Polymerization and colchicine binding. Two independent properties of tubulin

When stored frozen in 1 M sucrose and 1 mM GTP, tubulin loses polymerizing ability exponentially. Since addition of diethiothreitol does not change the decay half-life, this decrease in activity can not be attributed to disulfide bond formation. When tubulin is stored frozen in dithiothreitol and GTP only, the decay half-life increases by a factor of four, indicating that sucrose destabilizes polymerizing ability. Frozen storage in sucrose has the opposite effect on colchicine binding, which remains at 100% for 40 days. This temporal divergence indicates that colchine binding and polymerization are two independent properties of tubulin.     

Immunological detection of microtubule poison-induced conformational changes in tubulin

The interaction of tubulin-microtubule poison complexes with anti-tubulin antisera has been investigated using radioimmunoassay. The binding of the major antiserum used in this study to tubulin does not interfere with the binding of colchicine to the tubulin or affect the decay of the colchicine-binding activity of the tubulin. Conversely, if colchicine is incubated with the tubulin, forming tubulin-colchicine complexes, the tubulin-colchicine complexes are less efficient competitors for antibody-binding sites than tubulin alone. This is the result of the formation of specific colchicine-tubulin complexes, since tubulin, incubated with lumicolchicine or isocolchicine, behaves as if the tubulin were incubated alone in the radioimmunoassay. When tubulin is incubated with other microtubule poisons, podophyllotoxin or vinblastine, the tubulin-drug complexes have diminished ability to compete with tubulin as did the tubulin-colchicine complexes. These changes observed in the binding of tubulin-microtubule poison complexes to anti-tubulin antisera in a tubulin radioimmunoassay suggest that the binding of colchicine, podophyllotoxin, or vinblastine to tubulin induces subtle conformational changes on the surface of the tubulin dimer involving antigenic determinant sites.     

Effects of inhibitors of tubulin polymerization on GTP hydrolysis

The effects of a number of antimitotic drugs on the GTPase activity of tubulin were examined. The previously reported stimulation with colchicine and inhibition with podophyllotoxin and vinblastine wee confirmed. Maytansine, which competes with vinblastine in binding to tubulin, was comparable to the latter in inhibiting GTP hydrolysis. Nocodazole, which competes with colchicine in binding to tubulin, was significantly superior to colchicine in enhancing GTP hydrolysis. This superiority arose from the more rapid bindng of nocodazole to tubulin, as the two drugs had comparable activity when drug and tubulin were preincubated prior to the addition of GTP. Both colchicine and podophyllotoxin contain a trimethoxybenzene ring, while the closest structural analogy of nocodazole to colchicine includes the trimethoxybenzene ring. To explore this apparent paradox, we examined a number of simpler colchicine analogs for their effects on tubulin-dependent GTP hydrolysis. While tropolone was without effect, 3,4,5-trimethoxybenzaldehyde and 2,3,4-trimethoxybenzaldehyde stimulated the reaction. We therefore conclude that the trimethoxybenzene ring of colchicine is primarily responsible for the drug's stimulation of the GTPase activity of tubulin and that the inhibitory effect of podophyllotoxin must derive from the latter's tetrahydronaphthol moiety.     

Sulfhydryl groups of Mimosa pudica tubulin implicated in colchicine binding and polymerization in vitro.

The important characteristic of novel Mimosa pudica tubulin is its ability to bind colchicine only when dithiothreitol is included in the isolation buffer, indicating the involvement of sulfhydryl groups in colchicine binding. Modification of sulfhydryl groups by a sulfhydryl modifying agent also affects the normal assembly of tubulin into microtubules, as revealed by electron microscopic and spectrophotometric studies. The number of free sulfhydryl groups present in tubulin protein responsible for both colchicine binding and polymerization has been found to be 4, distributed in alpha and beta subunits, and is distinctly different from the number reported for animal tubulin.     

Interaction of 1-propargyl-5-chloropyrimidin-2-one (a metahalone) with rat brain tubulin

The mitotic inhibitor 1-propargyl-5-chloropyrimidin-2-one (a metahalone) was found to bind to DEAE-cellulose purified rat brain tubulin. A decrease in the fluorescence of 1-propargyl-5-chloropyrimidin-2-one was seen when the drug was incubated in the presence of increasing tubulin concentrations. The decrease in metahalone fluorescence was not affected by the addition of GTP, indicating drug interaction at other portions of the tubulin molecule than the nucleotide binding sites. Scatchard plot analysis following incubation of tubulin with 1-propargyl-5-chloro-[2-14C]pyrimidin-2-one revealed that 1 mol of metahalone bound to 1 mol of tubulin dimer with a measured association constant of 8.0 X 10(3) M-1. Double reciprocal plots of vincristine and colchicine binding to tubulin in the presence of 1-propargyl-5-chloropyrimidin-2-one showed that the metahalone competitively inhibited colchicine binding to tubulin but had no influence on vincristine binding. This conclusion was supported by gel filtration chromatography where an increase in unbound colchicine was measured when 1-propargyl-5-chloropyrimidin-2-one was present in an incubation mixture containing colchicine and tubulin. In the presence of 5 mM 1-propargyl-5-chloropyrimidin-2-one, tubulin self-aggregated into crystalline structures. The binding of 1-propargyl-5-chloropyrimidin-2-one to tubulin at or near the colchicine binding site may be responsible for the metaphase arresting characteristics of this drug.     

Human lymphocyte tubulin: Purification and characterization in normal and leukemic cells

Tubulin has been purified from human blood and tonsil lymphocytes. Using gel filtration, the molecular weight of human lymphocyte tubulin was estimated to be 119 000. The proteins was shown to consist of two subunits, with molecular weights of 61 000 and 58 000 comparable to the α and β polypeptides of human brain tubulin. A partial identity reaction was observed between lymphocyte tubulin and human tubulin when tested by double immunodiffusion against a rabbit anti-human brain tubulin antibody. In the presence of GTP, the purified protein polymerized to form microtubules. Tubulin was localized to the cell's juxtacentriolar region by immunofluorescence and electron microscopy. When assayed by a colchicine-binding assay corrected for time decay, the binding affinity was 1.50 ± 0.86 · 10⁶M^?1 and a level in normal lymphocytes of 1.21 · 10^?2 ± 0.79 g/g of soluble protein was determined. Since chronic lymphocytic leukemia lymphocytes have an anomalous capping behavior as well as an unusual susceptibility to colchicine toxicity, the properties and levels of tubulin were determined in these cells. Similar values were obtained for the level, decay rate, molecular weight, and K_a for colchicine as for normal lymphocytes. Chronic lymphocytic leukemia lymphocyte tubulin polymerized in a normal fashion. It thus appears that a decrease in the quantity or function of tubulin does not account for these anomalies in the chronic lymphocytic leukemia lymphocyte.     

THE MECHANISM OF ACTION OF COLCHICINE : Colchicine Binding Properties of Sea Urchin Sperm Tail Outer Doublet Tubulin

The thermal depolymerization procedure of Stephens (1970. J. Mol. Biol. 47:353) has been employed for solubilization of Strongylocentrotus purpuratus sperm tail outer doublet microtubules with the use of a buffer during solubilization which is of optimal pH and ionic strength for the preservation of colchicine binding activity of chick embryo brain tubulin. Colchicine binding values were corrected for first-order decay during heat solubilization at 50°C (t_½ = 5.4 min) and incubation with colchicine at 37°C in the presence of vinblastine sulfate (t_½ = 485 min). The colchicine binding properties of heat-solubilized outer doublet tubulin were qualitatively identical with those of other soluble forms of tubulin. The solubilized tubulin (mol wt, 115,000) bound 0.9 ± 0.2 mol of colchicine per mol of tubulin, with a binding constant of 6.3 x 10⁵ liters/mol at 37°C. The colchicine binding reaction was both time and temperature dependent, and the binding of colchicine was prevented in a competitive manner by podophyllotoxin (K_i = 1.3 x 10^-6 M). The first-order decay of colchicine binding activity was substantially decreased by the addition of the vinca alkaloids, vinblastine sulfate or vincristine sulfate, thus demonstrating the presence of a vinca alkaloid binding site(s) on the outer doublet tubulin. Tubulin contained within the assembled microtubules did not decay. Intact outer doublet microtubules bound less than 0.001 mol of colchicine per mol of tubulin contained in the microtubules, under conditions where soluble tubulin would have bound 1 mol of colchicine per mol of tubulin (saturating concentration of colchicine, no decay of colchicine binding activity). The presence of colchicine had no effect on the rate of solubilization of outer doublet microtubules during incubation at 37°C. Therefore, the colchicine binding site on tubulin is blocked (not available to bind colchicine) when the tubulin is in the assembled outer doublet microtubules.     

Purification of tubulin from bovine brain and its interaction with guanine nucleotides.

A rapid and sensitive assay for [3H]GTP binding activity of tubulin has been developed. This assay method is based on the quantitative retention of [3H]GTP. Tubulin complex on a nitrocellulose membrane filter. It was also found that bovine brain tubulin is markedly stablized by glycerol and GTP against denaturation. A large-scale purification of bovine brain tubulin was achieved using the new assay procedure and by the inclusion of glycerol and GTP in a buffer solution used for column chromatograph. The purified tubulin could be stored at -80degrees in the presence of glycerol and GTP for at least a year without any apprecialbe loss of [3H]GTP- and [3H]colchicine binding activities. The interaction of tubulin with guanine nucleotides was also studied using the nitorcellulose membrane filter procedure. It was found that the binding of [3H]GTP to tubulin with an empty exchangeable site proceeded promptly within k sec while the exchange of [3H]GTP- with a GTP-tubulin complex in which the exchangeable site had been occupied with unlabeled GTP occured more slowly. The dissociation constants for GTP and GDP at the exchangeable site of tubulin were determined as 0.5 times 10-6M and 1.9 times 10-6M, respectively. 5'-Guanylylimidodiphosphate could interact, although less strongly, with tubulin at this site, whereas the interaction of other nucleoside triphosphates includint ATP, CTP, UTP, and 5'-guanylyl methylenediphosphonate was very weak, if it occured at all. The presence of Mg2+ and a free sulfhydryl group was found to be essential for binding of [3H]GTP to tubulin. Ca2+ was found to replace Mg2+ in this binding reaction.     

Binding of dolastatin 10 to tubulin at a distinct site for peptide antimitotic agents near the exchangeable nucleotide and vinca alkaloid sites

Dolastatin 10, a potent antimitotic peptide from a marine animal, strongly inhibits microtubule assembly, tubulin-dependent GTP hydrolysis, and the binding of vinca alkaloids to tubulin. In studies of the binding of [3H]vincristine to the protein, with vinblastine as a control for competitive inhibition (Ki, 6.6 microM), we found that the macrolide antimitotic agents maytansine and rhizoxin were also competitive inhibitors (Ki values, 3.1 and 12 microM). Dolastatin 10 and an unrelated peptide antimitotic, phomopsin A, were more potent but noncompetitive inhibitors (Ki values, 1.4 and 2.8 microM). Since maytansine and, to a much lesser extent, vinblastine interfere with nucleotide exchange on tubulin, all drugs were examined for effects on nucleotide interactions at the exchangeable GTP site. Rhizoxin had effects intermediate between those of vinblastine and maytansine. Both peptides inhibited binding of radiolabeled GTP to tubulin even more strongly than did maytansine, but no drug displaced nucleotide from tubulin. The drugs were evaluated for stabilizing effects on the colchicine binding activity of tubulin. The peptides prevented loss of this activity, and vinblastine provided partial protection, while rhizoxin and maytansine did not stabilize tubulin. A tripeptide segment of dolastatin 10 also effectively inhibits tubulin polymerization and GTP hydrolysis. The tripeptide did not significantly inhibit either vincristine binding or nucleotide exchange, nor did it stabilize colchicine binding. These findings are rationalized in terms of a model with two distinct drug binding sites in close physical proximity to each other and to the exchangeable GTP site on beta-tubulin.     

Involvement of tryptophan residues in colchicine binding and the assembly of tubulin

Chemical modification of tubulin with 2-hydroxy-5-nitrobenzyl bromide, a reagent selective for tryptophan, inhibits tubulin's colchicine binding and in vitro assembly activities. Loss of colchicine binding shows a linear relationship with the modification of tryptophan residues, and is complete when not more than five residues are modified. GTP affords partial protection against this loss of colchicine binding. The in vitro assembly of tubulin is somewhat less sensitive, since microtubules are formed from tubulin dimers possessing 3–4 but not five modified residues. Furthermore, two of the eight tryptophans per dimer are reactive when tubulin is assembled into microtubules.     

Podophyllotoxin and nocodazole counter the effect of IKP104 on tubulin decay

Tubulin, the subunit protein of microtubules, undergoes a time-dependent loss of functional properties known as decay. We have previously shown that the drug 2-(4-fluorophenyl)-1-(2-chloro-3,5-dimethoxyphenyl)-3-methyl-6-phenyl-4(1H)-pyridinone (IKP104) accelerates decay, but that in the presence of colchicine, IKP104 becomes a stabilizer of tubulin. To see if this is due to conformational effects specific to colchicine or simply to occupancy at the colchicine site, we examined the effects of nocodazole and podophyllotoxin, two well-known competitive inhibitors of colchicine for binding to tubulin, on IKP104’s acceleration of decay. We found that podophyllotoxin abolished IKP104’s accelerating effect and, like colchicine, turned it into a stabilizer of tubulin. Nocodazole’s effects were similar to those of podophyllotoxin and colchicine, in that it abolished IKP104-induced enhancement of decay; however, in the presence of nocodazole, IKP104 caused little or no stabilization of tubulin. Since colchicine, nocodazole, and podophyllotoxin have very different interactions with tubulin, but all inhibit the IKP104-induced enhancement of decay, our findings suggest that this inhibition arises from occupancy of the colchicine site rather than from a direct conformational effect of these two drugs.     

Podophyllotoxin and nocodazole counter the effect of IKP104 on tubulin decay

Tubulin, the subunit protein of microtubules, undergoes a time-dependent loss of functional properties known as decay. We have previously shown that the drug 2-(4-fluorophenyl)-1-(2-chloro-3,5-dimethoxyphenyl)-3-methyl-6-phenyl-4(1H)-pyridinone (IKP104) accelerates decay, but that in the presence of colchicine, IKP104 becomes a stabilizer of tubulin. To see if this is due to conformational effects specific to colchicine or simply to occupancy at the colchicine site, we examined the effects of nocodazole and podophyllotoxin, two well-known competitive inhibitors of colchicine for binding to tubulin, on IKP104’s acceleration of decay. We found that podophyllotoxin abolished IKP104’s accelerating effect and, like colchicine, turned it into a stabilizer of tubulin. Nocodazole’s effects were similar to those of podophyllotoxin and colchicine, in that it abolished IKP104-induced enhancement of decay; however, in the presence of nocodazole, IKP104 caused little or no stabilization of tubulin. Since colchicine, nocodazole, and podophyllotoxin have very different interactions with tubulin, but all inhibit the IKP104-induced enhancement of decay, our findings suggest that this inhibition arises from occupancy of the colchicine site rather than from a direct conformational effect of these two drugs.     

Effect of antimitotic drugs on tubulin GTPase activity and self-assembly.

Microtubule inhibitors can be classified into two categories: 1) those which inhibit the polymerization-dependent GTPase activity of phosphocellulose-purified tubulin, but induce a significant polymerization-independent GTPase activity (e.g. colchicine, griseofulvine, daunorubicine); 2) those which inhibit the GTPase activity associated with tubulin polymerization and that induced by inhibitors of the first class (e.g. the vincaalkaloids and podophyllotoxin). The colchicine-stimulated GTPase activity of tubulin appears to be due to the tubulin.colchicine complex. This suggests that colchicine inhibits tubulin assembly by binding to a tubulin-tubulin interaction site required for the polymerization-dependent GTPase activity and induces by itself a tubulin conformational change that leads to polymerization-independent GTPase activity. Stoichiometry of inhibition by vinblastine of the colchicine-stimulated GTPase activity is 1:2. On the other hand, the inhibition by vinblastine of the tubulin self-assembly and of the polymerization-dependent GTPase activity is strongly substoichiometric at the beginning of the polymerization reaction, 1 vinblastine molecule inhibiting the ability of 10 tubulin dimers to polymerize and to hydrolyze the GTP. However, at the polymerization plateau, the inhibition effect by vinblastine appears to be lower, suggesting a selective action of vinblastine on the early stages of the polymerization reaction.     

Nucleotide-dependent bisANS binding to tubulin.

Non-covalent hydrophobic probes such as 5, 5'-bis(8-anilino-1-naphthalenesulfonate) (bisANS) have become increasingly popular to gain information about protein structure and conformation. However, there are limitations as bisANS binds non-specifically at multiple sites of many proteins. Successful use of this probe depends upon the development of binding conditions where only specific dye-protein interaction will occur. In this report, we have shown that the binding of bisANS to tubulin occurs instantaneously, specifically at one high affinity site when 1 mM guanosine 5'-triphosphate (GTP) is included in the reaction medium. Substantial portions of protein secondary structure and colchicine binding activity of tubulin are lost upon bisANS binding in absence of GTP. BisANS binding increases with time and occurs at multiple sites in the absence of GTP. Like GTP, other analogs, guanosine 5'-diphosphate, guanosine 5'-monophosphate and adenosine 5'-triphosphate, also displace bisANS from the lower affinity sites of tubulin. We believe that these multiple binding sites are generated due to the bisANS-induced structural changes on tubulin and the presence of GTP and other nucleotides protect those structural changes.     

Mechanism of binding of the new antimitotic drug MDL 27048 to the colchicine site of tubulin: equilibrium studies.

MDL 27048 [trans-1-(2,5-dimethoxyphenyl)-3-[4-(dimethylamino)phenyl]-2- methyl-2-propen-1-one] fluoresces when bound to tubulin but not in solution. This effect has been investigated and found to be mimicked by viscous solvents. Therefore, MDL 27048 appears to be a fluorescent compound whose intramolecular rotational relaxation varies as a function of microenvironment viscosity. The binding parameters of MDL 27048 to tubulin have been firmly established by fluorescence of the ligand, quenching of the protein fluorescence, and gel equilibrium chromatography. The apparent binding equilibrium constant was (2.75 +/- 0.45) x 10(6)M-1, and the binding site number was 0.81 +/- 0.12 (10 mM sodium phosphate-0.1 mM GTP, pH 7.0, at 25 degrees C). The binding is exothermic. The binding of MDL 27048 overlaps the colchicine and podophyllotoxin binding sites. Binding of MDL 27048 to the colchicine site was also measured by competition with MTC [2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cycloheptatrien-1-one] , a well-characterized reversibly binding probe of the colchicine site [Andreu et al. (1984) Biochemistry 23, 1742-1752; Bane et al., (1984) J. Biol. Chem. 259, 7391-7398]. In contrast with close analogues of colchicine, MDL 27048 and podophyllotoxin neither affected the far-ultraviolet circular dichroism spectrum of tubulin, within experimental error, nor induced tubulin GTPase activity. Like podophyllotoxin, an excess of MDL 27048 over tubulin induced no abnormal cooperative polymerization of tubulin, which is characteristic of colchicine binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

4',6-Diamidino-2-phenylindole, a fluorescent probe for tubulin and microtubules

A new fluorophor for tubulin which has permitted the monitoring of microtubule assembly in vitro is reported. DAPI (4',6-diamidino-2-phenylindole), a fluorophor already known as a DNA intercalator, was shown to bind specifically to a unique tubulin site as a dimer (KD(app) = 43 +/- 5 microM at 37 degrees C) or to tubulin associated in microtubules (KD(app) = 6 +/- 2 microM at 37 degrees C) with the same maximum enhancement in fluorescence. When tubulin polymerization was induced with GTP, the change in DAPI affinity for tubulin resulted in an enhancement of DAPI binding and, consequently, of fluorescence intensity. DAPI, whose binding site is different from that of colchicine, vinblastine, or taxol, did not interfere greatly with microtubule polymerization. It induced a slight diminution of the critical concentration for tubulin assembly due to a decrease in the depolymerizing rate constant. Moreover, DAPI did not interfere with GTP hydrolysis correlated with tubulin polymerization, but it decreased the GTPase activity at the steady state of tubulin assembly. Even at substoichiometric levels DAPI can be used to follow the kinetics of microtubule assembly.     

Evidence that the tightly bound magnesium in tubulin is associated with the N-site GTP

In an attempt to determine whether the tightly bound Mg2+ found in purified tubulin in associated with the N-site GTP or the E-site GDP or GTP, we removed the E-site nucleotide by several means: (i) alkaline phosphatase treatment; (ii) displacement using excess GMPPCP; and (iii) polymerizing tubulin in the presence of alkaline phosphatase and non-hydrolyzable analogues. The Mg2+ content remained equal to about 1 mol/mol tubulin under conditions where denaturation did not occur. Moreover, the Mg/GTP ratio always remained equal to 1. These results indicate that the Mg2+ is associated with the N-site GTP.     

Energy-transfer studies of the distance between the high-affinity metal binding site and the colchicine and 8-anilino-1-naphthalenesulfonic acid binding sites on calf brain tubulin

The high-affinity metal divalent cation Mg2+, associated with the exchangeable guanosine 5'-triphosphate (GTP) binding site (E site) on purified tubulin, has been replaced by the transition metal ion Co2+ on tubulin as well as on the tubulin-colchicine, tubulin-allocolchicine and tubulin-8-anilino-1-naphthalenesulfonic acid (tubulin-ANS) complexes. While pure native tubulin readily incorporated 0.8 atom of Co2+ per tubulin alpha-beta dimer, incorporation was reduced to 0.4 atom of Co2+ per mole of tubulin when it was complexed with colchicine, indicating that the conformational change induced in tubulin by the binding of colchicine leads to a reduced accessibility of the divalent cation binding site linked to the E site without necessarily changing the intrinsic binding constant. The fluorescence emission spectra of tubulin-bound colchicine, allocolchicine, and ANS displayed a strong overlap with the Co2+ absorption spectrum, identifying these as adequate donor-acceptor pairs. Fluorescence energy-transfer measurements were carried out between tubulin-bound colchicine (or allocolchicine) and ANS as donors and tubulin-complexed Co2+ as acceptor. It was found that the distance between the ANS and the high-affinity divalent cation binding sites is greater than 28 A, while that between the colchicine and the divalent cation binding sites is greater than 24 A.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for a distinct ligand binding site on tubulin discovered through inhibition by GDP of paclitaxel-induced tubulin assembly in the absence of exogenous GTP

GDP inhibits paclitaxel-induced tubulin assembly without GTP when the tubulin bears GDP in the exchangeable site (E-site). Initially, we thought inhibition was mediated through the E-site, since small amounts of GTP or Mg²⁺, which favors GTP binding to the E-site, reduced inhibition by GDP. We thought trace GTP released from the nonexchangeable site (N-site) by tubulin denaturation was required for polymer nucleation, but microtubule length was unaffected by GDP. Further, enhancing polymer nucleation reduced inhibition by GDP. Other mechanisms involving the E-site were eliminated experimentally. Upon finding that ATP weakly inhibited paclitaxel-induced assembly, we concluded that another ligand binding site was responsible for these inhibitory effects, and we found that GDP was not binding at the taxoid, colchicine, or vinca sites. There may therefore be a lower affinity site on tubulin to which GDP can bind distinct from the E- and N-sites, possibly on α-tubulin, based on molecular modeling studies.     

The interaction of phomopsin A with bovine brain tubulin

Phomopsin A is an anti-mitotic compound from the fungus Phomopsis leptostroniformis which is a potent inhibitor of microtubule assembly in vitro; like maytansine, it is known to compete with vinblastine for binding to tubulin (E. Lacey, J. A. Edgar, and C. C. J. Culvenor (1987) Biochem. Pharmacol. 36, 2133-2138). A major difference between the effects of maytansine and vinblastine is that vinblastine is a potent inhibitor of tubulin decay, whereas maytansine has little or no effect on decay. Since phomopsin A is structurally distinct from either maytansine or vinblastine, tubulin decay may be measured by either the time-dependent loss of the ability to bind to [3H]colchicine or the time-dependent increase in the binding of bis(8-anilinonaphthalene 1-sulfonate) (BisANS) to tubulin. By either method, phomopsin A was found to be a much stronger inhibitor of tubulin decay than is vinblastine or any other drug yet tested, and in fact, when decay is measured by the increase of BisANS binding, phomopsin A appears to stop the process entirely. This may prove to be useful in the determination of the higher-order structure of the tubulin molecule.