CYLD coordinates with EB1 to regulate microtubule dynamics and cell migration

Cylindromatosis (CYLD), a deubiquitinase involved in inflammation and tumorigenesis via the modulation of cell signaling, has recently been identified as a critical regulator of microtubule dynamics. CYLD has also been shown to stimulate cell migration and thereby contribute to normal physiological processes. However, it remains elusive how the regulation of microtubule dynamic properties by CYLD is connected to its role in mediating cell migration. In this study, we performed yeast 2-hybrid screening with CYLD as bait and identified 7 CYLD-interacting proteins, including end-binding protein 1 (EB1). The CYLD–EB1 interaction was confirmed both in cells and in vitro, and these 2 proteins colocalized at the plus ends of microtubules. Interestingly, the association of CYLD with EB1 was significantly increased upon the stimulation of cell migration. CYLD coordinated with EB1 to orchestrate tail retraction, centrosome reorientation, and leading-edge microtubule stabilization in migratory cells. In addition, CYLD acted in concert with EB1 to regulate microtubule assembly in vitro, microtubule nucleation at the centrosome, and microtubule growth at the cell periphery. These data provide mechanistic insights into the actions of CYLD in the regulation of microtubule dynamics and cell migration. These findings also support the notion that coordinated actions of microtubule-binding proteins are critical for microtubule-mediated cellular events.     

Peptide aptamers expressed in the secretory pathway interfere with cellular PrPSc formation

Prion diseases are rare and obligatory fatal neurodegenerative disorders caused by the accumulation of a misfolded isoform (PrPSc) of the host-encoded prion protein (PrPc). Prophylactic and therapeutic regimens against prion diseases are very limited. To extend such strategies we selected peptide aptamers binding to PrP from a combinatorial peptide library presented on the Escherichia coli thioredoxin A (trxA) protein as a scaffold. In a yeast two-hybrid screen employing full-length murine PrP (aa 23-231) as a bait we identified three peptide aptamers that reproducibly bind to PrP. Treatment of prion-infected cells with recombinantly expressed aptamers added to the culture medium abolished PrPSc conversion with an IC50 between 350 and 700 nM. For expression in eukaryotic cells, peptide aptamers were fused to an N-terminal signal peptide for entry of the secretory pathway. The C terminus was modified by a glycosyl-phosphatidyl-inositol-(GPI) anchoring signal, a KDEL retention motif and the transmembrane and cytosolic domain of LAMP-I, respectively. These peptide aptamers retained their binding properties to PrPc and, depending on peptide sequence and C-terminal modification, interfered with endogenous PrPSc conversion upon expression in prion-infected cells. Notably, infection of cell cultures could be prevented by expression of KDEL peptide aptamers. For the first time, we show that trxA-based peptide aptamers can be targeted to the secretory pathway, thereby not losing the affinity for their target protein. Beside their inhibitory effect on prion conversion, these molecules could be used as fundament for rational drug design.     

EB1 regulates microtubule dynamics and tubulin sheet closure in vitro

End binding 1 (EB1) is a plus-end-tracking protein (+TIP) that localizes to microtubule plus ends where it modulates their dynamics and interactions with intracellular organelles. Although the regulating activity of EB1 on microtubule dynamics has been studied in cells and purified systems, the molecular mechanisms involved in its specific activity are still unclear. Here, we describe how EB1 regulates the dynamics and structure of microtubules assembled from pure tubulin. We found that EB1 stimulates spontaneous nucleation and growth of microtubules, and promotes both catastrophes (transitions from growth to shrinkage) and rescues (reverse events). Electron cryomicroscopy showed that EB1 induces the initial formation of tubulin sheets, which rapidly close into the common 13-protofilament-microtubule architecture. Our results suggest that EB1 favours the lateral association of free tubulin at microtubule-sheet edges, thereby stimulating nucleation, sheet growth and closure. The reduction of sheet length at microtubule growing-ends together with the elimination of stressed microtubule lattices may account for catastrophes. Conversely, occasional binding of EB1 to the microtubule lattice may induce rescues.     

EB1 promotes microtubule dynamics by recruiting Sentin in Drosophila cells

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.     

MAP1B regulates microtubule dynamics by sequestering EB1/3 in the cytosol of developing neuronal cells

MAP1B, a structural microtubule (MT)‐associated protein highly expressed in developing neurons, plays a key role in neurite and axon extension. However, not all molecular mechanisms by which MAP1B controls MT dynamics during these processes have been revealed. Here, we show that MAP1B interacts directly with EB1 and EB3 (EBs), two core ‘microtubule plus‐end tracking proteins’ (+TIPs), and sequesters them in the cytosol of developing neuronal cells. MAP1B overexpression reduces EBs binding to plus‐ends, whereas MAP1B downregulation increases binding of EBs to MTs. These alterations in EBs behaviour lead to changes in MT dynamics, in particular overstabilization and looping, in growth cones of MAP1B‐deficient neurons. This contributes to growth cone remodelling and a delay in axon outgrowth. Together, our findings define a new and crucial role of MAP1B as a direct regulator of EBs function and MT dynamics during neurite and axon extension. Our data provide a new layer of MT regulation: a classical MAP, which binds to the MT lattice and not to the end, controls effective concentration of core +TIPs thereby regulating MTs at their plus‐ends.     

The EF-hand Ca2+-binding protein p22 plays a role in microtubule and endoplasmic reticulum organization and dynamics with distinct Ca2+-binding requirements

We have reported that p22, an N-myristoylated EF-hand Ca(2+)-binding protein, associates with microtubules and plays a role in membrane trafficking. Here, we show that p22 also associates with membranes of the early secretory pathway membranes, in particular endoplasmic reticulum (ER). On binding of Ca(2+), p22's ability to associate with membranes increases in an N-myristoylation-dependent manner, which is suggestive of a nonclassical Ca(2+)-myristoyl switch mechanism. To address the intracellular functions of p22, a digitonin-based "bulk microinjection" assay was developed to load cells with anti-p22, wild-type, or mutant p22 proteins. Antibodies against a p22 peptide induce microtubule depolymerization and ER fragmentation; this antibody-mediated effect is overcome by preincubation with the respective p22 peptide. In contrast, N-myristoylated p22 induces the formation of microtubule bundles, the accumulation of ER structures along the bundles as well as an increase in ER network formation. An N-myristoylated Ca(2+)-binding p22 mutant, which is unable to undergo Ca(2+)-mediated conformational changes, induces microtubule bundling and accumulation of ER structures along the bundles but does not increase ER network formation. Together, these data strongly suggest that p22 modulates the organization and dynamics of microtubule cytoskeleton in a Ca(2+)-independent manner and affects ER network assembly in a Ca(2+)-dependent manner.     

CLASP1 and CLASP2 bind to EB1 and regulate microtubule plus-end dynamics at the cell cortex

CLIP-associating protein (CLASP) 1 and CLASP2 are mammalian microtubule (MT) plus-end binding proteins, which associate with CLIP-170 and CLIP-115. Using RNA interference in HeLa cells, we show that the two CLASPs play redundant roles in regulating the density, length distribution and stability of interphase MTs. In HeLa cells, both CLASPs concentrate on the distal MT ends in a narrow region at the cell margin. CLASPs stabilize MTs by promoting pauses and restricting MT growth and shortening episodes to this peripheral cell region. We demonstrate that the middle part of CLASPs binds directly to EB1 and to MTs. Furthermore, we show that the association of CLASP2 with the cell cortex is MT independent and relies on its COOH-terminal domain. Both EB1- and cortex-binding domains of CLASP are required to promote MT stability. We propose that CLASPs can mediate interactions between MT plus ends and the cell cortex and act as local rescue factors, possibly through forming a complex with EB1 at MT tips.     

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

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

EB3 regulates microtubule dynamics at the cell cortex and is required for myoblast elongation and fusion

During muscle differentiation, myoblasts elongate and fuse into syncytial myotubes [1]. An early event during this process is the remodeling of the microtubule cytoskeleton, involving disassembly of the centrosome and, crucially, the alignment of microtubules into a parallel array along the long axis of the cell [2-5]. To further our understanding on how microtubules support myogenic differentiation, we analyzed the role of EB1-related microtubule-plus-end-binding proteins. We demonstrate that EB3 [6] is specifically upregulated upon myogenic differentiation and that knockdown of EB3, but not that of EB1, prevents myoblast elongation and fusion into myotubes. EB3-depleted cells show disorganized microtubules and fail to stabilize polarized membrane protrusions. Using live-cell imaging, we show that EB3 is necessary for the regulation of microtubule dynamics and microtubule capture at the cell cortex. Expression of EB1/EB3 chimeras on an EB3-depletion background revealed that myoblast fusion depends on two specific amino acids in the calponin-like domain of EB3, whereas the interaction sites with Clip-170 and CLASPs are dispensable. Our results suggest that EB3-mediated microtubule regulation at the cell cortex is a crucial step during myogenic differentiation and might be a general mechanism in polarized cell elongation.     

The microtubule plus-end proteins EB1 and dynactin have differential effects on microtubule polymerization

Several microtubule-binding proteins including EB1, dynactin, APC, and CLIP-170 localize to the plus-ends of growing microtubules. Although these proteins can bind to microtubules independently, evidence for interactions among them has led to the hypothesis of a plus-end complex. Here we clarify the interaction between EB1 and dynactin and show that EB1 binds directly to the N-terminus of the p150(Glued) subunit. One function of a plus-end complex may be to regulate microtubule dynamics. Overexpression of either EB1 or p150(Glued) in cultured cells bundles microtubules, suggesting that each may enhance microtubule stability. The morphology of these bundles, however, differs dramatically, indicating that EB1 and dynactin may act in different ways. Disruption of the dynactin complex augments the bundling effect of EB1, suggesting that dynactin may regulate the effect of EB1 on microtubules. In vitro assays were performed to elucidate the effects of EB1 and p150(Glued) on microtubule polymerization, and they show that p150(Glued) has a potent microtubule nucleation effect, whereas EB1 has a potent elongation effect. Overall microtubule dynamics may result from a balance between the individual effects of plus-end proteins. Differences in the expression and regulation of plus-end proteins in different cell types may underlie previously noted differences in microtubule dynamics.     

A novel localization pattern for an EB1-like protein links microtubule dynamics to endomembrane organization

A group of microtubule-associated proteins called +TIPs (plus end tracking proteins), including EB1 family proteins, label growing microtubule ends specifically in diverse organisms and are implicated in spindle dynamics, chromosome segregation, and directing microtubules toward cortical sites. Here, we report three new EB1-like proteins from Arabidopsis and provide the intracellular localization for AtEB1, which differs from all known EB1 proteins in having a very long acidic C-terminal tail. In marked contrast to other EB1 proteins, the GFP-AtEB1 fusion protein localizes not only to microtubule plus ends but also to motile, pleiomorphic tubulovesicular membrane networks that surround other organelles and frequently merge with the endoplasmic reticulum. AtEB1 behavior thus resembles that of +TIPs, such as the cytoplasmic linker protein CLIP-170, that are known to associate with and pull along membrane tubules in animal systems but for which homologs have not been identified in plants. In addition, though EB1 proteins are believed to stabilize microtubules, a different behavior is observed for AtEB1 where instead of stabilizing a microtubule it localizes to already stabilized regions on a microtubule. The dual localization pattern of AtEB1 suggests links between microtubule plus end dynamics and endomembrane organization during polarized growth of plant cells.     

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

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

Kif18B interacts with EB1 and controls astral microtubule length during mitosis

Regulation of microtubule (MT) dynamics is essential for proper spindle assembly and organization. Kinesin-8 family members are plus-end-directed motors that modulate plus-end MT dynamics by acting as MT depolymerases or as MT plus-end capping proteins. In this paper, we show that the human kinesin-8 Kif18B functions during mitosis to control astral MT organization. Kif18B is a MT plus-tip-tracking protein that localizes to the nucleus in interphase and is enriched at astral MT plus ends during early mitosis. Knockdown of Kif18B caused spindle defects, resulting in an increased number and length of MTs. A yeast two-hybrid screen identified an interaction of the C-terminal domain of Kif18B with the plus-end MT-binding protein EB1. EB1 knockdown disrupted Kif18B targeting to MT plus ends, indicating that EB1/Kif18B interaction is physiologically important. This interaction is direct, as the far C-terminal end of Kif18B is sufficient for binding to EB1 in vitro. Overexpression of this domain is sufficient for plus-end MT targeting in cells; however, targeting is enhanced by the motor domain, which cooperates with the tail to achieve proper Kif18B localization at MT plus ends. Our results suggest that Kif18B is a new MT dynamics regulatory protein that interacts with EB1 to control astral MT length.     

The Schizosaccharomyces pombe EB1 homolog Mal3p binds and stabilizes the microtubule lattice seam

End binding 1 (EB1) proteins are highly conserved regulators of microtubule dynamics. Using electron microscopy (EM) and high-resolution surface shadowing we have studied the microtubule-binding properties of the fission yeast EB1 homolog Mal3p. This allowed for a direct visualization of Mal3p bound on the surface of microtubules. Mal3p particles usually formed a single line on each microtubule along just one of the multiple grooves that are formed by adjacent protofilaments. We provide structural data showing that the alignment of Mal3p molecules coincides with the microtubule lattice seam as well as data suggesting that Mal3p not only binds but also stabilizes this seam. Accordingly, Mal3p stabilizes microtubules through a specific interaction with what is potentially the weakest part of the microtubule in a way not previously demonstrated. Our findings further suggest that microtubules exhibit two distinct reaction platforms on their surface that can independently interact with target structures such as microtubule-associated proteins, motors, kinetochores, or membranes.     

Drosophila RhoGEF2 associates with microtubule plus ends in an EB1-dependent manner

Members of the Rho/Rac/Cdc42 superfamily of GTPases and their upstream activators, guanine nucleotide exchange factors (GEFs) , have emerged as key regulators of actin and microtubule dynamics. In their GTP bound form, these proteins interact with downstream effector molecules that alter actin and microtubule behavior. During Drosophila embryogenesis, a Galpha subunit (Concertina) and a Rho-type guanine nucleotide exchange factor (DRhoGEF2) have been implicated in the dramatic epithelial-cell shape changes that occur during gastrulation and morphogenesis . Using Drosophila S2 cells as a model system, we show that DRhoGEF2 induces contractile cell shape changes by stimulating myosin II via the Rho1 pathway. Unexpectedly, we found that DRhoGEF2 travels to the cell cortex on the tips of growing microtubules by interaction with the microtubule plus-end tracking protein EB1. The upstream activator Concertina, in its GTP but not GDP bound form, dissociates DRhoGEF2 from microtubule tips and also causes cellular contraction. We propose that DRhoGEF2 uses microtubule dynamics to search for cortical subdomains of receptor-mediated Galpha activation, which in turn causes localized actomyosin contraction associated with morphogenetic movements during development.