtubule ends.

EB1 promotes microtubule dynamics by recruiting Sentin in Drosophila cells

The microtubule plus end regulator EB1 brings Sentin and possibly a microtubule polymerase to microtubule plus ends to promote microtubule dynamics.

Highly conserved EB1 family proteins bind to the growing ends of microtubules, recruit multiple cargo proteins, and are critical for making dynamic microtubules in vivo. However, it is unclear how these master regulators of microtubule plus ends promote microtubule dynamics. In this paper, we identify a novel EB1 cargo protein, Sentin. Sentin depletion in Drosophila melanogaster S2 cells, similar to EB1 depletion, resulted in an increase in microtubule pausing and led to the formation of shorter spindles, without displacing EB1 from growing microtubules. We demonstrate that Sentin’s association with EB1 was critical for its plus end localization and function. Furthermore, the EB1 phenotype was rescued by expressing an EBN-Sentin fusion protein in which the C-terminal cargo-binding region of EB1 is replaced with Sentin. Knockdown of Sentin attenuated plus end accumulation of Msps (mini spindles), the orthologue of XMAP215 microtubule polymerase. These results indicate that EB1 promotes dynamic microtubule behavior by recruiting the cargo protein Sentin and possibly also a microtubule polymerase to the microtubule tip.

Theoretical modeling of aging effects in microtubule dynamics

The microtubule (MT) network, an important part of the cytoskeleton, is constantly remodeled by alternating phases of growth and shrinkage of individual filaments. Plus-end tracking proteins (+TIPs) interact with the MT and in many cases alter its dynamics. Although it is established that some +TIPs modify MT dynamics by increasing rescues, the plus-end tracking mechanism is still under debate. We present a model for MT dynamics in which a rescue factor is dynamically added to the filament during growth. As a consequence, the filament shows aging behavior that should be experimentally accessible and thus allow one to exclude some hypothesized models regarding the inclusion of rescue factors at the MT plus end. This result is not limited to +TIPs and can be extended to any kind of mechanism shifting the parameters of dynamic instability. Additionally, we show that the cell geometry has a strong influence on the quantitative results.

Disease-associated mutations in the p150(Glued) subunit destabilize the CAP-gly domain

Point mutations within the CAP-gly domain of the p150(Glued) subunit of the dynactin complex have been identified in patients with distal spinal bulbar muscular atrophy (dSBMA) and Perry's syndrome. Herein, we show by CD and NMR experiments that each mutated CAP-gly domain is folded but less stable than the wild-type (WT) domain. We also demonstrate that the domains harboring these mutations bind to microtubules but fail to bind to EB1. These data indicate that these disease-associated, point mutations affect the stability of this domain and inhibit their interaction with EB1 but do not inhibit their interaction with microtubules.

Parallel genetic and proteomic screens identify Msps as a CLASP-Abl pathway interactor in Drosophila

Regulation of cytoskeletal structure and dynamics is essential for multiple aspects of cellular behavior, yet there is much to learn about the molecular machinery underlying the coordination between the cytoskeleton and its effector systems. One group of proteins that regulate microtubule behavior and its interaction with other cellular components, such as actin-regulatory proteins and transport machinery, is the plus-end tracking proteins (MT+TIPs). In particular, evidence suggests that the MT+TIP, CLASP, may play a pivotal role in the coordination of microtubules with other cellular structures in multiple contexts, although the molecular mechanism by which it functions is still largely unknown. To gain deeper insight into the functional partners of CLASP, we conducted parallel genetic and proteome-wide screens for CLASP interactors in Drosophila melanogaster. We identified 36 genetic modifiers and 179 candidate physical interactors, including 13 that were identified in both data sets. Grouping interactors according to functional classifications revealed several categories, including cytoskeletal components, signaling proteins, and translation/RNA regulators. We focused our initial investigation on the MT+TIP Minispindles (Msps), identified among the cytoskeletal effectors in both genetic and proteomic screens. Here, we report that Msps is a strong modifier of CLASP and Abl in the retina. Moreover, we show that Msps functions during axon guidance and antagonizes both CLASP and Abl activity. Our data suggest a model in which CLASP and Msps converge in an antagonistic balance in the Abl signaling pathway.

Effect of GFP tags on the localization of EB1 and EB1 fragments in vivo

EB1 is a microtubule plus-end tracking protein that plays a central role in the regulation of microtubule (MT) dynamics. GFP-tagged EB1 constructs are commonly used to study EB1 itself and also as markers of dynamic MT plus ends. To properly interpret these studies, it is important to understand the impact of tags on the behavior of EB1 and other proteins in vivo. To address this problem and improve understanding of EB1 function, we surveyed the localization of expressed EB1 fragments and investigated whether GFP tags alter these localizations. We found that neither N-terminal nor C-terminal tags are benign: tagged EB1 and EB1 fragments generally behave differently from their untagged counterparts. N-terminal tags significantly compromise the ability of expressed EB1 proteins to bind MTs and/or track MT plus ends, although they leave some MT-binding ability intact. C-terminally tagged EB1 constructs have localizations similar to the untagged constructs, initially suggesting that they are benign. However, most constructs tagged at either end cause CLIP-170 to disappear from MT plus ends. This effect is opposite to that of untagged full-length EB1, which recruits CLIP-170 to MTs. These observations demonstrate that although EB1-GFP can be a powerful tool for studying microtubule dynamics, it should be used carefully because it may alter the system that it is being used to study. In addition, some untagged fragments had unexpected localizations. In particular, an EB1 construct lacking the coiled-coil tracks MT plus ends, though weakly, providing evidence against the idea that EB1 +TIP behavior requires dimerization.

Regulation of microtubule dynamics by Bim1 and Bik1, the budding yeast members of the EB1 and CLIP-170 families of plus-end tracking proteins

Bim1 promotes microtubule assembly in vitro, primarily by decreasing the frequency of catastrophes. In contrast, Bik1 inhibits microtubule assembly by slowing growth and, consequently, promoting catastrophes. These proteins interact to form a complex that affects microtubule dynamics in much the same way as Bim1 alone.

Microtubule dynamics are regulated by plus-end tracking proteins (+TIPs), which bind microtubule ends and influence their polymerization properties. In addition to binding microtubules, most +TIPs physically associate with other +TIPs, creating a complex web of interactions. To fully understand how +TIPs regulate microtubule dynamics, it is essential to know the intrinsic biochemical activities of each +TIP and how +TIP interactions affect these activities. Here, we describe the activities of Bim1 and Bik1, two +TIP proteins from budding yeast and members of the EB1 and CLIP-170 families, respectively. We find that purified Bim1 and Bik1 form homodimers that interact with each other to form a tetramer. Bim1 binds along the microtubule lattice but with highest affinity for the microtubule end; however, Bik1 requires Bim1 for localization to the microtubule lattice and end. In vitro microtubule polymerization assays show that Bim1 promotes microtubule assembly, primarily by decreasing the frequency of catastrophes. In contrast, Bik1 inhibits microtubule assembly by slowing growth and, consequently, promoting catastrophes. Interestingly, the Bim1-Bik1 complex affects microtubule dynamics in much the same way as Bim1 alone. These studies reveal new activities for EB1 and CLIP-170 family members and demonstrate how interactions between two +TIP proteins influence their activities.

Nuclear-localized subtype of end-binding 1 protein regulates spindle organization in Arabidopsis

End-binding 1 (EB1) proteins are evolutionarily conserved plus-end-tracking proteins that localize to growing microtubule plus ends where they regulate microtubule dynamics and interactions with intracellular targets. Animal EB1 proteins have acidic C-terminal tails that might induce an autoinhibitory conformation. Although EB1 proteins with the same structural features occur in plants (EB1a and EB1b in Arabidopsis thaliana), a variant form (EB1c) is present that lacks the characteristic tail. We show that in Arabidopsis the tail region of EB1b, but not of EB1c, inhibits microtubule assembly in vitro. EB1a and EB1b form heterodimers with each other, but not with EB1c. Furthermore, the EB1 genes are expressed in various cell types of Arabidopsis, but the expression of EB1c is particularly strong in the meristematic cells where it is targeted to the nucleus by a nuclear localization signal in the C-terminal tail. Reduced expression of EB1c compromised the alignment of spindle and phragmoplast microtubules and caused frequent lagging of separating chromosomes at anaphase. Roots of the eb1c mutant were hypersensitive to a microtubule-disrupting drug and complete rescue of the mutant phenotype required the tail region of EB1c. These results suggest that a plant-specific EB1 subtype has evolved to function preferentially on the spindle microtubules by accumulating in the prophase nucleus.

The 90-kDa heat shock protein Hsp90 protects tubulin against thermal denaturation

Hsp90 and tubulin are among the most abundant proteins in the cytosol of eukaryotic cells. Although Hsp90 plays key roles in maintaining its client proteins in their active state, tubulin is essential for fundamental processes such as cell morphogenesis and division. Several studies have suggested a possible connection between Hsp90 and the microtubule cytoskeleton. Because tubulin is a labile protein in its soluble form, we investigated whether Hsp90 protects it against thermal denaturation. Both proteins were purified from porcine brain, and their interaction was characterized in vitro by using spectrophotometry, sedimentation assays, video-enhanced differential interference contrast light microscopy, and native polyacrylamide gel electrophoresis. Our results show that Hsp90 protects tubulin against thermal denaturation and keeps it in a state compatible with microtubule polymerization. We demonstrate that Hsp90 cannot resolve tubulin aggregates but that it likely binds early unfolding intermediates, preventing their aggregation. Protection was maximal at a stoichiometry of two molecules of Hsp90 for one of tubulin. This protection does not require ATP binding and hydrolysis by Hsp90, but it is counteracted by geldanamycin, a specific inhibitor of Hsp90.

Regulation of microtubule dynamic instability

Proper regulation of MT (microtubule) dynamics is essential for various vital processes, including the segregation of chromosomes, directional cell migration and differentiation. MT assembly and disassembly is modulated by a complex network of intracellular factors that co-operate or antagonize each other, are highly regulated in space and time and are thus attuned to the cell cycle and differentiation processes. While we only begin to appreciate how the concerted action of MT stabilizers and destabilizers shapes different MT patterns, a clear picture of how individual factors affect the MT structure is emerging. In this paper, we review the current knowledge about proteins that modulate MT dynamic instability.