Microtubule polymerization dynamics

Regulation of microtubule assembly by human EB1 family proteins

The EB1 family proteins are highly conserved microtubule-associated proteins. The EB1 protein in yeast has been shown to play an important role in regulating microtubule dynamics and chromosome segregation. Human EB1 family proteins include EB1, RP1 and EBF3. Although EB1 and RP1 have been shown to associate with microtubules, the subcellular localization of endogenous EBF3 had not been characterized. The function of human EB1 family proteins was also not clear. We therefore investigated the cellular localization of EBF3 and the regulation of microtubule organization by EB1 family proteins. As do EB1 and RP1, EBF3 was found to colocalize with microtubules, preferentially at their plus ends, throughout the cell cycle. Moreover, there was a very strong EBF3 signal at the centrosome in interphase cells and at the spindle poles in mitotic cells. When EB1 family proteins were overexpressed, they associated with the entire microtubule cytoskeleton. In addition, EB1 and EBF3 induced microtubule bundling in some cells overexpressing these proteins. These microtubule bundles were more resistant to nocodazole and were more acetylated than regular microtubules. Our results demonstrate for the first time that human EB1 family proteins could regulate microtubule assembly and stability.

A mechanism for nuclear positioning in fission yeast based on microtubule pushing

The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

Association of bovine papillomavirus type 1 with microtubules

Transport of BPV-1 virus from the cell membrane to the nucleus was studied in vitro in CV-1 cells. At reduced temperature (4 degrees C), BPV-1 binding to CV-1 cells was unaffected but there was no transport of virions across the cytosol. Electron microscopy showed BPV-1 virions in association with microtubules in the cytoplasm, a finding confirmed by co-immunoprecipitation of L1 protein and tubulin. Internalization of virus was unimpaired in cells treated with the microtubule-depolymerizing drug nocodazole but virions were retained in cytoplasmic vesicles and not transported to the nucleus. We conclude that a microtubule transport mechanism in CV-1 cells moves intact BPV-1 virions from the cell surface to the nuclear membrane.

Regulation of Op18 during spindle assembly in Xenopus egg extracts

Oncoprotein 18 (Op18) is a microtubule-destabilizing protein that is negatively regulated by phosphorylation. To evaluate the role of the three Op18 phosphorylation sites in Xenopus (Ser 16, 25, and 39), we added wild-type Op18, a nonphosphorylatable triple Ser to Ala mutant (Op18-AAA), and to mimic phosphorylation, a triple Ser to Glu mutant (Op18-EEE) to egg extracts and monitored spindle assembly. Op18-AAA dramatically decreased microtubule length and density, while Op18-EEE did not significantly affect spindle microtubules. Affinity chromatography with these proteins revealed that the microtubule-destabilizing activity correlated with the ability of Op18 to bind tubulin. Since hyperphosphorylation of Op18 is observed upon addition of mitotic chromatin to extracts, we reasoned that chromatin-associated proteins might play a role in Op18 regulation. We have performed a preliminary characterization of the chromatin proteins recruited to DNA beads, and identified the Xenopus polo-like kinase Plx1 as a chromatin-associated kinase that regulates Op18 phosphorylation. Depletion of Plx1 inhibits chromatin-induced Op18 hyperphosphorylation and spindle assembly in extracts. Therefore, Plx1 may promote microtubule stabilization and spindle assembly by inhibiting Op18.

Type I protein kinase a is localized to interphase microtubules and strongly associated with the mitotic spindle

We show here that type I protein kinase A is localized to microtubules during the entire cell cycle in epithelial (hepatoma, cervical carcinoma) and nonepithelial (myoblast) cell lines. The association of the type Ialpha regulatory subunit is very strong in all phases of mitosis, from prophase to cytokinesis. In interphase, the association appears weaker, reflecting perhaps a more dynamic molecular interaction. This regulatory subunit appears to recruit catalytic subunits as the latter are also associated with microtubules. BW1J hepatoma cells, stably transfected with either wild-type or mutant Ialpha regulatory subunit, are enriched in aberrant mitoses with multipolar spindles and in mono- or multinucleated giant cells. This suggests that type I protein kinase A could have a role in centrosome duplication and/or segregation, sister chromatid separation, or cytokinesis.

Cell cycle-dependent changes in microtubule dynamics in living cells expressing green fluorescent protein-alpha tubulin

LLCPK-1 cells were transfected with a green fluorescent protein (GFP)-alpha tubulin construct and a cell line permanently expressing GFP-alpha tubulin was established (LLCPK-1alpha). The mitotic index and doubling time for LLCPK-1alpha were not significantly different from parental cells. Quantitative immunoblotting showed that 17% of the tubulin in LLCPK-1alpha cells was GFP-tubulin; the level of unlabeled tubulin was reduced to 82% of that in parental cells. The parameters of microtubule dynamic instability were compared for interphase LLCPK-1alpha and parental cells injected with rhodamine-labeled tubulin. Dynamic instability was very similar in the two cases, demonstrating that LLCPK-1alpha cells are a useful tool for analysis of microtubule dynamics throughout the cell cycle. Comparison of astral microtubule behavior in mitosis with microtubule behavior in interphase demonstrated that the frequency of catastrophe increased twofold and that the frequency of rescue decreased nearly fourfold in mitotic compared with interphase cells. The percentage of time that microtubules spent in an attenuated state, or pause, was also dramatically reduced, from 73.5% in interphase to 11.4% in mitosis. The rates of microtubule elongation and rapid shortening were not changed; overall dynamicity increased 3.6-fold in mitosis. Microtubule release from the centrosome and a subset of differentially stable astral microtubules were also observed. The results provide the first quantitative measurements of mitotic microtubule dynamics in mammalian cells.

Clasps are CLIP-115 and -170 associating proteins involved in the regional regulation of microtubule dynamics in motile fibroblasts

CLIP-170 and CLIP-115 are cytoplasmic linker proteins that associate specifically with the ends of growing microtubules and may act as anti-catastrophe factors. Here, we have isolated two CLIP-associated proteins (CLASPs), which are homologous to the Drosophila Orbit/Mast microtubule-associated protein. CLASPs bind CLIPs and microtubules, colocalize with the CLIPs at microtubule distal ends, and have microtubule-stabilizing effects in transfected cells. After serum induction, CLASPs relocalize to distal segments of microtubules at the leading edge of motile fibroblasts. We provide evidence that this asymmetric CLASP distribution is mediated by PI3-kinase and GSK-3 beta. Antibody injections suggest that CLASP2 is required for the orientation of stabilized microtubules toward the leading edge. We propose that CLASPs are involved in the local regulation of microtubule dynamics in response to positional cues.

Novel G proteins, Rag C and Rag D, interact with GTP-binding proteins, Rag A and Rag B

Rag A/Gtr1p are G proteins and are known to be involved in the RCC1-Ran pathway. We employed the two-hybrid method using Rag A as the bait to identify proteins binding to Rag A, and we isolated two novel human G proteins, Rag C and Rag D. Rag C demonstrates homology with Rag D (81.1% identity) and with Gtr2p of Saccharomyces cerevisiae (46.1% identity), and it belongs to the Rag A subfamily of the Ras family. Rag C and Rag D contain conserved GTP-binding motifs (PM-1, -2, and -3) in their N-terminal regions. Recombinant glutathione S-transferase fusion protein of Rag C efficiently bound to both [(3)H]GTP and [(3)H]GDP. Rag A was associated with both Rag C and Rag D in their C-terminal regions where a potential leucine zipper motif and a coiled-coil structure were found. Rag C and D were associated with both the GDP and GTP forms of Rag A. Both Rag C and Rag D changed their subcellular localization, depending on the nucleotide-bound state of Rag A. In a similar way, the disruption of S. cerevisiae GTR1 resulted in a change in the localization of Gtr2p.

Protein kinase C activation promotes microtubule advance in neuronal growth cones by increasing average microtubule growth lifetimes

We describe a novel mechanism for protein kinase C regulation of axonal microtubule invasion of growth cones. Activation of PKC by phorbol esters resulted in a rapid, robust advance of distal microtubules (MTs) into the F-actin rich peripheral domain of growth cones, where they are normally excluded. In contrast, inhibition of PKC activity by bisindolylmaleimide and related compounds had no perceptible effect on growth cone motility, but completely blocked phorbol ester effects. Significantly, MT advance occurred despite continued retrograde F-actin flow-a process that normally inhibits MT advance. Polymer assembly was necessary for PKC-mediated MT advance since it was highly sensitive to a range of antagonists at concentrations that specifically interfere with microtubule dynamics. Biochemical evidence is presented that PKC activation promotes formation of a highly dynamic MT pool. Direct assessment of microtubule dynamics and translocation using the fluorescent speckle microscopy microtubule marking technique indicates PKC activation results in a nearly twofold increase in the typical lifetime of a MT growth episode, accompanied by a 1.7-fold increase and twofold decrease in rescue and catastrophe frequencies, respectively. No significant effects on instantaneous microtubule growth, shortening, or sliding rates (in either anterograde or retrograde directions) were observed. MTs also spent a greater percentage of time undergoing retrograde transport after PKC activation, despite overall MT advance. These results suggest that regulation of MT assembly by PKC may be an important factor in determining neurite outgrowth and regrowth rates and may play a role in other cellular processes dependent on directed MT advance.

Ran stimulates spindle assembly by altering microtubule dynamics and the balance of motor activities

The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindles. However, it is not known what aspects of the microtubule cytoskeleton are subject to regulation by Ran in mitosis. Here we show that Ran-GTP stimulates microtubule assembly by increasing the rescue frequency of microtubules three- to eightfold. In addition to changing microtubule dynamics, Ran-GTP also alters the balance of motor activities, partly as a result of an increase in the amount of motile Eg5, a plus-end-directed microtubule motor that is essential for spindle formation. Thus, Ran regulates multiple processes that are involved in spindle assembly.

Effects of stathmin inhibition on the mitotic spindle

Stathmin is a major cytosolic phosphoprotein that plays an important role in the regulation of microtubule dynamics during cell cycle progression. It has recently been proposed that the major function of stathmin is to promote depolymerization of the microtubules that make up the mitotic spindle. In this report, we tested the prediction that a deficiency in stathmin expression would result in constitutive stabilization of microtubules and lead to abnormalities in the organization of the mitotic spindle. Our studies demonstrate that antisense inhibition of stathmin expression in K562 erythroleukemic cells results in increased ratio of polymerized to depolymerized tubulin. These changes are associated with phenotypic abnormalities of the mitotic spindle and difficulty in completing mitosis. These studies also showed that inhibition of stathmin expression results in increased susceptibility of K562 leukemic cells to the pharmacological agents, like taxol, which are known to stabilize the mitotic spindle. In contrast, stathmin inhibition results in decreased sensitivity to vinblastine, an agent that destabilizes the mitotic spindle. Thus, our experimental findings are supportive of the model that stathmin is a microtubule-destabilizing factor that plays an important role in the regulation of the mitotic spindle. We also suggest a potential therapeutic approach for cancer based on the combination of stathmin inhibition with pharmacologic agents that stabilize the mitotic spindle.

Microtubule depolymerization facilitates contraction of vascular smooth muscle via increased activation of RhoA/Rho-kinase

The microtubule network is in a dynamic equilibrium between free and polymerized tubulin, with depolymerization resulting in increased cellular contractility (1-4). Originally, microtubule depolymerization was thought to facilitate contractile responses via the release of an internal, mechanical opposition to contraction. However, recent evidence suggests that depolymerization may also lead to the enhanced activity of various intracellular signaling proteins. The precise signaling pathway by which microtubule depolymerization facilitates vascular smooth muscle contraction is unknown. In non-vascular cells, depolymerization initiates stress fiber formation via increased activity of the small G-protein, RhoA (5-7). The role of this signaling candidate in a calcium-sensitizing contractile pathway is well established. We and others have found it tempting to speculate that RhoA mediates a contractile pathway enhanced by microtubule depolymerization. We further hypothesize the involvement of microtubule depolymerization (via RhoA and Rho-kinase) in the regulation of vascular smooth muscle contraction, with evidence of potential augmentations of this pathway contributing to the increased vasoconstrictor sensitivity seen in various hypertensive animal models.

Structural insights into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of /¿-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

DCAMKL1 encodes a protein kinase with homology to doublecortin that regulates microtubule polymerization

Doublecortin (DCX) is a microtubule-associated protein required for neuronal migration to the cerebral cortex. DCAMKL1 consists of an N terminus that is 65% similar to DCX throughout the entire length of DCX, but also contains an additional 360 amino acid C-terminal domain encoding a putative Ca(2+)/calmodulin-dependent protein kinase. The homology to DCX suggested that DCAMKL1 may regulate microtubules, as well as mediate a phosphorylation-dependent signal transduction pathway. Here we show that DCAMKL1 is expressed throughout the CNS and PNS in migrating neuronal populations and overlaps in its expression with DCX and microtubules. Purified DCAMKL1 associates with microtubules and stimulates polymerization of purified tubulin and the formation of aster-like microtubule structures. Overexpressed DCAMKL1 leads to striking microtubule bundling in cell lines and cultured primary neural cells. Time-lapse imaging of cells transfected with a DCAMKL1-green fluorescent protein fusion protein shows that the microtubules associated with the protein remain dynamic. DCAMKL1 also encodes a functional kinase capable of phosphorylating myelin basic protein and itself. However, elimination of the kinase activity of DCAMKL1 has no detectable effect on its microtubule polymerization activity. Because DCAMKL1 is coexpressed with DCX, the two proteins form a potentially mutually regulatory network linking calcium signaling and microtubule dynamics.

Microtubules don and doff their caps: dynamic attachments at plus and minus ends

Microtubule plus end dynamics, as well as interactions with the cell cortex and internal organelles, may be mediated by a 'plus end complex' of interacting proteins. Recent results suggest that centrosomes with different microtubule-releasing and anchoring properties underlie the development of various microtubule arrays.

Altered patterns of tubulin polymerization in dividing leaf cells of Chlorophyton comosum after a hyperosmotic treatment

•  Microtubule organization and tubulin polymerization in meristematic leaf cells of Chlorophyton comosum treated with an aqueous solution of 1 M mannitol, inducing plasmolysis, were examined with immunofluorescence and transmission electron microscopy. •  Hyperosmotic treatment induced disintegration of the interphase microtubule systems. Free tubulin, either liberated from the depolymerized microtubules or pre-existing as a nonassembled pool, was incorporated into a network of paracrystals. In most of the dividing cells, mitotic and cytokinetic microtubule systems were replaced by atypical spindle-like structures displaying bipolarity and atypical phragmoplasts, respectively. These atypical mitotic and cytokinetic structures consisted of large densely packed bundles of macrotubules (32 nm diameter) or macrotubules and paracrystals. Tubulin paracrystals also occurred in ectopic positions in plasmolysed mitotic and cytokinetic cells. Dividing cells displaying paracrystals only did not form atypical mitotic and cytokinetic apparatuses. •  Short hyperosmotic stress causes disintegration of all microtubule arrays in dividing cells of C. comosum. Free tubulin is incorporated into macrotubules and tubulin paracrystals. The latter exhibit definite periodicity and characteristic fine structure.

Microtubules in the fungal pathogen Ustilago maydis are highly dynamic and determine cell polarity

Many fungal pathogens undergo a yeast-hyphal transition during their pathogenic development that requires rearrangement of the cytoskeleton, followed by directed membrane traffic towards the growth region. The role of microtubules and their dynamic behavior during this process is not well understood. Here we set out to elucidate the organization, cellular role and in vivo dynamics of microtubules in the dimorphic phytopathogen Ustilago maydis. Hyphae and unbudded yeast-like cells of U. maydis contain bundles of spindle pole body-independent microtubules. At the onset of bud formation two spherical tubulin structures focus microtubules towards the growth region, suggesting that they support polar growth in G(2), while spindle pole body-nucleated astral microtubules participate in nuclear migration in M and early G(1). Conditional mutants of an essential alpha-tubulin gene from U. maydis, tub1, confirmed a role for interphase microtubules in determination of cell polarity and growth. Observation of GFP-Tub1 fusion protein revealed that spindle pole body-independent and astral microtubules are dynamic, with elongation and shrinkage rates comparable to those found in vertebrate systems. In addition, very fast depolymerization was measured within microtubule bundles. Unexpectedly, interphase microtubules underwent bending and rapid translocations within the cell, suggesting that unknown motor activities participate in microtubule organization in U. maydis. Movies available on-line: http://www.biologists.com/JCS/movies/jcs1792.html

Individual expression of recombinant alpha- and beta-tubulin from Haemonchus contortus: polymerization and drug effects

Three tubulin isotypes from the parasitic nematode Haemonchus contortus were individually expressed in Escherichia coli, purified, and induced to polymerize into microtubules in the absence of microtubule-associated proteins. The effect of different conditions on the rate of polymerization of pure tubulin was assessed. This is the first time that recombinant alpha-tubulin has been shown to be capable of polymerization into microtubule-like structures when incubated with recombinant beta-tubulin. In addition, the present study has shown that: (1) microtubule-associated proteins are not required for tubulin polymerization; and (2) pure beta-tubulin isotype, beta12-16, alone was capable of forming microtubule-like structures in the absence of alpha-tubulin. Polymerization of the recombinant invertebrate tubulin, as measured by a spectrophotometric assay, was found to be enhanced by a concentration of tubulin >0.25 mg/mL; temperature > or =20 degrees C; 2 mM GTP; glycerol; EGTA; and Mg(2+). Polymerization was inhibited by GTP (>2 mM) and albendazole. Calcium ions and a pH range of 6 to 8.5 had no measurable effect on polymerization. Individual isotypes of tubulin polymerized to approximately the same extent as an alpha-/beta-tubulin mixture. Samples of tubulin assembled under the above conditions for 60 min were also examined under a transmission electron microscope. Although the spectrophotometric assay indicated polymerization, it did not predict the structure of the polymer. In many cases tubulin sheets, folded sheets, and rings were observed in addition to, or instead of, microtubule-like structures.

Search, capture and signal: games microtubules and centrosomes play

Accurate distribution of the chromosomes in dividing cells requires coupling of cellular polarity cues with both the orientation of the mitotic spindle and cell cycle progression. Work in budding yeast has demonstrated that cytoplasmic dynein and the kinesin Kip3p define redundant pathways that ensure proper spindle orientation. Furthermore, it has been shown that the Kip3p pathway components Kar9p and Bim1p (Yeb1p) form a complex that provides a molecular link between cortical polarity cues and spindle microtubules. Recently, other studies indicated that the cortical localization of Kar9p depends upon actin cables and Myo2p, a type V myosin. In addition, a BUB2-dependent cell cycle checkpoint has been described that inhibits the mitotic exit network and cytokinesis until proper centrosome position is achieved. Combined, these studies provide molecular insight into how cells link cellular polarity, spindle position and cell cycle progression.

Importin beta is a mitotic target of the small GTPase Ran in spindle assembly

The GTPase Ran has recently been shown to stimulate microtubule polymerization in mitotic extracts, but its mode of action is not understood. Here we show that the mitotic role of Ran is largely mediated by the nuclear transport factor importin beta. Importin beta inhibits spindle formation in vitro and in vivo and sequesters an aster promoting activity (APA) that consists of multiple, independent factors. One component of APA is the microtubule-associated protein NuMA. NuMA and other APA components are discharged from importin beta by RanGTP and induce spindle-like structures in the absence of centrosomes, chromatin, or Ran. We propose that RanGTP functions in mitosis as in interphase by locally releasing cargoes from transport factors. In mitosis, this promotes spindle assembly by organizing microtubules in the vicinity of chromosomes.

The spindle: a dynamic assembly of microtubules and motors

In all eukaryotes, a microtubule-based structure known as the spindle is responsible for accurate chromosome segregation during cell division. Spindle assembly and function require localized regulation of microtubule dynamics and the activity of a variety of microtubule-based motor proteins. Recent work has begun to uncover the molecular mechanisms that underpin this process. Here we describe the structural and dynamic properties of the spindle, and introduce the current concepts regarding how a bipolar spindle is assembled and how it functions to segregate chromosomes.

Force-velocity relation for growing microtubules

Forces generated by microtubule polymerization and depolymerization are important for the biological functioning of cells. The mean growth velocity, V, under an opposing force, F, has been measured by; Science 278:856-860) for single microtubules growing in vitro, but their analysis of the data suggested that V decreased more rapidly with F than equilibrium (or "thermodynamic") theory predicted and entailed negative values for the dissociation rate and undefined (or unreasonable) values for the stall force, at which V vanishes. By contrast, considering the mean work done against the external load and allowing for load-distribution factors for the "on" and "off" rates, we find good agreement with a simple theory that yields a plausible stalling force. Although specific numerical results are sensitive to choice of fitting criteria, about 80% of the variation with load is carried by the "off" (or dissociation) rate, but, since that is small (in accordance with independent observations), the dominant force dependence comes from the "on" rate, which is associated with a displacement length, d(1), significantly longer than d(0) approximately 1/13(8.2 nm), the mean length increase per added tubulin dimer. Measuring the dispersion in length of the growing microtubules could provide a check. The theory implies that the stationary stall state (at V = 0) is not one of simple associative thermal equilibrium, as previously supposed; rather, it appears to be dissipative and kinetically controlled.

The catastrophe-promoting activity of ectopic Op18/stathmin is required for disruption of mitotic spindles but not interphase microtubules

Oncoprotein18/stathmin (Op18) is a microtubule (MT) destabilizing protein that is inactivated during mitosis by phosphorylation at four Ser-residues. Op18 has at least two functions; the N-terminal region is required for catastrophe-promotion (i.e., transition from elongation to shortening), while the C-terminal region is required to inhibit MT-polymerization rate in vitro. We show here that a "pseudophosphorylation" derivative of Op18 (i.e., four Ser- to Glu-substitutions at phosphorylation sites) exhibits a selective loss of catastrophe-promoting activity. This is contrasted to authentic phosphorylation, which efficiently attenuates all activities except tubulin binding. In intact cells, overexpression of pseudophosphorylated Op18, which is not phosphorylated by endogenous kinases, is shown to destabilize interphase MTs but to leave spindle formation untouched. To test if the mitotic spindle is sensitive only to the catastrophe-promoting activity of Op18 and resistant to C-terminally associated activities, N- and C-terminal truncations with defined activity-profiles were employed. The cell-cycle phenotypes of nonphosphorylatable mutants (i.e., four Ser- to Ala-substitutions) of these truncation derivatives demonstrated that catastrophe promotion is required for interference with the mitotic spindle, while the C-terminally associated activities are sufficient to destabilize interphase MTs. These results demonstrate that specific Op18 derivatives with defined activity-profiles can be used as probes to distinguish interphase and mitotic MTs.

The role of Xgrip210 in gamma-tubulin ring complex assembly and centrosome recruitment

The gamma-tubulin ring complex (gammaTuRC), purified from the cytoplasm of vertebrate and invertebrate cells, is a microtubule nucleator in vitro. Structural studies have shown that gammaTuRC is a structure shaped like a lock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the gammaTuRC. Consequently, the cap structure of the gammaTuRC is distal to the base of the microtubules, giving the end of the microtubule the shape of a pointed cap. Here, we report the cloning and characterization of a new subunit of Xenopus gammaTuRC, Xgrip210. We show that Xgrip210 is a conserved centrosomal protein that is essential for the formation of gammaTuRC. Using immunogold labeling, we found that Xgrip210 is localized to the ends of microtubules nucleated by the gammaTuRC and that its localization is more distal, toward the tip of the gammaTuRC-cap structure, than that of gamma-tubulin. Immunodepletion of Xgrip210 blocks not only the assembly of the gammaTuRC, but also the recruitment of gamma-tubulin and its interacting protein, Xgrip109, to the centrosome. These results suggest that Xgrip210 is a component of the gammaTuRC cap structure that is required for the assembly of the gammaTuRC.

The TACC domain identifies a family of centrosomal proteins that can interact with microtubules

We recently showed that the Drosophila transforming acidic coiled-coil (D-TACC) protein is located in the centrosome, interacts with microtubules, and is required for mitosis in the Drosophila embryo. There are three known human TACC proteins that share a conserved, C-terminal, coiled-coil region with D-TACC. These proteins have all been implicated in cancer, but their normal functions are unknown. We show that all three human TACC proteins are concentrated at centrosomes, but with very different characteristics: TACC1 is weakly concentrated at centrosomes during mitosis; TACC2 is strongly concentrated at centrosomes throughout the cell cycle; and TACC3 is strongly concentrated in a more diffuse region around centrosomes during mitosis. When the C-terminal TACC domain is overexpressed in HeLa cells, it forms large polymers in the cytoplasm that can interact with both microtubules and tubulin. The full-length TACC proteins form similar polymers when overexpressed, but their interaction with microtubules and tubulin is regulated during the cell cycle. At least one of the human TACC proteins appears to increase the number and/or stability of centrosomal microtubules when overexpressed during mitosis. Thus, the TACC domain identifies a family of centrosomal proteins that can interact with microtubules. This may explain the link between the TACC genes and cancer.

Changes in cytoskeletal organization in polyoma middle T antigen-transformed fibroblasts: involvement of protein phosphatase 2A and src tyrosine kinases

The major transforming activity of polyomavirus, middle T antigen, targets several cellular regulatory effectors including protein phosphatase 2A and src tyrosine kinases. Although transformed cells exhibit profound morphological changes, little is known about how middle T antigen-induced changes in the cellular regulatory environment specifically affect the cytoskeleton. We have investigated these changes in 10T(1/2) mouse fibroblasts transformed with polyoma middle T antigen. Immunofluorescence microscopy revealed that expression of middle T antigen (Pym T cells) depleted the stable (acetylated) microtubule array and increased the sensitivity of dynamic (tyrosinated) microtubules to nocodazole-induced disassembly. These effects were associated with a modest but statistically significant (P</=0.05) increase in recovery of protein phosphatase 2A activity with microtubules. Middle T antigen expression also depleted the normal cellular complement of actin stress fibers and focal adhesions, in parallel with changes in the distribution of src tyrosine kinases. Herbimycin A promoted recovery of paxillin and phosphotyrosine into nascent focal adhesion sites, in addition to restoring normal src tyrosine kinase distribution. However, herbimycin A did not restore actin stress fibers or parental-type microtubules to Pym T cells. We suggest that regulation of the microtubule array by middle T antigen may occur through direct effects including redistribution of protein phosphatase 2A as well as indirect effects such as altered interactions with actin-based stress fibers.

Multiple isoforms of the high molecular weight microtubule associated protein XMAP215 are expressed during development in Xenopus

We have cloned and sequenced cDNAs encoding two isoforms of XMAP215, a high molecular weight microtubule-associated protein identified in Xenopus eggs. XMAP215 is approximately 80% identical in amino acid sequence to the product of ch-TOG, a cDNA that is over expressed in certain human tumors [Charrasse et al., 1995: Eur J Biochem 234:406-413]. Northern and Western blots demonstrated that XMAP215 is expressed throughout development, from oogenesis to tadpole. We identified two XMAP215 transcripts differing only in the presence of a 108-bp sequence encoding a 36 amino acid insert. RT-PCR revealed that the transcripts encoding these two isoforms are expressed at distinct times during development: a transcript containing the insert (encoding XMAP215(M)) is expressed during oogenesis and is present through gastrulation. The second transcript (encoding XMAP215(Z)) lacks the 108-bp insert and is expressed from gastrulation onward. In situ hybridization demonstrated that XMAP215 transcripts are localized to the ectoderm of early embryos and in the developing nervous system during later development. These results suggest that XMAP215 plays important roles in at least two phases of development: (1) regulating the assembly of MTs during the rapid cell divisions after fertilization, and (2) regulating MT assembly during the development of the nervous system.

Microtubule dynamics: the view from the tip

Recent studies have suggested that proteins found at the tips of microtubules in vertebrate cells may play an important role in intracellular membrane transport processes. Evidence from fission yeast indicates that such proteins can also regulate microtubule dynamics.

Mutational analysis of op18/stathmin-tubulin-interacting surfaces. Binding cooperativity controls tubulin GTP hydrolysis in the ternary complex

Oncoprotein 18 (Op18) is a microtubule regulator that forms a ternary complex with two tubulin heterodimers. Dispersed regions of Op18 are involved in two-site cooperative binding and subsequent modulation of tubulin GTPase activity. Here we have analyzed specific determinants of Op18 that govern both stoichiometry and positive cooperativity in tubulin binding and consequent stimulatory and inhibitory effects on tubulin GTPase activity. The data revealed that the central and C-terminal regions of Op18 contain overlapping binding-motifs contacting both tubulin heterodimers, suggesting that these regions of Op18 are wedged into the previously noted 1-nm gap between the two longitudinally arranged tubulin heterodimers. Both the N- and C-terminal flanks adjacent to the central region are involved in stabilizing the ternary complex, but only the C-terminal flank does so by imposing positive binding cooperativity. Within the C-terminal flank, deletion of a 7-amino acid region attenuated positive binding cooperativity and resulted in a switch from stimulation to inhibition of tubulin GTP hydrolysis. This switch can be explained by attenuated binding cooperativity, because Op18 under these conditions may block longitudinal contact surfaces of single tubulins with consequent interference of tubulin-tubulin interaction-dependent GTP hydrolysis. Together, our results suggest that Op18 links two tubulin heterodimers via longitudinal contact surfaces to form a ternary GTPase productive complex.

Novel signaling pathways contributing to vascular changes in hypertension

In hypertension, increased peripheral resistance maintains elevated levels of arterial blood pressure. The increase in peripheral resistance results, in part, from abnormal constrictor and dilator responses and vascular remodeling. In this review, we consider four cellular signaling pathways as possible explanations for these abnormal vascular responses: (1) augmented signaling via the epidermal growth factor receptor to cause remodeling of the cerebrovasculature; (2) reduced sphingolipid signaling leading to blunted vasodilation and increased smooth muscle proliferation; (3) increased signaling via Rho/Rho kinase leading to enhanced vasoconstriction, and (4) a relative state of microtubular depolymerization favoring vasoconstriction in hypertension. These novel cell signaling pathways provide new pharmacological targets to reduce total peripheral vascular resistance in hypertension.

The new architectonics: an invitation to structural biology

The philosophy of art might offer an epistemological basis for talking about the complexity of biological molecules in a meaningful way. The analysis of artistic compositions requires the resolution of intrinsic tensions between disparate sensory categories-color, line and form-not unlike those encountered in looking at the surfaces of protein molecules, where charge, polarity, hydrophobicity, and shape compete for our attentions. Complex living systems exhibit behaviors such as contraction waves moving along muscle fibers, or shivers passing through the growth cones of migrating neurons, that are easy to describe with common words, but difficult to explain in terms of the language of chemistry. The problem follows from a lack of everyday experience with processes that move towards equilibrium by switching between crystalline order and chain-like disorder, a commonplace occurrence in the submicroscopic world of proteins. Since most of what is understood about protein function comes from studies of isolated macromolecules in solution, a serious gap exists between what we know and what we would like to know about organized biological systems. Closing this gap can be achieved by recognizing that protein molecules reside in gradients of Gibbs free energy, where local forces and movements can be large compared with Brownian motion. Architectonics, a term borrowed from the philosophical literature, symbolizes the eventual union of the structure of theories-how our minds construct the world-with the theory of structures-or how stability is maintained in the chaotic world of microsystems.

Microtubule binding of the drosophila DMAP-85 protein is regulated by phosphorylation in vitro

The phosphorylation of microtubule-associated proteins (MAPs) is thought to be a key factor in the regulation of microtubule (MT) stability. Previously we isolated DMAP-85, a Drosophila MAP shown to be associated with stable MTs. In this work we show that DMAP-85 phosphorylated in cell-free early embryo extracts is released from MTs. MPM-2 antibodies recognize the phosphorylated protein. In vitro, DMAP-85 can be phosphorylated by the mitotic kinase Polo affecting its binding to MTs and creating MPM-2 epitopes on the protein. The results suggest that phosphorylation of DMAP-85 might affect its MT stabilizing activity during early mitotic cycles.

Tea2p is a kinesin-like protein required to generate polarized growth in fission yeast

Cytoplasmic microtubules are critical for establishing and maintaining cell shape and polarity. Our investigations of kinesin-like proteins (klps) and morphological mutants in the fission yeast Schizosaccharomyces pombe have identified a kinesin-like gene, tea2(+), that is required for cells to generate proper polarized growth. Cells deleted for this gene are often bent during exponential growth and initiate growth from improper sites as they exit stationary phase. They have a reduced cytoplasmic microtubule network and display severe morphological defects in genetic backgrounds that produce long cells. The tip-specific marker, Tea1p, is mislocalized in both tea2-1 and tea2Delta cells, indicating that Tea2p function is necessary for proper localization of Tea1p. Tea2p is localized to the tips of the cell and in a punctate pattern within the cell, often coincident with the ends of cytoplasmic microtubules. These results suggest that this kinesin promotes microtubule growth, possibly through interactions with the microtubule end, and that it is important for establishing and maintaining polarized growth along the long axis of the cell.

Mechanisms of viral transport in the cytoplasm

Analogous to the spread of viruses within the host animal during pathogenesis, from their site of entry to distant sites via the bloodstream, lymphatic system and nervous system, there is also movement within infected cells. As cytoplasmic diffusion only operates within very small volumes, active membrane traffic and cytosolic transport of viral genome-protein complexes are required, which involve both the actin and microtubule cytoskeleton.

The fusome organizes the microtubule network during oocyte differentiation in Drosophila

Differentiation of the Drosophila oocyte takes place in a cyst of 16 interconnected germ cells and is dependent on a network of microtubules that becomes polarized as differentiation progresses (polarization). We have investigated how the microtubule network polarizes using a GFP-tubulin construct that allows germ-cell microtubules to be visualized with greater sensitivity than in previous studies. Unexpectedly, microtubules are seen to associate with the fusome, an asymmetric germline-specific organelle, which elaborates as cysts form and undergoes complex changes during cyst polarization. This fusome-microtubule association occurs periodically during late interphases of cyst divisions and then continuously in 16-cell cysts that have entered meiotic prophase. As meiotic cysts move through the germarium, microtubule minus ends progressively focus towards the center of the fusome, as visualized using a NOD-lacZ marker. During this same period, discrete foci rich in gamma tubulin that very probably correspond to migrating cystocyte centrosomes also associate with the fusome, first on the fusome arms and then in its center, subsequently moving into the differentiating oocyte. The fusome is required for this complex process, because microtubule network organization and polarization are disrupted in hts(1) mutant cysts, which lack fusomes. Our results suggest that the fusome, a specialized membrane-skeletal structure, which arises in early germ cells, plays a crucial role in polarizing 16-cell cysts, at least in part by interacting with microtubules and centrosomes.

Microtubule organization and the effects of GFP-tubulin expression in dictyostelium discoideum

We have labeled microtubules in living Dictyostelium amoebae by incorporation of a GFP-alpha-tubulin fusion protein. The GFP-alpha-tubulin incorporates into microtubules and, as reported by others [Neujahr et al., 1998], the labeled microtubules are highly motile. Electron microscopy (EM) analysis of the distribution and organization of microtubules in the amoebae shows that some cytoplasmic microtubules form close associations. These associations could allow motor proteins attached to one microtubule to walk along an adjacent microtubule and thus generate some of the observed motility. Protein blot analysis indicates that the GFP-alpha-tubulin incorporates into microtubules at a lower efficiency than does the endogenous alpha-tubulin. EM and immunofluorescence (IF) analyses suggest that the GFP-alpha-tubulin interferes with microtubule nucleation. We have also observed an increased sensitivity of the GFP-alpha-tubulin expressing cells to blue light, as compared to wild-type cells. These results suggest that although GFP-alpha-tubulin can be used as a marker for microtubules in living cells, the use of this marker is not recommended for certain types of studies such as assembly dynamics.

CLIP170-like tip1p spatially organizes microtubular dynamics in fission yeast

Rod-shaped fission yeast cells grow in a polarized manner, and unlike budding yeast, the correct positioning of the growth sites at cell ends requires interphase microtubules. Here we describe a microtubule guidance mechanism that orients microtubules in the intracellular space along the long axis of the cell, guiding them to their target region at the cell ends. This mechanism involves tip1p, a CLIP170-like protein that localizes to distal tips of cytoplasmic microtubules. In the absence of tip1p, microtubular catastrophe is no longer restricted to cell ends but occurs when microtubules reach any region of the cellular cortex. Thus, tip1p enables microtubules to discriminate different cortical regions and regulates their dynamics accordingly.

Surface topography of microtubule walls decorated with monomeric and dimeric kinesin constructs

The surface topography of opened-up microtubule walls (sheets) decorated with monomeric and dimeric kinesin motor domains was investigated by freeze-drying and unidirectional metal shadowing. Electron microscopy of surface-shadowed specimens produces images with a high signal/noise ratio, which enable a direct observation of surface features below 2 nm detail. Here we investigate the inner and outer surface of microtubules and tubulin sheets with and without decoration by kinesin motor domains. Tubulin sheets are flattened walls of microtubules, keeping lateral protofilament contacts intact. Surface shadowing reveals the following features: (i) when the microtubule outside is exposed the surface relief is dominated by the bound motor domains. Monomeric motor constructs generate a strong 8 nm periodicity, corresponding to the binding of one motor domain per alpha-beta-tubulin heterodimer. This surface periodicity largely disappears when dimeric kinesin motor domains are used for decoration, even though it is still visible in negatively stained or frozen hydrated specimens. This could be explained by disorder in the binding of the second (loosely tethered) kinesin head, and/or disorder in the coiled-coil tail. (ii) Both surfaces of undecorated sheets or microtubules, as well as the inner surface of decorated sheets, reveal a strong 4 nm repeat (due to the periodicity of tubulin monomers) and a weak 8 nm repeat (due to slight differences between alpha- and beta-tubulin). The differences between alpha- and beta-tubulin on the inner surface are stronger than expected from cryo-electron microscopy of unstained microtubules, indicating the existence of tubulin subdomain-specific surface properties that reflect the surface corrugation and hence metal deposition during evaporation. The 16 nm periodicity visible in some negatively stained specimens (caused by the pairing of cooperatively bound kinesin dimers) is not detected by surface shadowing.

Helical tubes of FtsZ from Methanococcus jannaschii

Bacterial cell division depends on the formation of a cytokinetic ring structure, the Z-ring. The bacterial tubulin homologue FtsZ is required for Z-ring formation. FtsZ assembles into various polymeric forms in vitro, indicating a structural role in the septum of bacteria. We have used recombinant FtsZ1 protein from M. jannaschii to produce helical tubes and sheets with high yield using the GTP analogue GMPCPP [guanylyl-(alpha,beta)-methylene-diphosphate]. The sheets appear identical to the previously reported Ca++-induced sheets of FtsZ from M. jannaschii that were shown to consist of 'thick'-filaments in which two protofilaments run in parallel. Tubes assembled either in Ca++ or in GMPCPP contain filaments whose dimensions indicate that they could be equivalent to the 'thick'-filaments in sheets. Some tubes are hollow but others are filled by additional protein density. Helical FtsZ tubes differ from eukaryotic microtubules in that the filaments curve around the filament axis with a pitch of approximately 430 A for Ca++-induced tubes or 590 - 620 A for GMPCPP. However, their assembly in vitro as well-ordered polymers over distances comparable to the inner circumference of a bacterium may indicate a role in vivo. Their size and stability make them suitable for use in motility assays.

Novel combination chemotherapy in the treatment of non-small cell lung cancer

Treatment of patients with advanced non-small cell lung cancer (NSCLC) remains a vexing problem and long-term survival beyond 5 years is extremely rare. Five new agents, paclitaxel, docetaxel, vinorelbine, gemcitabine and irinotecan, have been introduced for the treatment of NSCLC and investigated extensively both preclinically and clinically. Monotherapy with one of these agents has produced survival benefits over the best supportive care in Phase III studies. Combination chemotherapy with a new agent and platinum produced a higher response rate than conventional cisplatin-based chemotherapy and improved survival was observed in some randomised trials. There was little difference in efficacy and toxicity between the chemotherapeutic regimens with a new agent and a platinum in Phase III trials, suggesting the clinical utility of these regimens is similar. Many trials have focused on regimens containing two new agents, with or without platinum. Preliminary results of Phase III trials of three drug combinations versus two drug combinations suggested the former to be more promising, in terms of response rates and survival. Whether the era of platinum-based chemotherapy in the treatment of NSCLC should continue or not must be determined by Phase III trials, evaluating the use of a platinum agent with one of the new agent combinations. These aggressive chemotherapeutic combinations will hopefully improve survival and quality of life for patients with advanced NSCLC.

TOR signaling regulates microtubule structure and function

The functional diversity and structural heterogeneity of microtubules are largely determined by microtubule-associated proteins (MAPs) [1] [2]. Bik1p (bilateral karyogamy defect protein) is one of the MAPs required for microtubule assembly, stability and function in cell processes such as karyogamy and nuclear migration and positioning in the yeast Saccharomyces cerevisiae [3]. The macrocyclic immunosuppressive antibiotic rapamycin, complexed with its binding protein FKBP12, binds to and inhibits the target of rapamycin protein (TOR) in yeast [4] [5]. We report here that TOR physically interacts with Bik1p, the yeast homolog of human CLIP-170/Restin [6] [7]. Inhibition of TOR by rapamycin significantly affects microtubule assembly, elongation and stability. This function of TOR is independent of new protein synthesis. Rapamycin also causes defects in spindle orientation, nuclear movement and positioning, karyogamy and chromosomal stability, defects also found in the bikDelta mutant. Our data suggest a role for TOR signaling in regulating microtubule stability and function, possibly through Bik1p.

The interaction of TOGp with microtubules and tubulin

TOGp is the human homolog of XMAP215, a Xenopus microtubule-associated protein that promotes rapid microtubule assembly at plus ends. These proteins are thought to be critical for microtubule assembly and/or mitotic spindle formation. To understand how TOGp interacts with the microtubule lattice, we cloned full-length TOGp and various truncations for expression in a reticulocyte lysate system. Based on microtubule co-pelleting assays, the microtubule binding domain is contained within a basic 600-amino acid region near the N terminus, with critical domains flanking a region homologous to the microtubule binding domain found in the related proteins Stu2p (S. cerevisiae) and Dis1 (S. pombe). Both full-length TOGp and the N-terminal fragment show enhanced binding to microtubule ends. Full-length TOGp also binds altered polymer lattice structures including parallel protofilament sheets, antiparallel protofilament sheets induced with zinc ions, and protofilament rings, suggesting that TOGp binds along the length of individual protofilaments. The C-terminal region of TOGp has a low affinity for microtubule polymer but binds tubulin dimer. We propose a model to explain the microtubule-stabilizing and/or assembly-promoting functions of the XMAP215/TOGp family of microtubule-associated proteins based on the binding properties we have identified.

Feedback interactions between cell-cell adherens junctions and cytoskeletal dynamics in newt lung epithelial cells

To test how cell-cell contacts regulate microtubule (MT) and actin cytoskeletal dynamics, we examined dynamics in cells that were contacted on all sides with neighboring cells in an epithelial cell sheet that was undergoing migration as a wound-healing response. Dynamics were recorded using time-lapse digital fluorescence microscopy of microinjected, labeled tubulin and actin. In fully contacted cells, most MT plus ends were quiescent; exhibiting only brief excursions of growth and shortening and spending 87.4% of their time in pause. This contrasts MTs in the lamella of migrating cells at the noncontacted leading edge of the sheet in which MTs exhibit dynamic instability. In the contacted rear and side edges of these migrating cells, a majority of MTs were also quiescent, indicating that cell-cell contacts may locally regulate MT dynamics. Using photoactivation of fluorescence techniques to mark MTs, we found that MTs in fully contacted cells did not undergo retrograde flow toward the cell center, such as occurs at the leading edge of motile cells. Time-lapse fluorescent speckle microscopy of fluorescently labeled actin in fully contacted cells revealed that actin did not flow rearward as occurs in the leading edge lamella of migrating cells. To determine if MTs were required for the maintenance of cell-cell contacts, cells were treated with nocodazole to inhibit MTs. After 1-2 h in either 10 microM or 100 nM nocodazole, breakage of cell-cell contacts occurred, indicating that MT growth is required for maintenance of cell-cell contacts. Analysis of fixed cells indicated that during nocodazole treatment, actin became reduced in adherens junctions, and junction proteins alpha- and beta-catenin were lost from adherens junctions as cell-cell contacts were broken. These results indicate that a MT plus end capping protein is regulated by cell-cell contact, and in turn, that MT growth regulates the maintenance of adherens junctions contacts in epithelia.

Dynamics of interphase microtubules in Schizosaccharomyces pombe

BACKGROUND

Microtubules in interphase Schizosaccharomyces pombe are essential for maintaining the linear growth habit of these cells. The dynamics of assembly and disassembly of these microtubules are so far uncharacterised.

RESULTS

Live cell confocal imaging of alpha1 tubulin tagged with enhanced green fluorescent protein revealed longitudinally oriented, dynamically unstable interphase microtubule assemblies (IMAs). The IMAs were uniformly bright along their length apart from a zone of approximately doubly intense fluorescence commonly present close to their centres. The ends of each IMA switched from growth ( approximately 3.0 microm/min) to shrinkage ( approximately 4.5 microm/min) at 1.0 events per minute and from shrinkage to growth at 1.9 events per minute, and the two ends were equivalently dynamic, suggesting equivalent structure. We accordingly propose a symmetrical model for microtubule packing within the IMAs, in which microtubules are plus ends out and overlap close to the equator of the cell. IMAs may contain multiple copies of this motif; if so, then within each IMA end, the microtubule ends must synchronise catastrophe and rescue. When both ends of an IMA lodge in the hemispherical cell ends, the IMAs start to bend under compression and their overall growth rate is inhibited about twofold. Similar microtubule dynamics were observed in cells ranging in size from half to twice normal length. Patterned photobleaching indicated no detectable treadmilling or microtubule sliding during interphase.

CONCLUSIONS

The consequence of the mechanisms described is continuous recruitment of microtubule ends to the ends of growing cells, supporting microtubule-based transport into the cell ends and qualitatively accounting for the essential role for microtubules in directing linear cell growth in S. pombe.

COBRA-1, a rationally-designed epoxy-THF containing compound with potent tubulin depolymerizing activity as a novel anticancer agent

A novel mono-THF containing synthetic anticancer drug, COBRA-1, was designed for targeting a previously unrecognized unique narrow binding cavity on the surface of alpha-tubulin. COBRA-1 inhibited GTP-induced tubulin polymerization in cell-free tubulin turbidity assays. Treatment of human breast cancer and brain tumor (glioblastoma) cells with COBRA-1 caused destruction of microtubule organization and apoptosis. Like other microtubule-interfering agents, COBRA-1 activated the proapoptotic c-Jun N-terminal kinase (JNK) signal transduction pathway, as evidenced by rapid induction of c-jun expression.