Epothilone and paclitaxel: unexpected differences in promoting the assembly and stabilization of yeast microtubules

Paclitaxel (Taxol) and the epothilones are antimitotic agents that promote the assembly of mammalian tubulin and stabilization of microtubules. The epothilones competitively inhibit the binding of paclitaxel to mammalian brain tubulin, suggesting that the two types of compounds share a common binding site in tubulin, despite the lack of structural similarities. It is known that paclitaxel does not stabilize microtubules formed in vitro from Saccharomyces cerevisiae tubulin; thus, it would be expected that the epothilones would not affect yeast microtubules. However, we found that epothilone A and B do stimulate the formation of microtubules from purified yeast tubulin. In addition, epothilone B severely dampens the dynamics of yeast microtubules in vitro in a manner similar to the effect of paclitaxel on mammalian microtubules. We used current models describing paclitaxel and epothilone binding to mammalian beta-tubulin to explain why paclitaxel apparently fails to bind to yeast tubulin. We propose that three amino acid substitutions in the N-terminal region and at position 227 in yeast beta-tubulin weaken the interaction of the 3'-benzamido group of paclitaxel with the protein. These results also indicate that mutagenesis of yeast tubulin could help define the sites of interaction with paclitaxel and the epothilones.     

A ubiquitous beta-tubulin disrupts microtubule assembly and inhibits cell proliferation

Vertebrate tubulin is encoded by a multigene family that produces distinct gene products, or isotypes, of both the alpha- and beta-tubulin subunits. The isotype sequences are conserved across species supporting the hypothesis that different isotypes subserve different functions. To date, however, most studies have demonstrated that tubulin isotypes are freely interchangeable and coassemble into all classes of microtubules. We now report that, in contrast to other isotypes, overexpression of a mouse class V beta-tubulin cDNA in mammalian cells produces a strong, dose-dependent disruption of microtubule organization, increased microtubule fragmentation, and a concomitant reduction in cellular microtubule polymer levels. These changes also disrupt mitotic spindle assembly and block cell proliferation. Consistent with diminished microtubule assembly, there is an increased tolerance for the microtubule stabilizing drug, paclitaxel, which is able to reverse many of the effects of class V beta-tubulin overexpression. Moreover, transfected cells selected in paclitaxel exhibit increased expression of class V beta-tubulin, indicating that this isotype is responsible for the drug resistance. The results show that class V beta-tubulin is functionally distinct from other tubulin isotypes and imparts unique properties on the microtubules into which it incorporates.     

Interactions of antimitotic peptides and depsipeptides with tubulin

Tubulin is the target for an ever increasing number of structurally unusual peptides and depsipeptides isolated from a wide range of organisms. Since tubulin is the subunit protein of microtubules, the compounds are usually potently toxic to mammalian cells. Without exception, these (depsi)peptides disrupt cellular microtubules and prevent spindle formation. This causes cells to accumulate at the G2/M phase of the cell cycle through inhibition of mitosis. In biochemical assays, the compounds inhibit microtubule assembly from tubulin and suppress microtubule dynamics at low concentrations. Most of the (depsi)peptides inhibit the binding of Catharanthus alkaloids to tubulin in a noncompetitive manner, GTP hydrolysis by tubulin, and nucleotide turnover at the exchangeable GTP site on beta-tubulin. In general, the (depsi)peptides induce the formation of tubulin oligomers of aberrant morphology. In all cases tubulin rings appear to be formed, but these rings differ in diameter, depending on the (depsi)peptide present during their formation.     

Equilibrium studies of a fluorescent paclitaxel derivative binding to microtubules

A fluorescent derivative of paclitaxel, 3'-N-m-aminobenzamido-3'-N-debenzamidopaclitaxel (N-AB-PT), has been prepared in order to probe paclitaxel-microtubule interactions. Fluorescence spectroscopy was used to quantitatively assess the association of N-AB-PT with microtubules. N-AB-PT was found equipotent with paclitaxel in promoting microtubule polymerization. Paclitaxel and N-AB-PT underwent rapid exchange with each other on microtubules assembled from GTP-, GDP-, and GMPCPP-tubulin. The equilibrium binding parameters for N-AB-PT to microtubules assembled from GTP-tubulin were derived through fluorescence titration. N-AB-PT bound to two types of sites on microtubules (K(d1) = 61 +/- 7.0 nM and K(d2) = 3.3 +/- 0.54 microM). The stoichiometry of each site was less than one ligand per tubulin dimer in the microtubule (n(1) = 0.81 +/- 0.03 and n(2) = 0.44 +/- 0.02). The binding experiments were repeated after exchanging the GTP for GDP or for GMPCPP. It was found that N-AB-PT bound to a single site on microtubules assembled from GDP-tubulin with a dissociation constant of 2.5 +/- 0.29 microM, and that N-AB-PT bound to a single site on microtubules assembled from GMPCPP-tubulin with a dissociation constant of 15 +/- 4.0 nM. It therefore appears that microtubules contain two types of binding sites for paclitaxel and that the binding site affinity for paclitaxel depends on the nucleotide content of tubulin. It has been established that paclitaxel binding does not inhibit GTP hydrolysis and microtubules assembled from GTP-tubulin in the presence of paclitaxel contain almost exclusively GDP at the E-site. We propose that although all the subunits of the microtubule at steady state are the same "GDP-tubulin-paclitaxel", they are formed through two paths: paclitaxel binding to a tubulin subunit before its E-site GTP hydrolysis is of high affinity, and paclitaxel binding to a tubulin subunit containing hydrolyzed GDP at its E-site is of low affinity.     

Cyclostreptin derivatives specifically target cellular tubulin and further map the paclitaxel site

Cyclostreptin is the first microtubule-stabilizing agent whose mechanism of action was discovered to involve formation of a covalent bond with tubulin. Treatment of cells with cyclostreptin irreversibly stabilizes their microtubules because cyclostreptin forms a covalent bond to β-tubulin at either the T220 or the N228 residue, located at the microtubule pore or luminal taxoid binding site, respectively. Because of its unique mechanism of action, cyclostreptin overcomes P-glycoprotein-mediated multidrug resistance in tumor cells. We used a series of reactive cyclostreptin analogues, 6-chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and [(14)C-acetyl]-8-acetyl-cyclostreptin, to characterize the cellular target of the compound and to map the binding site. The three analogues were cytotoxic and stabilized microtubules in both sensitive and multidrug resistant tumor cells. In both types of cells, we identified β-tubulin as the only or the predominantly labeled cellular protein, indicating that covalent binding to microtubules is sufficient to prevent drug efflux mediated by P-glycoprotein. 6-Chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and 8-acetyl-cyclostreptin labeled both microtubules and unassembled tubulin at a single residue of the same tryptic peptide of β-tubulin as was labeled by cyclostreptin (219-LTTPTYGDLNHLVSATMSGVTTCLR-243), but labeling with the analogues occurred at different positions of the peptide. 8-Acetyl-cyclostreptin reacted with either T220 or N228, as did the natural product, while 8-chloroacetyl-cyclostreptin formed a cross-link to C241. Finally, 6-chloroacetyl-cyclostreptin reacted with any of the three residues, thus labeling the pathway for cyclostreptin-like compounds, leading from the pore where these compounds enter the microtubule to the luminal binding pocket.     

Microtubule regulation in mitosis: tubulin phosphorylation by the cyclin-dependent kinase Cdk1

The activation of the cyclin-dependent kinase Cdk1 at the transition from interphase to mitosis induces important changes in microtubule dynamics. Cdk1 phosphorylates a number of microtubule- or tubulin-binding proteins but, hitherto, tubulin itself has not been detected as a Cdk1 substrate. Here we show that Cdk1 phosphorylates beta-tubulin both in vitro and in vivo. Phosphorylation occurs on Ser172 of beta-tubulin, a site that is well conserved in evolution. Using a phosphopeptide antibody, we find that a fraction of the cell tubulin is phosphorylated during mitosis, and this tubulin phosphorylation is inhibited by the Cdk1 inhibitor roscovitine. In mitotic cells, phosphorylated tubulin is excluded from microtubules, being present in the soluble tubulin fraction. Consistent with this distribution in cells, the incorporation of Cdk1-phosphorylated tubulin into growing microtubules is impaired in vitro. Additionally, EGFP-beta3-tubulin(S172D/E) mutants that mimic phosphorylated tubulin are unable to incorporate into microtubules when expressed in cells. Modeling shows that the presence of a phosphoserine at position 172 may impair both GTP binding to beta-tubulin and interactions between tubulin dimers. These data indicate that phosphorylation of tubulin by Cdk1 could be involved in the regulation of microtubule dynamics during mitosis.     

beta-Tubulin C354 mutations that severely decrease microtubule dynamics do not prevent nuclear migration in yeast

Microtubule dynamics are influenced by interactions of microtubules with cellular factors and by changes in the primary sequence of the tubulin molecule. Mutations of yeast beta-tubulin C354, which is located near the binding site of some antimitotic compounds, reduce microtubule dynamicity greater than 90% in vivo and in vitro. The resulting intrinsically stable microtubules allowed us to determine which, if any, cellular processes are dependent on dynamic microtubules. The average number of cytoplasmic microtubules decreased from 3 in wild-type to 1 in mutant cells. The single microtubule effectively located the bud site before bud emergence. Although spindles were positioned near the bud neck at the onset of anaphase, the mutant cells were deficient in preanaphase spindle alignment along the mother-bud axis. Spindle microtubule dynamics and spindle elongation rates were also severely depressed in the mutants. The pattern and extent of cytoplasmic microtubule dynamics modulation through the cell cycle may reveal the minimum dynamic properties required to support growth. The ability to alter intrinsic microtubule dynamics and determine the in vivo phenotype of cells expressing the mutant tubulin provides a critical advance in assessing the dynamic requirements of an essential gene function.     

Localization of the antimitotic peptide and depsipeptide binding site on beta-tubulin

Several naturally occurring peptides and depsipeptides which include the cryptophycins, dolastatin 10, hemiasterlin, and phomopsin A have been found to be potent antimitotic agents, causing cell death at picomolar or low nanomolar concentrations. These compounds inhibit microtubule growth, modulate the dynamics of microtubules, and induce the self-association of tubulin dimers into single-walled rings and spirals. These peptides exhibit mutual competitive inhibition in binding to beta-tubulin, while noncompetitively inhibiting the binding of vinblastine and vincristine to beta-tubulin. Despite the abundance of biochemical information, the details of their molecular interactions with tubulin are not known. In this study, using a combination of molecular dynamics simulations and molecular docking studies, a common binding site for cryptophycin 1, cryptophycin 52, dolastatin 10, hemiasterlin, and phomopsin A on beta-tubulin has been identified. Application of these same methods to alpha-tubulin indicated no interaction between alpha-tubulin and any of the peptides. On the basis of the docking results, a model for the mechanism of microtubule disruption and formation of aberrant nonmicrotubule structures is proposed. Both the active site and mechanism of microtubule depolymerization predictions are in good agreement with experimental findings.     

Tau induces ring and microtubule formation from alphabeta-tubulin dimers under nonassembly conditions

Tau is a neuronal microtubule-associated protein that plays a central role in many cellular processes, both physiological and pathological, such as axons stabilization and Alzheimer's disease. Despite extensive studies, very little is known about the detailed molecular basis of tau binding to microtubules. We used the four-repeat recombinant htau40 and tubulin dimers to show for the first time that tau is able to induce both microtubule and ring formation from 6S alphabeta tubulin in phosphate buffer without added magnesium (nonassembly conditions). The amount of microtubules or rings formed was protein concentration-, temperature-, and nucleotide-dependent. By means of biophysical approaches, we showed that tau binds to tubulin without global-folding change, detectable by circular dichroism. We also demonstrated that the tau-tubulin interaction follows a ligand-mediated elongation process, with two tau-binding site per tubulin dimer. Moreover, using a tubulin recombinant alpha-tubulin C-terminal fragment (404-451) and a beta-tubulin C-terminal fragment (394-445), we demonstrated the involvement of both of these tubulin regions in tau binding. From this model system, we gain new insight into the mechanisms by which tau binds to tubulin and induces microtubule formation.     

The microtubule stabilizing agent laulimalide does not bind in the taxoid site,kills cells resistant to paclitaxel and epothilones,and may not require its epoxide moiety for activity

Laulimalide is a cytotoxic natural product that stabilizes microtubules. The compound enhances tubulin assembly, and laulimalide is quantitatively comparable to paclitaxel in its effects on the reaction. Laulimalide is also active in P-glycoprotein overexpressing cells, while isolaulimalide, a congener without the drug's epoxide moiety, was reported to have negligible cytotoxic and biochemical activity [Mooberry et al. (1999) Cancer Res. 59, 653-660]. We report here that laulimalide binds at a site on tubulin polymer that is distinct from the taxoid site. We found that laulimalide, while as active as paclitaxel, epothilone A, and eleutherobin in promoting the assembly of cold-stable microtubules, was unable to inhibit the binding of radiolabeled paclitaxel or of 7-O-[N-(2,7-difluoro-4'-fluoresceincarbonyl)-L-alanyl]paclitaxel, a fluorescent paclitaxel derivative, to tubulin. Confirming this observation, we demonstrated that microtubules formed in the presence of both laulimalide and paclitaxel contained near-molar quantities, relative to tubulin, of both drugs. Laulimalide was active against cell lines resistant to paclitaxel or epothilones A and B on the basis of mutations in the M40 human beta-tubulin gene. We also report that a laulimalide analogue lacking the epoxide moiety, while less active than laulimalide in biochemical and cellular systems, is probably more active than isolaulimalide. Further exploration of the role of the epoxide in the interaction of laulimalide with tubulin is therefore justified.     

Cellular studies reveal mechanistic differences between taccalonolide A and paclitaxel

Taccalonolide A is a microtubule stabilizer that has cellular effects almost identical to paclitaxel. However, biochemical studies show that, unlike paclitaxel, taccalonolide A does not enhance purified tubulin polymerization or bind tubulin/microtubules. Mechanistic studies aimed at understanding the nature of the differences between taccalonolide A and paclitaxel were conducted. Our results show that taccalonolide A causes bundling of interphase microtubules at concentrations that cause antiproliferative effects. In contrast, the concentration of paclitaxel that initiates microtubule bundling is 31-fold higher than its IC₅₀. Taccalonolide A''s effects are further differentiated from paclitaxel in that it is unable to enhance the polymerization of tubulin in cellular extracts. This finding extends previous biochemical results with purified brain tubulin to demonstrate that taccalonolide A requires more than tubulin and a full complement of cytosolic proteins to cause microtubule stabilization. Reversibility studies were conducted and show that the cellular effects of taccalonolide A persist after drug washout. In contrast, other microtubule stabilizers, including paclitaxel and laulimalide, demonstrate a much higher degree of cellular reversibility in both short-term proliferation and long-term clonogenic assays. The propensity of taccalonolide A to alter interphase microtubules at antiproliferative concentrations as well as its high degree of cellular persistence may explain why taccalonolide A is more potent in vivo than would be expected from cellular studies. The close linkage between the microtubule bundling and antiproliferative effects of taccalonolide A is of interest given the recent hypothesis that the effects of microtubule targeting agents on interphase microtubules might play a prominent role in their clinical anticancer efficacy.Key words: taccalonolide, paclitaxel, microtubule stabilizer, microtubule targeted agent, tubulin, microtubule, laulimalide, antimitotic agent, drug persistence     

Covalent binding of the benzamide RH-4032 to tubulin in suspension-cultured tobacco cells and its application in a cell-based competitive-binding assay

The benzamide, RH-4032, was found to be a potent antimicrotubule agent in tobacco (Nicotiana tabacum) cells. It strongly inhibited root growth and produced swollen club-shaped roots, an accumulation of cells in arrested metaphase, and loss of microtubules. RH-4032 inhibited the in vitro assembly of bovine tubulin into microtubules, with inhibition requiring a relatively long incubation period. Treatment of tobacco suspension-cultured cells or isolated bovine tubulin with [(14)C]RH-4032, and analysis of radiolabeled protein revealed a highly specific covalent attachment to beta-tubulin. Binding of [(3)H]RH-4032 in tobacco suspension-cultured cells was shown to be saturable and to be influenced by pre-incubation of the cells with various antimicrotubule agents: Binding of [(3)H]RH-4032 was inhibited by the benzamides, pronamide and zarilamide, the N-phenylcarbamate, chlorpropham, and the microtubule-stabilizing drug, paclitaxel, whereas trifluralin and amiprophosmethyl were not inhibitory. A common characteristic of agents that cause microtubule disassembly was a slight enhancement of [(3)H]RH-4032 binding at low concentrations, which did not occur with the microtubule-stabilizing agent paclitaxel. For structural analogs of RH-4032 and various N-phenylcarbamates, it was shown that the ability to inhibit binding of [(3)H]RH-4032 was correlated with the ability to inhibit tobacco root elongation. The results suggest a common binding site on beta-tubulin for RH-4032, pronamide, zarilamide, and chlorpropham, which is distinct from the binding site(s) for trifluralin and amiprophosmethyl. RH-4032 provides a unique approach to studying effects of antimicrotubule agents on plant cells by allowing competitive tubulin binding assays to be conducted in whole cells.     

Cellular studies reveal mechanistic differences between taccalonolide A and paclitaxel

Taccalonolide A is a microtubule stabilizer that has cellular effects almost identical to paclitaxel. However, biochemical studies show that, unlike paclitaxel, taccalonolide A does not enhance purified tubulin polymerization or bind tubulin/microtubules. Mechanistic studies aimed at understanding the nature of the differences between taccalonolide A and paclitaxel were conducted. Our results show that taccalonolide A causes bundling of interphase microtubules at concentrations that cause antiproliferative effects. In contrast, the concentration of paclitaxel that initiates microtubule bundling is 31-fold higher than its IC₅₀. Taccalonolide A’s effects are further differentiated from paclitaxel in that it is unable to enhance the polymerization of tubulin in cellular extracts. This finding extends previous biochemical results with purified brain tubulin to demonstrate that taccalonolide A requires more than tubulin and a full complement of cytosolic proteins to cause microtubule stabilization. Reversibility studies were conducted and show that the cellular effects of taccalonolide A persist after drug washout. In contrast, other microtubule stabilizers, including paclitaxel and laulimalide, demonstrate a much higher degree of cellular reversibility in both short-term proliferation and long-term clonogenic assays. The propensity of taccalonolide A to alter interphase microtubules at antiproliferative concentrations as well as its high degree of cellular persistence may explain why taccalonolide A is more potent in vivo than would be expected from cellular studies. The close linkage between the microtubule bundling and antiproliferative effects of taccalonolide A is of interest given the recent hypothesis that the effects of microtubule targeting agents on interphase microtubules might play a prominent role in their clinical anticancer efficacy.     

Tubulin assembly, taxoid site binding, and cellular effects of the microtubule-stabilizing agent dictyostatin

(-)-Dictyostatin is a sponge-derived, 22-member macrolactone natural product shown to cause cells to accumulate in the G2/M phase of the cell cycle, with changes in intracellular microtubules analogous to those observed with paclitaxel treatment. Dictyostatin also induces assembly of purified tubulin more rapidly than does paclitaxel, and nearly as vigorously as does dictyostatin's close structural congener, (+)-discodermolide (Isbrucker et al. (2003), Biochem. Pharmacol. 65, 75-82). We used synthetic (-)-dictyostatin to study its biochemical and cytological activities in greater detail. The antiproliferative activity of dictyostatin did not differ greatly from that of paclitaxel or discodermolide. Like discodermolide, dictyostatin retained antiproliferative activity against human ovarian carcinoma cells resistant to paclitaxel due to beta-tubulin mutations and caused conversion of cellular soluble tubulin pools to microtubules. Detailed comparison of the abilities of dictyostatin and discodermolide to induce tubulin assembly demonstrated that the compounds had similar potencies. Dictyostatin inhibited the binding of radiolabeled discodermolide to microtubules more potently than any other compound examined, and dictyostatin and discodermolide had equivalent activity as inhibitors of the binding of both radiolabeled epothilone B and paclitaxel to microtubules. These results are consistent with the idea that the macrocyclic structure of dictyostatin represents the template for the bioactive conformation of discodermolide.     

Pleckstrin homology domains of phospholipase C-gamma1 directly interact with beta-tubulin for activation of phospholipase C-gamma1 and reciprocal modulation of beta-tubulin function in microtubule assembly

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) has two pleckstrin homology (PH) domains, an N-terminal domain and a split PH domain. Here we show that pull down of NIH3T3 cell extracts with PLC-gamma1 PH domain-glutathione S-transferase fusion proteins, followed by matrix-assisted laser desorption ionization-time of flight-mass spectrometry, identified beta-tubulin as a binding protein of both PLC-gamma1 PH domains. Tubulin is a main component of microtubules and mitotic spindle fibers, which are composed of alpha- and beta-tubulin heterodimers in all eukaryotic cells. PLC-gamma1 and beta-tubulin colocalized in the perinuclear region in COS-7 cells and cotranslocated to the plasma membrane upon agonist stimulation. Membrane-targeted translocation of depolymerized tubulin by agonist stimulation was also supported by immunoprecipitation analyses. The phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolyzing activity of PLC-gamma1 was substantially increased in the presence of purified tubulin in vitro, whereas the activity was not promoted by bovine serum albumin, suggesting that beta-tubulin activates PLC-gamma1. Furthermore, indirect immunofluorescent microscopy showed that PLC-gamma1 was highly concentrated in mitotic spindle fibers, suggesting that PLC-gamma1 is involved in spindle fiber formation. The effect of PLC-gamma1 in microtubule formation was assessed by overexpression and silencing PLC-gamma1 in COS-7 cells, which resulted in altered microtubule dynamics in vivo. Cells overexpressing PLC-gamma1 showed higher microtubule densities than controls, whereas PLC-gamma1 silencing with small interfering RNAs led to decreased microtubule network densities as compared with control cells. Taken together, our results suggest that PLC-gamma1 and beta-tubulin transmodulate each other, i.e. that PLC-gamma1 modulates microtubule assembly by beta-tubulin, and beta-tubulin promotes PLC-gamma1 activity.     

Nordihydroguaiaretic acid,of a new family of microtubule-stabilizing agents,shows effects differentiated from paclitaxel

Nordihydroguaiaretic acid (NDGA) protected microtubules in NRK cells from depolymerization caused by structurally and functionally diverse drugs such as nocodazole, colchicine, vinblastine, and ilimaquinone. Hitherto reported drugs, although structurally unrelated to paclitaxel, stabilize microtubules in a way similar to that of paclitaxel and compete for paclitaxel binding to tubulin. However, NDGA had activity toward microtubules different from the effects of paclitaxel. In NRK cells, paclitaxel caused microtubule bundle formation in the presence and absence of microtubule-depolymerizing drugs. However, microtubule bundle did not form, and microtubules radiated from the microtubule-organizing center, in cells treated with NDGA. Acceleration of tubulin polymerization in vitro by paclitaxel was strong but that by NDGA was weak. Microtubules polymerized in vitro in the presence of paclitaxel, but not those polymerized in the presence of NDGA, resisted the effects of cold. NDGA seemed to bind to tubulin, but did not compete for [3H]paclitaxel binding to tubulin. These observations indicate that NDGA belongs to a novel family of microtubule-stabilizing drugs.     

Discodermolide interferes with the binding of tau protein to microtubules

We investigated whether discodermolide, a novel antimitotic agent, affects the binding to microtubules of tau protein repeat motifs. Like taxol, the new drug reduces the proportion of tau that pellets with microtubules. Despite their differing structures, discodermolide, taxol and tau repeats all bind to a site on beta-tubulin that lies within the microtubule lumen and is crucial in controlling microtubule assembly. Low concentrations of tau still bind strongly to the outer surfaces of preformed microtubules when the acidic C-terminal regions of at least six tubulin dimers are available for interaction with each tau molecule; otherwise binding is very weak.     

Essential role of tubulin-folding cofactor D in microtubule assembly and its association with microtubules in fission yeast.

The main structural components of microtubules are alpha- and beta-tubulins. A group of proteins called cofactors are crucial in the formation of assembly-competent tubulin molecules in vitro. Whilst an in vitro role is emerging for these cofactors, their biological functions in vivo remain to be established. In order to understand the fundamental mechanisms that determine cell polarity, we have screened for fission yeast mutants with altered polarity. Here we show that alp1+ encodes a homologue of cofactor D and executes a function essential for cell viability. A temperature-sensitive alp1 mutant shows a variety of defects including abnormal mitoses, loss of microtubule structures, displacement of the nucleus, altered growth polarity and asymmetrical cell division. Overexpression of Alp1 is lethal in wild-type cells, resulting in altered cell shape, but is rescued by co-overexpression of beta-tubulin. Alp1 co-localizes with microtubules, both interphase arrays and mitotic spindles. Furthermore, Alp1 binds to and co-sediments with taxol (paclitaxel)-stabilized porcine microtubules. Our results suggest that, in addition to a function in the folding of beta-tubulin, cofactor D may play a vital role in microtubule-dependent processes as a microtubule-associated protein.     

Perturbation of microtubule polymerization by quercetin through tubulin binding: a novel mechanism of its antiproliferative activity

The dietary flavonoid quercetin has a broad range of biological activities, including potent antitumor activity against several types of tumors. Recently, it has been shown that quercetin inhibits cancer cells proliferation by depleting cellular microtubules and perturbing cellular microtubule functions. However, the direct interactions of quercetin with tubulin and microtubules have not been examined so far. Here, we found that quercetin inhibited polymerization of microtubules and depolymerized microtubules made from purified tubulin in vitro. The binding of quercetin with tubulin was studied using quercetin fluorescence and intrinsic tryptophan fluorescence of tubulin. Quercetin bound to tubulin at a single site with a dissociation constant of 5-7 microM, and it specifically inhibited colchicine binding to tubulin but did not bind at the vinblastine site. In addition, quercetin perturbed the secondary structure of tubulin, and the binding of quercetin stimulated the intrinsic GTPase activity of soluble tubulin. Further, quercetin stabilized tubulin against decay and protected two cysteine residues of tubulin toward chemical modification by 5,5'-dithiobis-2-nitrobenzoic acid. Our data demonstrated that the binding of quercetin to tubulin induces conformational changes in tubulin and a mechanism through which quercetin could perturb microtubule polymerization dynamics has been proposed. The data suggest that quercetin inhibits cancer cells proliferation at least in part by perturbing microtubule functions through tubulin binding.     

Cellular Effects of Curcumin on Plasmodium falciparum Include Disruption of Microtubules

Curcumin has been widely investigated for its myriad cellular effects resulting in reduced proliferation of various eukaryotic cells including cancer cells and the human malaria parasite Plasmodium falciparum. Studies with human cancer cell lines HT-29, Caco-2, and MCF-7 suggest that curcumin can bind to tubulin and induce alterations in microtubule structure. Based on this finding, we investigated whether curcumin has any effect on P. falciparum microtubules, considering that mammalian and parasite tubulin are 83% identical. IC₅₀ of curcumin was found to be 5 µM as compared to 20 µM reported before. Immunofluorescence images of parasites treated with 5 or 20 µM curcumin showed a concentration-dependent effect on parasite microtubules resulting in diffuse staining contrasting with the discrete hemispindles and subpellicular microtubules observed in untreated parasites. The effect on P. falciparum microtubules was evident only in the second cycle for both concentrations tested. This diffuse pattern of tubulin fluorescence in curcumin treated parasites was similar to the effect of a microtubule destabilizing drug vinblastine on P. falciparum. Molecular docking predicted the binding site of curcumin at the interface of alpha and beta tubulin, similar to another destabilizing drug colchicine. Data from predicted drug binding is supported by results from drug combination assays showing antagonistic interactions between curcumin and colchicine, sharing a similar binding site, and additive/synergistic interactions of curcumin with paclitaxel and vinblastine, having different binding sites. This evidence suggests that cellular effects of curcumin are at least, in part, due to its perturbing effect on P. falciparum microtubules. The action of curcumin, both direct and indirect, on P. falciparum microtubules is discussed.