CLASP promotes microtubule rescue by recruiting tubulin dimers to the microtubule

Control of microtubule organization and dynamics: two ends in the limelight

Microtubules have fundamental roles in many essential biological processes, including cell division and intracellular transport. They assemble and disassemble from their two ends, denoted the plus end and the minus end. Significant advances have been made in our understanding of microtubule plus-end-tracking proteins (+TIPs) such as end-binding protein 1 (EB1), XMAP215, selected kinesins and dynein. By contrast, information on microtubule minus-end-targeting proteins (-TIPs), such as the calmodulin-regulated spectrin-associated proteins (CAMSAPs) and Patronin, has only recently started to emerge. Here, we review our current knowledge of factors, including microtubule-targeting agents, that associate with microtubule ends to control the dynamics and function of microtubules during the cell cycle and development.

Nucleation and Dynamics of Golgi-derived Microtubules

Integrity of the Golgi apparatus requires the microtubule (MT) network. A subset of MTs originates at the Golgi itself, which in this case functions as a MT-organizing center (MTOC). Golgi-derived MTs serve important roles in post-Golgi trafficking, maintenance of Golgi integrity, cell polarity and motility, as well as cell type-specific functions, including neurite outgrowth/branching. Here, we discuss possible models describing the formation and dynamics of Golgi-derived MTs. How Golgi-derived MTs are formed is not fully understood. A widely discussed model implicates that the critical step of the process is recruitment of molecular factors, which drive MT nucleation (γ-tubulin ring complex, or γ-TuRC), to the Golgi membrane via specific scaffolding interactions. Based on recent findings, we propose to introduce an additional level of regulation, whereby MT-binding proteins and/or local tubulin dimer concentration at the Golgi helps to overcome kinetic barriers at the initial nucleation step. According to our model, emerging MTs are subsequently stabilized by Golgi-associated MT-stabilizing proteins. We discuss molecular factors potentially involved in all three steps of MT formation. To preserve proper cell functioning, a balance must be maintained between MT subsets at the centrosome and the Golgi. Recent work has shown that certain centrosomal factors are important in maintaining this balance, suggesting a close connection between regulation of centrosomal and Golgi-derived MTs. Finally, we will discuss potential functions of Golgi-derived MTs based on their nucleation site location within a Golgi stack.

PKA antagonizes CLASP-dependent microtubule stabilization to re-localize Pom1 and buffer cell size upon glucose limitation

Cells couple growth with division and regulate size in response to nutrient availability. In rod-shaped fission yeast, cell-size control occurs at mitotic commitment. An important regulator is the DYRK-family kinase Pom1, which forms gradients from cell poles and inhibits the mitotic activator Cdr2, itself localized at the medial cortex. Where and when Pom1 modulates Cdr2 activity is unclear as Pom1 medial cortical levels remain constant during cell elongation. Here we show that Pom1 re-localizes to cell sides upon environmental glucose limitation, where it strongly delays mitosis. This re-localization is caused by severe microtubule destabilization upon glucose starvation, with microtubules undergoing catastrophe and depositing the Pom1 gradient nucleator Tea4 at cell sides. Microtubule destabilization requires PKA/Pka1 activity, which negatively regulates the microtubule rescue factor CLASP/Cls1/Peg1, reducing CLASP's ability to stabilize microtubules. Thus, PKA signalling tunes CLASP's activity to promote Pom1 cell side localization and buffer cell size upon glucose starvation.

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.

CLASP2 Has Two Distinct TOG Domains That Contribute Differently to Microtubule Dynamics

CLIP-associated proteins CLASPs are mammalian microtubule (MT) plus-end tracking proteins (+TIPs) that promote MT rescue in vivo. Their plus-end localization is dependent on other +TIPs, EB1 and CLIP-170, but in the leading edge of the cell, CLASPs display lattice-binding activity. MT association of CLASPs is suggested to be regulated by multiple TOG (tumor overexpressed gene) domains and by the serine-arginine (SR)-rich region, which contains binding sites for EB1. Here, we report the crystal structures of the two TOG domains of CLASP2. Both domains consist of six HEAT repeats, which are similar to the canonical paddle-like tubulin-binding TOG domains, but have arched conformations. The degrees and directions of curvature are different between the two TOG domains, implying that they have distinct roles in MT binding. Using biochemical, molecular modeling and cell biological analyses, we have investigated the interactions between the TOG domains and αβ-tubulin and found that each domain associates differently with αβ-tubulin. Our findings suggest that, by varying the degrees of domain curvature, the TOG domains may distinguish the structural conformation of the tubulin dimer, discriminate between different states of MT dynamic instability and thereby function differentially as stabilizers of MTs.

Chromatids segregate without centrosomes during Caenorhabditis elegans mitosis in a Ran- and CLASP-dependent manner

Laser ablation of centrosomes in one-cell Caenorhabditis elegans embryos shows that chromatids can segregate independently of centrosomes and also independently of the activity of kinetochore microtubules during mitosis. CLASP and RanGTP are required to generate this centrosome-independent force, whereas SPD-1 and BMK-1 act as brakes to oppose it.

During mitosis, chromosomes are connected to a microtubule-based spindle. Current models propose that displacement of the spindle poles and/or the activity of kinetochore microtubules generate mechanical forces that segregate sister chromatids. Using laser destruction of the centrosomes during Caenorhabditis elegans mitosis, we show that neither of these mechanisms is necessary to achieve proper chromatid segregation. Our results strongly suggest that an outward force generated by the spindle midzone, independently of centrosomes, is sufficient to segregate chromosomes in mitotic cells. Using mutant and RNAi analysis, we show that the microtubule-bundling protein SPD-1/MAP-65 and BMK-1/kinesin-5 act as a brake opposing the force generated by the spindle midzone. Conversely, we identify a novel role for two microtubule-growth and nucleation agents, Ran and CLASP, in the establishment of the centrosome-independent force during anaphase. Their involvement raises the interesting possibility that microtubule polymerization of midzone microtubules is continuously required to sustain chromosome segregation during mitosis.

Drosophila melanogaster mini spindles TOG3 utilizes unique structural elements to promote domain stability and maintain a TOG1- and TOG2-like tubulin-binding surface

Microtubule-associated proteins regulate microtubule (MT) dynamics spatially and temporally, which is essential for proper formation of the bipolar mitotic spindle. The XMAP215 family is comprised of conserved microtubule-associated proteins that use an array of tubulin-binding tumor overexpressed gene (TOG) domains, consisting of six (A-F) Huntingtin, elongation factor 3, protein phosphatase 2A, target of rapamycin (HEAT) repeats, to robustly increase MT plus-end polymerization rates. Recent work showed that TOG domains have differentially conserved architectures across the array, with implications for position-dependent TOG domain tubulin binding activities and function within the XMAP215 MT polymerization mechanism. Although TOG domains 1, 2, and 4 are well described, structural and mechanistic information characterizing TOG domains 3 and 5 is outstanding. Here, we present the structure and characterization of Drosophila melanogaster Mini spindles (Msps) TOG3. Msps TOG3 has two unique features as follows: the first is a C-terminal tail that stabilizes the ultimate four HEAT repeats (HRs), and the second is a unique architecture in HR B. Structural alignments of TOG3 with other TOG domain structures show that the architecture of TOG3 is most similar to TOG domains 1 and 2 and diverges from TOG4. Docking TOG3 onto recently solved Stu2 TOG1· and TOG2·tubulin complex structures suggests that TOG3 uses similarly conserved tubulin-binding intra-HEAT loop residues to engage α- and β-tubulin. This indicates that TOG3 has maintained a TOG1- and TOG2-like TOG-tubulin binding mode despite structural divergence. The similarity of TOG domains 1-3 and the divergence of TOG4 suggest that a TOG domain array with polarized structural diversity may play a key mechanistic role in XMAP215-dependent MT polymerization activity.

The contribution of αβ-tubulin curvature to microtubule dynamics

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

Microtubules in plants

Microtubules (MTs) are highly conserved polar polymers that are key elements of the eukaryotic cytoskeleton and are essential for various cell functions. αβ-tubulin, a heterodimer containing one structural GTP and one hydrolysable and exchangeable GTP, is the building block of MTs and is formed by the sequential action of several molecular chaperones. GTP hydrolysis in the MT lattice is mechanistically coupled with MT growth, thus giving MTs a metastable and dynamic nature. MTs adopt several distinct higher-order organizations that function in cell division and cell morphogenesis. Small molecular weight compounds that bind tubulin are used as herbicides and as research tools to investigate MT functions in plant cells. The de novo formation of MTs in cells requires conserved γ-tubulin-containing complexes and targeting/activating regulatory proteins that contribute to the geometry of MT arrays. Various MT regulators and tubulin modifications control the dynamics and organization of MTs throughout the cell cycle and in response to developmental and environmental cues. Signaling pathways that converge on the regulation of versatile MT functions are being characterized.

Glycogen synthase kinase 3β orchestrates microtubule remodeling in compensatory glomerular adaptation to podocyte depletion

Reminiscent of neural repair, following podocyte depletion, remnant-surviving podocytes exhibit a considerable adaptive capacity to expand and cover the denuded renal glomerular basement membrane. Microtubules, one of the principal cytoskeletal components of podocyte major processes, play a crucial role in podocyte morphogenesis and podocyte process outgrowth, branching, and elongation. Here, we demonstrated that the microtubule-associated proteins Tau and collapsin response mediator protein (CRMP) 2, key regulators of microtubule dynamics, were abundantly expressed by glomerular podocytes in vivo and in vitro, interacted with glycogen synthase kinase (GSK)3β, and served as its putative substrates. GSK3β overactivity induced by adriamycin injury or by a constitutively active mutant of GSK3β augmented phosphorylation of Tau and CRMP2, concomitant with microtubule depolymerization, cell body shrinkage, and shortening of podocyte processes. Conversely, inhibition of GSK3β by a dominant negative mutant or by lithium, a Food and Drug Administration-approved neuroprotective mood stabilizer, diminished Tau and CRMP2 phosphorylation, resulting in microtubule polymerization, podocyte expansion, and lengthening of podocyte processes. In a mouse model of adriamycin-induced podocyte depletion and nephropathy, delayed administration of a single low dose of lithium attenuated proteinuria and ameliorated progressive glomerulosclerosis despite no correction of podocytopenia. Mechanistically, lithium therapy obliterated GSK3β overactivity, mitigated phosphorylation of Tau and CRMP2, and enhanced microtubule polymerization and stabilization in glomeruli in adriamycin-injured kidneys, associated with elongation of podocyte major processes. Collectively, our findings suggest that the GSK3β-dictated podocyte microtubule dynamics might serve as a novel therapeutic target to reinforce the compensatory glomerular adaptation to podocyte loss.

CLASPs are required for proper microtubule localization of end-binding proteins

Microtubule (MT) plus-end tracking proteins (+TIPs) preferentially localize to MT plus ends. End-binding proteins (EBs) are master regulators of the +TIP complex; however, it is unknown whether EBs are regulated by other +TIPs. Here, we show that cytoplasmic linker-associated proteins (CLASPs) modulate EB localization at MTs. In CLASP-depleted cells, EBs localized along the MT lattice in addition to plus ends. The MT-binding region of CLASP was sufficient for restoring normal EB localization, whereas neither EB-CLASP interactions nor EB tail-binding proteins are involved. In vitro assays revealed that CLASP directly functions to remove EB from MTs. Importantly, this effect occurs specifically during MT polymerization, but not at preformed MTs. Increased GTP-tubulin content within MTs in CLASP-depleted cells suggests that CLASPs facilitate GTP hydrolysis to reduce EB lattice binding. Together, these findings suggest that CLASPs influence the MT lattice itself to regulate EB and determine exclusive plus-end localization of EBs in cells.

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 TOGL domain specifically targets yeast CLASP to kinetochores to stabilize kinetochore microtubules

The two different TOGL domains of the budding yeast CLASP Stu1 are responsible for its distinct mitotic activities, and these activities are only partially mediated by tight microtubule binding.

Cytoplasmic linker–associated proteins (CLASPs) are proposed to function in cell division based on their ability to bind tubulin via arrayed tumor overexpressed gene (TOG)–like (TOGL) domains. Structure predictions suggest that CLASPs have at least two TOGL domains. We show that only TOGL2 of Saccharomyces cerevisiae CLASP Stu1 binds to tubulin and is required for polymerization of spindle microtubules (MTs) in vivo. In contrast, TOGL1 recruits Stu1 to kinetochores (KTs), where it is essential for the stability and tension-dependent regulation of KT MTs. Stu1 is also recruited to spindle MTs by different mechanisms depending on the mitotic phase: in metaphase, Stu1 binds directly to the MT lattice, whereas in anaphase, it is localized indirectly to the spindle midzone. In both phases, the activity of TOGL2 is essential for interpolar MT stability, whereas TOGL1 is not involved. Thus, the two TOGL domains of yeast CLASP have different activities and execute distinct mitotic functions.

The kinetochore

A critical requirement for mitosis is the distribution of genetic material to the two daughter cells. The central player in this process is the macromolecular kinetochore structure, which binds to both chromosomal DNA and spindle microtubule polymers to direct chromosome alignment and segregation. This review will discuss the key kinetochore activities required for mitotic chromosome segregation, including the recognition of a specific site on each chromosome, kinetochore assembly and the formation of kinetochore-microtubule connections, the generation of force to drive chromosome segregation, and the regulation of kinetochore function to ensure that chromosome segregation occurs with high fidelity.

In situ imaging in C. elegans reveals developmental regulation of microtubule dynamics

Microtubules (MTs) are cytoskeletal polymers that undergo dynamic instability, the stochastic transition between growth and shrinkage phases. MT dynamics are required for diverse cellular processes and, while intrinsic to tubulin, are highly regulated. However, little is known about how MT dynamics facilitate or are regulated by tissue biogenesis and differentiation. We imaged MT dynamics in a smooth muscle-like lineage in intact developing Caenorhabditis elegans. All aspects of MT dynamics change significantly as stem-like precursors exit mitosis and, secondarily, as they differentiate. We found that suppression, but not enhancement, of dynamics perturbs differentiated muscle function in vivo. Distinct ensembles of MT-associated proteins are specifically required for tissue biogenesis versus tissue function. A CLASP family MT stabilizer and the depolymerizing kinesin MCAK are differentially required for MT dynamics in the precursor or differentiated cells, respectively. All of these multidimensional phenotypic comparisons were facilitated by a data display method called the diamond graph.

Microtubule networks for plant cell division

During cytokinesis the cytoplasm of a cell is divided to form two daughter cells. In animal cells, the existing plasma membrane is first constricted and then abscised to generate two individual plasma membranes. Plant cells on the other hand divide by forming an interior dividing wall, the so-called cell plate, which is constructed by localized deposition of membrane and cell wall material. Construction starts in the centre of the cell at the locus of the mitotic spindle and continues radially towards the existing plasma membrane. Finally the membrane of the cell plate and plasma membrane fuse to form two individual plasma membranes. Two microtubule-based cytoskeletal networks, the phragmoplast and the pre-prophase band (PPB), jointly control cytokinesis in plants. The bipolar microtubule array of the phragmoplast regulates cell plate deposition towards a cortical position that is templated by the ring-shaped microtubule array of the PPB. In contrast to most animal cells, plants do not use centrosomes as foci of microtubule growth initiation. Instead, plant microtubule networks are striking examples of self-organizing systems that emerge from physically constrained interactions of dispersed microtubules. Here we will discuss how microtubule-based activities including growth, shrinkage, severing, sliding, nucleation and bundling interrelate to jointly generate the required ordered structures. Evidence mounts that adapter proteins sense the local geometry of microtubules to locally modulate the activity of proteins involved in microtubule growth regulation and severing. Many of the proteins and mechanisms involved have roles in other microtubule assemblies as well, bestowing broader relevance to insights gained from plants.

Plk1 phosphorylates CLIP-170 and regulates its binding to microtubules for chromosome alignment

The microtubule (MT) cytoskeleton is essential for cellular morphogenesis, cell migration, and cell division. MT organization is primarily mediated by a variety of MT-associated proteins. Among these proteins, plus-end-tracking proteins (+TIPs) are evolutionarily conserved factors that selectively accumulate at growing MT plus ends. Cytoplasmic linker protein (CLIP)-170 is a +TIP that associates with diverse proteins to determine the behavior of MT ends and their linkage to intracellular structures, including mitotic chromosomes. However, how CLIP-170 activity is spatially and temporally controlled is largely unknown. Here, we show that phosphorylation at Ser312 in the third serine-rich region of CLIP-170 is increased during mitosis. Polo-like kinase 1 (Plk1) is responsible for this phosphorylation during the mitotic phase of dividing cells. In vitro analysis using a purified CLIP-170 N-terminal fragment showed that phosphorylation by Plk1 diminishes CLIP-170 binding to the MT ends and lattice without affecting binding to EB3. Furthermore, we demonstrate that during mitosis, stable kinetochore/MT attachment and subsequent chromosome alignment require CLIP-170 and a proper phosphorylation/dephosphorylation cycle at Ser312. We propose that CLIP-170 phosphorylation by Plk1 regulates proper chromosome alignment by modulating the interaction between CLIP-170 and MTs in mitotic cells and that CLIP-170 activity is stringently controlled by its phosphorylation state, which depends on the cellular context.

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.

Cryptic no longer: arrays of CLASP1 TOG domains

CLASP proteins play crucial roles in regulating microtubules. In this issue of Structure, Leano and colleagues show that an essential and previously cryptic domain of CLASP is a TOG domain with unusual features that might explain its unique functions.

Microtubule organization and microtubule-associated proteins in plant cells

Plants have unique microtubule (MT) arrays, cortical MTs, preprophase band, mitotic spindle, and phragmoplast, in the processes of evolution. These MT arrays control the directions of cell division and expansion especially in plants and are essential for plant morphogenesis and developments. Organizations and functions of these MT arrays are accomplished by diverse MT-associated proteins (MAPs). This review introduces 10 of conserved MAPs in eukaryote such as γ-TuC, augmin, katanin, kinesin, EB1, CLASP, MOR1/MAP215, MAP65, TPX2, formin, and several plant-specific MAPs such as CSI1, SPR2, MAP70, WVD2/WDL, RIP/MIDD, SPR1, MAP18/PCaP, EDE1, and MAP190. Most of the studies cited in this review have been analyzed in the particular model plant, Arabidopsis thaliana. The significant knowledge of A. thaliana is the important established base to understand MT organizations and functions in plants.

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.

The role of dynamic instability in microtubule organization

Microtubules are one of the three major cytoskeletal components in eukaryotic cells. Heterodimers composed of GTP-bound α- and β-tubulin molecules polymerize to form microtubule protofilaments, which associate laterally to form a hollow microtubule. Tubulin has GTPase activity and the GTP molecules associated with β-tubulin molecules are hydrolyzed shortly after being incorporated into the polymerizing microtubules. GTP hydrolysis alters the conformation of the tubulin molecules and drives the dynamic behavior of microtubules. Periods of rapid microtubule polymerization alternate with periods of shrinkage in a process known as dynamic instability. In plants, dynamic instability plays a key role in determining the organization of microtubules into arrays, and these arrays vary throughout the cell cycle. In this review, we describe the mechanisms that regulate microtubule dynamics and underlie dynamic instability, and discuss how dynamic instability may shape microtubule organization in plant cells.

Reconstituting dynamic microtubule polymerization regulation by TOG domain proteins

Microtubules (MTs) polymerize from soluble αβ-tubulin and undergo rapid dynamic transitions to depolymerization at their ends. Microtubule-associated regulator proteins modulate polymerization dynamics in vivo by altering microtubule plus end conformations or influencing αβ-tubulin incorporation rates. Biochemical reconstitution of dynamic MT polymerization can be visualized with total internal reflection fluorescence (TIRF) microscopy using purified MT regulators. This approach has provided extensive details on the regulation of microtubule dynamics. Here, I describe a general approach to reconstitute MT dynamic polymerization with TOG domain microtubule regulators from the XMAP215/Dis1 and CLASP families using TIRF microscopy. TIRF imaging strategies require nucleation of microtubule polymerization from surface-attached, stabilized MTs. The approaches described here can be used to study the mechanism of a wide variety of microtubule regulatory proteins.

Protein 4.1R binds to CLASP2 and regulates dynamics, organization and attachment of microtubules to the cell cortex

The microtubule (MT) cytoskeleton is essential for many cellular processes, including cell polarity and migration. Cortical platforms, formed by a subset of MT plus-end-tracking proteins, such as CLASP2, and non-MT binding proteins such as LL5β, attach distal ends of MTs to the cell cortex. However, the mechanisms involved in organizing these platforms have not yet been described in detail. Here we show that 4.1R, a FERM-domain-containing protein, interacts and colocalizes with cortical CLASP2 and is required for the correct number and dynamics of CLASP2 cortical platforms. Protein 4.1R also controls binding of CLASP2 to MTs at the cell edge by locally altering GSK3 activity. Furthermore, in 4.1R-knockdown cells MT plus-ends were maintained for longer in the vicinity of cell edges, but instead of being tethered to the cell cortex, MTs continued to grow, bending at cell margins and losing their radial distribution. Our results suggest a previously unidentified role for the scaffolding protein 4.1R in locally controlling CLASP2 behavior, CLASP2 cortical platform turnover and GSK3 activity, enabling correct MT organization and dynamics essential for cell polarity.

Epiblast integrity requires CLASP and Dystroglycan-mediated microtubule anchoring to the basal cortex

Amniote epiblast cells differentiate into mesoderm and endoderm lineages during gastrulation through a process called epithelial-to-mesenchymal transition (EMT). Molecular regulation of gastrulation EMT is poorly understood. Here we show that epiblast epithelial status was maintained by anchoring microtubules to the basal cortex via CLIP-associated protein (CLASP), a microtubule plus-end tracking protein, and Dystroglycan, a transmembrane protein that bridges the cytoskeleton and basement membrane (BM). Mesoderm formation required down-regulation of CLASP and Dystroglycan, and reducing CLASP activity in pregastrulation epiblast cells caused ectopic BM breakdown and disrupted epiblast integrity. These effects were mediated through the CLASP-binding partner LL5. Live-imaging using EB1-enhanced GFP (eGFP) revealed that reducing CLASP and LL5 levels in the epiblast destabilized basal microtubules. We further show that Dystroglycan is localized to basolateral membrane in epiblast cells. Basal but not lateral localization of Dystroglycan was regulated by CLASP. We propose that epiblast-BM interaction requires CLASP- and Dystroglycan-mediated cortical microtubule anchoring, the disruption of which initiates gastrulation EMT.

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.

Molecular mechanisms of Tau binding to microtubules and its role in microtubule dynamics in live cells

Despite extensive studies, the molecular mechanisms of Tau binding to microtubules (MTs) and its consequences on MT stability still remain unclear. It is especially true in cells where the spatiotemporal distribution of Tau-MT interactions is unknown. Using Förster resonance energy transfer (FRET), we showed that the Tau-MT interaction was distributed along MTs in periodic hotspots of high and low FRET intensities. Fluorescence recovery after photobleaching (FRAP) revealed a two-phase exchange of Tau with MTs as a rapid diffusion followed by a slower binding phase. A real-time FRET assay showed that high FRET occurred simultaneously with rescue and pause transitions at MT ends. To further explore the functional interaction of Tau with MTs, the binding of paclitaxel (PTX), tubulin acetylation induced by trichostatin A (TSA), and the expression of non-acetylatable tubulin were used. With PTX and TSA, FRAP curves best fitted a single phase with a long time constant, whereas with non-acetylatable α-tubulin, curves best fitted a two phase recovery. Upon incubation with PTX and TSA, the number of high and low FRET hotspots decreased by up to 50% and no hotspot was observed during rescue and pause transitions. In the presence of non-acetylatable α-tubulin, a 34% increase in low FRET hotspots occurred, and our real-time FRET assay revealed that low FRET hotspots appeared with MTs recovering growth. In conclusion, we have identified, by FRET and FRAP, a discrete Tau-MT interaction, in which Tau could induce conformational changes of MTs, favoring recovery of MT self-assembly.

Micropattern-controlled local microtubule nucleation, transport, and mesoscale organization

Microtubule organization in living cells is determined by spatial control of microtubule nucleation, their dynamic properties, and transport by molecular motors. Here, we establish a new micropattern-guided method for controlling local microtubule nucleation by spatially confined immobilization of a microtubule polymerase and show that these nucleated microtubules can be transported and organized in space by motor proteins. This assay provides a new platform for deciphering the principles underlying mesoscale microtubule organization.

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.

Building complexity: insights into self-organized assembly of microtubule-based architectures

Successful completion of diverse cellular functions, such as mitosis, positioning organelles, and assembling cilia, depends on the proper assembly of microtubule-based structures. While essentially all of the proteins needed to assemble these structures are now known, we cannot explain how even simple features such as size and shape are determined. As steps toward filling this knowledge gap, there have been several recent efforts toward reconstituting, with purified proteins, the basic structural motifs that recur in diverse cytoskeletal arrays. We discuss these studies and highlight how they shed light on the self-organized assembly of complex and dynamic cytoskeleton-based cellular structures.

Microtubule plus-end tracking protein CLASP2 regulates neuronal polarity and synaptic function

Microtubule organization and dynamics are essential during axon and dendrite formation and maintenance in neurons. However, little is known about the regulation of microtubule dynamics during synaptic development and function in mammalian neurons. Here, we present evidence that the microtubule plus-end tracking protein CLASP2 (cytoplasmic linker associated protein 2) is a key regulator of axon and dendrite outgrowth that leads to functional alterations in synaptic activity and formation. We found that CLASP2 protein levels steadily increase throughout neuronal development in the mouse brain and are specifically enriched at the growth cones of extending neurites. The short-hairpin RNA-mediated knockdown of CLASP2 in primary mouse neurons decreased axon and dendritic length, whereas overexpression of human CLASP2 caused the formation of multiple axons, enhanced dendritic branching, and Golgi condensation, implicating CLASP2 in neuronal morphogenesis. In addition, the CLASP2-induced morphological changes led to significant functional alterations in synaptic transmission. CLASP2 overexpression produced a large increase in spontaneous miniature event frequency that was specific to excitatory neurotransmitter release. The changes in presynaptic activity produced by CLASP2 overexpression were accompanied by increases in presynaptic terminal circumference, total synapse number, and a selective increase in presynaptic proteins that are involved in neurotransmitter release. Also, we found a smaller increase in miniature event amplitude that was accompanied by an increase in postsynaptic surface expression of GluA1 receptor localization. Together, these results provide evidence for involvement of the microtubule plus-end tracking protein CLASP2 in cytoskeleton-related mechanisms underlying neuronal polarity and interplay between microtubule stabilization and synapse formation and activity.

Microtubule catastrophe and rescue

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

Calcium tips the balance: a microtubule plus end to lattice binding switch operates in the carboxyl terminus of BPAG1n4

Microtubules (MTs) are integral to numerous cellular functions, such as cell adhesion, differentiation and intracellular transport. Their dynamics are largely controlled by diverse MT-interacting proteins, but the signalling mechanisms that regulate these interactions remain elusive. In this report, we identify a rapid, calcium-regulated switch between MT plus end interaction and lattice binding within the carboxyl terminus of BPAG1n4. This switch is EF-hand dependent, and mutations of the EF-hands abolish this dynamic behaviour. Our study thus uncovers a new, calcium-dependent regulatory mechanism for a spectraplakin, BPAG1n4, at the MT plus end.

Move in for the kill: motile microtubule regulators

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

+TIPs: SxIPping along microtubule ends

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

Fission yeast Alp14 is a dose-dependent plus end-tracking microtubule polymerase

Alp14, a XMAP215 orthologue in fission yeast, is a microtubule (MT) polymerase. It tracks growing MT plus ends and regulates the polymerization state of tubulin by cycling between a tubulin dimer–bound cytoplasmic state and a MT polymerase state that promotes rapid MT assembly.

XMAP215/Dis1 proteins are conserved tubulin-binding TOG-domain proteins that regulate microtubule (MT) plus-end dynamics. Here we show that Alp14, a XMAP215 orthologue in fission yeast, Schizosaccharomyces pombe, has properties of a MT polymerase. In vivo, Alp14 localizes to growing MT plus ends in a manner independent of Mal3 (EB1). alp14-null mutants display short interphase MTs with twofold slower assembly rate and frequent pauses. Alp14 is a homodimer that binds a single tubulin dimer. In vitro, purified Alp14 molecules track growing MT plus ends and accelerate MT assembly threefold. TOG-domain mutants demonstrate that tubulin binding is critical for function and plus end localization. Overexpression of Alp14 or only its TOG domains causes complete MT loss in vivo, and high Alp14 concentration inhibits MT assembly in vitro. These inhibitory effects may arise from Alp14 sequestration of tubulin and effects on the MT. Our studies suggest that Alp14 regulates the polymerization state of tubulin by cycling between a tubulin dimer–bound cytoplasmic state and a MT polymerase state that promotes rapid MT assembly.

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.

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

Interplay between microtubule dynamics and intracellular organization

Microtubules are hollow tubes essential for many cellular functions such as cell polarization and migration, intracellular trafficking and cell division. They are polarized polymers composed of α and β tubulin that are, in most cells, nucleated at the centrosome at the center of the cell. Microtubule plus-ends are oriented towards the periphery of the cell and explore the cytoplasm in a very dynamic manner. Microtubule alternate between phases of growth and shrinkage in a manner described as dynamic instability. Their dynamics is highly regulated by multiple factors: tubulin post-translational modifications such as detyrosination or acetylation, and microtubule-associated proteins, among them the plus-tip tracking proteins. This regulation is necessary for microtubule functions in the cell. In this review, we will focus on the role of microtubules in intracellular organization. After an overview of the mechanisms responsible for the regulation of microtubule dynamics, the major roles of microtubules dynamics in organelle positioning and organization in interphase cells will be discussed. Conversely, the role of certain organelles, like the nucleus and the Golgi apparatus as microtubule organizing centers will be reviewed. We will then consider the role of microtubules in the establishment and maintenance of cell polarity using few examples of cell polarization: epithelial cells, neurons and migrating cells. In these cells, the microtubule network is reorganized and undergoes specific and local regulation events; microtubules also participate in the intracellular reorganization of different organelles to ensure proper cell differentiation.

GSK3 controls axon growth via CLASP-mediated regulation of growth cone microtubules

Suppression of glycogen synthase kinase 3 (GSK3) activity in neurons yields pleiotropic outcomes, causing both axon growth promotion and inhibition. Previous studies have suggested that specific GSK3 substrates, such as adenomatous polyposis coli (APC) and collapsin response mediator protein 2 (CRMP2), support axon growth by regulating the stability of axonal microtubules (MTs), but the substrate(s) and mechanisms conveying axon growth inhibition remain elusive. Here we show that CLIP (cytoplasmic linker protein)-associated protein (CLASP), originally identified as a MT plus end-binding protein, displays both plus end-binding and lattice-binding activities in nerve growth cones, and reveal that the two MT-binding activities regulate axon growth in an opposing manner: The lattice-binding activity mediates axon growth inhibition induced by suppression of GSK3 activity via preventing MT protrusion into the growth cone periphery, whereas the plus end-binding property supports axon extension via stabilizing the growing ends of axonal MTs. We propose a model in which CLASP transduces GSK3 activity levels to differentially control axon growth by coordinating the stability and configuration of growth cone MTs.

Regulation of cell migration by dynamic microtubules

Microtubules define the architecture and internal organization of cells by positioning organelles and activities, as well as by supporting cell shape and mechanics. One of the major functions of microtubules is the control of polarized cell motility. In order to support the asymmetry of polarized cells, microtubules have to be organized asymmetrically themselves. Asymmetry in microtubule distribution and stability is regulated by multiple molecular factors, most of which are microtubule-associated proteins that locally control microtubule nucleation and dynamics. At the same time, the dynamic state of microtubules is key to the regulatory mechanisms by which microtubules regulate cell polarity, modulate cell adhesion and control force-production by the actin cytoskeleton. Here, we propose that even small alterations in microtubule dynamics can influence cell migration via several different microtubule-dependent pathways. We discuss regulatory factors, potential feedback mechanisms due to functional microtubule-actin crosstalk and implications for cancer cell motility.

The microtubule lattice and plus-end association of Drosophila Mini spindles is spatially regulated to fine-tune microtubule dynamics

Individual microtubules (MTs) exhibit dynamic instability, a behavior in which they cycle between phases of growth and shrinkage while the total amount of MT polymer remains constant. Dynamic instability is promoted by the conserved XMAP215/Dis1 family of microtubule-associated proteins (MAPs). In this study, we conducted an in vivo structure-function analysis of the Drosophila homologue Mini spindles (Msps). Msps exhibits EB1-dependent and spatially regulated MT localization, targeting to microtubule plus ends in the cell interior and decorating the lattice of growing and shrinking microtubules in the cell periphery. RNA interference rescue experiments revealed that the NH(2)-terminal four TOG domains of Msps function as paired units and were sufficient to promote microtubule dynamics and EB1 comet formation. We also identified TOG5 and novel inter-TOG linker motifs that are required for targeting Msps to the microtubule lattice. These novel microtubule contact sites are necessary for the interplay between the conserved TOG domains and inter-TOG MT binding that underlies the ability of Msps to promote MT dynamic instability.

AKAP220 protein organizes signaling elements that impact cell migration

Cell movement requires the coordinated reception, integration, and processing of intracellular signals. We have discovered that the protein kinase A anchoring protein AKAP220 interacts with the cytoskeletal scaffolding protein IQGAP1 to influence cell motility. AKAP220/IQGAP1 networks receive and integrate calcium and cAMP second messenger signals and position signaling enzymes near their intended substrates at leading edges of migrating cells. IQGAP1 supports calcium/calmodulin-dependent association of factors that modulate microtubule dynamics. AKAP220 suppresses GSK-3β and positions this kinase to allow recruitment of the plus-end microtubule tracking protein CLASP2. Gene silencing of AKAP220 alters the rate of microtubule polymerization and the lateral tracking of growing microtubules and retards cell migration in metastatic human cancer cells. This reveals an unappreciated role for this anchored kinase/microtubule effector protein network in the propagation of cell motility.

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.

Regulation of microtubule dynamics by TOG-domain proteins XMAP215/Dis1 and CLASP

The molecular mechanisms by which microtubule-associated proteins (MAPs) regulate the dynamic properties of microtubules (MTs) are still poorly understood. We review recent advances in our understanding of two conserved families of MAPs, the XMAP215/Dis1 and CLASP family of proteins. In vivo and in vitro studies show that XMAP215 proteins act as microtubule polymerases at MT plus ends to accelerate MT assembly, and CLASP proteins promote MT rescue and suppress MT catastrophe events. These are structurally related proteins that use conserved TOG domains to recruit tubulin dimers to MTs. We discuss models for how these proteins might use these individual tubulin dimers to regulate dynamic behavior of MT plus 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.