TheDrosophilaCLASP homologue, Mast/Orbit regulates the dynamic behaviour of interphase microtubules by promoting the pause state

Construction and validation of co-expression vector for rice alpha tubulin and microtubule associated protein respectively fused with fluorescent proteins

Microtubule (MT) consists of α-tubulin and β-tubulin. The dynamic instability regulated by various microtubule associated proteins (MAPs) is essential for MT functions. To analyze the interaction between tubulin/MT and MAP in vivo, we usually need tubulin and MAP co-expressed. Here, we constructed a dual-transgene vector expressing rice (Oryza sativa) α-tubulin and MAP simultaneously. To construct this vector, plant expression vector pCambia1301 was used as the plasmid backbone and Gibson assembly cloning technology was used. We first fused and cloned the GFP fragment, α-tubulin open reading frame (ORF), and NOS terminator into the vector pCambia1301 to construct the p35S::GFP-α-tubulin vector that expressed GFP-α-tubulin fusion protein. Subsequently, we fused and cloned the CaMV 35S promoter, mCherry fragment, and NOS terminator into the p35S::GFP-α-tubulin vector to generate the universal dual-transgene expression vector (p35S::GFP-α-tubulin-p35S::mCherry vector). With the p35S::GFP-α-tubulin-p35S::mCherry vector, MAP ORF can be cloned into the site of 5' or 3' terminus of mCherry to co-express GFP-α-tubulin and MAP-mCherry/mCherry-MAP. To validate the availability and universality of the dual-transgene expression vector, a series of putative rice MAP genes including GL7, OsKCBP, OsCLASP, and OsMOR1 were cloned into the vector respectively, transformed into Agrobacterium tumefaciens strain, and expressed in Nicotiana benthamiana leaves. The results indicated that all of the MAPs were co-expressed with α-tubulin and localized to MTs, validating the availability and universality of the vector and that GL7, OsKCBP, OsCLASP, and OsMOR1 might be MAPs. The application of the co-expression vector constructed by us would facilitate studies on the interaction between tubulin/MT and MAP in tobacco transient expression systems or transgenic rice.

Mechanical coupling coordinates microtubule growth

During mitosis, kinetochore-attached microtubules form bundles (k-fibers) in which many filaments grow and shorten in near-perfect unison to align and segregate each chromosome. However, individual microtubules grow at intrinsically variable rates, which must be tightly regulated for a k-fiber to behave as a single unit. This exquisite coordination might be achieved biochemically, via selective binding of polymerases and depolymerases, or mechanically, because k-fiber microtubules are coupled through a shared load that influences their growth. Here, we use a novel dual laser trap assay to show that microtubule pairs growing in vitro are coordinated by mechanical coupling. Kinetic analyses show that microtubule growth is interrupted by stochastic, force-dependent pauses and indicate persistent heterogeneity in growth speed during non-pauses. A simple model incorporating both force-dependent pausing and persistent growth speed heterogeneity explains the measured coordination of microtubule pairs without any free fit parameters. Our findings illustrate how microtubule growth may be synchronized during mitosis and provide a basis for modeling k-fiber bundles with three or more microtubules, as found in many eukaryotes.

More is different: Reconstituting complexity in microtubule regulation

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

Mechanical coupling coordinates microtubule growth

During mitosis, kinetochore-attached microtubules form bundles (k-fibers) in which many filaments grow and shorten in near-perfect unison to align and segregate each chromosome. However, individual microtubules grow at intrinsically variable rates, which must be tightly regulated for a k-fiber to behave as a single unit. This exquisite coordination might be achieved biochemically, via selective binding of polymerases and depolymerases, or mechanically, because k-fiber microtubules are coupled through a shared load that influences their growth. Here, we use a novel dual laser trap assay to show that microtubule pairs growing in vitro are coordinated by mechanical coupling. Kinetic analyses show that microtubule growth is interrupted by stochastic, force-dependent pauses and indicate persistent heterogeneity in growth speed during non-pauses. A simple model incorporating both force-dependent pausing and persistent growth speed heterogeneity explains the measured coordination of microtubule pairs without any free fit parameters. Our findings illustrate how microtubule growth may be synchronized during mitosis and provide a basis for modeling k-fiber bundles with three or more microtubules, as found in many eukaryotes.

CLASPs stabilize the pre-catastrophe intermediate state between microtubule growth and shrinkage

Cytoplasmic linker-associated proteins (CLASPs) regulate microtubules in fundamental cellular processes. CLASPs stabilize dynamic microtubules by suppressing microtubule catastrophe and promoting rescue, the switch-like transitions between growth and shrinkage. How CLASPs specifically modulate microtubule transitions is not understood. Here, we investigate the effects of CLASPs on the pre-catastrophe intermediate state of microtubule dynamics, employing distinct microtubule substrates to mimic the intermediate state. Surprisingly, we find that CLASP1 promotes the depolymerization of stabilized microtubules in the presence of GTP, but not in the absence of nucleotide. This activity is also observed for CLASP2 family members and a minimal TOG2-domain construct. Conversely, we find that CLASP1 stabilizes unstable microtubules upon tubulin dilution in the presence of GTP. Strikingly, our results reveal that CLASP1 drives microtubule substrates with vastly different inherent stabilities into the same slowly depolymerizing state in a nucleotide-dependent manner. We interpret this state as the pre-catastrophe intermediate state. Therefore, we conclude that CLASPs suppress microtubule catastrophe by stabilizing the intermediate state between growth and shrinkage.

MAPping tubulin mutations

Microtubules are filamentous structures that play a critical role in a diverse array of cellular functions including, mitosis, nuclear translocation, trafficking of organelles and cell shape. They are composed of α/β-tubulin heterodimers which are encoded by a large multigene family that has been implicated in an umbrella of disease states collectively known as the tubulinopathies. De novo mutations in different tubulin genes are known to cause lissencephaly, microcephaly, polymicrogyria, motor neuron disease, and female infertility. The diverse clinical features associated with these maladies have been attributed to the expression pattern of individual tubulin genes, as well as their distinct Functional repertoire. Recent studies, however, have highlighted the impact of tubulin mutations on microtubule-associated proteins (MAPs). MAPs can be classified according to their effect on microtubules and include polymer stabilizers (e.g., tau, MAP2, doublecortin), destabilizers (e.g., spastin, katanin), plus-end binding proteins (e.g., EB1-3, XMAP215, CLASPs) and motor proteins (e.g., dyneins, kinesins). In this review we analyse mutation-specific disease mechanisms that influence MAP binding and their phenotypic consequences, and discuss methods by which we can exploit genetic variation to identify novel MAPs.

Modelling microtubule dynamic instability: Microtubule growth, shortening and pause

Microtubules (MTs) are protein polymers found in all eukaryotic cells. They are crucial for normal cell development, providing structural support for the cell and aiding in the transportation of proteins and organelles. In order to perform these functions, MTs go through periods of relatively slow polymerization (growth) and very fast depolymerization (shortening), where the switch from growth to shortening is called a catastrophe and the switch from shortening to growth is called a rescue. Although MT dynamic instability has traditionally been described solely in terms of growth and shortening, MTs have been shown to pause for extended periods of time, however the reason for pausing is not well understood. Here, we present a new mathematical model to describe MT dynamics in terms of growth, shortening, and pausing. Typically, MT dynamics are defined by four key parameters which include the MT growth rate, shortening rate, frequency of catastrophe, and the frequency of rescue. We derive a mathematical expression for the catastrophe frequency in the presence of pausing, as well as expressions to describe the total time that MTs spend in a state of growth and pause. In addition to exploring MT dynamics in a control-like setting, we explore the implicit effect of stabilizing MT associated proteins (MAPs) and stabilizing and destabilizing chemotherapeutic drugs that target MTs on MT dynamics through variations in model parameters.

Quantification of Microtubule Stutters: Dynamic Instability Behaviors that are Strongly Associated with Catastrophe

Microtubules (MTs) are cytoskeletal fibers that undergo dynamic instability (DI), a remarkable process involving phases of growth and shortening separated by stochastic transitions called catastrophe and rescue. Dissecting DI mechanism(s) requires first characterizing and quantifying these dynamics, a subjective process that often ignores complexity in MT behavior. We present a Statistical Tool for Automated Dynamic Instability Analysis (STADIA) that identifies and quantifies not only growth and shortening, but also a category of intermediate behaviors that we term “stutters.” During stutters, the rate of MT length change tends to be smaller in magnitude than during typical growth or shortening phases. Quantifying stutters and other behaviors with STADIA demonstrates that stutters precede most catastrophes in our in vitro experiments and dimer-scale MT simulations, suggesting that stutters are mechanistically involved in catastrophes. Related to this idea, we show that the anticatastrophe factor CLASP2γ works by promoting the return of stuttering MTs to growth. STADIA enables more comprehensive and data-driven analysis of MT dynamics compared with previous methods. The treatment of stutters as distinct and quantifiable DI behaviors provides new opportunities for analyzing mechanisms of MT dynamics and their regulation by binding proteins.

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

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

CLASPs at a glance

CLIP-associating proteins (CLASPs) form an evolutionarily conserved family of regulatory factors that control microtubule dynamics and the organization of microtubule networks. The importance of CLASP activity has been appreciated for some time, but until recently our understanding of the underlying molecular mechanisms remained basic. Over the past few years, studies of, for example, migrating cells, neuronal development, and microtubule reorganization in plants, along with in vitro reconstitutions, have provided new insights into the cellular roles and molecular basis of CLASP activity. In this Cell Science at a Glance article and the accompanying poster, we will summarize some of these recent advances, emphasizing how they impact our current understanding of CLASP-mediated microtubule regulation.

Structures of TOG1 and TOG2 from the human microtubule dynamics regulator CLASP1

Tubulin-binding TOG domains are found arrayed in a number of proteins that regulate microtubule dynamics. While much is known about the structure and function of TOG domains from the XMAP215 microtubule polymerase family, less in known about the TOG domain array found in animal CLASP family members. The animal CLASP TOG array promotes microtubule pause, potentiates rescue, and limits catastrophe. How structurally distinct the TOG domains of animal CLASP are from one another, from XMAP215 family TOG domains, and whether a specific order of structurally distinct TOG domains in the TOG array is conserved across animal CLASP family members is poorly understood. We present the x-ray crystal structures of Homo sapiens (H.s.) CLASP1 TOG1 and TOG2. The structures of H.s. CLASP1 TOG1 and TOG2 are distinct from each other and from the previously determined structure of Mus musculus (M.m.) CLASP2 TOG3. Comparative analyses of CLASP family TOG domain structures determined to date across species and paralogs supports a conserved CLASP TOG array paradigm in which structurally distinct TOG domains are arrayed in a specific order. H.s. CLASP1 TOG1 bears structural similarity to the free-tubulin binding TOG domains of the XMAP215 family but lacks many of the key tubulin-binding determinants found in XMAP215 family TOG domains. This aligns with studies that report that animal CLASP family TOG1 domains cannot bind free tubulin or microtubules. In contrast, animal CLASP family TOG2 and TOG3 domains have reported microtubule-binding activity but are structurally distinct from the free-tubulin binding TOG domains of the XMAP215 family. H.s. CLASP1 TOG2 has a convex architecture, predicted to engage a hyper-curved tubulin state that may underlie its ability to limit microtubule catastrophe and promote rescue. M.m. CLASP2 TOG3 has unique structural elements in the C-terminal half of its α-solenoid domain that our modeling studies implicate in binding to laterally-associated tubulin subunits in the microtubule lattice in a mode similar to, yet distinct from those predicted for the XMAP215 family TOG4 domain. The potential ability of the animal CLASP family TOG3 domain to engage lateral tubulin subunits may underlie the microtubule rescue activity ascribed to the domain. These findings highlight the structural diversity of TOG domains within the CLASP family TOG array and provide a molecular foundation for understanding CLASP-dependent effects on microtubule dynamics.

Insulin Induces Microtubule Stabilization and Regulates the Microtubule Plus-end Tracking Protein Network in Adipocytes

Insulin-stimulated glucose uptake is known to involve microtubules, although the function of microtubules and the microtubule-regulating proteins involved in insulin action are poorly understood. CLASP2, a plus-end tracking microtubule-associated protein (+TIP) that controls microtubule dynamics, was recently implicated as the first +TIP associated with insulin-regulated glucose uptake. Here, using protein-specific targeted quantitative phosphoproteomics within 3T3-L1 adipocytes, we discovered that insulin regulates phosphorylation of the CLASP2 network members G2L1, MARK2, CLIP2, AGAP3, and CKAP5 as well as EB1, revealing the existence of a previously unknown microtubule-associated protein system that responds to insulin. To further investigate, G2L1 interactome studies within 3T3-L1 adipocytes revealed that G2L1 coimmunoprecipitates CLASP2 and CLIP2 as well as the master integrators of +TIP assembly, the end binding (EB) proteins. Live-cell total internal reflection fluorescence microscopy in adipocytes revealed G2L1 and CLASP2 colocalize on microtubule plus-ends. We found that although insulin increases the number of CLASP2-containing plus-ends, insulin treatment simultaneously decreases CLASP2-containing plus-end velocity. In addition, we discovered that insulin stimulates redistribution of CLASP2 and G2L1 from exclusive plus-end tracking to "trailing" behind the growing tip of the microtubule. Insulin treatment increases α-tubulin Lysine 40 acetylation, a mechanism that was observed to be regulated by a counterbalance between GSK3 and mTOR, and led to microtubule stabilization. Our studies introduce insulin-stimulated microtubule stabilization and plus-end trailing of +TIPs as new modes of insulin action and reveal the likelihood that a network of microtubule-associated proteins synergize to coordinate insulin-regulated microtubule dynamics.

CLASP Suppresses Microtubule Catastrophes through a Single TOG Domain

The dynamic instability of microtubules plays a key role in controlling their organization and function, but the cellular mechanisms regulating this process are poorly understood. Here, we show that cytoplasmic linker-associated proteins (CLASPs) suppress transitions from microtubule growth to shortening, termed catastrophes, including those induced by microtubule-destabilizing agents and physical barriers. Mammalian CLASPs encompass three TOG-like domains, TOG1, TOG2, and TOG3, none of which bind to free tubulin. TOG2 is essential for catastrophe suppression, whereas TOG3 mildly enhances rescues but cannot suppress catastrophes. These functions are inhibited by the C-terminal domain of CLASP2, while the TOG1 domain can release this auto-inhibition. TOG2 fused to a positively charged microtubule-binding peptide autonomously accumulates at growing but not shrinking ends, suppresses catastrophes, and stimulates rescues. CLASPs suppress catastrophes by stabilizing growing microtubule ends, including incomplete ones, preventing their depolymerization and promoting their recovery into complete tubes. TOG2 domain is the key determinant of these activities.

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CLASPs potently suppress microtubule catastrophes induced by different mechanisms

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CLASPs act by stabilizing growing microtubule ends, including incomplete ones

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CLASP2 TOG-like domain, TOG2, is necessary and sufficient for catastrophe inhibition

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TOG2 fused to a positively charged peptide accumulates at growing microtubule ends

CLASP promotes stable tethering of endoplasmic microtubules to the cell cortex to maintain cytoplasmic stability in Arabidopsis meristematic cells

Following cytokinesis in plants, Endoplasmic MTs (EMTs) assemble on the nuclear surface, forming a radial network that extends out to the cell cortex, where they attach and incorporate into the cortical microtubule (CMT) array. We found that in these post-cytokinetic cells, the MT-associated protein CLASP is enriched at sites of EMT-cortex attachment, and is required for stable EMT tethering and growth into the cell cortex. Loss of EMT-cortex anchoring in clasp-1 mutants results in destabilized EMT arrays, and is accompanied by enhanced mobility of the cytoplasm, premature vacuolation, and precocious entry into cell elongation phase. Thus, EMTs appear to maintain cells in a meristematic state by providing a structural scaffold that stabilizes the cytoplasm to counteract actomyosin-based cytoplasmic streaming forces, thereby preventing premature establishment of a central vacuole and rapid cell elongation.

Human CLASP2 specifically regulates microtubule catastrophe and rescue

Cytoplasmic linker-associated proteins (CLASPs) are microtubule-associated proteins essential for microtubule regulation in many cellular processes. However, the molecular mechanisms underlying CLASP activity are not understood. Here, we use purified protein components and total internal reflection fluorescence microscopy to investigate the effects of human CLASP2 on microtubule dynamics in vitro. We demonstrate that CLASP2 suppresses microtubule catastrophe and promotes rescue without affecting the rates of microtubule growth or shrinkage. Strikingly, when CLASP2 is combined with EB1, a known binding partner, the effects on microtubule dynamics are strongly enhanced. We show that synergy between CLASP2 and EB1 is dependent on a direct interaction, since a truncated EB1 protein that lacks the CLASP2-binding domain does not enhance CLASP2 activity. Further, we find that EB1 targets CLASP2 to microtubules and increases the dwell time of CLASP2 at microtubule tips. Although the temporally averaged microtubule growth rates are unaffected by CLASP2, we find that microtubules grown with CLASP2 display greater variability in growth rates. Our results provide insight into the regulation of microtubule dynamics by CLASP proteins and highlight the importance of the functional interplay between regulatory proteins at dynamic microtubule ends.

Structure of the ACF7 EF-Hand-GAR Module and Delineation of Microtubule Binding Determinants

Spectraplakins are large molecules that cross-link F-actin and microtubules (MTs). Mutations in spectraplakins yield defective cell polarization, aberrant focal adhesion dynamics, and dystonia. We present the 2.8 Å crystal structure of the hACF7 EF1-EF2-GAR MT-binding module and delineate the GAR residues critical for MT binding. The EF1-EF2 and GAR domains are autonomous domains connected by a flexible linker. The EF1-EF2 domain is an EFβ-scaffold with two bound Ca²⁺ ions that straddle an N-terminal α helix. The GAR domain has a unique α/β sandwich fold that coordinates Zn²⁺. While the EF1-EF2 domain is not sufficient for MT binding, the GAR domain is and likely enhances EF1-EF2-MT engagement. Residues in a conserved basic patch, distal to the GAR domain's Zn²⁺-binding site, mediate MT binding.

A microtubule dynamics reconstitutional convention

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

Five factors can reconstitute all three phases of microtubule polymerization dynamics

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

Microtubule encounter-based catastrophe in Arabidopsis cortical microtubule arrays

BACKGROUND

The cortical microtubules (CMTs) that line the plasma membrane of interphase plant cells are extensively studied owing to their importance in forming cell walls, and their usefulness as a model system for the study of MT dynamic instability and acentrosomal MT organization. CMTs influence the orientation and structure of cellulose microfibrils in the cell wall by cooperatively forming arrays of varied patterns from parallel to netted. These CMT patterns are controlled by the combined activities of MT dynamic instability and MT-MT interactions. However, it is an open question as to how CMT patterns may feedback to influence CMT dynamics and interactions.

RESULTS

To address this question, we investigated the effects of CMT array patterning on encounter-based CMT catastrophe, which occurs when one CMT grows into another and is unable to cross over. We hypothesized that the varied CMT angles present in disordered (mixed CMTs) arrays will create more opportunities for MT-MT interactions, and thus increase encounter-based catastrophe rates and distribution. Using live-cell imaging of Arabidopsis cotyledon and leaf epidermal cells, we found that roughly 87% of catastrophes occur via the encounter-based mechanism, with the remainder occurring without encounter (free). When comparing ordered (parallel) and disordered (mixed orientation) CMT arrays, we found that disordered configurations show higher proportions of encounter-based catastrophe relative to free. Similarly, disordered CMT arrays have more catastrophes in general than ordered arrays. Encounter-based catastrophes were associated with frequent and sustained periods of pause prior to depolymerization, and CMTs with tight anchoring to the plasma membrane were more prone to undergo encounter-based catastrophe than weakly-attached ones. This suggests that encounter-based catastrophe has a mechanical basis, wherein MTs form physical barriers to one another. Lastly, we show that the commonly used measure of catastrophe frequencies (Fcat) can also be influenced by CMT ordering and plasma membrane anchoring.

CONCLUSIONS

Our observations add a new layer of complexity to our current understanding of MT organization in plants, showing that not only do individual CMT dynamics influence CMT array organization, but that CMT organization itself has a strong effect on the behavior of individual MTs.

TOG Proteins Are Spatially Regulated by Rac-GSK3β to Control Interphase Microtubule Dynamics

Microtubules are regulated by a diverse set of proteins that localize to microtubule plus ends (+TIPs) where they regulate dynamic instability and mediate interactions with the cell cortex, actin filaments, and organelles. Although individual +TIPs have been studied in depth and we understand their basic contributions to microtubule dynamics, there is a growing body of evidence that these proteins exhibit cross-talk and likely function to collectively integrate microtubule behavior and upstream signaling pathways. In this study, we have identified a novel protein-protein interaction between the XMAP215 homologue in Drosophila, Mini spindles (Msps), and the CLASP homologue, Orbit. These proteins have been shown to promote and suppress microtubule dynamics, respectively. We show that microtubule dynamics are regionally controlled in cells by Rac acting to suppress GSK3β in the peripheral lamellae/lamellipodium. Phosphorylation of Orbit by GSK3β triggers a relocalization of Msps from the microtubule plus end to the lattice. Mutation of the Msps-Orbit binding site revealed that this interaction is required for regulating microtubule dynamic instability in the cell periphery. Based on our findings, we propose that Msps is a novel Rac effector that acts, in partnership with Orbit, to regionally regulate microtubule dynamics.

Crescerin uses a TOG domain array to regulate microtubules in the primary cilium

Primary cilia are critical organelles involved in development, sensation, and signaling. Crescerin, a conserved protein family in ciliated and flagellated eukaryotes, uses a TOG domain array with tubulin polymerization activity to regulate cilia microtubules and facilitate proper cilia length, ultrastructure, and function.

Eukaryotic cilia are cell-surface projections critical for sensing the extracellular environment. Defects in cilia structure and function result in a broad range of developmental and sensory disorders. However, mechanisms that regulate the microtubule (MT)-based scaffold forming the cilia core are poorly understood. TOG domain array–containing proteins ch-TOG and CLASP are key regulators of cytoplasmic MTs. Whether TOG array proteins also regulate ciliary MTs is unknown. Here we identify the conserved Crescerin protein family as a cilia-specific, TOG array-containing MT regulator. We present the crystal structure of mammalian Crescerin1 TOG2, revealing a canonical TOG fold with conserved tubulin-binding determinants. Crescerin1's TOG domains possess inherent MT-binding activity and promote MT polymerization in vitro. Using Cas9-triggered homologous recombination in Caenorhabditis elegans, we demonstrate that the worm Crescerin family member CHE-12 requires TOG domain–dependent tubulin-binding activity for sensory cilia development. Thus, Crescerin expands the TOG domain array–based MT regulatory paradigm beyond ch-TOG and CLASP, representing a distinct regulator of cilia structure.

Emergent Properties of the Metaphase Spindle

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

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.

A cryptic TOG domain with a distinct architecture underlies CLASP-dependent bipolar spindle formation

CLASP is a key regulator of microtubule (MT) dynamics and bipolar mitotic spindle structure with CLASP mutants displaying a distinctive monopolar spindle phenotype. It has been postulated that cryptic TOG domains underlie CLASP’s ability to regulate MT dynamics. Here, we report the crystal structure of a cryptic TOG domain (TOG2) from human CLASP1, demonstrating the presence of a TOG array in the CLASP family. Strikingly, CLASP1 TOG2 exhibits a convex architecture across the tubulin-binding surface that contrasts with the flat tubulin-binding surface of XMAP215 family TOG domains. Mutations in key conserved TOG2 determinants abrogate the ability of CLASP mutants to rescue bipolar spindle formation in Drosophila cells depleted of endogenous CLASP. These findings highlight the common mechanistic use of TOG domains in XMAP215 and CLASP families to regulate MT dynamics and suggest that differential TOG domain architecture may confer distinct functions to these critical cytoskeletal regulators.

Xenopus cytoplasmic linker-associated protein 1 (XCLASP1) promotes axon elongation and advance of pioneer microtubules

Dynamic microtubules (MTs) are required for neuronal guidance, in which axons extend directionally toward their target tissues. We found that depletion of the MT-binding protein Xenopus cytoplasmic linker-associated protein 1 (XCLASP1) or treatment with the MT drug Taxol reduced axon outgrowth in spinal cord neurons. To quantify the dynamic distribution of MTs in axons, we developed an automated algorithm to detect and track MT plus ends that have been fluorescently labeled by end-binding protein 3 (EB3). XCLASP1 depletion reduced MT advance rates in neuronal growth cones, very much like treatment with Taxol, demonstrating a potential link between MT dynamics in the growth cone and axon extension. Automatic tracking of EB3 comets in different compartments revealed that MTs increasingly slowed as they passed from the axon shaft into the growth cone and filopodia. We used speckle microscopy to demonstrate that MTs experience retrograde flow at the leading edge. Microtubule advance in growth cone and filopodia was strongly reduced in XCLASP1-depleted axons as compared with control axons, but actin retrograde flow remained unchanged. Instead, we found that XCLASP1-depleted growth cones lacked lamellipodial actin organization characteristic of protrusion. Lamellipodial architecture depended on XCLASP1 and its capacity to associate with MTs, highlighting the importance of XCLASP1 in actin-microtubule interactions.

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.

CLASPs function redundantly to regulate astral microtubules in the C. elegans embryo

Microtubule dynamics are thought to play an important role in regulating microtubule interactions with cortical force generating motor proteins that position the spindle during asymmetric cell division. CLASPs are microtubule-associated proteins that have a conserved role in regulating microtubule dynamics in diverse cell types. Caenorhabditis elegans has three CLASP homologs in its genome. CLS-2 is known to localize to kinetochores and is needed for chromosome segregation at meiosis and mitosis; however CLS-1 and CLS-3 have not been reported to have any role in embryonic development. Here, we show that depletion of CLS-2 in combination with either CLS-1 or CLS-3 results in defects in nuclear rotation, maintenance of spindle length, and spindle displacement in the one-cell embryo. Polarity is normal in these embryos, but reduced numbers of astral microtubules reach all regions of the cortex at the time of spindle positioning. Analysis of the microtubule plus-end tracker EB1 also revealed a reduced number of growing microtubules reaching the cortex in CLASP depleted embryos, but the polymerization rate of astral microtubules was not slower than in wild type. These results indicate that C. elegans CLASPs act partially redundantly to regulate astral microtubules and position the spindle during asymmetric cell division. Further, we show that these spindle pole-positioning roles are independent of the CLS-2 binding proteins HCP-1 and HCP-2.

Identification of genes that promote or antagonize somatic homolog pairing using a high-throughput FISH-based screen

The pairing of homologous chromosomes is a fundamental feature of the meiotic cell. In addition, a number of species exhibit homolog pairing in nonmeiotic, somatic cells as well, with evidence for its impact on both gene regulation and double-strand break (DSB) repair. An extreme example of somatic pairing can be observed in Drosophila melanogaster, where homologous chromosomes remain aligned throughout most of development. However, our understanding of the mechanism of somatic homolog pairing remains unclear, as only a few genes have been implicated in this process. In this study, we introduce a novel high-throughput fluorescent in situ hybridization (FISH) technology that enabled us to conduct a genome-wide RNAi screen for factors involved in the robust somatic pairing observed in Drosophila. We identified both candidate "pairing promoting genes" and candidate "anti-pairing genes," providing evidence that pairing is a dynamic process that can be both enhanced and antagonized. Many of the genes found to be important for promoting pairing are highly enriched for functions associated with mitotic cell division, suggesting a genetic framework for a long-standing link between chromosome dynamics during mitosis and nuclear organization during interphase. In contrast, several of the candidate anti-pairing genes have known interphase functions associated with S-phase progression, DNA replication, and chromatin compaction, including several components of the condensin II complex. In combination with a variety of secondary assays, these results provide insights into the mechanism and dynamics of somatic pairing.

Multiple domains of human CLASP contribute to microtubule dynamics and organization in vitro and in Xenopus egg extracts

Cytoplasmic linker associated proteins (CLASPs) comprise a class of microtubule (MT) plus end-binding proteins (+TIPs) that contribute to the dynamics and organization of MTs during many cellular processes, among them mitosis. Human CLASP proteins contain multiple MT-binding domains, including tumor over-expressed gene (TOG) domains, and a Ser-x-Ile-Pro (SxIP) motif known to target some +TIPs though interaction with end-binding protein 1 (EB1). However, how individual domains contribute to CLASP function is poorly understood. We generated full-length recombinant human CLASP1 and a series of truncation mutants and found that two N-terminal TOG domains make the strongest contribution to MT polymerization and bundling, but also identified a third TOG domain that further contributes to CLASP activity. Plus end tracking by CLASP requires the SxIP motif and interaction with EB1. The C-terminal coiled-coil domain mediates dimerization and association with many other factors, including the kinetochore motor centromere protein E (CENP-E), and the chromokinesin Xkid. Only the full-length protein was able to rescue spindle assembly in Xenopus egg extracts depleted of endogenous CLASP. Deletion of the C-terminal domain caused aberrant MT polymerization and dramatic spindle phenotypes, even with small amounts of added protein, indicating that proper localization of CLASP activity is essential to control MT polymerization during mitosis. © 2012 Wiley Periodicals, Inc

A CLASP-modulated cell edge barrier mechanism drives cell-wide cortical microtubule organization in Arabidopsis

It is well known that the parallel order of microtubules in the plant cell cortex defines the direction of cell expansion, yet it remains unclear how microtubule orientation is controlled, especially on a cell-wide basis. Here we show through 4D imaging and computational modelling that plant cell polyhedral geometry provides spatial input that determines array orientation and heterogeneity. Microtubules depolymerize when encountering sharp cell edges head-on, whereas those oriented parallel to those sharp edges remain. Edge-induced microtubule depolymerization, however, is overcome by the microtubule-associated protein CLASP, which accumulates at specific cell edges, enables microtubule growth around sharp edges and promotes formation of microtubule bundles that span adjacent cell faces. By computationally modelling dynamic 'microtubules on a cube' with edges differentially permissive to microtubule passage, we show that the CLASP-edge complex is a 'tuneable' microtubule organizer, with the inherent flexibility to generate the numerous cortical array patterns observed in nature.

A PAR-1-dependent orientation gradient of dynamic microtubules directs posterior cargo transport in the Drosophila oocyte

Cytoskeletal organization is central to establishing cell polarity in various cellular contexts, including during messenger ribonucleic acid sorting in Drosophila melanogaster oocytes by microtubule (MT)-dependent molecular motors. However, MT organization and dynamics remain controversial in the oocyte. In this paper, we use rapid multichannel live-cell imaging with novel image analysis, tracking, and visualization tools to characterize MT polarity and dynamics while imaging posterior cargo transport. We found that all MTs in the oocyte were highly dynamic and were organized with a biased random polarity that increased toward the posterior. This organization originated through MT nucleation at the oocyte nucleus and cortex, except at the posterior end of the oocyte, where PAR-1 suppressed nucleation. Our findings explain the biased random posterior cargo movements in the oocyte that establish the germline and posterior.

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.

Control of mitotic spindle length

The mitotic spindle accurately segregates genetic instructions by moving chromosomes to spindle poles (anaphase A) and separating the poles (anaphase B) so that, in general, the chromosomes and poles are positioned near the centers of the nascent daughter cell products of each cell division. Because the size of different types of dividing cells, and thus the spacing of their daughter cell centers, can vary significantly, the length of the metaphase or postanaphase B spindle often scales with cell size. However, significant exceptions to this scaling rule occur, revealing the existence of cell size–independent, spindle-associated mechanisms of spindle length control. The control of spindle length reflects the action of mitotic force-generating mechanisms, and its study may illuminate general principles by which cells regulate the size of internal structures. Here we review molecules and mechanisms that control spindle length, how these mechanisms are deployed in different systems, and some quantitative models that describe the control of spindle length.

CLASP promotes microtubule rescue by recruiting tubulin dimers to the microtubule

Spatial regulation of microtubule (MT) dynamics contributes to cell polarity and cell division. MT rescue, in which a MT stops shrinking and reinitiates growth, is the least understood aspect of MT dynamics. Cytoplasmic Linker Associated Proteins (CLASPs) are a conserved class of MT-associated proteins that contribute to MT stabilization and rescue in vivo. We show here that the Schizosaccharomyces pombe CLASP, Cls1p, is a homodimer that binds an alphabeta-tubulin heterodimer through conserved TOG-like domains. In vitro, CLASP increases MT rescue frequency, decreases MT catastrophe frequency, and moderately decreases MT disassembly rate. CLASP binds stably to the MT lattice, recruits tubulin, and locally promotes rescues. Mutations in the CLASP TOG domains demonstrate that tubulin binding is critical for its rescue activity. We propose a mechanism for rescue in which CLASP-tubulin dimer complexes bind along the MT lattice and reverse MT depolymerization with their bound tubulin dimer.

A central role for polyamines in microtubule assembly in cells

Owing to preferential electrostatic adsorption of multivalent cations on highly anionic surfaces, natural multivalent polyamines and especially quadrivalent spermine can be considered as potential regulators of the complex dynamical properties of anionic MTs (microtubules). Indeed, the C-terminal tails of tubulin display many negative residues in a row which should enable the formation of a correlated liquid-like phase of multivalent counterions on its surface. Although it is known that polyamine counterions promote MT assembly in vitro, little is known about the relevance of this interaction in vivo. In the present study, we have explored the relationship between polyamine levels and MT assembly in HeLa and epithelial NRK (normal rat kidney) cells using DFMO (α-difluoromethylornithine), an irreversible inhibitor of ornithine decarboxylase, and APCHA [N-(3-aminopropyl)-N-cyclohexylamine], a spermine synthase inhibitor. Under conditions of intracellular polyamine depletion, the MT network is clearly disrupted and the MT mass decreases. Addition of spermine to polyamine-depleted cells reverses this phenotype and rapidly promotes the extensions of the MT network. Finally, we show that polyamine levels modulate the coating of MTs with MAP4 (MT-associated protein 4), an MT-stabilizing protein, and the spatial distribution of EB1 (end-binding protein 1), an MT plus-end-binding protein. In addition, polyamines favour the formation of gap junctions in NRK cells, a process which requires MT extensions at the cell periphery. The present study provides a basis for a better understanding of the role played by polyamines in MT assembly and establishes polyamine metabolism as a potential cellular target for modulating MT functions.

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.

Clasp-mediated microtubule bundling regulates persistent motility and contact repulsion in Drosophila macrophages in vivo

Drosophila melanogaster macrophages are highly migratory cells that lend themselves beautifully to high resolution in vivo imaging experiments. By expressing fluorescent probes to reveal actin and microtubules, we can observe the dynamic interplay of these two cytoskeletal networks as macrophages migrate and interact with one another within a living organism. We show that before an episode of persistent motility, whether responding to developmental guidance or wound cues, macrophages assemble a polarized array of microtubules that bundle into a compass-like arm that appears to anticipate the direction of migration. Whenever cells collide with one another, their microtubule arms transiently align just before cell-cell repulsion, and we show that forcing depolymerization of microtubules by expression of Spastin leads to their defective polarity and failure to contact inhibit from one another. The same is true in orbit/clasp mutants, indicating a pivotal role for this microtubule-binding protein in the assembly and/or functioning of the microtubule arm during polarized migration and contact repulsion.

{gamma}-Tubulin ring complexes regulate microtubule plus end dynamics

Independently of their nucleation activity, γ-tubulin ring complex proteins localize along microtubules, limiting catastrophe events during interphase.

γ-Tubulin is critical for the initiation and regulation of microtubule (MT) assembly. In Drosophila melanogaster, it acts within two main complexes: the γ-tubulin small complex (γ-TuSC) and the γ-tubulin ring complex (γ-TuRC). Proteins specific of the γ-TuRC, although nonessential for viability, are required for efficient mitotic progression. Until now, their role during interphase remained poorly understood. Using RNA interference in Drosophila S2 cells, we show that the γ-TuRC is not critical for overall MT organization. However, depletion of any component of this complex results in an increase of MT dynamics. Combined immunofluorescence and live imaging analysis allows us to reveal that the γ-TuRC localizes along interphase MTs and that distal γ-tubulin spots match with sites of pause or rescue events. We propose that, in addition to its role in nucleation, the γ-TuRC associated to MTs may regulate their dynamics by limiting catastrophes.

Dynein and Mast/Orbit/CLASP have antagonistic roles in regulating kinetochore-microtubule plus-end dynamics

Establishment and maintenance of the mitotic spindle requires the balanced activity of microtubule-associated proteins and motors. In this study we have addressed how the microtubule plus-end tracking protein Mast/Orbit/CLASP and cytoplasmic dynein regulate this process in Drosophila melanogaster embryos and S2 cells. We show that Mast accumulates at kinetochores early in mitosis, which is followed by a poleward streaming upon microtubule attachment. This leads to a reduction of Mast levels at kinetochores during metaphase and anaphase that depends largely on the microtubule minus end-directed motor cytoplasmic dynein. Surprisingly, we also found that co-depletion of Dynein rescues spindle bipolarity in Mast-depleted cells, while restoring normal microtubule poleward flux. Our results suggest that Mast and Dynein have antagonistic roles in the local regulation of microtubule plus-end dynamics at kinetochores, which are important for the maintenance of spindle bipolarity and normal spindle length.

Mechanisms for maintaining microtubule bundles

The dynamics of microtubules (MTs) are crucial to many of their functions. Certain MT structures, such as the mitotic spindle apparatus, exhibit high MT turnover yet maintain their mass stably through long periods of time. Here, we highlight what are emerging as two important mechanisms for maintaining MT bundles: the first, MT nucleation from pre-existing MTs by means of gamma-tubulin-containing complexes; and the second, MT 'rescue' by the stabilizing protein CLASP. As examples, we describe recent advances in understanding the assembly and maintenance of simple MT bundles in fission yeast and plant cells, which have implications for the bundles of the animal mitotic spindle.

A multicomponent assembly pathway contributes to the formation of acentrosomal microtubule arrays in interphase Drosophila cells

In animal cells, centrosomes nucleate microtubules that form polarized arrays to organize the cytoplasm. Drosophila presents an interesting paradox however, as centrosome-deficient mutant animals develop into viable adults. To understand this discrepancy, we analyzed behaviors of centrosomes and microtubules in Drosophila cells, in culture and in vivo, using a combination of live-cell imaging, electron microscopy, and RNAi. The canonical model of the cycle of centrosome function in animal cells states that centrosomes act as microtubule-organizing centers throughout the cell cycle. Unexpectedly, we found that many Drosophila cell-types display an altered cycle, in which functional centrosomes are only present during cell division. On mitotic exit, centrosomes disassemble producing interphase cells containing centrioles that lack microtubule-nucleating activity. Furthermore, steady-state interphase microtubule levels are not changed by codepleting both gamma-tubulins. However, gamma-tubulin RNAi delays microtubule regrowth after depolymerization, suggesting that it may function partially redundantly with another pathway. Therefore, we examined additional microtubule nucleating factors and found that Mini-spindles, CLIP-190, EB1, or dynein RNAi also delayed microtubule regrowth; surprisingly, this was not further prolonged when we codepleted gamma-tubulins. Taken together, these results modify our view of the cycle of centrosome function and reveal a multi-component acentrosomal microtubule assembly pathway to establish interphase microtubule arrays in Drosophila.

Tracking the ends: a dynamic protein network controls the fate of microtubule tips

Microtubule plus-end tracking proteins (+TIPs) are a diverse group of evolutionarily conserved cellular factors that accumulate at the ends of growing microtubules. They form dynamic networks through the interaction of a limited set of protein modules, repeat sequences and linear motifs that bind to each other with moderate affinities. +TIPs regulate different aspects of cell architecture by controlling microtubule dynamics, microtubule interactions with cellular structures and signalling factors, and the forces that are exerted on microtubule networks.