The Chemical Characterization of Calf Brain Microtubule Protein Subunits

Cosolvent and Crowding Effects on the Temperature- and Pressure-Dependent Dissociation Process of the α/β-Tubulin Heterodimer

Tubulin is one of the main components of the cytoskeleton of eukaryotic cells. The formation of microtubules depends strongly on environmental and solution conditions, and has been found to be among the most pressure sensitive processes in vivo. We explored the effects of different types of cosolvents, such as trimethylamine-N-oxide (TMAO), sucrose and urea, and crowding agents to mimic cell-like conditions, on the temperature and pressure stability of the building block of microtubules, i. e. the α/β-tubulin heterodimer. To this end, fluorescence and FTIR spectroscopy, differential scanning and pressure perturbation calorimetry as well as fluorescence anisotropy and correlation spectroscopies were applied. The pressure and temperature of dissociation of α/β-tubulin as well as the underlying thermodynamic parameters upon dissociation, such as volume and enthalpy changes, have been determined for the different solution conditions. The temperature and pressure of dissociation of the α/β-tubulin heterodimer and hence its stability increases dramatically in the presence of TMAO and the nanocrowder sucrose. We show that by adjusting the levels of compatible cosolutes and crowders, cells are able to withstand deteriorating effects of pressure even up to the kbar-range.

Proscillaridin A exerts anti-tumor effects through GSK3β activation and alteration of microtubule dynamics in glioblastoma

Glioblastoma (GBM) is characterized by highly aggressive growth and invasive behavior. Due to the highly lethal nature of GBM, new therapies are urgently needed and repositioning of existing drugs is a promising approach. We have previously shown the activity of Proscillaridin A (ProA), a cardiac glycoside inhibitor of the Na(+)/K(+) ATPase (NKA) pump, against proliferation and migration of GBM cell lines. ProA inhibited tumor growth in vivo and increased mice survival after orthotopic grafting of GBM cells. This study aims to decipher the mechanism of action of ProA in GBM tumor and stem-like cells. ProA displayed cytotoxic activity on tumor and stem-like cells grown in 2D and 3D culture, but not on healthy cells as astrocytes or oligodendrocytes. Even at sub-cytotoxic concentration, ProA impaired cell migration and disturbed EB1 accumulation at microtubule (MT) plus-ends and MT dynamics instability. ProA activates GSK3β downstream of NKA inhibition, leading to EB1 phosphorylation on S155 and T166, EB1 comet length shortening and MT dynamics alteration, and finally inhibition of cell migration and cytotoxicity. Similar results were observed with digoxin. Therefore, we disclosed here a novel pathway by which ProA and digoxin modulate MT-governed functions in GBM tumor and stem-like cells. Altogether, our results support ProA and digoxin as potent candidates for drug repositioning in GBM.

Exploring the influence of natural cosolvents on the free energy and conformational landscape of filamentous actin and microtubules

Natural osmolytes have a significant influence on the temperature- and pressure-dependent stability of filamentous actin and microtubules.

Tubulin Dimer Reversible Dissociation: AFFINITY, KINETICS, AND DEMONSTRATION OF A STABLE MONOMER

Tubulins are evolutionarily conserved proteins that reversibly polymerize and direct intracellular traffic. Of the tubulin family only αβ-tubulin forms stable dimers. We investigated the monomer-dimer equilibrium of rat brain αβ-tubulin using analytical ultracentrifugation and fluorescence anisotropy, observing tubulin in virtually fully monomeric and dimeric states. Monomeric tubulin was stable for a few hours and exchanged into preformed dimers, demonstrating reversibility of dimer dissociation. Global analysis combining sedimentation velocity and fluorescence anisotropy yielded Kd = 84 (54-123) nm Dimer dissociation kinetics were measured by analyzing the shape of the sedimentation boundary and by the relaxation of fluorescence anisotropy following rapid dilution of labeled tubulin, yielding koff in the range 10(-3)-10(-2) s(-1) Thus, tubulin dimers reversibly dissociate with moderately fast kinetics. Monomer-monomer association is much less sensitive than dimer-dimer association to solution changes (GTP/GDP, urea, and trimethylamine oxide).

Quantitative analysis of tau-microtubule interaction using FRET

The interaction between the microtubule associated protein, tau and the microtubules is investigated. A fluorescence resonance energy transfer (FRET) assay was used to determine the distance separating tau to the microtubule wall, as well as the binding parameters of the interaction. By using microtubules stabilized with Flutax-2 as donor and tau labeled with rhodamine as acceptor, a donor-to-acceptor distance of 54 ± 1 Å was found. A molecular model is proposed in which Flutax-2 is directly accessible to tau-rhodamine molecules for energy transfer. By titration, we calculated the stoichiometric dissociation constant to be equal to 1.0 ± 0.5 µM. The influence of the C-terminal tails of αβ-tubulin on the tau-microtubule interaction is presented once a procedure to form homogeneous solution of cleaved tubulin has been determined. The results indicate that the C-terminal tails of α- and β-tubulin by electrostatic effects and of recruitment seem to be involved in the binding mechanism of tau.

Macromolecular interaction of halichondrin B analogues eribulin (E7389) and ER-076349 with tubulin by analytical ultracentrifugation

Halichondrin B is an antimitotic drug that inhibits microtubule assembly. To understand the molecular details of its interaction with tubulin, we investigated the binding of two halichondrin B analogues, eribulin (previously, ER-086526, E7389) and ER-076349, to tubulin by quantitative analytical ultracentrifugation. Eribulin is currently undergoing phase III clinical trials for cancer; ER-076349 is a closely related analogue with C.35 hydroxyl instead of C.35 primary amine [Towle, M. J., et al. (2001) Cancer Res. 61, 1013]. Below the critical concentration for microtubule assembly and in the presence of GDP, tubulin undergoes weak self-association into short curved oligomers. Eribulin inhibits this oligomer formation 4-6-fold, while ER-076349 slightly stimulates oligomer formation by 2-fold. This is in contrast to vinblastine which strongly stimulates large spiral polymers by 1000-fold under these same conditions. Vinblastine-induced spiral formation is strongly inhibited by both eribulin and ER-076349. Colchicine binding to the intradimer interface has no significant effect on small oligomer formation or the inhibitory activity of eribulin on this process. These results suggest that halichondrin B analogues bind to the interdimer interface or to the beta-subunit alone, disrupt polymer stability, and compete with vinblastine-induced spiral formation. Stathmin is known to form a tight 1:2 complex with tubulin. Eribulin strongly inhibits formation of the 1:2 stathmin-tubulin complex (>3.3 kcal/mol), while ER-076349 weakens formation of the 1:2 complex by approximately 1.9 kcal/mol. These results suggest that eribulin is a global inhibitor of tubulin polymer formation, disrupting tubulin-tubulin contacts at the interdimer interface. ER-076349 also perturbs tubulin-tubulin contacts, but in a more polymer specific manner, reflecting adaptability of the interdimer interface to drug and polymer polymorphism. These results suggest halichondrin B analogues exhibit unique tubulin-based activities that may underlie the clinical utility of these compounds.

Human microtubule-associated-protein tau regulates the number of protofilaments in microtubules: a synchrotron x-ray scattering study

Microtubules (MTs), a major component of the eukaryotic cytoskeleton, are 25 nm protein nanotubes with walls comprised of assembled protofilaments built from alphabeta heterodimeric tubulin. In neural cells, different isoforms of the microtubule-associated-protein (MAP) tau regulate tubulin assembly and MT stability. Using synchrotron small angle x-ray scattering (SAXS), we have examined the effects of all six naturally occurring central nervous system tau isoforms on the assembly structure of taxol-stabilized MTs. Most notably, we found that tau regulates the distribution of protofilament numbers in MTs as reflected in the observed increase in the average radius R(MT) of MTs with increasing Phi, the tau/tubulin-dimer molar ratio. Within experimental scatter, the change in R(MT) seems to be isoform independent. Significantly, R(MT) was observed to rapidly increase for 0 < Phi < 0.2 and saturate for Phi between 0.2-0.5. Thus, a local shape distortion of the tubulin dimer on tau binding, at coverages much less than a monolayer, is spread collectively over many dimers on the scale of protofilaments. This implies that tau regulates the shape of protofilaments and thus the spontaneous curvature C(o)(MT) of MTs leading to changes in the curvature C(MT) (=1/R(MT)). An important biological implication of these findings is a possible allosteric role for tau where the tau-induced shape changes of the MT surface may effect the MT binding activity of other MAPs present in neurons. Furthermore, the results, which provide insight into the regulation of the elastic properties of MTs by tau, may also impact biomaterials applications requiring radial size-controlled nanotubes.

A novel chalcone derivative which acts as a microtubule depolymerising agent and an inhibitor of P-gp and BCRP in in-vitro and in-vivo glioblastoma models

BACKGROUND

Over the past decades, in spite of intensive search, no significant increase in the survival of patients with glioblastoma has been obtained. The role of the blood-brain barrier (BBB) and especially the activity of efflux pumps belonging to the ATP Binding Cassette (ABC) family may, in part, explain this defect.

METHODS

The in-vitro activities of JAI-51 on cell proliferation were assessed by various experimental approaches in four human and a murine glioblastoma cell lines. Using drug exclusion assays and flow-cytometry, potential inhibitory effects of JAI-51 on P-gp and BCRP were evaluated in sensitive or resistant cell lines. JAI-51 activity on in-vitro microtubule polymerization was assessed by tubulin polymerization assay and direct binding measurements by analytical ultracentrifugation. Finally, a model of C57BL/6 mice bearing subcutaneous GL26 glioblastoma xenografts was used to assess the activity of the title compound in vivo. An HPLC method was designed to detect JAI-51 in the brain and other target organs of the treated animals, as well as in the tumours.

RESULTS

In the four human and the murine glioblastoma cell lines tested, 10 muM JAI-51 inhibited proliferation and blocked cells in the M phase of the cell cycle, via its activity as a microtubule depolymerising agent. This ligand binds to tubulin with an association constant of 2 x 105 M-1, overlapping the colchicine binding site. JAI-51 also inhibited the activity of P-gp and BCRP, without being a substrate of these efflux pumps. These in vitro studies were reinforced by our in vivo investigations of C57BL/6 mice bearing GL26 glioblastoma xenografts, in which JAI-51 induced a delay in tumour onset and a tumour growth inhibition, following intraperitoneal administration of 96 mg/kg once a week. In accordance with these results, JAI-51 was detected by HPLC in the tumours of the treated animals. Moreover, JAI-51 was detected in the brain, showing that the molecule is also able to cross the BBB.

CONCLUSION

These in vitro and in vivo data suggest that JAI-51 could be a good candidate for a new treatment of tumours of the CNS. Further investigations are in progress to associate the title compound chemotherapy to radiotherapy in a rat model.

Modulation of microtubule assembly by the HIV-1 Tat protein is strongly dependent on zinc binding to Tat

Background

During HIV-1 infection, the Tat protein plays a key role by transactivating the transcription of the HIV-1 proviral DNA. In addition, Tat induces apoptosis of non-infected T lymphocytes, leading to a massive loss of immune competence. This apoptosis is notably mediated by the interaction of Tat with microtubules, which are dynamic components essential for cell structure and division. Tat binds two Zn²⁺ ions through its conserved cysteine-rich region in vitro, but the role of zinc in the structure and properties of Tat is still controversial.

Results

To investigate the role of zinc, we first characterized Tat apo- and holo-forms by fluorescence correlation spectroscopy and time-resolved fluorescence spectroscopy. Both of the Tat forms are monomeric and poorly folded but differ by local conformational changes in the vicinity of the cysteine-rich region. The interaction of the two Tat forms with tubulin dimers and microtubules was monitored by analytical ultracentrifugation, turbidity measurements and electron microscopy. At 20°C, both of the Tat forms bind tubulin dimers, but only the holo-Tat was found to form discrete complexes. At 37°C, both forms promoted the nucleation and increased the elongation rates of tubulin assembly. However, only the holo-Tat increased the amount of microtubules, decreased the tubulin critical concentration, and stabilized the microtubules. In contrast, apo-Tat induced a large amount of tubulin aggregates.

Conclusion

Our data suggest that holo-Tat corresponds to the active form, responsible for the Tat-mediated apoptosis.

Large scale purification of brain tubulin with the modified Weisenberg procedure

This method is a modification of the initial procedure employed to purify tubulin from mammalian brain. It consists of tissue homogenization, elimination of cell membranes, ammonium sulfate fractionation, and batch anion exchange, followed by selective precipitation with magnesium chloride. Half gram of electrophoretically homogenous, active, concentrated calf brain tubulin is typically purified in 9 h, dialyzed overnight, and stored under liquid nitrogen. Prior to use the protein is equilibrated in the experimental buffer and its concentration measured. This tubulin preparation has been very extensively characterized. Frozen aliquots have been found to retain microtubule assembly activity after 10 yr of storage.

Synthesis and biological activity of 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin

The trisubstituted purine myoseverin has been recently identified as a novel inhibitor of microtubule assembly. To analyze the effects of modifying its heterocyclic skeleton, we prepared 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin and compared their biological activities. Rearrangement of nitrogen atoms in the heterocycle changes the affinity of the compounds to purified tubulin, as demonstrated by the results of polymerization assays, and affects the proliferation of cancer cell lines. Surprisingly, compound E2GG, a pyrazolo[4,3-d]pyrimidine analogue of myoseverin, displayed inhibitory activity towards both tubulin polymerization and the activity of cyclin-dependent kinases 1, 2 and 7. Such a dual specificity-inhibitor offers a starting point for developing a novel class of antiproliferative agents.

Microtubule protofilament number is modulated in a stepwise fashion by the charge density of an enveloping layer

Microtubules are able to adjust their protofilament (PF) number and, as a consequence, their dynamics and function, to the assembly conditions and presence of cofactors. However, the principle behind such variations is poorly understood. Using synchrotron x-ray scattering and transmission electron microscopy, we studied how charged membranes, which under certain conditions can envelop pre-assembled MTs, regulate the PF number of those MTs. We show that the mean PF number, <N>, is modulated primarily by the charge density of the membranes. <N> decreases in a stepwise fashion with increasing membrane charge density. <N> does not depend on the membrane-protein stoichiometry or the solution ionic strength. We studied the effect of taxol and found that <N> increases logarithmically with taxol/tubulin stoichiometry. We present a theoretical model, which by balancing the electrostatic and elastic interactions in the system accounts for the trends in our findings and reveals an effective MT bending stiffness of order 10-100 k(B)T/nm, associated with the observed changes in PF number.

Cationic membranes complexed with oppositely charged microtubules: hierarchical self-assembly leading to bio-nanotubes

The self-assembly of microtubules and charged membranes has been studied, using x-ray diffraction and electron microscopy. Polyelectrolyte lipid complexes usually form structures templated by the lipid phase, when the polyelectrolyte curvature is much larger than the membrane spontaneous curvature. When the polyelectrolyte curvature approaches the membrane spontaneous curvature, as in microtubules, two types of new structures emerge. Depending on the conditions, vesicles either adsorb onto the microtubule, forming a 'beads on a rod' structure, or coat the microtubule, which now forms the template. Tubulin oligomers then coat the external lipid layer, forming a lipid protein nanotube. The tubulin oligomer coverage at the external layer is determined by the membrane charge density. The energy barrier between the beads on a rod and the lipid-protein nanotube states depends on the membrane bending rigidity and membrane charge density. By controlling the lipid/tubulin stoichiometry we can switch between lipid-protein nanotubes with open ends to lipid-protein nanotubes with closed end with lipid cups. This forms the basis for controlled drug encapsulation and release.

Cationic liposome-microtubule complexes: pathways to the formation of two-state lipid-protein nanotubes with open or closed ends

Intermolecular interactions between charged membranes and biological polyelectrolytes, tuned by physical parameters, which include the membrane charge density and bending rigidity, the membrane spontaneous curvature, the biopolymer curvature, and the overall charge of the complex, lead to distinct structures and morphologies. The self-assembly of cationic liposome-microtubule (MT) complexes was studied, using synchrotron x-ray scattering and electron microscopy. Vesicles were found to either adsorb onto MTs, forming a "beads on a rod" structure, or undergo a wetting transition and coating the MT. Tubulin oligomers then coat the external lipid layer, forming a tunable lipid-protein nanotube. The beads on a rod structure is a kinetically trapped state. The energy barrier between the states depends on the membrane bending rigidity and charge density. By controlling the cationic lipid/tubulin stoichiometry it is possible to switch between two states of nanotubes with either open ends or closed ends with lipid caps, a process that forms the basis for controlled chemical and drug encapsulation and release.

A comparison of weight average and direct boundary fitting of sedimentation velocity data for indefinite polymerizing systems

Analysis of sedimentation velocity data for indefinite self-associating systems is often achieved by fitting of weight average sedimentation coefficients (s(20,w)) However, this method discriminates poorly between alternative models of association and is biased by the presence of inactive monomers and irreversible aggregates. Therefore, a more robust method for extracting the binding constants for indefinite self-associating systems has been developed. This approach utilizes a set of fitting routines (SedAnal) that perform global non-linear least squares fits of up to 10 sedimentation velocity experiments, corresponding to different loading concentrations, by a combination of finite element simulations and a fitting algorithm that uses a simplex convergence routine to search parameter space. Indefinite self-association is analyzed with the software program isodesfitter, which incorporates user provided functions for sedimentation coefficients as a function of the degree of polymerization for spherical, linear and helical polymer models. The computer program hydro was used to generate the sedimentation coefficient values for the linear and helical polymer assembly mechanisms. Since this curve fitting method directly fits the shape of the sedimenting boundary, it is in principle very sensitive to alternative models and the presence of species not participating in the reaction. This approach is compared with traditional fitting of weight average data and applied to the initial stages of Mg(2+)-induced tubulin self-associating into small curved polymers, and vinblastine-induced tubulin spiral formation. The appropriate use and limitations of the methods are discussed.

The glutamine-rich region of the HIV-1 Tat protein is involved in T-cell apoptosis

Human immunodeficiency virus (HIV) infection and the progression to AIDS are characterized by the depletion of CD4(+) T-cells. HIV-1 infection leads to apoptosis of uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated, in part, by the HIV-1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells. We chemically synthesized two 86-residue subtype D Tat proteins, Ug05RP and Ug11LTS, from two Ugandan patients who were clinically categorized as either rapid progressor or long-term survivor, with non-conservative mutations located essentially in the glutamine-rich region. Structural heterogeneities were revealed by CD, which translate into differing trans-activational and apoptotic effects. CD data analysis and molecular modeling indicated that the short alpha-helix observed in subtype D Tat proteins from rapid progressor patients such as Tat Mal and Tat Ug05RP is not present in Ug11LTS. We show that Tat Ug05RP is more efficient than Tat Ug11LTS in its trans-activational role and in inducing apoptosis in binding tubulin via the mitochondrial pathway. The glutamine-rich region of Tat appears to be involved in the Tat-mediated apoptosis of T-cells.

Reversible unfolding of FtsZ cell division proteins from archaea and bacteria. Comparison with eukaryotic tubulin folding and assembly

The stability, refolding, and assembly properties of FtsZ cell division proteins from Methanococcus jannaschii and Escherichia coli have been investigated. Their guanidinium chloride unfolding has been studied by circular dichroism spectroscopy. FtsZ from E. coli and tubulin released the bound guanine nucleotide, coinciding with an initial unfolding stage at low denaturant concentrations, followed by unfolding of the apoprotein. FtsZ from M. jannaschii released its nucleotide without any detectable secondary structural change. It unfolded in an apparently two-state transition at larger denaturant concentrations. Isolated FtsZ polypeptide chains were capable of spontaneous refolding and GTP-dependent assembly. The homologous eukaryotic tubulin monomers misfold in solution, but fold within the cytosolic chaperonin CCT. Analysis of the extensive tubulin loop insertions in the FtsZ/tubulin common core and of the intermolecular contacts in model microtubules and tubulin-CCT complexes shows a loop insertion present at every element of lateral protofilament contact and at every contact of tubulin with CCT (except at loop T7). The polymers formed by purified FtsZ have a distinct limited protofilament association in comparison with microtubules. We propose that the loop insertions of tubulin and its CCT-assisted folding coevolved with the lateral association interfaces responsible for extended two-dimensional polymerization into microtubule polymers.

Mechanisms of tubulin modification by phosphatidylcholine hydroperoxides

The interaction of lipid peroxides with cellular proteins has been postulated to contribute to cellular aging. A potential target for such effects is tubulin, the building block of microtubules. We examined the concentration-dependent effects of phosphatidylcholine hydroperoxides on the ability of tubulin to polymerize into microtubules. The results demonstrated that even very low concentrations of peroxides were sufficient to interfere with the tubulin and, therefore, the microtubule function. Decreased tubulin activity (as measured by tubulin GTPase activity) showed correlation with the modification of methionine and cysteine in tubulin and a change in the tubulin conformational state as indicated by fluorescence and ultraviolet spectroscopic measurements. As no effect on electric conductivity was observed, indicating that modulation of ionic binding was not involved, the interaction mechanism may be a hydrophobic one.

The lipid peroxidation product 4-hydroxynonenal inhibits neurite outgrowth, disrupts neuronal microtubules, and modifies cellular tubulin

Oxidative stress is believed to be an important factor in the development of age-related neurodegenerative diseases such as Alzheimer's disease (AD). The CNS is enriched in polyunsaturated fatty acids and is therefore particularly vulnerable to lipid peroxidation. Indeed, accumulation of lipid peroxidation products has been demonstrated in affected regions in brains of AD patients. Another feature of AD is a change in neuronal microtubule organization. A possible causal relationship between lipid peroxidation products and changes in neuronal cell motility and cytoskeleton has not been investigated. We show here that 4-hydroxy-2(E)-nonenal (HNE), a major product of lipid peroxidation, inhibits neurite outgrowth and disrupts microtubules in Neuro 2A cells. The effect of HNE on microtubules was rapid, being observed after incubation times as short as 15 min. HNE can react with target proteins by forming either Michael adducts or pyrrole adducts. 4-Oxononanal, an HNE analogue that can form only pyrrole adducts but not Michael adducts, had no effect on the microtubules. This suggests that the HNE-induced disruption of microtubules occurs via Michael addition. We also show that cellular tubulin is one of the major proteins modified by HNE and that the HNE adduction to tubulin occurs via Michael addition. Inhibition of neurite outgrowth, disruption of microtubules, and tubulin modification were observed at pathologically relevant HNE concentrations and were not accompanied by cytotoxicity. Our results show that these are proximal effects of HNE that may contribute to cytoskeletal alterations that occur in AD.

Heat-shock protein 90 (hsp90) binds in vitro to tubulin dimer and inhibits microtubule formation

Hsp90 interacts with steroid hormone receptors, protein kinases, and cytoskeletal proteins. The mode of action of hsp90 on microtubules and tubulin has not been investigated. Using isolated purified hsp90 and isolated tubulin, we demonstrated in vitro by difference absorption and fluorescence spectroscopy that hsp90 bound to tubulin with an apparent affinity constant of 5 x 10(5) M-1, assuming an apparent stoichiometry of 1 at 25 degrees C. Using microcalorimetry, we found a delta H of -9.8 +/- 0.8 kJ.mol-1. The binding of hsp90 to tubulin was confirmed by a sedimentation assay. Moreover, we showed that hsp90 inhibited tubulin polymerisation.

The susceptibility of pure tubulin to high magnetic fields: a magnetic birefringence and x-ray fiber diffraction study

The orientational behavior of microtubules assembled in strong magnetic fields has been studied. It is shown that when microtubules are assembled in a magnetic field, they align with their long axis parallel to the magnetic field. The effect of several parameters known to affect the microtubule assembly are investigated with respect to their effect on the final degree of alignment. Aligned samples of hydrated microtubules suitable for low-resolution x-ray fiber diffraction experiments have been produced, and the results obtained from the fiber diffraction experiments have been compared with the magnetic birefringence experiments. Comparisons with earlier fiber diffraction work and small-angle x-ray solution scattering experiments have been made.

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.

Tubulin-tyrosine ligase catalyzes covalent binding of 3-fluoro-tyrosine to tubulin: kinetic and [19F]NMR studies

The use of 3-fluoro-tyrosine as an alternative substrate for the enzyme tubulin:tyrosine ligase which catalyzes the incorporation of tyrosine into the alpha-tubulin subunit was investigated. The incorporation of tyrosine into tubulin was inhibited competitively by 3-fluoro-tyrosine with an apparent Ki of approximately 25 microM. The affinity for this analog was similar to that of tyrosine, confirming that the hydrogen at position 3 of the aromatic ring is not essential for the reaction catalyzed by TTLase. The incorporation of 3-fluoro-tyrosine into the C-terminus of the alpha-tubulin subunit was demonstrated through [19F]NMR spectroscopy. The 3-fluoro-tyrosine signal at -58.6 ppm (trifluoroacetic acid as external standard), with a bandwidth of 24.7 Hz presented a chemical shift of 0.75 ppm upfield and an enlargement in the bandwidth (30.5 Hz) when incorporated into tubulin. These results strongly suggest that this amino acid is exposed to the solvent in tubulin. Tubulin covalently labeled with 3-fluoro-tyrosine was competent to polymerize into microtubules. The use of fluorinated tubulin in [19F]NMR spectroscopy for studying questions concerning protein conformation and interactions will be discussed.

Molecular interaction of tubulin with 1-deaza-7,8-dihydropteridines: a comparative study of enantiomers NSC 613862 (S) and NSC 613863 (R) by Raman and Fourier transform infrared spectroscopy

Pre-resonance Raman spectroscopy has been applied to compare the vibrational modes of the R and S chiral isomers of 1-deaza-7,8-dihydropteridine when they are bound to tubulin. The main Raman bands are due to the chromophore and are coupled with the pi-pi electronic transition of C = C and C = N vibrational stretching. On binding to tubulin, the Raman spectra of both isomers are modified. However, the modifications induced are different for each isomer. The Raman bands due to C = C stretching from the phenyl ring are more strongly modified for the bound R isomer than for the S isomer. This leads us to suggest that R and S isomers differ in terms of their orientation in front of the binding locus of tubulin. In fact, with respect to the orientation of the bulky methyl group, the chromophore of the R isomer is more likely to be positioned against the external surface of either tubulin or GTPase proteins, while that of the S isomer is likely to be positioned away from the surface. The conformational changes induced in tubulin by R and S isomers have also been studied by Fourier transform infrared spectroscopy and by the analysis of amide I and II absorption bands. Both enantiomers induce similar minor changes to the tubulin secondary structure, corresponding to a decrease in the disordered alpha-helical content and accompanied by an increase in the undefined conformation content.

Comparative study of the colchicine binding site and the assembly of fish and mammalian microtubule proteins

Isolated microtubules from cod (Gadus morhua) are apparently more stable to colchicine than bovine microtubules. In order to further characterize this difference, the effect of the colchicine analogue 2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cyclo heptatrien-1-one (MTC) was studied on assembly, as measured by turbidity and sedimentation analysis, and on polymer morphology. MTC has the advantage to bind fast and reversible to the colchicine binding site of tubulin even at low temperatures. It was found to bind to one site in cod brain tubulin, with affinity (6.5 +/- 1.5) x 10(5)M-1 at both low or high temperature, similarly to bovine brain tubulin. However, the effect of the binding differed. At substoichiometric concentrations of MTC bovine brain microtubule assembly was almost completely inhibited, while less effect was seen on the mass of polymerized cod microtubule proteins. A preformed bovine tubulin-colchicine complex inhibited the assembly of both cod and bovine microtubules at substoichiometric concentrations, but the effect on the assembly of cod microtubules was less. At higher concentrations (5 x 10(-5) to 1 x 10(-3) M), MTC induced a large amount of cold-stable spirals of cod proteins, whereas abnormal polymers without any defined structure were formed from bovine proteins. Spirals of cod microtubule proteins were only formed in the presence of microtubule associated proteins (MAPs), indicating that the morphological effect of MTC can be modulated by MAPs. The effects of colchicine and MTC differed. At 10(-5) M colchicine no spirals were formed, while at 10(-4) M and 10(-3) M, a mixture of spirals and aggregates was found. The morphology of the spirals differed both from vinblastine spirals and from the spirals previously found when cod microtubule proteins polymerize in the presence of high Ca2+ concentrations. The present data show that even if the colchicine binding site is conserved between many different species, the bindings have different effects which seem to depend on intrinsic properties of the different tubulins.

Energy transfer studies of the distances between the colchicine, ruthenium red, and bisANS binding sites on calf brain tubulin

Fluorescence energy transfer experiments were performed in order to measure the spatial separation between the colchine and Ruthenium Red binding sites, the high-affinity bisANS and Ruthenium Red sites, and the allocolchicine and high-affinity bisANS sites on calf brain tubulin. Energy transfer was observed between both colchicine and allocolchicine and Ruthenium Red, resulting in a distance of 40-45 A between these sites on the tubulin molecule. No detectable energy transfer could be observed when allocolchicine was used as fluorescence donor and bisANS as acceptor or when bisANS was used as donor and Ruthenium Red as acceptor. This indicates that the distance of separation between the allocolchicine and bisANS sites is greater than 50 A, while that between the bisANS and Ruthenium Red sites is greater than 72 A. On the basis of these and previous distance measurements (Ward & Timasheff, 1988), two triangles of binding sites have been defined (colchicine-bisANS-E-site and colchicine-bisANS-Ruthenium Red). Since the dihedral angle between them is not known, a schematic model has been drawn with all the sites located in a single plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Cooperative multiple binding of bisANS and daunomycin to tubulin

The binding of daunomycin and bisANS to tubulin was studied by direct equilibrium techniques. Both ligands generated abnormal Scatchard plots. Their concave-downward nature indicated positive cooperativity. The data conform to tubulin possessing ca. 35 daunomycin binding sites with a binding constant of 570-1430 M-1. The binding of bisANS is characterized by 1 strong binding site (KA = 4.5 x 10(5) M-1) and 40-50 lower affinity sites. Hill plots of both showed low degrees of cooperativity (m = 1.8 for daunomycin and 2.3 for bisANS). A detailed analysis was carried out of the cooperativity of binding of daunomycin to tubulin. Concentration differences spectra and sedimentation velocity analysis of daunomycin showed that this molecule undergoes self-association in the drug concentration range used in the binding study. The low level of polymerization (approximately tetramer), however, indicated that this could not be the source of the observed cooperativity between 35 molecules. Both the shape and concentration dependence of the daunomycin concentration difference spectra were strikingly similar to those generated on the binding of daunomycin to tubulin, which indicates the stacking of daunomycin in both cases. The observed Scatchard plot of the binding was found to be consistent with a process that involves in part ligand-ligand interactions when complexed to tubulin. Examination of the binding of bisANS in the presence of daunomycin revealed a strong increase of bisANS binding to tubulin, which suggests a loosening of tubulin structure with the exposure of new sites as these ligands bind. The mutual interaction between the two ligands in dilute solution was demonstrated by difference spectroscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

The colchicine-induced GTPase activity of tubulin: state of the product. Activation by microtubule-promoting cosolvents

Colchicine induces a weak assembly-independent GTPase activity in calf brain tubulin [David-Pfeuty, T., Erickson, H. P., & Pantaloni, D. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 5372-5376; Andreu, J. M., & Timasheff, S. N. (1981) Arch. Biochem. Biophys. 211, 151-157]. Kinetic analysis shows a turnover number of 2 x 10(-4) s-1 in 0.01 M sodium phosphate and 4 mM MgCl2, pH 7.0, with an apparent Km for GTP of 10 microM. This activity, which requires Mg2+ ions and attains a plateau at 4 mM MgCl2, is independent of pH over the pH range of 6.6-7.4. This GTPase activity was induced by all colchicine analogues that contain rings A and C (or C'), the strength varying in a manner parallel to the free energy of binding of the ligand. The specific GTPase activity was found to be independent of the tubulin-colchicine complex concentration over the range of 2-20 microM. Sedimentation velocity examination of the product of the reaction showed that GDP-tubulin-colchicine generated by hydrolysis of the E-site GTP was indistinguishable from that produced by nucleotide exchange at the site, the protein assuming the "curved" conformation in both cases. Steady-state kinetic analysis in the presence of high concentrations of microtubule-inducing cosolvent additives revealed an increase in kcat/Km of up to 1 order of magnitude that followed the order poly(ethylene glycol) 6000 (PEG-6000 > PEG-1000 = 2-methyl-2,4- pentanediol > sucrose > L-glutamate > glycerol = PEG-200 > betaine, with no apparent change in Km.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Chemically synthesized 182-235 segment of tau protein and analogue peptides are efficient in vitro microtubule assembly inducers of low apparent sequence specificity

A 54-amino acid peptide reproducing the first and second repeats and intervening spacer sequence of the tubulin binding motif (residues 182-235) of murine tau protein, and several congeners representing different degrees of sequence scrambling have been prepared by solid phase methods and fully characterized chemically. These double-repeat peptides have been shown to induce microtubule formation at concentrations about one order of magnitude lower than single-repeat controls, under conditions close to the critical concentration needed for tubulin self-assembly. On the other hand, partial loss of microtubule-inducing capacity was observed for peptides with primary structures increasingly disorganized with respect to the canonical peptide. These results call into question the assumption that a high degree of primary structure specificity is involved in the tau-tubulin interaction leading to in vitro microtubule formation.

Low resolution structure of microtubules in solution

The structure of microtubules has been characterized to 3 nm resolution employing time-resolved X-ray scattering. This has revealed detailed structural features of microtubules not observed before in solution. The polymerization of highly purified tubulin, induced by the antitumour drug taxol, has been employed as a microtubule model system. This assembly reaction requires Mg2+, is optimal at a 1:1 taxol to tubulin heterodimer molar ratio, proceeds with GTP or GDP and is intrinsically reversible. The X-ray scattering profiles are consistent with identical non-globular alpha and beta-tubulin monomers ordered within the known helical surface lattice of microtubules. Purified tubulin-taxol microtubules have a smaller mean diameter (approx. 22 nm) than those induced by microtubule associated proteins or glycerol (approx. 24 nm), but nearly identical wall substructure to the resolution of the measurements. This is because the majority of the former consist of only 12 protofilaments instead of the typical 13 protofilaments, as confirmed by electron microscopy of thin-sectioned, negatively stained and ice-embedded taxol microtubules. It may be concluded that taxol induces a slight reduction of the lateral contact curvature between tubulin monomers. The main fringe pattern observed in cryo-electron micrographs is consistent with a simple 12 protofilament 3-start skewed lattice model. Cylindrical closure of this lattice can be achieved by tilting the lattice 0.8 degrees with respect to the microtubule axis. The closure implies a discontinuity in the type of lateral contacts between the tubulin monomers (regardless of whether these are of the -alpha-beta- or the -alpha-alpha-/-beta-beta- type), which indicates that lateral contacts and the subunit specificity of taxol binding are, to a large degree, equivalent.

Aging of tubulin at neutral pH: the destabilizing effect of vinca alkaloids

The effect of the vinca alkaloid drugs, vincristine, vinblastine, catharanthine, and vindoline, on the aging process of tubulin has been examined. It was found that addition of vincristine or vinblastine accelerated by a factor of 3-3.5 the transformation of tubulin from the 5.8 S alpha-beta-tubulin dimer to paucidisperse polymers, with an average sedimentation coefficient of 9 S, previously observed in the absence of drugs (V. Prakash and S. N. Timasheff, 1982, J. Mol. Biol. 160, 499-515). This transformation of tubulin from 5.8 S to "9 S" followed pseudo-first-order kinetics whether the starting protein was predominantly dimeric (i.e., at low drug concentration) or self-associated into the reversible linear polymers induced by the vinca alkaloid drugs at high drug concentration (G. C. Na and S. N. Timasheff, 1980, Biochemistry 19, 1355-1365; V. Prakash and S. N. Timasheff, 1985, Biochemistry 24, 5004-5010). Identical kinetics were found in a fluorescence examination of the loss by tubulin of its ability to bind colchicine specifically, indicating that the rate determining step is a protein conformational change that induces a major change in the far uv circular dichroism spectrum of tubulin. The found lack of an effect of dithiothreitol on the aging and aggregation processes is consistent with the irreversible aggregation being due to the intermolecular coalescence of nonpolar patches on the protein. The observations that vincristine binds to aged tubulin and that the aging of tubulin is accompanied by quenching of the tryptophan fluorescence similar to that which occurs on the binding of the vinca drugs has led to the proposal that the vinca alkaloids stabilize the aged conformation of the protein by interacting with nonpolar regions that may be related to the aggregation sites.

Thermodynamics of reversible monomer-dimer association of tubulin

The equilibrium between the rat brain tubulin alpha beta dimer and the dissociated alpha and beta monomers has been studied by analytical ultracentrifugation with use of a new method employing short solution columns, allowing rapid equilibration and hence short runs, minimizing tubulin decay. Simultaneous analysis of the equilibrium concentration distributions of three different initial concentrations of tubulin provides clear evidence of a single equilibrium characterized by an association constant, Ka, of 4.9 X 10(6) M-1 (Kd = 2 X 10(-7) M) at 5 degrees, corresponding to a standard free energy change on association delta G degrees = -8.5 kcal mol-1. Colchicine and GDP both stabilize the dimer against dissociation, increasing the Ka values (at 4.5 degrees C) to 20 X 10(6) and 16 X 10(6) M-1, respectively. Temperature dependence of association was examined with multiple three-concentration runs at temperatures from 2 to 30 degrees C. The van't Hoff plot was linear, yielding positive values for the enthalpy and entropy changes on association, delta S degrees = 38.1 +/- 2.4 cal deg-1 mol-1 and delta H degrees = 2.1 +/- 0.7 kcal mol-1, and a small or zero value for the heat capacity change on association, delta C p degrees. The entropically driven association of tubulin monomers is discussed in terms of the suggested importance of hydrophobic interactions to the stability of the monomer association and is compared to the thermodynamics of dimer polymerization.

Possible regulation of the in vitro assembly of bovine brain tubulin by the bovine thioredoxin system

Microtubule assembly in vitro and in vivo is highly sensitive to a variety of sulfhydryl-reactive reagents, raising the question of the possible existence of a physiological sulfhydryl-mediated system for regulating microtubule assembly. However, the specific reagents which have previously been used to inhibit microtubule assembly in vitro are either nonphysiological or, if physiological, effective only at concentrations much higher than their physiological ones. Because of reports of association in vivo between microtubules and the sulfhydryl-reactive proteins thioredoxin and thioredoxin reductase, we decided to examine the interaction in vitro between microtubules and the thioredoxin system, comprising thioredoxin, thioredoxin reductase and NADPH. At pH 6.8, both the mammalian and the Escherichia coli thioredoxin systems inhibited microtubule assembly by 4-35% (19 +/- 9%) by reducing one intra-subunit disulfide bond in the tubulin dimer. The thioredoxin-reducible disulfide of the tubulin dimer remains protected from thioredoxin in the assembled microtubules. Thioredoxin or thioredoxin reductase alone, or together in the absence of NADPH, were incapable of either reducing tubulin or inhibiting microtubule assembly. Microtubules formed from reduced tubulin were found to be stable and morphologically identical to those obtained from native tubulin dimers. Since the components of the thioredoxin system were used at concentrations similar to their physiological ones, our results suggest a potential role of the thioredoxin system in regulation of microtubule assembly in vivo.

Tubulin sulfhydryl groups as probes and targets for antimitotic and antimicrotubule agents

The sulfhydryl groups of tubulin are highly reactive entities. The reactivity of the sulfhydryl groups is sensitive to the presence of tubulin ligands, making these groups excellent probes for the interaction of tubulin with ligands. When tubulin is reacted with N,N'-ethylenebis-(iodoacetamide), two intrachain cross-links form in the beta subunit. Formation of one of these cross-links is completely blocked by colchicine, podophyllotoxin, and nocodazole; formation of the other is blocked completely by maytansine, phomopsin A and GTP and partly by Vinca alkaloids. Different ligands also differ in their effect on the rate of alkylation of tubulin with iodo[14C]acetamide, with vinblastine and phomopsin A being strong inhibitors and maytansine having very little effect. Oxidation of certain key sulfhydryl groups can inhibit microtubule assembly. One of these sulfhydryl groups appears to be cys239, but there are others not yet identified. Sulfhydryl-oxidizing agents also interfere with microtubule-mediated processes in vivo, raising the question of the existence of a physiological regulator of microtubule assembly. Potential physiological regulators have been examined to see if they can control microtubule assembly in vitro at their physiological concentrations. Of the ones that have been examined, thioredoxin and thioredoxin reductase are much better candidates for being physiological regulators than are either cystamine or glutathione.

Tubulin assembly probed with antibodies to synthetic peptides

Antibodies to synthetic peptides from the alpha and beta-tubulin sequences were employed to study zones of this protein active in microtubule assembly. In purified calf brain tubulin, six short sequences, selected according to their hydrophilicity and conservation, were found to be accessible to their affinity-purified immunoglobulin G (IgG) antibodies, in a competition radioimmunoassay performed under non-assembly native conditions. This indicated that the six sequences are exposed on the surface of the tubulin alpha beta heterodimer. IgG antibodies to the alpha(430-443) and beta(412-431) sequences perturbed substoichiometrically the assembly of purified tubulin, inducing microtubule bundling and the formation of opened up structures. These positions, which are close to the C termini, were accessible to the anti-peptide antibodies in taxol-induced microtubules, Zn2(+)-induced tubulin sheets, Mg2(+)-induced tubulin rings and in PtK2 cell microtubules. This, together with the comparison of the sizes and gross shapes of the antibody probes and microtubules, suggested that these sequences might be located at the protruding parts of the protofilaments. Antibodies to positions alpha(155-168) did not react with microtubules, while the equivalent zone beta(153-165) was accessible. The alpha(214-226) and beta(241-256) sequences were antigenically occluded in the taxol microtubules, Zn2(+)-induced sheets and Mg2(+)-induced ring arrays, as well as in native microtubules from PtK2 cells, though they became reactive by fixation. This result strongly suggested that these two zones are close to tubulin-tubulin contact sites. A working model is proposed in which the positions alpha(214-226) and beta(241-256) are close to the axial contacts between heterodimers, which lead to protofilament formation, while the positions alpha(241-256) and beta(214-226) are suggested to be related to the alpha-beta binding interface within the heterodimer.

C-terminal cleavage of tubulin by subtilisin enhances ring formation

Following cleavage of alpha- and beta-tubulin C termini, under mild conditions we observed that microtubule-related polymers were formed, and also that ring aggregates were abundant. These ring aggregates were clearly detected by turbidity and electron microscope studies under standard assembly conditions. It was found that removal of the C-terminal fragments of tubulin (phosphocellulose-tubulin or Weisenberg tubulin) markedly favored Mg2(+)-induced ring formation. Binding of GDP to the exchangeable nucleotide site of cleaved tubulin further enhanced ring formation. The cleaved tubulin-GDP ring aggregates could be classified into three types: aggregates without apparent order, bidimensionally ordered ring aggregates, and stacks of rings. Temperature had little effect on the formation of these ring aggregates; however, they were very sensitive to ionic strength.

4-Hydroxynonenal interacts with tubulin by reacting with its functional -SH groups

4-Hydroxynonenal, which is one of the most important products of lipid peroxidation, alters microtubular organization and structure in 3T3 fibroblasts. Changes in cell shape and the disappearance of microtubules are observed by immunofluorescence after incubation with the aldehyde, and the colchicine binding activity of tubulin from 3T3 cells is modified. Moreover, the aldehyde determines a decrease in the ability of purified tubulin to polymerize and to bind colchicine. These effects may be related to the interaction of the aldehyde with functional -SH groups of tubulin which are necessary for protein integrity and functions. Indeed, the addition of cysteine protects against the damaging effects of the aldehyde.