Microtubule polymerization dynamics

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.

Structure of the gamma-tubulin ring complex: a template for microtubule nucleation

The gamma-tubulin ring complex (gammaTuRC) is a protein complex of relative molecular mass approximately 2.2 x 10(6) that nucleates microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the structure of isolated Drosophila gammaTuRCs and the ends of microtubules nucleated by gammaTuRCs and by centrosomes. We show that the gammaTuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap. A similar capped ring is also visible at one end of microtubules grown from isolated gammaTuRCs and from centrosomes. Antibodies against gamma-tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a template-mediated mechanism for microtubule nucleation by the gammaTuRC.

Dictyostelium DdCP224 is a microtubule-associated protein and a permanent centrosomal resident involved in centrosome duplication

A cDNA encoding a 224-kDa Dictyostelium discoideum centrosomal protein (DdCP224) was isolated by immunoscreening. DdCP224 was detected at the centrosome and, more weakly, along microtubules throughout the entire cell cycle. Centrosomal localization does not require microtubules, suggesting that DdCP224 is a genuine centrosomal component. DdCP224 exhibits sequence identity to a weakly conserved class of microtubule-associated proteins including human TOGp and yeast Stu2p. Stu2p has a size of only approximately 100 kDa and corresponds to the N-terminal half of DdCP224. The functions of the N- and C-terminal halves of DdCP224 were investigated in the corresponding GFP-fusion mutants. Surprisingly, the N-terminal construct showed only cytosolic localization, whereas the C-terminal construct localized exclusively to the centrosome. This is unexpected because Stu2p is localized at the spindle pole body. Full-length DdCP224-GFP was present both at centrosomes and along microtubules. Furthermore, it bound to microtubules in vitro, unlike the two truncated mutants. Thus centrosome binding is determined by the C-terminal half and microtubule binding may require the interaction of the N- and C-terminal halves. Interestingly, cells expressing full-length DdCP224-GFP exhibit supernumerary centrosomes and show a cytokinesis defect, suggesting that DdCP224 plays an important role in centrosome duplication. These features are unique among the known centrosomal proteins.

Microtubules switch occasionally into unfavorable configurations during elongation

Tubulin assembles to form a range of structures that differ by their protofilament and monomer helix-start numbers. The microtubule lattice is believed to accommodate these different configurations by skewing the protofilaments so that the lateral interactions between tubulin subunits are maintained. Here, we present the characterization of 14 types of microtubules, including six novel ones, through an extensive analysis of microtubules assembled in vitro from pure tubulin. Although the six new types represented only 1 % of the total length of the population examined ( approximately 17 mm), they define the limits of microtubule structure and assembly. Protofilament skewing is restricted to within +/-2 degrees. Outside this range, the restoring force induced by the skewed protofilaments is compensated by a longitudinal shift (less than +/-0.2 nm) between adjacent protofilaments. Configurations with theoretical protofilament skew angles larger than +/-4 degrees or that necessitate larger modifications of the microtubule surface lattice were not observed. Analysis of the microtubule types distribution reveals that it is sharply peaked around the less skewed conformations. These results indicate that both the flexibility of the protofilaments and the strength of their lateral interactions restrict the range of structures assembled. They also demonstrate that growing microtubules can occasionally switch into energetically unfavorable configurations, a behavior that may account for the stochastic nature of catastrophes.

Upstream and downstream of ran GTPase

Among the Ras family, Ran is a unique small G protein. It does not have a lipid modification motif at the C-terminus to bind to the membrane, which is often observed within the Ras family. Ran may therefore interact with a wide range of proteins in various intracellular locations. This means that Ran could play many different roles like nucleocytoplasmic transport, microtubule assembly and so on. All of the Ran functions should be regulated by RanGEF and RanGAP. It is an interesting issue why RCC1, a RanGEF, is localized in the nucleus and RanGAP1/Ran1p in the cytoplasm. It is possible that RCC1 checks the state of chromosomal DNA replication and transfers it to the downstream events through Ran; thereby, RCC1 would be involved in coupling the spatial localization of cellular macromolecules with the cell cycle progression. In this context, Ran will be a very important cell cycle mediator. There is yet another G protein cascade, Gtr1-Gtr2, which interacts with the Ran cycle.

Magnesium-induced linear self-association of the FtsZ bacterial cell division protein monomer. The primary steps for FtsZ assembly

The bacterial cell division protein FtsZ from Escherichia coli has been purified with a new calcium precipitation method. The protein contains one GDP and one Mg(2+) bound, it shows GTPase activity, and requires GTP and Mg(2+) to polymerize into long thin filaments at pH 6.5. FtsZ, with moderate ionic strength and low Mg(2+) concentrations, at pH 7.5, is a compact and globular monomer. Mg(2+) induces FtsZ self-association into oligomers, which has been studied by sedimentation equilibrium over a wide range of Mg(2+) and FtsZ concentrations. The oligomer formation mechanism is best described as an indefinite self-association, with binding of an additional Mg(2+) for each FtsZ monomer added to the growing oligomer, and a slight gradual decrease of the affinity of addition of a protomer with increasing oligomer size. The sedimentation velocity of FtsZ oligomer populations is compatible with a linear single-stranded arrangement of FtsZ monomers and a spacing of 4 nm. It is proposed that these FtsZ oligomers and the polymers formed under assembly conditions share a similar axial interaction between monomers (like in the case of tubulin, the eukaryotic homolog of FtsZ). Similar mechanisms may apply to FtsZ assembly in vivo, but additional factors, such as macromolecular crowding, nucleoid occlusion, or specific interactions with other cellular components active in septation have to be invoked to explain FtsZ assembly into a division ring.

NEM tubulin inhibits microtubule minus end assembly by a reversible capping mechanism

Although microtubule (MT) dynamic instability is thought to depend on the guanine nucleotide (GTP vs GDP) bound to the beta-tubulin of the terminal subunit(s), the MT minus end exhibits dynamic instability even though the terminal beta-tubulin is always crowned by GTP-alpha-tubulin. As an approach toward understanding how dynamic instability occurs at the minus end, we investigated the effects of N-ethylmaleimide-modified tubulin (NTb) on elongation and rapid shortening of individual MTs. NTb preferentially inhibits minus end assembly when combined with unmodified tubulin (PCTb), but the mechanism of inhibition is unknown. Here, video-enhanced differential interference contrast microscopy was used to observe the effects of NTb on MTs assembled from PCTb onto axoneme fragments. MTs were exposed to mixtures of PCTb (25 microM) and NTb (labeled on approximately 1 Cys per monomer) in which the NTb/PCTb ratio varied from 0.025 to 1. The NTb/PCTb mixture had a slight inhibitory effect on the plus end elongation rate, but significantly inhibited or completely arrested minus end elongation. For the majority of mixtures that were assayed (0.1-1 NTb/PCTb ratio), minus end MT length remained constant until the NTb/PCTb mixture was replaced. Replacement with PCTb allowed elongation to proceed, whereas replacement with buffer or NTb caused minus ends to shorten. Taken together, the results indicate that NTb associates with both plus and minus ends and that NTb acts to reversibly cap minus ends only when PCTb is also present. Low-resolution mapping of labeled Cys residues, along with previous experiments with other Cys-reactive compounds, suggests that modification of beta-tubulin Cys(239) may be associated with the capping action of NTb.

Orbit, a novel microtubule-associated protein essential for mitosis in Drosophila melanogaster

We describe a Drosophila gene, orbit, that encodes a conserved 165-kD microtubule-associated protein (MAP) with GTP binding motifs. Hypomorphic mutations in orbit lead to a maternal effect resulting in branched and bent mitotic spindles in the syncytial embryo. In the larval central nervous system, such mutants have an elevated mitotic index with some mitotic cells showing an increase in ploidy. Amorphic alleles show late lethality and greater frequencies of hyperploid mitotic cells. The presence of cells in the hypomorphic mutant in which the chromosomes can be arranged, either in a circular metaphase or an anaphase-like configuration on monopolar spindles, suggests that polyploidy arises through spindle and chromosome segregation defects rather than defects in cytokinesis. A role for the Orbit protein in regulating microtubule behavior in mitosis is suggested by its association with microtubules throughout the spindle at all mitotic stages, by its copurification with microtubules from embryonic extracts, and by the finding that the Orbit protein directly binds to MAP-free microtubules in a GTP-dependent manner.

Focusing-in on microtubules

A good approximation of the atomic structure of a microtubule has been derived from docking the high-resolution structure of tubulin, solved by electron crystallography, into lower resolution maps of whole microtubules. Some structural interactions with other molecules, including nucleotides, drugs, motor proteins and microtubule-associated proteins, can now be predicted.

Dissecting cellular processes using small molecules: identification of colchicine-like, taxol-like and other small molecules that perturb mitosis

BACKGROUND

Understanding the molecular mechanisms of complex cellular processes requires unbiased means to identify and to alter conditionally gene products that function in a pathway of interest. Although random mutagenesis and screening (forward genetics) provide a useful means to this end, the complexity of the genome, long generation time and redundancy of gene function have limited their use with mammalian systems. We sought to develop an analogous process using small molecules to modulate conditionally the function of proteins. We hoped to identify simultaneously small molecules that may serve as leads for the development of therapeutically useful agents.

RESULTS

We report the results of a high-throughput, phenotype-based screen for identifying cell-permeable small molecules that affect mitosis of mammalian cells. The predominant class of compounds that emerged directly alters the stability of microtubules in the mitotic spindle. Although many of these compounds show the colchicine-like property of destabilizing microtubules, one member shows the taxol-like property of stabilizing microtubules. Another class of compounds alters chromosome segregation by novel mechanisms that do not involve direct interactions with microtubules.

CONCLUSIONS

The identification of structurally diverse small molecules that affect the mammalian mitotic machinery from a large library of synthetic compounds illustrates the use of chemical genetics in dissecting an essential cellular pathway. This screen identified five compounds that affect mitosis without directly targeting microtubules. Understanding the mechanism of action of these compounds, along with future screening efforts, promises to help elucidate the molecular mechanisms involved in chromosome segregation during mitosis.

A mutation in gamma-tubulin alters microtubule dynamics and organization and is synthetically lethal with the kinesin-like protein pkl1p

Mitotic segregation of chromosomes requires spindle pole functions for microtubule nucleation, minus end organization, and regulation of dynamics. gamma-Tubulin is essential for nucleation, and we now extend its role to these latter processes. We have characterized a mutation in gamma-tubulin that results in cold-sensitive mitotic arrest with an elongated bipolar spindle but impaired anaphase A. At 30 degrees C cytoplasmic microtubule arrays are abnormal and bundle into single larger arrays. Three-dimensional time-lapse video microscopy reveals that microtubule dynamics are altered. Localization of the mutant gamma-tubulin is like the wild-type protein. Prediction of gamma-tubulin structure indicates that non-alpha/beta-tubulin protein-protein interactions could be affected. The kinesin-like protein (klp) Pkl1p localizes to the spindle poles and spindle and is essential for viability of the gamma-tubulin mutant and in multicopy for normal cell morphology at 30 degrees C. Localization and function of Pkl1p in the mutant appear unaltered, consistent with a redundant function for this protein in wild type. Our data indicate a broader role for gamma-tubulin at spindle poles in regulating aspects of microtubule dynamics and organization. We propose that Pkl1p rescues an impaired function of gamma-tubulin that involves non-tubulin protein-protein interactions, presumably with a second motor, MAP, or MTOC component.

Non-hydrolysable GTP-gamma-S stabilizes the FtsZ polymer in a GDP-bound state

FtsZ, a tubulin homologue, forms a cytokinetic ring at the site of cell division in prokaryotes. The ring is thought to consist of polymers that assemble in a strictly GTP-dependent way. GTP, but not guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), has been shown to induce polymerization of FtsZ, whereas in vitro Ca2+ is known to inhibit the GTP hydrolysis activity of FtsZ. We have studied FtsZ dynamics at limiting GTP concentrations in the presence of 10 mM Ca2+. GTP and its non-hydrolysable analogue GTP-gamma-S bind FtsZ with similar affinity, whereas the non-hydrolysable analogue guanylyl-imidodiphosphate (GMP-PNP) is a poor substrate. Preformed FtsZ polymers can be stabilized by GTP-gamma-S and are destabilized by GDP. As more than 95% of the nucleotide associated with the FtsZ polymer is in the GDP form, it is concluded that GTP hydrolysis by itself does not trigger FtsZ polymer disassembly. Strikingly, GTP-gamma-S exchanges only a small portion of the FtsZ polymer-bound GDP. These data suggest that FtsZ polymers are stabilized by a small fraction of GTP-containing FtsZ subunits. These subunits may be located either throughout the polymer or at the polymer ends, forming a GTP cap similar to tubulin.

Disruption of microtubules in living cells by tyrphostin AG-1714

Tyrphostin AG-1714 and several related molecules with the general structure of nitro-benzene malononitrile (BMN) disrupt microtubules in a large variety of cultured cells. This process can be inhibited by the stabilization of microtubules with taxol or by pretreatment of the cells with pervanadate, which inhibits tyrosine phosphatases and increases the overall levels of phosphotyrosine in cells. Unlike other microtubule-disrupting drugs such as nocodazole or colchicine, tyrphostin AG-1714 does not interfere with microtubule polymerization or stability in vitro, suggesting that the effect of this tyrphostin on microtubules is indirect. These results imply an involvement of protein tyrosine phosphorylation in the regulation of overall microtubule dynamics. Tyrphostins of AG-1714 type could thus be powerful tools for the identification of such microtubule regulatory pathways.

Op18/stathmin caps a kinked protofilament-like tubulin tetramer

Oncoprotein 18/stathmin (Op18), a regulator of microtubule dynamics, was recombinantly expressed and its structure and function analysed. We report that Op18 by itself can fold into a flexible and extended alpha-helix, which is in equilibrium with a less ordered structure. In complex with tubulin, however, all except the last seven C-terminal residues of Op18 are tightly bound to tubulin. Digital image analysis of Op18:tubulin electron micrographs revealed that the complex consists of two longitudinally aligned alpha/beta-tubulin heterodimers. The appearance of the complex was that of a kinked protofilament-like structure with a flat and a ribbed side. Deletion mapping of Op18 further demonstrated that (i) the function of the N-terminal part of the molecule is to 'cap' tubulin subunits to ensure the specificity of the complex and (ii) the complete C-terminal alpha-helical domain of Op18 is necessary and sufficient for stable Op18:tubulin complex formation. Together, our results suggest that besides sequestering tubulin, the structural features of Op18 enable the protein specifically to recognize microtubule ends to trigger catastrophes.

Adenomatous polyposis coli (APC) protein moves along microtubules and concentrates at their growing ends in epithelial cells

Adenomatous polyposis coli (APC) tumor suppressor protein has been shown to be localized near the distal ends of microtubules (MTs) at the edges of migrating cells. We expressed green fluorescent protein (GFP)-fusion proteins with full-length and deletion mutants of Xenopus APC in Xenopus epithelial cells, and observed their dynamic behavior in live cells. During cell spreading and wound healing, GFP-tagged full-length APC was concentrated as granules at the tip regions of cellular extensions. At higher magnification, APC appeared to move along MTs and concentrate as granules at the growing plus ends. When MTs began to shorten, the APC granules dropped off from the MT ends. Immunoelectron microscopy revealed that fuzzy structures surrounding MTs were the ultrastructural counterparts for these GFP signals. The COOH-terminal region of APC was targeted to the growing MT ends without forming granular aggregates, and abruptly disappeared when MTs began to shorten. The APC lacking the COOH-terminal region formed granular aggregates that moved along MTs toward their plus ends in an ATP-dependent manner. These findings indicated that APC is a unique MT-associated protein that moves along selected MTs and concentrates at their growing plus ends through their multiple functional domains.

Microtubule dynamics and tubulin interacting proteins

Microtubule dynamics are crucial in generation of the mitotic spindle. During the transition from interphase to mitosis, there is an increase in the frequency of microtubule catastrophes. Recent work has identified two proteins, Op 18/stathmin and XKCM1, which can promote microtubule catastrophes in vitro and in cells or extracts. Although both of these proteins share the ability to bind tubulin dimers, their mechanisms of action in destabilizing microtubules are distinct.

The effects of cell passages on the cell morphology and the outcome of herpes simplex virus type 1 infection

Because cell cultures are essential in biological research which involves the analysis of virus morphogenesis, this study focused on examining the significance of cell passages. Human embryonic lung fibroblasts (MRC-5) at passage (P) 27 were seeded twice a week to P 32, P 40, and P 48, when just at confluence and then infected with herpes simplex virus type 1 (HSV-1). The structure of the non-virus-infected (MOCK) and HSV-1 infected cells, the amount of cellular infectious virus particles and the capability to express HSV-1 glycoproteins C (gC-1) and D (gD-1) were investigated by phase-contrast and immunofluorescence light microscopy, immunogold cryosection EM, plaque assays, immunoblots, and total protein assays. Modified cell structure including fragmentation of tubulin fibers were visible in MOCK from P 38 onwards. The quantity of vimentin remained unchanged while actin accumulated and beta-tubulin decreased in HSV-1 infected late P cells compared to early P cultures. Cells of high P counts contained significantly fewer infectious virus particles, very likely of lower virulence, and their expression of gC-1 and gD-1 were concordantly reduced. These observations indicate that the number of cell P must be considered in order to reproduce results of cell biology and viral morphogenesis. The MRC-5 cells ought not to be passaged more than ten times beyond P 27 in the laboratory.

D-TACC: a novel centrosomal protein required for normal spindle function in the early Drosophila embryo

We identify Drosophila TACC (D-TACC) as a novel protein that is concentrated at centrosomes and interacts with microtubules. We show that D-TACC is essential for normal spindle function in the early embryo; if D-TACC function is perturbed by mutation or antibody injection, the microtubules emanating from centrosomes in embryos are short and chromosomes often fail to segregate properly. The C-terminal region of D-TACC interacts, possibly indirectly, with microtubules, and can target a heterologous fusion protein to centrosomes and microtubules in embryos. This C-terminal region is related to the mammalian transforming, acidic, coiled-coil-containing (TACC) family of proteins. The function of the TACC proteins is unknown, but the genes encoding the known TACC proteins are all associated with genomic regions that are rearranged in certain cancers. We show that at least one of the mammalian TACC proteins appears to be associated with centrosomes and microtubules in human cells. We propose that this conserved C-terminal 'TACC domain' defines a new family of microtubule-interacting proteins.

LIN-5 is a novel component of the spindle apparatus required for chromosome segregation and cleavage plane specification in Caenorhabditis elegans

Successful divisions of eukaryotic cells require accurate and coordinated cycles of DNA replication, spindle formation, chromosome segregation, and cytoplasmic cleavage. The Caenorhabditis elegans gene lin-5 is essential for multiple aspects of cell division. Cells in lin-5 null mutants enter mitosis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation, and cytokinesis. Despite these defects, cells exit from mitosis without delay and progress through subsequent rounds of DNA replication, centrosome duplication, and abortive mitoses. In addition, early embryos that lack lin-5 function show defects in spindle positioning and cleavage plane specification. The lin-5 gene encodes a novel protein with a central coiled-coil domain. This protein localizes to the spindle apparatus in a cell cycle- and microtubule-dependent manner. The LIN-5 protein is located at the centrosomes throughout mitosis, at the kinetochore microtubules in metaphase cells, and at the spindle during meiosis. Our results show that LIN-5 is a novel component of the spindle apparatus required for chromosome and spindle movements, cytoplasmic cleavage, and correct alternation of the S and M phases of the cell cycle.

Characterization of a microtubule assembly inhibitor from Xenopus oocytes

The dynamic properties of microtubules (MTs) are important for a wide variety of cellular processes, including cell division and morphogenesis. MT assembly and disassembly in vivo are regulated by cellular factors that influence specific parameters of MT dynamics. Here, we describe the characterization of a previously reported MT assembly inhibitor activity from Xenopus oocytes [Gard and Kirschner, 1987: J. Cell Biol. 105:2191-2201]. Video microscopy measurements reveal that the inhibitor specifically decreases the plus end growth rate of MTs and increases the critical concentration for tubulin. However, catastrophe frequency, rescue frequency, and shrinkage rates are not affected by the activity. Chromatography on Mono Q and hydroxyapatite columns has shown that the activity cofractionates with a subpopulation of tubulin. This tubulin subpopulation and the MT assembly inhibitor activity also co-migrate with a large S value (25-30S) on sucrose gradients. The high molecular weight tubulin complex and the MT assembly inhibitor activity are both developmentally regulated and disappear after oocyte maturation with progesterone.