Isolation of microtubule protein from mammalian brain frozen for extended periods of time

Interaction of Colchicine-Site Ligands With the Blood Cell-Specific Isotype of β-Tubulin—Notable Affinity for Benzimidazoles

Tubulin, the main component of microtubules, is an α-β heterodimer that contains one of multiple isotypes of each monomer. Although the isotypes of each monomer are very similar, the beta tubulin isotype found in blood cells is significantly divergent in amino acid sequence compared to other beta tubulins. This isotype, beta class VI, coded by human gene TUBB1, is found in hematologic cells and is recognized as playing a role in platelet biogenesis and function. Tubulin from the erythrocytes of the chicken Gallus gallus contains almost exclusively βVI tubulin. This form of tubulin has been reported to differ from brain tubulin in binding of colchicine-site ligands, previously thought to be a ubiquitous characteristic of tubulin from higher eukaryotes. In this study, we sought to gain a better understanding of the structure-activity relationship of the colchicine site of this divergent isotype, using chicken erythrocyte tubulin (CeTb) as the model. We developed a fluorescence-based assay to detect binding of drugs to the colchicine site and used it to study the interaction of 53 colchicine-site ligands with CeTb. Among the ligands known to bind at this site, most colchicine derivatives had lower affinity for CeTb compared to brain tubulin. Remarkably, many of the benzimidazole class of ligands shows increased affinity for CeTb compared to brain tubulin. Because the colchicine site of human βVI tubulin is very similar to that of chicken βVI tubulin, these results may have relevance to the effect of anti-cancer agents on hematologic tissues in humans.

Taurine Is Covalently Incorporated into Alpha-Tubulin

Taurine is the most abundant free amino acid in the human body. It is found in relatively high concentrations (1-10 mM) in many animal tissues but not in plants. It has been studied since the early 1800s but has not been found to be covalently incorporated into proteins in any animal tissue. Taurine has been found in only one macromolecular complex as a post-transcriptional modification to mitochondrial tRNA. Tubulin is the subunit of microtubules found in all eukaryotic species and almost all eukaryotic cells and subject to numerous post-translational modifications (PTMs). An important PTM on α-tubulin is the removal and re-ligation of the final carboxyl residue, tyrosine. We here demonstrate that taurine can be covalently incorporated at the C-terminal end of alpha-tubulin in avian erythrocytes in a reaction that requires the de-tyrosination PTM and prevents the re-tyrosination PTM. Further, this is, to our knowledge, the first instance of taurine incorporation into a large protein.

All tubulins are not alike: Heterodimer dissociation differs among different biological sources

Tubulin, the subunit of microtubules, is a noncovalent heterodimer composed of one α- and one β-tubulin monomer. Both tubulins are encoded by multiple genes or composed of different isotypes, which are differentially expressed in different tissues and in development. Tubulin αβ dimers are found throughout the eukaryotes and, although very similar, are known to differ among organisms. We seek to investigate tubulins from different tissues and different organisms for a basic physical characteristic: heterodimer stability and monomer exchange between heterodimers. We previously showed that mammalian brain tubulin heterodimers reversibly dissociate, following the mass action law. Dissociation yields native monomers that can exchange with added tubulin to form new heterodimers. Here, we compared the dissociation of tubulins from multiple sources, including mammalian (rat) brain, cultured human cells (HeLa cells), chicken brain, chicken erythrocytes, and the protozoan Leishmania We used fluorescence-detected analytical ultracentrifugation to measure tubulin dissociation over a >1000-fold range in concentration and found that tubulin heterodimers from different biological sources differ in K_d by as much as 150-fold under the same conditions. Furthermore, when fluorescent tracer tubulins from various sources were titrated with unlabeled tubulin from a single source (rat brain tubulin), heterologous dimerization occurred, exhibiting similar affinities, in some cases binding even more strongly than with autologous tubulin. These results provide additional insight into the regulation of heterodimer formation of tubulin from different biological sources, revealing that monomer exchange appears to contribute to the sorting of α- and β-tubulin monomers that associate following tubulin folding.

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).

Measurement of in vitro microtubule polymerization by turbidity and fluorescence

Tubulin polymerization may be conveniently monitored by the increase in turbidity (optical density, or OD) or by the increase in fluorescence intensity of diamidino-phenylindole. The resulting data can be a quantitative measure of microtubule (MT) assembly, but some care is needed in interpretation, especially of OD data. Buffer formulations used for the assembly reaction significantly influence the polymerization, both by altering the critical concentration for polymerization and by altering the exact polymer produced-for example, by increasing the production of sheet polymers in addition to MT. Both the turbidity and the fluorescence methods are useful for demonstrating the effect of MT-stabilizing or -destabilizing additives.

Production of reliable MALDI spectra with quality threshold clustering of replicates

We present the first application of the quality threshold (QT) clustering algorithm to mass spectrometry (MS) data. The unique abilities of QT clustering to yield precision nodes that are commensurate with the mass measurement precision of the instrument are exploited to generate a consensus spectrum out of multiple replicate spectra. The spectral dot product and confidence intervals are used as a tool for evaluating the similarity and reproducibility between the consensus and replicates. The method is equally applicable to high and low resolution measurements. This paper demonstrates applications to linear spectra from a matrix assisted laser desorption ionization (MALDI) time of flight (TOF) instrument as well as peptide fragmentation data obtained from a TOF/TOF after unimolecular decomposition. The advantages of clustering to mitigate the inherent precision the shortcomings of MALDI data are discussed.

Isolating tubulin from nonneural sources

Tubulin is a highly conserved, negatively charged protein that is found in essentially all eukaryotic cells. These properties ensure that isolation protocols successful in one system will likely work, with a few modifications, in most systems. Tubulin has been isolated most frequently from mammalian brain, and the main difference encountered in other systems versus brain is that tubulin is much less abundant in nearly all other sources than it is in brain. This means that attempting to purify tubulin by direct polymerization from a homogenate will often fail or be quite inefficient. However, the conservation of negative charge on tubulin means that an initial ion exchange step can be used to both purify and concentrate the protein from most systems. Polymerization-competent tubulin can usually be obtained by inducing polymerization in the salt eluate from the ion exchange step. We describe protocols for this procedure and describe its application to a number of vertebrate, fungal, protozoal, and plant sources.

Direct measurement of energy fluxes from mitochondria into cytoplasm in permeabilized cardiac cells in situ: some evidence for Mitochondrial Interactosome

The aim of this study was to measure energy fluxes from mitochondria in isolated permeabilized cardiomyocytes. Respiration of permeabilized cardiomyocytes and mitochondrial membrane potential were measured in presence of MgATP, pyruvate kinase - phosphoenolpyruvate and creatine. ATP and phosphocreatine concentrations in medium surrounding cardiomyocytes were determined. While ATP concentration did not change in time, mitochondria effectively produced phosphocreatine (PCr) with PCr/O(2) ratio equal to 5.68 +/- 0.14. Addition of heterodimeric tubulin to isolated mitochondria was found to increase apparent Km for exogenous ADP from 11 +/- 2 microM to 330 +/- 47 microM, but creatine again decreased it to 23 +/- 6 microM. These results show directly that under physiological conditions the major energy carrier from mitochondria into cytoplasm is PCr, produced by mitochondrial creatine kinase (MtCK), which functional coupling to adenine nucleotide translocase is enhanced by selective limitation of permeability of mitochondrial outer membrane within supercomplex ATP Synthasome-MtCK-VDAC-tubulin, Mitochondrial Interactosome.

Nitrosoureas inhibit the stathmin-mediated migration and invasion of malignant glioma cells

Malignant gliomas are the most common primary intrinsic brain tumors and are highly lethal. The widespread migration and invasion of neoplastic cells from the initial site of tumor formation into the surrounding brain render these lesions refractory to definitive surgical treatment. Stathmin, a microtubule-destabilizing protein that mediates cell cycle progression, can also regulate directed cell movement. Nitrosoureas, traditionally viewed as DNA alkylating agents, can also covalently modify proteins such as stathmin. We therefore sought to establish a role for stathmin in malignant glioma cell motility, migration, and invasion and determine the effects of nitrosoureas on these cell movement-related processes. Scratch wound-healing recovery, Boyden chamber migration, Matrigel invasion, and organotypic slice invasion assays were performed before and after the down-regulation of cellular stathmin levels and in the absence and presence of sublethal nitrosourea ([1-(2-chloroethyl)-3-cyclohexyl-l-nitrosourea]; CCNU) concentrations. We show that decreases in stathmin expression lead to significant decreases in malignant glioma cell motility, migration, and invasion. CCNU, at a concentration of 10 micromol/L, causes similar significant decreases, even in the absence of any effects on cell viability. The direct inhibition of stathmin by CCNU is likely a contributing factor. These findings suggest that the inhibition of stathmin expression and function may be useful in limiting the spread of malignant gliomas within the brain, and that nitrosoureas may have therapeutic benefits in addition to their antiproliferative effects.

Regulation of respiration in brain mitochondria and synaptosomes: restrictions of ADP diffusion in situ, roles of tubulin, and mitochondrial creatine kinase

The role of ubiquitous mitochondrial creatine kinase (uMtCK) reaction in regulation of mitochondrial respiration was studied in purified preparations of rat brain synaptosomes and mitochondria. In permeabilized synaptosomes, apparent Km for exogenous ADP, Km (ADP), in regulation of respiration in situ was rather high (110 +/- 11 microM) in comparison with isolated brain mitochondria (9 +/- 1 microM). This apparent Km for ADP observed in isolated mitochondria in vitro dramatically increased to 169 +/- 52 microM after their incubation with 1 muM of dimeric tubulin showing that in rat brain, particularly in synaptosomes, mitochondrial outer membrane permeability for ADP, and ATP may be restricted by tubulin binding to voltage dependent anion channel (VDAC). On the other hand, in synaptosomes apparent Km (ADP) decreased to 25 +/- 1 microM in the presence of 20 mM creatine. To fully understand this effect of creatine on kinetics of respiration regulation, complete kinetic analysis of uMtCK reaction in isolated brain mitochondria was carried out. This showed that oxidative phosphorylation specifically altered only the dissociation constants for MgATP, by decreasing that from ternary complex MtCK.Cr.MgATP (K (a)) from 0.13 +/- 0.02 to 0.018 +/- 0.007 mM and that from binary complex MtCK.MgATP (K (ia)) from 1.1 +/- 0.29 mM to 0.17 +/- 0.07 mM. Apparent decrease of dissociation constants for MgATP reflects effective cycling of ATP and ADP between uMtCK and adenine nucleotide translocase (ANT). These results emphasize important role and various pathophysiological implications of the phosphocreatine-creatine kinase system in energy transfer in brain cells, including synaptosomes.

Evaluating reproducibility and similarity of mass and intensity data in complex spectra--applications to tubulin

We present a data processing approach based on the spectral dot product for evaluating spectral similarity and reproducibility. The method introduces 95% confidence intervals on the spectral dot product to evaluate the strength of spectral correlation; it is the only calculation described to date that accounts for both the non-normal sampling distribution of the dot product and the number of peaks the spectra have in common. These measures of spectral similarity allow for the recursive generation of a consensus spectrum, which incorporates the most consistent features from statistically similar replicate spectra. Taking the spectral dot product and 95% confidence intervals between consensus spectra from different samples yields the similarity between these samples. Applying the data analysis scheme to replicates of brain tubulin CNBr peptides enables a robust comparison of tubulin isotype expression and post-translational modification patterns in rat and cow brains.

Association of the CYP1B1*3 allele with survival in patients with prostate cancer receiving docetaxel

Using a single nucleotide polymorphism association study in 52 men with prostate cancer receiving docetaxel, we found that individuals carrying two copies of the CYP1B1*3 polymorphic variant had a poor prognosis after docetaxel-based therapies compared with individuals carrying at least one copy of the CYP1B1*1 allele (30.6 versus 12.8 months; P=0.0004). The association between CYP1B1*3 and response to therapy was not observed in similar subjects receiving non-taxane-based therapy (P=0.18). The systemic clearance of docetaxel was also unrelated to CYP1B1 genotype status (P=0.39), indicating that the association of CYP1B1*3 with clinical response is not due to docetaxel metabolism. To explain these results, we hypothesized that an indirect gene-drug interaction was interfering with the primary mechanism of action of docetaxel, tubulin polymerization. We therefore conducted tubulin polymerization experiments with taxanes in the presence or absence of certain CYP1B1 estrogen metabolites, which are known to bind to nucleophilic sites in proteins and DNA, that revealed the primary estrogen metabolite of CYP1B1, 4-hydroxyestradiol (4-OHE2), when oxidized to estradiol-3,4-quinone strongly inhibits tubulin polymerization. The 4-OHE2 is also formed more readily by the protein encoded by the CYP1B1*3 allele, validating further our data in patients. Furthermore, estradiol-3,4-quinone reacted in vitro with docetaxel to form the 4-OHE2-docetaxel adduct. This pilot study provides evidence that CYP1B1*3 may be an important marker for estimating docetaxel efficacy in patients with prostate cancer. This link is likely associated with CYP1B1*3 genotype-dependent estrogen metabolism.

Structure–activity-relationship studies of conformationally restricted analogs of combretastatin A-4 derived from SU5416

A series of combretastatin A-4 analogs derived from the ATP competitive, VEGF receptor tyrosine kinase inhibitor, SU5416 were synthesized. The cytotoxic effects of the analogs were evaluated against PC-3 and MDA-MB-231 cancer cell lines, as well as their abilities to inhibit tubulin polymerization. Results are compared to those of compound 1, our lead compound previously reported.

A thalidomide analogue with in vitro antiproliferative, antimitotic, and microtubule-stabilizing activities

We discovered a thalidomide analogue [5-hydroxy-(2,6-diisopropylphenyl)-1H-isoindole-1,3-dione (5HPP-33)] with antiproliferative activity against nine cancer cell lines in vitro. Flow cytometric analyses showed that the compound caused G2-M arrest, which occurred mainly at the mitotic phase. In addition, immunofluorescence microscopy and in vitro tubulin polymerization studies showed that 5HPP-33 has antimicrotubule activity with a paclitaxel-like mode of action. It is effective against four different paclitaxel-resistant cell lines. Thus, 5HPP-33 represents a potential antitumor agent.

Identification of 12Cysbeta on tubulin as the binding site of tubulyzine

We have undertaken quantitative binding site studies in order to identify the binding site of the known microtubule destabilizing agents, the tubulyzines, in the tubulin dimer. Two different approaches were employed that utilized the tubulyzines and their derivatives. The first approach was based on a chemical affinity labeling method using tubulyzine affinity derivatives, and the second approach employed the mass spectrometric measurement of the differential reactivity of cysteines using the tubulyzines and monobromobimane. Based on overlapping data from these two approaches, we propose that the tubulyzines bind at the guanosine-5'-triphosphate binding site of beta-tubulin. Interestingly, we also show that the tubulyzines' binding to tubulin induces a conformational change in tubulin that prevents further interaction of the 239Cysbeta with other reagents.

Selective antimicrotubule activity of N1-phenyl-3,5-dinitro-N4,N4-di-n-propylsulfanilamide (GB-II-5) against kinetoplastid parasites

Analogs of the antimitotic herbicide oryzalin (3,5-dinitro-N4,N4-di-n-propylsulfanilamide) were recently prepared that were more potent in vitro than the parent compound against the kinetoplastid parasite Leishmania donovani (Bioorg Med Chem Lett 12:2395-2398, 2002). In the present work, we show that the most active molecule in the group, N1-phenyl-3,5-dinitro-N4,N4-di-n-propylsulfanilamide (GB-II-5), is a potent, selective antimitotic agent against kinetoplastid parasites. GB-II-5 possesses IC50 values of 0.41 and 0.73 microM in vitro against two strains of the related parasite Trypanosoma brucei but is much less toxic to J774 murine macrophages and PC3 prostate cancer cells, exhibiting IC50 values of 29 and 35 microM against these lines, respectively. Selectivity is also observed for GB-II-5 with purified leishmanial and mammalian tubulin. The assembly of 15 microM leishmanial tubulin is completely inhibited by 10 microM GB-II-5, whereas 40 microM GB-II-5 inhibits the assembly of 15 microM porcine brain tubulin by only 17%. In cultured L. donovani and T. brucei, treatment with 5 and 0.5 microM GB-II-5, respectively, causes a striking increase in the fraction of G2M cells compared with control. Given the potency and selectivity of this agent against kinetoplastid tubulin, GB-II-5 emerges as an exciting new antitrypanosomal and antileishmanial lead compound.

The effects of Tubulin denaturation on the characterization of its polymerization behavior

We report here upon a simulation study examining the effect of a dynamic mode of tubulin denaturation upon the kinetic and thermodynamic characterisation of the polymer formed for two idealized models of a tubulin polymerization reaction: (i) an irreversibly polymerizing system; and (ii) a reversibly polymerizing system. The effects of each denaturation mode upon the two model systems behavior are highlighted by interpretation of the data in terms of the classical Oosawa reversible condensation polymerization model. We reveal here findings which suggest that the measurement strategy in concert with Tubulin's instability over the time course of the experiment may bias the results obtained so as to make an irreversible system's behavior conform to the equilibrium model or alternatively distort the results obtained from a truly reversible system to produce values of the critical concentration quite seriously in error. It was also found that Tubulin denaturation may seriously distort parameter estimates gained from a kinetic characterization of the system (e.g. nucleus size and growth rate constant).

Use of small-angle neutron scattering to study tubulin polymers

Small-angle neutron scattering has been used to examine taxol-stabilized microtubules and other tubulin samples in both H(2)O and D(2)O buffers. Measurements were made at pH/pD values between 6.0 and 7.8, and observed scattered intensities, I(Q), have been interpreted in terms of multicomponent models of microtubules and related tubulin polymers. A semiquantitative curve fitting procedure has been used to estimate the relative amounts of the supramolecular components of the samples. At both pH and pD 7.0 and above, the tubulin polymers are seen to be predominantly microtubules. Although in H(2)O buffer the polymer distribution is little changed as the pH varies, when pD is lowered the samples appear to contain an appreciable amount of sheetlike structures and the average microtubule protofilament number increases from ca. 12.5 at pD > or = approximately 7.0 to ca. 14 at pD approximately 6.0. Such structural change indicates that analysis of microtubule solutions based on H(2)O/D(2)O contrast variation must be performed with caution, especially at lower pH/pD.

Charge variants of tubulin, tubulin S, membrane-bound and palmitoylated tubulin from brain and pheochromocytoma cells

Isoelectric focusing (IEF) of only approximately 1 microg of rat brain tubulin yields 27-30 distinct charge variants in the pH range of 4.5-5.4 with band separations of 0.01-0.02 pH units as detected by silver staining. Variants can be efficiently transferred from the immobilized gradient strip to polyvinylidene difluoride (PVDF) membranes for reaction with monoclonal antibodies. C-terminal-directed antibodies to alpha- and beta-tubulin yield patterns similar to N-terminal-directed antibodies. Removal of the acidic C-termini with subtilisin to form tubulin S increases the pI values by approximately 1 pH unit, leads to a loss in the isoelectric distinction between the alpha- and beta-tubulin variants seen by N-terminal-directed antibodies, and abolishes reactions with all beta-variants and all but three alpha variants by C-terminal-directed antibodies (TU-04 and TU-14). Many, but not all, of the variants are substrates for autopalmitoylation of rat brain tubulin. The distribution of isoelectric variants differs between cytoplasm and membrane fractions from PC12 pheochromocytoma cells. A potential role for different variants is suggested.

2',3'-Cyclic nucleotide 3'-phosphodiesterase: a membrane-bound, microtubule-associated protein and membrane anchor for tubulin

2',3'-Cyclic nucleotide-3'-phosphodiesterase (CNP) is firmly associated with tubulin from brain tissue and FRTL-5 thyroid cells as demonstrated by copolymerization with microtubules through several warm/cold cycles, the presence of CNP activity in purified tubulin preparations, and identical behavior during various extraction procedures. CNP acts as a microtubule-associated protein in promoting microtubule assembly at low mole ratios. This activity resides in the C terminus of the enzyme, which, by itself, promotes microtubule assembly at higher mole ratios. Phosphorylation of CNP interferes with its assembly-promoting activity, as does deletion of the C terminus, which leads to abnormal microtubule distribution in the cell. Submembranous colocalization of the proteins and CNP-dependent microtubule organization suggest that CNP is a membrane-bound microtubule-associated protein that can link tubulin to membranes and may regulate cytoplasmic microtubule distribution.

p53 is associated with cellular microtubules and is transported to the nucleus by dynein

Here we show that p53 protein is physically associated with tubulin in vivo and in vitro, and that it localizes to cellular microtubules. Treatment with vincristine or paclitaxel before DNA-damage or before leptomycin B treatment reduces nuclear accumulation of p53 and expression of mdm2 and p21. Overexpression of dynamitin or microinjection of anti-dynein antibody before DNA damage abrogates nuclear accumulation of p53. Our results indicate that transport of p53 along microtubules is dynein-dependent. The first 25 amino acids of p53 contain the residues that are essential for binding to microtubules. We propose that functional microtubules and the dynein motor protein participate in transport of p53 and facilitate its accumulation in the nucleus after DNA damage.

HIV-1 rev depolymerizes microtubules to form stable bilayered rings

We describe a novel interaction between HIV-1 Rev and microtubules (MTs) that results in the formation of bilayered rings that are 44-49 nm in external diameter, 3.4-4.2 MD (megadaltons) in mass, and have 28-, 30-, or 32-fold symmetry. Ring formation is not sensitive to taxol, colchicine, or microtubule-associated proteins, but requires Mg(2+) and is inhibited by maytansine. The interaction involves the NH(2)-terminal domain of Rev and the face of tubulin exposed on the exterior of the MTs. The NH(2)-terminal half of Rev has unexpected sequence similarity to the tubulin-binding portion of the catalytic/motor domains of the microtubule-destabilizing Kin I kinesins. We propose a model wherein binding of Rev dimers to MTs at their ends causes segments of two neighboring protofilaments to peel off and close into rings, circumferentially containing 14, 15, or 16 tubulin heterodimers, with Rev bound on the inside. Rev has a strong inhibitory effect on aster formation in Xenopus egg extracts, demonstrating that it can interact with tubulin in the presence of normal levels of cellular constituents. These results suggest that Rev may interact with MTs to induce their destabilization, a proposition consistent with the previously described disruption of MTs after HIV-1 infection.

FR901228 causes mitotic arrest but does not alter microtubule polymerization

FR901228, a natural cyclic depsipeptide, shows high cytotoxicity against human cancer cell lines (low nM IC50 values). Cells exposed to FR901228 arrest with G1 or G2/M DNA content; S phase is depleted. G2/M cells include cells arrested in mitosis. We wished to understand the mitotic arrest by this compound. Mitotic arrest is often due to interference with microtubules and COMPARE testing in the NCI drug screen indicated a possible taxane-like mechanism. Testing of FR901228 for tubulin binding or alteration of in vitro MT assembly failed to reveal any effect. Likewise, examination of cellular microtubules following exposure to FR901228 did not reveal any change. Similar G2/M accumulation was observed in MCF7, MCF10 and PC3 cells. About 50% of G2/M cells were mitotic and contained microtubule spindles. Mitotic cells peaked at about 14-16 h drug exposure and declined to near 0% by 24-30 h. The block was at prometaphase, with numerous chromosomes unattached to the spindle. We conclude that FR901228 induces formation of aberrant spindles probably by interfering with chromosome attachment, causing mitotic accumulation without affecting mitotic microtubules.

Preparation and properties of pure tubulin S

Limited proteolysis of the tubulin dimer (alphabeta) by subtilisin occurs more rapidly with beta than with alpha tubulin. This leads to the formation of an intermediate hybrid dimer, alphabeta(s), before both C termini are cleaved to form tubulin S(alpha(s)beta(s)). The three forms of tubulin usually coexist in subtilisin-treated preparations and such cross-contamination can be reliably detected only by running SDS-polyacrylamide gels well beyond expulsion of the dye front. Previously published preparations have not ruled out such contamination or have formed poorly reversible polymers. Because ion exchange separation incurred substantial protein losses, we have developed a new protocol for rapid preparation of tubulin S (alpha(s)beta(s), free of alphabeta or alphabeta(s)) that is based on proteolysis at low ionic strength. This increases the relative rate of C terminal cleavage of beta tubulin. The product forms sheets, bundles, or rings that are depolymerized by cold, salt, and podophyllotoxin, partially depolymerized by Ca2+, and has a decreased critical concentration for polymerization that can be further decreased by taxol. We have also found a method for forming nearly pure alphabeta(s) dimers by using methods that retard proteolysis of the C terminus of alpha tubulin.

Purification, characterization, and drug susceptibility of tubulin from Leishmania

Past work suggests that tubulin from kinetoplastid parasites may present an excellent drug target. To explore this possibility, tubulin was purified on a milligram scale from Leishmania mexicana amazonensis promastigotes by sonication, DEAE-Sepharose chromatography, and one cycle of assembly-disassembly. Purified leishmanial tubulin is recognized by commercially available anti-tubulin antibodies and displays concentration dependent assembly in vitro. The vinca site agents vinblastine, maytansine, and rhizoxin bind to leishmanial tubulin as assessed by the quenching of intrinsic tubulin fluorescence and the alteration of the proteins reactivity with the sulfhydryl-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid). They also interfere with the assembly of leishmanial tubulin at low micromolar concentrations. Electrophilic compounds such as phenyl arsenoxide and 4-chloro-3,5-dinitro-alpha,alpha,alpha-trifluorotoluene (chloralin), which are of interest as traditional and experimental antiparasitic agents, respectively, inhibit the assembly of leishmanial tubulin in vitro as well. Colchicine-site agents and trifluralin, on the other hand, have little or no effect on leishmanial tubulin in these assays. Maytansine, taxol, and the electrophiles block the growth of Leishmania donovani amastigote-like forms in vitro at low ( <1 microM) concentrations, while colchicine site agents, trifluralin, vinblastine, and rhizoxin are at least two orders of magnitude less toxic to the parasite.

The C terminus of beta-tubulin regulates vinblastine-induced tubulin polymerization

Oligoanions such as sodium triphosphate or GTP prevent and/or reverse vinblastine-induced polymerization of tubulin. We now show that the anions of glutamate-rich extreme C termini of tubulin are similarly involved in the regulation of the vinblastine effect. Cleavage of the C termini by limited proteolysis with subtilisin enhances vinblastine-induced tubulin polymerization and abolishes the anion effect. Only the beta-tubulin C terminus needs to be removed to achieve these changes and the later cleavage of the alpha-tubulin C terminus has little additional effect. In fact, vinblastine concentrations >20 microM block cleavage of the alpha-tubulin C terminus in the polymer, whereas cleavage of the beta-tubulin C terminus proceeds unimpeded over the time used. The vinblastine effect on tubulin polymerization is also highly pH-dependent between pH 6.5 and 7.5; this is less marked, but not absent, after subtilisin treatment. A working model is proposed wherein an anionic domain proximal to the extreme C terminus must interact with a cationic domain to permit vinblastine to promote polymerization. Both exogenous and extreme C-terminal anions compete for the cationic domain with the proximal anionic domain to prevent vinblastine-induced polymerization. We conclude that the electrostatic regulation of tubulin polymerization induced by vinblastine resides primarily in the beta-tubulin C terminus but that additional regulation proximal in the tubulin molecule also plays a role.

Vinblastine-induced formation of tubulin polymers is electrostatically regulated and nucleated

Vinblastine promotes tubulin polymerization as measured by turbidity at 400 nm. Unlike microtubule assembly, this mode of polymerization does not require GTP and, in fact, GTP inhibits polymerization, as do other G nucleotides in the potency order: GtetraP > or = GTP > GDP > GMP > no nucleotide. Inhibition is not nucleoside-specific as ATP, ADP, and CTP also inhibit, and inorganic oligophosphates are as inhibitory as nucleotides in the order tetraphosphate approximately triphosphate > pyrophosphate >> phosphate. Inhibition of polymerization is a rough function of the number of anionic charges and can be mimicked by suramin or tartrate. It is not due to sequestration of magnesium or to debinding of vinblastine. The anion-induced decrease in turbidity generation is reflected in the amount of tubulin that is pelletable, but even in the absence of turbidity significant pelletable tubulin persists which can be assessed by 90 degree light scattering. Formation of this polymer is less sensitive to anions. Shearing of GTP-inhibited and vinblastine-induced samples promotes turbidity and addition of seeds made from vinblastine polymers leads to rapid increases in turbidity in a concentration-dependent manner. Adjustment of the vinblastine concentration permits the demonstration of a latent period for polymerization that can be shortened by polymer seeds. Vinblastine-induced polymerization shows a critical concentration, and, in the presence of GTP, two distinct critical concentrations can be identified. We conclude that charge-charge interactions play a significant role in the formation of vinblastine-induced polymers, and that their formation is a two-step process resembling a nucleation/elongation mechanism.

Dissociation of tubulin assembly-inhibiting and aggregation-promoting activities by a vinblastine derivative

A fluorescent vinblastine analogue, vinblastine-4'-anthranilate (Antvin), that binds to the vinca site on tubulin, inhibits tubulin assembly but does not lead to spiral or other large aggregate formation at concentrations up to 1.6 mM. As judged by turbidity, 90 degrees light scattering and fluorescence anisotropy, little aggregation could be detected. This is in marked contrast to vinblastine and suggests that inhibition of assembly and aggregate formation can be dissociated from each other by suitable substitution in vinblastine.

Identification of a 48-kDa prenylated protein that associates with microtubules as 2',3'-cyclic nucleotide 3'-phosphodiesterase in FRTL-5 cells

In an effort to study the nature of tubulin attachment to membranes, we have previously observed that after blocking prenylation in FRTL-5 thyroid cells, the microtubules become disconnected from the plasma membrane region [Bifulco M. et al. (1983) J. Cell. Physiol. 155, 340-348]. In this study we show that several [3H]mevalonate labeled proteins in FRTL-5 cells associate with membrane and cytoskeleton and, among these, we describe the presence of a 48-kDa prenylated protein, identified by immunoprecipitation as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), that associates with microtubules. This latter association persists through several polymerization/depolymerization cycles, whereas other prenylated proteins are lost. It is suggested that CNP can be a novel microtubule-associated protein (MAP) and a promising candidate as a membrane anchor for microtubules.

Cation selective promotion of tubulin polymerization by alkali metal chlorides

A role for charge-based interactions in protein stability at the monomer or dimer level is well known. We show here that such interactions can also be important for the higher-order structures of microtubule assembly. Alkali metal chlorides increase the rate of polymerization of pure tubulin driven by either taxol or dimethyl sulfoxide. The effect is cation selective, exhibiting a sequence Na+ > K+ > Li+ > Cs+, with optimal concentrations for Na+ at approximately 160 mM. Hofmeister anion effects are additive with these rate stimulations. Sodium is less potent than guanidinium ion stimulation reported previously, but produces a larger fraction of normal microtubules. Alkali metal cations lower the critical concentration by a factor of approximately 2, produce cold reversible polymers whose formation is sensitive to podophyllotoxin inhibition, increase the fraction of polymers present as microtubules from approximately 0.9 to 0.99, and reverse or prevent urea-induced depolymerization of microtubules. In the presence of microtubule-associated proteins, the promotion of polymerization is no longer cation selective. In the polymerization of tubulin S, in which the acidic C termini of both monomers have been cleaved, the cation enhancement is markedly decreased, although selective persists. Because the selectivity sequence is similar to that of the coil/helix transition of polyglutamic acid, we suggest that a major part, although not all, of the cation selective enhancement of polymerization results from shielding of the glutamate-rich C termini of the tubulin monomers.

Localization of the vinblastine-binding site on beta-tubulin

A fluorescent vinblastine derivative, vinblastine-4'-anthranilate, has been shown to inhibit polymerization of rat brain tubulin (IC50 = 4.8 microM). Binding of the drug to tubulin increases fluorescence intensity, causes a small emission blue shift, and has a quantum yield of 0.037. Fluorescence increases as a function of drug concentration, with a high affinity site and an undetermined number of lower affinity sites. Photolabeling, by exciting the fluorescent drug-tubulin complex at the absorption maximum of anthranilate, yields a covalent adduct confined to beta-tubulin. Its formation is specific in that it is blocked by maytansine or vinblastine. Tryptic hydrolysis identifies a single fluorescent beta-peptide coinciding with residues 175-213. The interactions between various ligands at this central portion of beta-tubulin are discussed.

Charge-shielding and the "paradoxical" stimulation of tubulin polymerization by guanidine hydrochloride

Low concentrations of guanidine hydrochloride (GuHCl) increase the rate (and to a lesser degree, the extent) of tubulin polymerization as assessed by light scattering. Maximum enhancement occurs at 120-160 mM GuHCl followed by decreases at higher GuHCl. The latent period is decreased, and there is a 3-4 fold reduction in the critical concentration of polymerization. Electronmicrographs reveal microtubules in the controls and an increasing fraction of total polymers present as aberrant microtubules as the GuHCl concentration is increased from 20 to 100 mM. The GuHCl effect is markedly reduced, but not abolished, in tubulin S (in which the anionic C termini of both monomers have been removed). The GuHCl-induced polymerization has an absolute requirement for GTP and taxol or DMSO, is very sensitive to podophyllotoxin inhibition, and can overcome urea-mediated inhibition of polymerization. Guanidinium analogues mimic the GuHCl effect roughly as a function of the number of potential hydrogen bonds. The anions of the guanidine salts superimpose their inhibitory action on the guanidinium cation effect according to the lyotropic series. At higher GuHCl concentrations (peak effect 500-700 mM), a different polymer (type II) is formed that is GTP and taxol independent, but whose polymerization is retarded but not prevented by podophyllotoxin. Its structure resembles the fibrillar network seen in unfolding intermediates of other proteins. We conclude that both charge and hydrogen-bonding ability are major contributors to the GuHCl-induced promotion of tubulin polymerization, and that charge-shielding is likely to be the basis for this effect.

Local unfolding and the stepwise loss of the functional properties of tubulin

Tubulin exhibits a number of characteristic functions that can be used to identify it. They include the ability to polymerize to microtubules, GTPase activity, and the binding of numerous antimitotic drugs and fluorophores. These functions can be differentially modified by low (0.1-1.0M) urea concentrations, and such urea-induced modifications are stable over time periods of minutes to hours. These intermediate states suggest the existence of restricted regions in the protein each of which is associated with a function and its own urea sensitivity. In order of decreasing sensitivity to urea these effects are decreased rate of polymerization of tubulin to microtubules > decreased extent of polymerization approximately decreased GTPase activity > enhanced fluorescence of a rapidly binding analogue of colchicine-MTPT [2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone] approximately decreased proteolysis by trypsin (after alpha Arg339) and by chymotrypsin (after beta Tyr281) > enhanced fluorescence of 1-anilino-8-naphthalenesulfonic acid (ANS). Additional evidence for the independent behavior of the restricted regions stems from the markedly different time dependence of the response to urea. These low urea concentrations do not induce significant changes in tryptophan fluorescence, suggesting that the observed effects are due to local unfolding. At higher urea concentrations (2-4 M), the enhanced fluorescence of the ligands is abolished; MTPT fluorescence decreases at lower urea concentrations than ANS fluorescence. Moreover, tubulin becomes highly susceptible to proteolysis at multiple sites, and tryptophan emission shows a red-shift, as expected. Multistep unfolding in response to denaturants has been reported for some other proteins. Tubulin appears to be an extreme example of such local responses that proceed under milder conditions than the global transition to the unfolded state.

Molecular mechanism of colchicine action: induced local unfolding of beta-tubulin

Colchicine, the classic antimitotic poison, disrupts cell division by preventing proper assembly of microtubules in the mitotic spindle. Colchicine is known to act by binding to tubulin, the heterodimeric subunit of microtubules. How this binding to tubulin changes the structure of the protein and results in polymerization poisoning has not been characterized. The structural locus of spectroscopically detected conformational changes induced by colchicine is unknown. We report here that colchicine induces the unfolding of a small region in the carboxyl-terminal region of beta-tubulin, around Arg-390. This unfolding is detected by proteolysis with trypsin and chymotrypsin. Chymotrypsin cleaves this region after Phe-389, and trypsin cleaves after Lys-392. The unfolded region appears to be the carboxyl end of an amphipathic helix in the absence of colchicine, and we propose that this unfolding prevents contacts necessary for assembly. Our results suggest that beta-tubulin is exposed on the growing end of the microtubule, which provides a mechanism for coupling GTP hydrolysis to polymerization.