Nucleotide hydrolysis in cytoskeletal assembly

Bottom-Up Synthetic Biology Using Cell-Free Protein Synthesis

Technical advances in biotechnology have greatly accelerated the development of bottom-up synthetic biology. Unlike top-down approaches, bottom-up synthetic biology focuses on the construction of a minimal cell from scratch and the application of these principles to solve challenges. Cell-free protein synthesis (CFPS) systems provide minimal machinery for transcription and translation, from either a fractionated cell lysate or individual purified protein elements, thus speeding up the development of synthetic cell projects. In this review, we trace the history of the cell-free technique back to the first in vitro fermentation experiment using yeast cell lysate. Furthermore, we summarized progresses of individual cell mimicry modules, such as compartmentalization, gene expression regulation, energy regeneration and metabolism, growth and division, communication, and motility. Finally, current challenges and future perspectives on the field are outlined.

Steady-state EB cap size fluctuations are determined by stochastic microtubule growth and maturation

Growing microtubules are protected from depolymerization by the presence of a GTP or GDP/Pi cap. End-binding proteins of the EB1 family bind to the stabilizing cap, allowing monitoring of its size in real time. The cap size has been shown to correlate with instantaneous microtubule stability. Here we have quantitatively characterized the properties of cap size fluctuations during steady-state growth and have developed a theory predicting their timescale and amplitude from the kinetics of microtubule growth and cap maturation. In contrast to growth speed fluctuations, cap size fluctuations show a characteristic timescale, which is defined by the lifetime of the cap sites. Growth fluctuations affect the amplitude of cap size fluctuations; however, cap size does not affect growth speed, indicating that microtubules are far from instability during most of their time of growth. Our theory provides the basis for a quantitative understanding of microtubule stability fluctuations during steady-state growth.

Microtubule aging probed by microfluidics-assisted tubulin washout

Microtubule aging—the decrease of stability with age—is an interesting but mechanistically not understood property of microtubules. It constrains possible mechanisms of catastrophe induction and is believed to be crucial for length regulation. New in vitro experiments and model fits provide insight into the origin of microtubule aging.

Microtubules switch stochastically between phases of growth and shrinkage. The molecular mechanism responsible for the end of a growth phase, an event called catastrophe, is still not understood. The probability for a catastrophe to occur increases with microtubule age, putting constraints on the possible molecular mechanism of catastrophe induction. Here we used microfluidics-assisted fast tubulin washout experiments to induce microtubule depolymerization in a controlled manner at different times after the start of growth. We found that aging can also be observed in this assay, providing valuable new constraints against which theoretical models of catastrophe induction can be tested. We found that the data can be quantitatively well explained by a simple kinetic threshold model that assumes an age-dependent broadening of the protective cap at the microtubule end as a result of an evolving tapered end structure; this leads to a decrease of the cap density and its stability. This analysis suggests an intuitive picture of the role of morphological changes of the protective cap for the age dependence of microtubule stability.

Plasma membrane calcium ATPase activity is regulated by actin oligomers through direct interaction

As recently described by our group, plasma membrane calcium ATPase (PMCA) activity can be regulated by the actin cytoskeleton. In this study, we characterize the interaction of purified G-actin with isolated PMCA and examine the effect of G-actin during the first polymerization steps. As measured by surface plasmon resonance, G-actin directly interacts with PMCA with an apparent 1:1 stoichiometry in the presence of Ca(2+) with an apparent affinity in the micromolar range. As assessed by the photoactivatable probe 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, the association of PMCA to actin produced a shift in the distribution of the conformers of the pump toward a calmodulin-activated conformation. G-actin stimulates Ca(2+)-ATPase activity of the enzyme when incubated under polymerizing conditions, displaying a cooperative behavior. The increase in the Ca(2+)-ATPase activity was related to an increase in the apparent affinity for Ca(2+) and an increase in the phosphoenzyme levels at steady state. Although surface plasmon resonance experiments revealed only one binding site for G-actin, results clearly indicate that more than one molecule of G-actin was needed for a regulatory effect on the pump. Polymerization studies showed that the experimental conditions are compatible with the presence of actin in the first stages of assembly. Altogether, these observations suggest that the stimulatory effect is exerted by short oligomers of actin. The functional interaction between actin oligomers and PMCA represents a novel regulatory pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca(2+) homeostasis.

Cytoskeleton in mast cell signaling

Mast cell activation mediated by the high affinity receptor for IgE (FcεRI) is a key event in allergic response and inflammation. Other receptors on mast cells, as c-Kit for stem cell factor and G protein-coupled receptors (GPCRs) synergistically enhance the FcεRI-mediated release of inflammatory mediators. Activation of various signaling pathways in mast cells results in changes in cell morphology, adhesion to substrate, exocytosis, and migration. Reorganization of cytoskeleton is pivotal in all these processes. Cytoskeletal proteins also play an important role in initial stages of FcεRI and other surface receptors induced triggering. Highly dynamic microtubules formed by αβ-tubulin dimers as well as microfilaments build up from polymerized actin are affected in activated cells by kinases/phosphatases, Rho GTPases and changes in concentration of cytosolic Ca(2+). Also important are nucleation proteins; the γ-tubulin complexes in case of microtubules or Arp 2/3 complex with its nucleation promoting factors and formins in case of microfilaments. The dynamic nature of microtubules and microfilaments in activated cells depends on many associated/regulatory proteins. Changes in rigidity of activated mast cells reflect changes in intermediate filaments build up from vimentin. This review offers a critical appraisal of current knowledge on the role of cytoskeleton in mast cells signaling.

Structural model of a complex between the heterotrimeric G protein, Gsalpha, and tubulin

A number of studies have demonstrated interplay between the cytoskeleton and G protein signaling. Many of these studies have determined a specific interaction between tubulin, the building block of microtubules, and G proteins. The alpha subunits of some heterotrimeric G proteins, including Gsalpha, have been shown to interact strongly with tubulin. Binding of Galpha to tubulin results in increased dynamicity of microtubules due to activation of GTPase of tubulin. Tubulin also activates Gsalpha via a direct transfer of GTP between these molecules. Structural insight into the interaction between tubulin and Gsalpha was required, and was determined, in this report, through biochemical and molecular docking techniques. Solid phase peptide arrays suggested that a portion of the amino terminus, alpha2-beta4 (the region between switch II and switch III) and alpha3-beta5 (just distal to the switch III region) domains of Gsalpha are important for interaction with tubulin. Molecular docking studies revealed the best-fit models based on the biochemical data, showing an interface between the two molecules that includes the adenylyl cyclase/Gbetagamma interaction regions of Gsalpha and the exchangeable nucleotide-binding site of tubulin. These structural models explain the ability of tubulin to facilitate GTP exchange on Galpha and the ability of Galpha to activate tubulin GTPase.

Mechanical stress induced mechanism of microtubule catastrophes

Microtubules assembled in vitro from pure tubulin can switch occasionally from growing to shrinking states or resume assembly, an unusual behavior termed "dynamic instability of microtubule growth". Its origin remains unclear and several models have been proposed, including occasional switching of the microtubules into energetically unfavorable configurations during assembly. In this study, we have asked whether the excess energy accumulated in these configurations would be of sufficient magnitude to destabilize the capping region that must exist at the end of growing microtubules. For this purpose, we have analyzed the frequency distribution of microtubules assembled in vitro from pure tubulin, and modeled the different mechanical constraints accumulated in their wall. We find that the maximal excess energy that the microtubule lattice can store is in the order of 11 kBT per dimer. Configurations that require distortions up to approximately 20 kBT are allowed at the expense of a slight conformational change, and larger distortions are not observed. Modeling of the different elastic deformations suggests that the excess energy is essentially induced by protofilament skewing, microtubule radial curvature change and inter-subunit shearing, distortions that must destabilize further the tubulin subunits interactions. These results are consistent with the hypothesis that unfavorable closure events may trigger the catastrophes observed at low tubulin concentration in vitro. In addition, we propose a novel type of representation that describes the stability of microtubule assembly systems, and which might be of considerable interest to study the effects of stabilizing and destabilizing factors on microtubule structure and dynamics.

Three- and four-repeat tau regulate the dynamic instability of two distinct microtubule subpopulations in qualitatively different manners. Implications for neurodegeneration

The microtubule-associated protein tau is implicated in the pathogenesis of many neurodegenerative diseases, including fronto-temporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), in which both RNA splicing and amino acid substitution mutations in tau cause dominantly inherited early onset dementia. RNA-splicing FTDP-17 mutations alter the wild-type approximately 50:50 3-repeat (3R) to 4-repeat (4R) tau isoform ratio, usually resulting in an excess of 4R tau. To examine further how splicing mutations might cause dysfunction by misregulation of microtubule dynamics, we used video microscopy to determine the in vitro behavior of individual microtubules stabilized by varying amounts of human 4R and 3R tau. At low tau:tubulin ratios (1:55 and 1:45), all 3R isoforms reduced microtubule growth rates relative to the no-tau control, whereas all 4R isoforms increased them; however, at a high tau:tubulin ratio (1:20), both 4R and 3R tau increased the growth rates. Further analysis revealed two distinct subpopulations of growing microtubules in the absence of tau. Increasing concentrations of both 4R and 3R tau resulted in an increase in the size of the faster growing subpopulation of microtubules; however, 4R tau caused a redistribution to the faster growing subpopulation at lower tau:tubulin ratios than 3R tau. This modulation of discrete growth rate subpopulations by tau suggests that tau causes a conformational shift in the microtubule resulting in altered dynamics. Quantitative and qualitative differences observed between 4R and 3R tau are consistent with a "microtubule misregulation" model in which abnormal tau isoform expression results in the inability to properly regulate microtubule dynamics, leading to neuronal death and dementia.

An intermediate form of ADP-F-actin

With yeast actin, contrary to other actins, filament formation, ATP hydrolysis, and Pi release are concurrent at low actin concentrations, the condition usually employed to assess actin polymerization. This observation leads to a question concerning the conformation of the filament barbed end that might be recognized by specific actin-binding proteins. To try to detect possible new actin polymer conformations that might be intermediate in the pathway leading to mature F-actin, we monitored the change in intrinsic tryptophan fluorescence of yeast and muscle actins polymerized at pH 6 to accelerate the rate of filament formation. This allowed temporal resolution of the Pi release process from the slower process of polymerization. With both actins, we detected a biphasic instead of the usual monophasic fluorescence change, a rapid decrease that tracks with filament formation followed by a slower rebound (the second phase). This second phase postpolymerization conformational change requires Pi release and occurs nearly coincident with its release. The addition of Pi causes this second phase response to disappear, and the inclusion of Pi during polymerization prevents its appearance. At pH 7.5, with higher yeast actin concentrations to accelerate polymerization, a two-phase fluorescence change is also observed. In this case, the second phase change lags substantially behind Pi release. Pi release could also be resolved from polymer formation. V159N yeast actin, hypothesized previously as remaining in a postpolymerization ATP-like state, exhibits the same two-phase intrinsic tryptophan fluorescence behavior as wild-type yeast actin. Together, these observations demonstrate the presence of an intermediate filament state between ADP-Pi and mature ADP-F-actin.

Microtubule's conformational cap

The molecular mechanisms that allow elongation of the unstable microtubule lattice remain unclear. It is usually thought that the GDP-liganded tubulin lattice is capped by a small layer of GTP- or GDP-Pi-liganded molecules, the so called "GTP-cap". Here, we point-out that the elastic properties of the microtubule lattice cause a difference in stability between the elongating tubulin sheet and the completed microtubule wall. The implications of our observations for microtubule structure and dynamics are discussed.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

GTP-yeast actin

Because of the apparently greater conformational flexibility of yeast versus muscle actin and the ability of other members in the actin protein superfamily to efficiently use both ATP and GTP, we assessed the ability of yeast actin to function with GTP. Etheno-ATP exchange studies showed that the binding of GTP to yeast actin is about 1/9 as tight as that of ATP in contrast to the 1/1,240 ratio for muscle actin. Proteolysis of GTP-bound G-yeast actin suggests that the conformation of subdomain 2 is very much like that of ATP-bound actin, but CD studies show that GTP-bound actin is less thermostable than ATP-bound actin. GTP-actin polymerizes with an apparent critical concentration of 1.5 microm, higher than that of ATP-actin (0.3 microm) although filament structures observed by electron microscopy were similar. Yeast actin hydrolyzes GTP in a polymerization-dependent manner, and GTP-bound F-actin decorates with the myosin S1. Conversion of Phe(306) in the nucleotide binding site to the Tyr found in muscle actin raised the nucleotide discrimination ratio from the 1/9 of wild-type actin to 1/125. This result agrees with modeling that predicts that removal of the Tyr hydroxyl will create a space for the C2 amino group of the GTP guanine.

Cancer cell motility--on the road from c-erbB-2 receptor steered signaling to actin reorganization

Cell migration depends mainly on actin polymerization and intracellular organization, which are influenced by a vast variety of actin binding proteins (ABPs). Regulation of ABP activity is mediated by second messengers such as phosphoinositides and calcium. Signaling via these second messengers is initiated and regulated by membrane receptors, e.g., receptor tyrosine kinases (RTKs), and by adhesion molecule interactions (e.g., integrins and selectins) and focal adhesion kinases. A major role in steering second-messenger signaling and thus in actin cytoskeleton reorganization and motility of cancer cells is played by the RTK c-erbB-2. This occurs through a number of signaling pathways which involve mainly enzymes, e.g., phospholipase Cgamma1 and GTPases, which modify signaling molecules. Furthermore large multiprotein complexes including actin-related protein 2/3, Wiskott-Aldrich syndrome protein, profilin, and capping protein among others play an important role in regulating actin reorganization. The complex picture of the mode of actin reorganization, which is involved in tumor cell migration, is slowly emerging from the mists of cellular signaling pathways, but this is still by no means a clear view.

Conformational and dynamic differences between actin filaments polymerized from ATP- or ADP-actin monomers

Conformational and dynamic properties of actin filaments polymerized from ATP- or ADP-actin monomers were compared by using fluorescence spectroscopic methods. The fluorescence intensity of IAEDANS attached to the Cys(374) residue of actin was smaller in filaments from ADP-actin than in filaments from ATP-actin monomers, which reflected a nucleotide-induced conformational difference in subdomain 1 of the monomer. Radial coordinate calculations revealed that this conformational difference did not modify the distance of Cys(374) from the longitudinal filament axis. Temperature-dependent fluorescence resonance energy transfer measurements between donor and acceptor molecules on Cys(374) of neighboring actin protomers revealed that the inter-monomer flexibility of filaments assembled from ADP-actin monomers were substantially greater than the one of filaments from ATP-actin monomers. Flexibility was reduced by phalloidin in both types of filaments.

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.

Stepwise reconstitution of interphase microtubule dynamics in permeabilized cells and comparison to dynamic mechanisms in intact cells

Microtubules in permeabilized cells are devoid of dynamic activity and are insensitive to depolymerizing drugs such as nocodazole. Using this model system we have established conditions for stepwise reconstitution of microtubule dynamics in permeabilized interphase cells when supplemented with various cell extracts. When permeabilized cells are supplemented with mammalian cell extracts in the presence of protein phosphatase inhibitors, microtubules become sensitive to nocodazole. Depolymerization induced by nocodazole proceeds from microtubule plus ends, whereas microtubule minus ends remain inactive. Such nocodazole-sensitive microtubules do not exhibit subunit turnover. By contrast, when permeabilized cells are supplemented with Xenopus egg extracts, microtubules actively turn over. This involves continuous creation of free microtubule minus ends through microtubule fragmentation. Newly created minus ends apparently serve as sites of microtubule depolymerization, while net microtubule polymerization occurs at microtubule plus ends. We provide evidence that similar microtubule fragmentation and minus end-directed disassembly occur at the whole-cell level in intact cells. These data suggest that microtubule dynamics resembling dynamics observed in vivo can be reconstituted in permeabilized cells. This model system should provide means for in vitro assays to identify molecules important in regulating microtubule dynamics. Furthermore, our data support recent work suggesting that microtubule treadmilling is an important mechanism of microtubule turnover.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

The active GTP- and ground GDP-liganded states of tubulin are distinguished by the binding of chiral isomers of ethyl 5-amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl carbamate

NSC 613862 (S)-(-) and NSC 613863 (R)-(+) are the two chiral isomers of ethyl-5-amino-2-methyl-1,2-dihydro-3-phenylpyrido[3, 4-b]pyrazin-7-yl carbamate. Both compounds bind to tubulin in a region that overlaps the colchicine site. They induce formation of abnormal polymers from purified GTP-Mg-tubulin, the active assembly form of tubulin, in glycerol-free buffer with magnesium [De Ines, C., Leynadier, D., Barasoain, I., Peyrot, V., Garcia, P., Briand, C., Rener, G. A., and Temple, C., Jr. (1994) Cancer Res. 54, 75-84]. In this study, we observed that the S-isomer can promote polymerization of GDP-tubulin, the inactive assembly-incompetent form of tubulin, into nonmicrotubular structures at a critical protein concentration of 1 mg/mL (12 mM MgCl2). Neither the R-isomer nor colchicine have this ability. By electron microscopy, these tubulin polymers showed the same poorly defined filamentous structure when GDP-tubulin or GTP-Mg-tubulin were used. By HPLC measurements, we demonstrated that a dissociated GTP hydrolysis and exchange of nucleotide occurred during the isomer-induced abnormal assembly. Both isomers inhibited the Mg2+-induced tubulin self-association leading to 42 S double ring formation from GTP-Mg-tubulin or GDP-tubulin. Measurement of their binding under nonassociation conditions revealed a 3-fold decrease in the apparent equilibrium binding constant of the R-isomer to GDP-tubulin relative to GTP-Mg-tubulin. For the S-isomer, the decrease in the binding constant was less pronounced. Binding data, analyzed in terms of a system of linked conformational and association equilibria, provide evidence that the active ("straight") rather than the inactive ("curved") conformation of tubulin differentially recognizes these ligands. Whereas binding of colchicine to tubulin is well-known to induce GTP hydrolysis, this is the first case in which the interaction of a ligand with the colchicine site is shown to be sensitive to the presence of GDP or GTP at the distant nucleotide binding site.

Control of the structural stability of the tubulin dimer by one high affinity bound magnesium ion at nucleotide N-site

Tubulin liganded with GTP at the N-site in the alpha-subunit and with GDP at the E-site in the beta-subunit (GDP-tubulin) reversibly binds one high affinity Mg2+ cation (Kb = 1.1 x 10(7) M-1), whereas tubulin liganded with GTP at both subunits (GTP-tubulin) binds one more high affinity Mg2+. The two cation binding loci are identified as nucleotide sites N and E, respectively. Mg2+ at the N-site controls the stability and structure of the alphabeta-tubulin dimer. Mg2+ dissociation is followed by the slow release of bound nucleotide and functional inactivation. Mg2+ bound to the N-site significantly increases the thermal stability of the GDP-tubulin dimer (by 10 degrees C and approximately 50 kcal mol-1 of experimental enthalpy change). However, the thermal stability of Mg2+-liganded GDP- and GTP-tubulin is the same. Mg2+ binding to the N-site is linked to the alphabeta-dimer formation. The binding of Mg2+ to the alpha-subunit communicates a marked enhancement of fluorescence to a colchicine analogue bound to the beta-subunit. Colchicine, in turn, thermally stabilizes Mg2+-depleted tubulin. The tubulin properties described would be simply explained if the N-site and the colchicine site are at the alpha-beta dimerization interface. It follows that the E-site would be at the beta-end of the tubulin dimer, consistent with the known functional role of the E nucleotide gamma-phosphate and coordinated cation controlling microtubule stability.

Hydrolysis of GTP associated with the formation of tubulin oligomers is involved in microtubule nucleation

Hydrolysis of GTP is known to accompany microtubule assembly. Here we show that hydrolysis of GTP is also associated with the formation of linear oligomers of tubulin, which are precursors (prenuclei) in microtubule assembly. The hydrolysis of GTP on these linear oligomers inhibits the lateral association of GTP-tubulin that leads to the formation of a bidimensional lattice. Therefore GTP hydrolysis interferes with the nucleation of microtubules. Linear oligomers are also formed in mixtures of GTP-tubulin and GDP-tubulin. The hydrolysis of GTP associated with heterologous interactions between GTP-tubulin and GDP-tubulin in the cooligomer takes place at a threefold faster rate than upon homologous interactions between GTP-tubulins. The implication of these results in a model of vectorial GTP hydrolysis in microtubule assembly is discussed.

Estimation of the diffusion-limited rate of microtubule assembly

Microtubule assembly is a complex process with individual microtubules alternating stochastically between extended periods of assembly and disassembly, a phenomenon known as dynamic instability. Since the discovery of dynamic instability, molecular models of assembly have generally assumed that tubulin incorporation into the microtubule lattice is primarily reaction-limited. Recently this assumption has been challenged and the importance of diffusion in microtubule assembly dynamics asserted on the basis of scaling arguments, with tubulin gradients predicted to extend over length scales exceeding a cell diameter, approximately 50 microns. To assess whether individual microtubules in vivo assemble at diffusion-limited rates and to predict the theoretical upper limit on the assembly rate, a steady-state mean-field model for the concentration of tubulin about a growing microtubule tip was developed. Using published parameter values for microtubule assembly in vivo (growth rate = 7 microns/min, diffusivity = 6 x 10(-12) m2/s, tubulin concentration = 10 microM), the model predicted that the tubulin concentration at the microtubule tip was approximately 89% of the concentration far from the tip, indicating that microtubule self-assembly is not diffusion-limited. Furthermore, the gradients extended less than approximately 50 nm (the equivalent of about two microtubule diameters) from the microtubule tip, a distance much less than a cell diameter. In addition, a general relation was developed to predict the diffusion-limited assembly rate from the diffusivity and bulk tubulin concentration. Using this relation, it was estimated that the maximum theoretical assembly rate is approximately 65 microns/min, above which tubulin can no longer diffuse rapidly enough to support faster growth.

Xenopus actin depolymerizing factor/cofilin (XAC) is responsible for the turnover of actin filaments in Listeria monocytogenes tails

In contrast to the slow rate of depolymerization of pure actin in vitro, populations of actin filaments in vivo turn over rapidly. Therefore, the rate of actin depolymerization must be accelerated by one or more factors in the cell. Since the actin dynamics in Listeria monocytogenes tails bear many similarities to those in the lamellipodia of moving cells, we have used Listeria as a model system to isolate factors required for regulating the rapid actin filament turnover involved in cell migration. Using a cell-free Xenopus egg extract system to reproduce the Listeria movement seen in a cell, we depleted candidate depolymerizing proteins and analyzed the effect that their removal had on the morphology of Listeria tails. Immunodepletion of Xenopus actin depolymerizing factor (ADF)/cofilin (XAC) from Xenopus egg extracts resulted in Listeria tails that were approximately five times longer than the tails from undepleted extracts. Depletion of XAC did not affect the tail assembly rate, suggesting that the increased tail length was caused by an inhibition of actin filament depolymerization. Immunodepletion of Xenopus gelsolin had no effect on either tail length or assembly rate. Addition of recombinant wild-type XAC or chick ADF protein to XAC-depleted extracts restored the tail length to that of control extracts, while addition of mutant ADF S3E that mimics the phosphorylated, inactive form of ADF did not reduce the tail length. Addition of excess wild-type XAC to Xenopus egg extracts reduced the length of Listeria tails to a limited extent. These observations show that XAC but not gelsolin is essential for depolymerizing actin filaments that rapidly turn over in Xenopus extracts. We also show that while the depolymerizing activities of XAC and Xenopus extract are effective at depolymerizing normal filaments containing ADP, they are unable to completely depolymerize actin filaments containing AMPPNP, a slowly hydrolyzible ATP analog. This observation suggests that the substrate for XAC is the ADP-bound subunit of actin and that the lifetime of a filament is controlled by its nucleotide content.

Calcium and gadolinium ions stimulate the GTPase activity of purified chicken brain tubulin through a conformational change

Ca2+ and Gd3+ stimulated the GTPase activity of chicken brain tubulin 13- and 26-fold, respectively. Mg2+, Tb3+, and Na+ had no effect. This GTPase activity showed a saturation behavior with Ca2+ and Gd3+ with a maximal activity of 0.26 +/- 0.026 and 1.15 +/- 0.78 nmol min-1 per mg of tubulin and semisaturation constants, expressed as the concentration of the cation needed for 50% of saturation, of 0.32 +/- 0.18 and 0.011 +/- 0.007 mM, respectively. In the presence of Ca2+, the GTPase activity was proportional to tubulin concentration in the range 0.9-31.8 microM. The semisaturation constants for the inhibition of tubulin polymerization and for the depolymerization of microtubules by Ca2+ were 0.71 +/- 0.1 and 0.049 +/- 0.043 mM, respectively. The similarity of the Ca2+ semisaturation constants for inhibition of tubulin assembly and stimulation of the GTPase activity suggests that these processes are correlated. These results support the hypothesis that the GTPase activity is related to but not directly involved in the mechanism of inhibition of Ca2+ -dependent tubulin assembly. This inhibition could be better explained by the formation of a nonfunctional conformational state of tubulin induced by Ca2+ that is responsible for the GTPase activity. Quenching of the intrinsic fluorescence of tryptophan induced by Ca2+ showed an apparent dissociation constant of 0.14 +/- 0.005 mM, in the range of values determined through tubulin polymerization inhibition or through the induction of GTPase activity by Ca2+. Acrylamide-induced quenching of the intrinsic fluorescence showed values of the Stern-Volmer constants of 5.4 +/- 0.12 and 5.0 +/- 0.15 M-1 in the absence and presence of Ca2+, respectively. These results support the hypothesis that the inhibition of tubulin polymerization and the induction of the GTPase activity by Ca2+ is mediated by a conformational change. Ca2+ failed to induce depolymerization of GDP-AIF4-microtubules; this could be explained by a model in which Ca-tubulin is unable to assemble into microtubules and the rate of dissociation of GDP-Pi-tubulin from the microtubule ends is extremely slow compared with the rate of GDP-subunit dissociation, supporting the concept that the GTP- and GDP-Pi-tubulin cap at the ends of microtubules regulates their dynamic instability.

Force generation by microtubule assembly/disassembly in mitosis and related movements

In this article, we review the dynamic nature of the filaments (microtubules) that make up the labile fibers of the mitotic spindle and asters, we discuss the roles that assembly and disassembly of microtubules play in mitosis, and we consider how such assembling and disassembling polymer filaments can generate forces that are utilized by the living cell in mitosis and related movements.

Expression of the nerve growth factor gene is controlled by the microtubule network

Colchicine, nocodazol, and vinblastine, three microtubule-disrupting drugs, were shown to increase the levels of both nerve growth factor (NGF) mRNA and cell-secreted NGF protein in L929 cells, with levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or amyloid precursor protein (APP) mRNAs remaining unaffected. Northern blot analysis demonstrated that colchicine also increased NGF mRNA levels in rat primary astrocytes and mouse skin fibroblasts. The specificity of the effects observed was assessed by the fact that the microtubule-stabilizing agent Taxotere, a semisynthetic compound structurally related to taxol, suppressed the effects of colchicine, whereas lumicolchicine, a colchicine derivative that has no action on the microtubule network, had no influence on NGF expression. Likewise, the disruption of the microfilament network by cytochalasin B did not increase NGF mRNA levels in L929 cells. Furthermore, the increase in NGF gene expression observed following microtubule disruption depended on a cascade of events involving at least one protein kinase, which is not down-regulated by phorbol ester, and on a pertussis toxin sensitive step. These results support the concept that tubulin and/or the microtubule cytoskeleton play an active role in the regulation of the NGF gene.

Assembly of purified GDP-tubulin into microtubules induced by taxol and taxotere: reversibility, ligand stoichiometry, and competition

Purified tubulin fully liganded to GDP at the exchangeable nucleotide binding site has been prepared by a new direct nucleotide exchange procedure. This normally inactive GDP-tubulin is driven to assemble into microtubules by the binding of the antitumor drug taxol or its more soluble side-chain analogue Taxotere in Mg(2+)-containing buffer, and it disassembles by cooling the solution. Therefore this ligand-induced equilibrium microtubule assembly system dispenses with the requirement of a gamma-phosphate-metal cation ligand bound at the nucleotide site for tubulin to be active. GDP-tubulin can also form characteristic pseudo-ordered aggregates of double rings. These aggregates dissociate upon warming or by addition of GTP. Back-substitution of the nucleotide gamma-phosphate permits glycerol-induced assembly without taxol and reduces the critical protein concentration required for drug-induced microtubule assembly by a factor of 2.6 +/- 0.1. The ligand-induced assembly is maximal at taxol or Taxotere concentrations equimolar with tubulin, and both drugs bind to assembled tubulin with a stoichiometry of 0.99 +/- 0.04 ligand per alpha beta dimer. Taxotere apparently competes with taxol for the same binding site, with 1.9 +/- 0.1 times larger effective affinity. Similarly, the Taxotere-induced assembly of GDP-tubulin or GTP-tubulin proceeds with a critical protein concentration 2.1 +/- 0.1 times smaller than with taxol.

Cooperative effects on filament stability in actin modified at the C-terminus by substitution or truncation

We have studied the contribution of the C-terminus of actin to filament stability by chemical modification and limited proteolysis. Formation of mixed disulfides of the penultimate C-terminal cysteine residue 374 with various low-molecular-mass thiols resulted in filament destabilization, as reflected by an increase in critical concentration and steady-state ATPase activity. These effects were fully reversed by the addition of phalloidin. Both the destabilization by glutathionylation and the reversal of it by phalloidin exhibited a high degree of cooperativity; half-maximal destabilization required the modification of four out of five actin subunits, and half-maximal restabilization by phalloidin was already reached when only one out of 20 actin subunits was complexed. C-terminal truncation by limited trypsinolysis of filamentous actin resulted in a similar destabilization of the polymer, as shown by a 2-3-fold increase in the steady-state ATPase activity. This effect was likewise cooperative and could be reversed by phalloidin. Since truncation of the C-terminus of actin has an effect on stability similar to that of chemical modification with bulky substituents, the possibility can be excluded that, in the latter case, destabilization was caused by steric hindrance. Rather, it seems that the highly conserved C-terminal part of actin plays an active role in establishing a tight contact between neighbouring subunits.

Control of microtubule dynamics and length by cyclin A- and cyclin B-dependent kinases in Xenopus egg extracts

In eukaryotic cells, the onset of mitosis involves cyclin molecules which interact with proteins of the cdc2 family to produce active kinases. In vertebrate cells, cyclin A dependent kinases become active in S- and pro-phases, whereas a cyclin B-dependent kinase is mostly active in metaphase. It has recently been shown that, when added to Xenopus egg extracts, bacterially produced A- and B-type cyclins associate predominantly with the same kinase catalytic subunit, namely p34cdc2, and induce its histone H1 kinase activity with different kinetics. Here, we show that in the same cell free system, both the addition of cyclin A and cyclin B changes microtubule behavior. However, the cyclin A-dependent kinase does not induce a dramatic shortening of centrosome-nucleated microtubules whereas the cyclin B-dependent kinase does, as previously reported. Analysis of the parameters of microtubule dynamics by fluorescence video microscopy shows that the dramatic shortening induced by the cyclin B-dependent kinase is correlated with a several fold increase in catastrophe frequency, an effect not observed with the cyclin A-dependent kinase. Using a simple mathematical model, we show how the length distributions of centrosome-nucleated microtubules relate to the four parameters that describe microtubule dynamics. These four parameters define a threshold between unlimited microtubule growth and the establishment of steady-state dynamics, which implies that well defined steady-state length distributions can be produced by regulating precisely the respective values of the dynamical parameters. Moreover, the dynamical model predicts that increasing catastrophe frequency is more efficient than decreasing the rescue frequency to reduce the average steady state length of microtubules. These theoretical results are quantitatively confirmed by the experimental data.

The structure of the F-actin filament and the actin molecule

A consensus view on the three-dimensional structure of the F-actin filament and the relative strength of the intersubunit contacts in the filament has been established from an atomic filament model and recent three-dimensional reconstructions from electron micrographs of F-actin filaments. Functional implications of recent structural and biochemical data indicating a rather dynamic filament structure are discussed.

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.

Involvement of microtubules in modifications associated with cellular aging

Microtubules are ubiquitous cellular components involved in the control of cell structure and functions, such as cell division, regulation of shape and polarity, intracellular transport, etc. Consequently, any alteration affecting them in structure or function has a good chance of affecting the cell and generally leads to cell dysfunctions. This has been shown for instance, after treatment with microtubule-interacting drugs. Cellular aging is also characterized by the appearance of various cell dysfunctions, but the possible involvement of the microtubules in the aging process, although a rather tempting hypothesis, has not yet been extensively investigated. In this paper, I will first rapidly review the different components that build, organize and control the microtubules in normal cells, independently of the aging process. I will then consider the possible involvement of the microtubules in the aging process, more particularly in models of cells aging in vitro and in aging neuronal cells, which have been the most extensively investigated. There is some evidence for alterations in the microtubule organization both in cells aging in vitro and in the aging brain. But the interpretation of these data awaits further experiments, taking into account the latest progress in tubulin genetics and in microtubule biochemistry. Microtubules could also represent one of the cellular targets affected after signal transduction and could thus be involved in the resulting cellular responses. This hypothesis will be discussed, as it offers new insights into the regulation of microtubule organization, dynamics and functions in normal cells, which will be worthwhile to investigate during the aging process.