Exchangeable GTP binding site of beta-tubulin. Identification of cysteine 12 as the major site of cross-linking by direct photoaffinity labeling

In silico analysis of TUBA4A mutations in Amyotrophic Lateral Sclerosis to define mechanisms of microtubule disintegration

Amyotrophic lateral sclerosis (ALS) is an inexorably progressive and degenerative disorder of motor neurons with no currently-known cure. Studies to determine the mechanism of neurotoxicity and the impact of ALS-linked mutations (SOD1, FUS, TARDP, C9ORF72, PFN1, TUBA4A and others) have greatly expanded our knowledge of ALS disease mechanisms and have helped to identify potential targets for ALS therapy. Cellular pathologies (e.g., aggregation of mutant forms of SOD1, TDP43, FUS, Ubiqulin2, PFN1, and C9ORF72), mitochondrial dysfunction, neuroinflammation, and oxidative damage are major pathways implicated in ALS. Nevertheless, the selective vulnerability of motor neurons remains unexplained. The importance of tubulins for long-axon infrastructure, and the special morphology and function of motor neurons, underscore the central role of the cytoskeleton. The recent linkage of mutations to the tubulin α chain, TUBA4A, to familial and sporadic cases of ALS provides a new investigative opportunity to shed light on both mechanisms of ALS and the vulnerability of motor neurons. In the current study we investigate TUBA4A, a structural microtubule protein with mutations causal to familial ALS, using molecular-dynamic (MD) modeling of protein structure to predict the effects of each mutation and its overall impact on GTP binding, chain stability, tubulin assembly, and aggregation propensity. These studies predict that each of the reported mutations will cause notable structural changes to the TUBA4A (α chain) tertiary protein structure, adversely affecting its physical properties and functions. Molecular docking and MD simulations indicate certain α chain mutations (e.g. K430N, R215C, and W407X) may cause structural deviations that impair GTP binding, and plausibly prevent or destabilize tubulin polymerization. Furthermore, several mutations (including R320C and K430N) confer a significant increase in predicted aggregation propensity of TUBA4A mutants relative to wild-type. Taken together, these in silico modeling studies predict structural perturbations and disruption of GTP binding, culminating in failure to form a stable tubulin heterocomplex, which may furnish an important pathogenic mechanism to trigger motor neuron degeneration in ALS.

Identification of protein succination as a novel modification of tubulin

Protein succination is a stable post-translational modification that occurs when fumarate reacts with cysteine residues to generate 2SC [S-(2-succino)cysteine]. We demonstrate that both α- and β-tubulin are increasingly modified by succination in 3T3-L1 adipocytes and in the adipose tissue of db/db mice. Incubation of purified tubulin from porcine brain with fumarate (50 mM) or the pharmacological compound DMF (dimethylfumarate, 500 μM) inhibited polymerization up to 35% and 59% respectively. Using MS we identified Cys347α, Cys376α, Cys12β and Cys303β as sites of succination in porcine brain tubulin and the relative abundance of succination at these cysteine residues increased in association with fumarate concentration. The increase in succination after incubation with fumarate altered tubulin recognition by an anti-α-tubulin antibody. Succinated tubulin in adipocytes cultured in high glucose compared with normal glucose also had reduced reactivity with the anti-α-tubulin antibody; suggesting that succination may interfere with tubulin-protein interactions. DMF reacted rapidly with 11 of the 20 cysteine residues in the αβ-tubulin dimer, decreased the number of free thiols and inhibited the proliferation of 3T3-L1 fibroblasts. Our data suggest that inhibition of tubulin polymerization is an important undocumented mechanism of action of DMF. Taken together, our results demonstrate that succination is a novel post-translational modification of tubulin and suggest that extensive modification by fumarate, either physiologically or pharmacologically, may alter microtubule dynamics.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

Microtubules are subcellular nanotubes composed of α- and β-tubulin that arise from microtubule nucleation sites, mainly composed of γ-tubulin complexes [corrected]. Cell wall encased plant cells have evolved four distinct microtubule arrays that regulate cell division and expansion. Microtubule-associated proteins, the so called MAPs, construct, destruct and reorganize microtubule arrays thus regulating their spatiotemporal transitions during the cell cycle. By physically binding to microtubules and/or modulating their functions, MAPs control microtubule dynamic instability and/or interfilament cross talk. We survey the recent analyses of Arabidopsis MAPs such as MAP65, MOR1, CLASP, katanin, TON1, FASS, TRM, TAN1 and kinesins in terms of their effects on microtubule array organizations and plant development.

Analysis of Beta-Tubulin Gene Exon 4 Mutations in Advanced Stage III or IV Gastric Cancer

The mechanisms that cause chemoresistance of gastric cancer have yet to be elucidated. Taxanes and promising agents that were recently approved for treatment of advanced or recurrent gastric cancer. Mutations of beta-tubulin, which is a target of taxianes, have been shown to confer chemoresistance against these agents. The aim of the present study is to investigate the presence of mutations of the beta-tubulin in gastric cancer tissues. Sixty-six patients with advanced stage III or IV gastric cancer patients enrolled in this study. Paired samples of gastric cancer tissue and normal mucosa were obtained by endoscopy. The guanosine 5'-triphosphate (GTP)-binding site in exon 4 of the beta-tubulin gene was examined by polymerase chain reaction single-strand conformational polymorphism (PCR-SSCP) analysis, followed by sequencing of the products with abnormally shifted bands. SSCP analysis showed abnormal bands upstream of the GTP-binding site in 7 of the 66 patients, but sequence analysis found no nucleotide substitutions in these patients. Three variant bands were also detected down stream of the the GTP-binding site, but the sequences of the 3 products corresponded to those of two independent pseudogenes. Thus, none of the tumor samples showed mutation of the beta-tubulin exon 4 GTP-binding site. In conclusion, these findings suggest that mutations of the beta-tubulin gene are rare and are unlikely to be an important cause of taxane resistance to taxians.

Molecular cloning and biochemical characterization of alpha- and beta-tubulin from potato plants (Solanum tuberosum L.)

Few studies have investigated microtubules from plants that host pathogenic fungi. Considerable efforts are underway to find an antimitotic agent against plant pathogens like Phytophthora infestans. However, screening the effects of antifungal agents on plant tubulin in vivo or using purified native microtubule in vitro is a time consuming process. A recombinant, correctly folded, microtubule-like structure forming tubulin could accelerate research in this area. In this study, we cloned full length cDNAs isolated from potato leaves using reverse-transcribed polymerase chain reaction (RT-PCR). Solanum tuberosum (Stub) alpha-tubulin and beta-tubulin were predicted to encode 449 and 451 amino acid long proteins with molecular masses of 57 kDa and 60 kDa, respectively. Average yields of alpha- and beta-tubulin were 2.0-3.5 mg l(-1) and 1.3-3.0 mg l(-1) of culture, respectively. The amino acids, His6, Glu198, and Phe170 involved in benomyl sensitivity were conserved in Stub tubulin. The dimerization of tubulin monomers was confirmed by western blot analysis. When combined under appropriate conditions, these recombinant alpha- and beta-tubulins were capable of polymerizing into microtubules. Accessibility of cysteine residues of tubulin revealed that important ligand binding sites were folded correctly. This recombinant tubulin could serve as a control of phytotoxicity of selected antimitotic fungicide compounds during in vitro screening experiments.

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 Mg(2+), 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 alpha-tubulin, based on molecular modeling studies.

Identification and characterization of DNA-binding proteins by mass spectrometry

Mass spectrometry is the most sensitive and specific analytical technique available for protein identification and quantification. Over the past 10 years, by the use of mass spectrometric techniques hundreds of previously unknown proteins have been identified as DNA-binding proteins that are involved in the regulation of gene expression, replication, or DNA repair. Beyond this task, the applications of mass spectrometry cover all aspects from sequence and modification analysis to protein structure, dynamics, and interactions. In particular, two new, complementary ionization techniques have made this possible: matrix-assisted laser desorption/ionization and electrospray ionization. Their combination with different mass-over-charge analyzers and ion fragmentation techniques, as well as specific enzymatic or chemical reactions and other analytical techniques, has led to the development of a broad repertoire of mass spectrometric methods that are now available for the identification and detailed characterization of DNA-binding proteins. These techniques, how they work, what their requirements and limitations are, and selected examples that document their performance are described and discussed in this chapter.

Thiol-disulphide interchange in tubulin: kinetics and the effect on polymerization

All 20 cysteine residues are accessible to disulphide reagents in the tubulin dimer, whereas only four are accessible in taxol-stabilized microtubules. Reaction rates with disulphide reagents are a function of the reagent, are decreased by G nucleotides, and increased with increase in pH and urea. With transient (stop-flow) kinetics, DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] and 2,2'-dithiodipyridine progress curves cannot be fitted by the sum of exponential terms based only on classes of cysteines. The mixed disulphide products react further to form both intra- and intermonomer disulphide bonds that can be reversed by reducing agents. With MMTS (methyl methanethiosulphonate) or ODNB (n-octyl-dithio-2-nitrobenzoate), virtually no protein-protein disulphide bonds are formed and the ODNB reaction can be given as the sum of three exponential terms with pseudo-first-order rate constants of 0.206, 0.069 and 0.010 s(-1) at pH 6.5, suggesting three classes of thiol reactivities. Limited cysteine substitution leads to only small changes in tryptophan or CD spectra, whereas complete substitution leads to loss of the helix content. MMTS-induced loss of SH groups leads to progressive increases in the critical concentration and loss of polymerization competence that can be reversed by assembly promoters such as higher protein concentration, taxol or high ionic strength. Under such conditions, the substituted tubulin forms protofilament-based structures such as microtubules, open tubules, sheets and/or bundles.

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.

Two photoaffinity analogues of the tripeptide, hemiasterlin, exclusively label alpha-tubulin

A synthetic analogue of the tripeptide hemiasterlin, designated HTI-286, depolymerizes microtubules, is a poor substrate for P-glycoprotein, and inhibits the growth of paclitaxel-resistant tumors in xenograft models. Two radiolabeled photoaffinity analogues of HTI-286, designated 4-benzoyl-N,beta,beta-trimethyl-l-phenylalanyl-N(1)-[(1S,2E)-3-carboxy-1-isopropylbut-2-enyl]-N(1),3-dimethyl-l-valinamide (probe 1) and N,beta,beta-trimethyl-l-phenylalanyl-4-benzoyl-N-[(1S,2E)-3-carboxy-1-isopropyl-2-butenyl]-N,beta,beta-trimethyl-l-phenylalaninamide (probe 2), were made to help identify HTI-286 binding sites in tubulin. HTI-286, probe 1, and probe 2 had similar affinities for purified tubulin [apparent K(D(app)) = 0.2-1.1 microM], inhibited polymerization of purified tubulin approximately 80%, and were potent inhibitors of cell growth (IC(50) = 1.0-22 nM). Both radiolabeled probes labeled exclusively alpha-tubulin. Labeling by [(3)H]probe 1 was inhibited by probe 1, HTI-286, vinblastine, or dolastatin 10 (another peptide antimitotic agent that depolymerizes microtubules) but was either unaffected or enhanced (at certain temperatures) by colchicine or paclitaxel. [(3)H]Probe 1 also labeled exclusively tubulin in cytosolic extracts of whole cells. The major, if not exclusive, contact site for probe 1 was mapped to residues 314-339 of alpha-tubulin and corresponds to the sheet 8 and helix 10 region. This region is known to (1) have longitudinal interactions with beta-tubulin across the interdimer interface, (2) have lateral interactions with adjacent protofilaments, and (3) contact the N-terminal region of stathmin, a protein that induces depolymerization of tubulin. Binding of probe 1 to this region may alter the conformation of tubulin outside the labeling domain, since enzymatic removal of the C-terminus of only alpha-tubulin by subtilisin after, but not before, photolabeling is blocked by probe 1. These results suggest that hemiasterlin is in close contact with alpha-tubulin and may span the interdimer interface so that it contacts the vinblastine- and dolastatin 10-binding sites believed to be in beta-tubulin. In addition, we speculate that antimitotic peptides mimic the interaction of stathmin with tubulin.

Mass spectrometric measurement of differential reactivity of cysteine to localize protein-ligand binding sites. Application to tubulin-binding drugs

A new method for localizing binding sites of noncovalent drugs on proteins is presented. We have developed an accurate and high-throughput method based on the mass spectrometric measurement of differential reaction yield of cysteine alkylation (MS-DRC). This method, essentially a semiquantitative footprinting approach, is applicable to any type of ligand targeting cysteine-rich proteins because the method measures the reactivity change of each cysteine toward an alkylating agent instead of monitoring the drug itself. Thus, no modification of the drug is needed. In this study, the method is evaluated using tubulin as a model system. Tubulin and drug-treated tubulin were alkylated separately with several alkylating reagents, followed by proteolysis and high-performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS) and HPLC-MS. Relative alkylation yields of each cysteine toward the reagents were measured by mass spectrometric quantitation. The reaction yields of each cysteine of two samples were compared to detect a particular cysteine (or cysteines) for which reaction yield was markedly decreased following drug binding. Monobromobimane (mBrB) showed the highest differential.Thus, the MS-DRC method with mBrB was evaluated with various tubulin agents, including the covalent agent T138067 and the noncovalent agents colchicine, podophyllotoxin, and 2-methoxyestradiol. Conformational changes induced by drug binding, as well as sites of direct binding, may be identified.

Cells resistant to HTI-286 do not overexpress P-glycoprotein but have reduced drug accumulation and a point mutation in α-tubulin

HTI-286, a synthetic analogue of hemiasterlin, depolymerizes microtubules and is proposed to bind at the Vinca peptide site in tubulin. It has excellent in vivo antitumor activity in human xenograft models, including tumors that express P-glycoprotein, and is in phase II clinical evaluation. To identify potential mechanisms of resistance induced by HTI-286, KB-3-1 epidermoid carcinoma cells were exposed to increasing drug concentrations. When maintained in 4.0 nmol/L HTI-286, cells had 12-fold resistance to HTI-286. Cross-resistance was observed to other Vinca peptide-binding agents, including hemiasterlin A, dolastatin-10, and vinblastine (7- to 28-fold), and DNA-damaging drugs, including Adriamycin and mitoxantrone (16- to 57-fold), but minimal resistance was seen to taxanes, epothilones, or colchicine (1- to 4-fold). Resistance to HTI-286 was retained when KB-HTI-resistant cells were grown in athymic mice. Accumulation of [3H]HTI-286 was lower in cells selected in intermediate (2.5 nmol/L) and high (4.0 nmol/L) concentrations of HTI-286 compared with parental cells, whereas accumulation of [14C]paclitaxel was unchanged. Sodium azide treatment partially reversed low HTI-286 accumulation, suggesting involvement of an ATP-dependent drug pump. KB-HTI-resistant cells did not overexpress P-glycoprotein, breast cancer resistance protein (BCRP/ABCG2/MXR), MRP1, or MRP3. No mutations were found in the major β-tubulin isoform. However, 4.0 nmol/L HTI-286-selected cells had a point mutation in α-tubulin that substitutes Ser for Ala12 near the nonexchangeable GTP-binding site of α-tubulin. KB-HTI-resistant cells removed from drug became less resistant to HTI-286, no longer had low HTI-286 accumulation, and retained the Ala12 mutation. These data suggest that HTI-286 resistance may be partially mediated by mutation of α-tubulin and by an ATP-binding cassette drug pump distinct from P-glycoprotein, ABCG2, MRP1, or MRP3.

Effects of purinenucleotide analogues on microtubule assembly

This minireview summarizes the syntheses of various purinenucleotide analogues and their effects on microtubule (Mt) assembly. 27 analogues were so far synthesized and, together with 3 analogues commercially available (ITP, XTP and dGTP), their effects on Microtubule assembly were investigated. The positions C2, C6, C8, and ribose moiety of purine nucleotides were modified or substituted. It was found that the microenvironments of the purine base and ribose moiety are important for the nucleotides to support Mt assembly. Introduction of amino group into position C2 of ATP, formation of 2-amino ATP, caused Mt assembly substantially. 2-Amino deoxy ATP and deoxy GTP are more potent than GTP in supporting assembly. The introduction of reactive thiol group into C6 (6-SH-GTP) largely reduces the activity of the analogue to support assembly. However, sequestering reactivity of the thiol group by association with methyl group largely recovers the ability of the analogue to promote assembly. Free rotation of the glycosidic linkage was found to be also innevitable in promoting assembly, as the introduction of sulfur atom between C8 of the purine base and C2' of the ribose moiety (formation of 8,2'-S-cyclo purine nucleotides) caused total inhibition. Purinenucleoside triphosphate supports assembly better than GTP but the deoxy-type analogues are totally inhibitory. 2-Amino-8-hydroxy ATP and other analogues support assembly much better than does GTP. However, their diphosphate analogues are totally incapable of supporting assembly. Introduction of a bulky fluorescent probes into C3' can be made to visualize the fluorescent signal in assembled Mts. Together with the suggestions proposed from electron chrystallography of zinc-induced tubulin sheets, interactions of the purine base and ribose moieties with surrounding amino acid residues are discussed.

Direct photoaffinity labeling by dolastatin 10 of the amino-terminal peptide of beta-tubulin containing cysteine 12

Tubulin with bound [5-3H]dolastatin 10 was exposed to ultraviolet light, and 8-10% of the bound drug cross-linked to the protein, most of it specifically. The primary cross-link was to the peptide spanning amino acid residues 2-31 of beta-tubulin, but the specific amino acid could not be identified. Indirect studies indicated that cross-link formation occurred between cysteine 12 and the thiazole moiety of dolastatin 10. An equipotent analog of dolastatin 10, lacking the thiazole ring, did not form an ultraviolet light-induced cross-link to beta-tubulin. Preillumination of tubulin with ultraviolet light, known to induce cross-link formation between cysteine 12 and exchangeable site nucleotide, inhibited the binding of [5-3H]dolastatin 10 and cross-link formation more potently than it inhibited the binding of colchicine or vinblastine to tubulin. Conversely, binding of dolastatin 10 to tubulin inhibited formation of the cross-link between cysteine 12 and the exchangeable site nucleotide. Dithiothreitol inhibited formation of the beta-tubulin/dolastatin 10 cross-link but not the beta-tubulin/exchangeable site nucleotide cross-link. Modeling studies revealed a highly favored binding site for dolastatin 10 at the + end of beta-tubulin in proximity to the exchangeable site GDP. Computational docking of an energy-minimized dolastatin 10 conformation at this site placed the thiazole ring of dolastatin 10 8-9 A from the sulfur atom of cysteine 12. Dolastatin 15 and cryptophycin 1 could also be docked into positions that overlapped more extensively with the docked dolastatin 10 than with each other. This result was consistent with the observed binding properties of these peptides.

The local electrostatic environment determines cysteine reactivity of tubulin

Of the 20 cysteines of rat brain tubulin, some react rapidly with sulfhydryl reagents, and some react slowly. The fast reacting cysteines cannot be distinguished with [14C]iodoacetamide, N-[(14)C]ethylmaleimide, or IAEDANS ([5-((((2-iodoacetyl)amino)ethyl)amino) naphthalene-1-sulfonic acid]), since modification to mole ratios 1 cysteine/dimer always leads to labeling of 6-7 cysteine residues. These have been identified as Cys-305alpha, Cys-315alpha, Cys-316alpha, Cys-347alpha, Cys-376alpha, Cys-241beta, and Cys-356beta by mass spectroscopy and sequencing. This lack of specificity can be ascribed to reagents that are too reactive; only with the relatively inactive chloroacetamide could we identify Cys-347alpha as the most reactive cysteine of tubulin. Using the 3.5-A electron diffraction structure, it could be shown that the reactive cysteines were within 6.5 A of positively charged arginines and lysines or the positive edges of aromatic rings, presumably promoting dissociation of the thiol to the thiolate anion. By the same reasoning the inactivity of a number of less reactive cysteines could be ascribed to inhibition of modification by negatively charged local environments, even with some surface-exposed cysteines. We conclude that the local electrostatic environment of cysteine is an important, although not necessarily the only, determinant of its reactivity.

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.

Analysis of protein-nucleic acid interactions by photochemical cross-linking and mass spectrometry

Photochemical cross-linking is a commonly used method for studying the molecular details of protein-nucleic acid interactions. Photochemical cross-linking aids in defining nucleic acid binding sites of proteins via subsequent identification of cross-linked protein domains and amino acid residues. Mass spectrometry (MS) has emerged as a sensitive and efficient analytical technique for determination of such cross-linking sites in proteins. The present review of the field describes a number of MS-based approaches for the characterization of cross-linked protein-nucleic acid complexes and for sequencing of peptide-nucleic acid heteroconjugates. The combination of photochemical cross-linking and MS provides a fast screening method to gain insights into the overall structure and formation of protein-oligonucleotide complexes. Because the analytical methods are continuously refined and protein structural data are rapidly accumulating in databases, we envision that many protein-nucleic acid assemblies will be initially characterized by combinations of cross-linking methods, MS, and computational molecular modeling.

Mutagenesis of beta-tubulin cysteine residues in Saccharomyces cerevisiae: mutation of cysteine 354 results in cold-stable microtubules

Cysteine residues play important roles in the control of tubulin function. To determine which of the six cysteine residues in beta-tubulin are critical to tubulin function, we mutated the cysteines in Saccharomyces cerevisiae beta-tubulin individually to alanine and serine residues. Of the twelve mutations, only three produced significant effects: C12S, C354A, and C354S. The C12S mutation was lethal in the haploid, but the C12A mutation had no observable phenotype. Based on interactive views of the electron crystallographic structure of tubulin, we suggest that substitution of serine for cysteine at this position has a destabilizing effect on the interaction of tubulin with the exchangeable GTP. The two C354 mutations, although not lethal, produced dramatic effects on microtubules and cellular processes that require microtubules. The C354 mutant cells had decreased growth rates, a slowed mitosis, increased resistance to benomyl, and impaired nuclear migration and spindle assembly. The C354A mutation produced a more severe phenotype than the C354S mutation: the haploid cells had chromosome segregation defects, only 50% of cells in a culture were viable, and a significant percentage of the cells were misshapened. Cytoplasmic microtubules in the C354S and C354A cells were longer than in the control strain and spindle structures appeared shorter and thicker. Both cytoplasmic and spindle microtubules in the two C354 mutants were extremely stable to cold temperature. After 24 h at 4 degrees C, the microtubules were still present and, in fact, very long and thick tubulin polymers had formed. Evidence exists to indicate that the C354 residue in mammalian tubulin is near the colchicine binding site and the electron crystal structure of tubulin places the residue at the interface between the alpha- and beta-subunits. The sulfhydryl group is situated in a polar environment, which may explain why the alanine mutation is more severe than the serine mutation. When the C12S and the two C354 mutations were made in a diploid strain, the mutated tubulin was incorporated into microtubules and the resulting heterozygotes had phenotypes that were intermediate between those of the mutated haploids and the wild-type strains. The results suggest that the C12 and C354 residues play important roles in the structure and function of tubulin.

Sequence analysis and immunofluorescence study of alpha- and beta-tubulins in Reticulomyxa filosa: implications of the high degree of beta2-tubulin divergence

We have cloned and sequenced 2 alpha- and 2 beta-tubulin isoforms from the giant freshwater amoeba Reticulomyxa filosa. The microtubules of this organism exhibit some unusual properties, including the highest rates of assembly and disassembly known and the inability to be stabilized by taxol. The cloned alpha-tubulins show a high degree of identity when compared to an alpha-tubulin consensus sequence. The beta-tubulins, however, are more divergent, the beta2-tubulin being the most unusual beta-tubulin found so far. The deduced amino acid sequence of beta2 shows 55% identity to a beta-tubulin consensus sequence. It also features 51 unique exchanges which cluster in the C-terminal half of the molecule. Several unique exchanges and two insertions occur in regions adjacent to, or directly implicated in, conserved beta-tubulin functions. A phylogenetic analysis places the beta-tubulins of R. filosa in the vicinity of beta-tubulins from fungi and slime molds. Monoclonal and polyclonal antibodies raised against R. filosa tubulins show that the electrophoretic mobility of alpha- and beta-tubulins is reversed with respect to tubulins from most other sources. Immunofluorescence experiments reveal a ubiquitous distribution of both beta-tubulins in the amoebal network. Our observations suggest possible links between the aberrant primary structure of the beta2-tubulin and the unusual properties of R. filosa microtubules.

NEM tubulin inhibits microtubule minus end assembly by a reversible capping mechanism

Although microtubule (MT) dynamic instability is thought to depend on the guanine nucleotide (GTP vs GDP) bound to the beta-tubulin of the terminal subunit(s), the MT minus end exhibits dynamic instability even though the terminal beta-tubulin is always crowned by GTP-alpha-tubulin. As an approach toward understanding how dynamic instability occurs at the minus end, we investigated the effects of N-ethylmaleimide-modified tubulin (NTb) on elongation and rapid shortening of individual MTs. NTb preferentially inhibits minus end assembly when combined with unmodified tubulin (PCTb), but the mechanism of inhibition is unknown. Here, video-enhanced differential interference contrast microscopy was used to observe the effects of NTb on MTs assembled from PCTb onto axoneme fragments. MTs were exposed to mixtures of PCTb (25 microM) and NTb (labeled on approximately 1 Cys per monomer) in which the NTb/PCTb ratio varied from 0.025 to 1. The NTb/PCTb mixture had a slight inhibitory effect on the plus end elongation rate, but significantly inhibited or completely arrested minus end elongation. For the majority of mixtures that were assayed (0.1-1 NTb/PCTb ratio), minus end MT length remained constant until the NTb/PCTb mixture was replaced. Replacement with PCTb allowed elongation to proceed, whereas replacement with buffer or NTb caused minus ends to shorten. Taken together, the results indicate that NTb associates with both plus and minus ends and that NTb acts to reversibly cap minus ends only when PCTb is also present. Low-resolution mapping of labeled Cys residues, along with previous experiments with other Cys-reactive compounds, suggests that modification of beta-tubulin Cys(239) may be associated with the capping action of NTb.

Paclitaxel resistance in non-small-cell lung cancer associated with beta-tubulin gene mutations

PURPOSE

The mechanisms that cause chemoresistance in non-small-cell lung cancer (NSCLC) patients have yet to be clearly elucidated. Paclitaxel is a tubulin-disrupting agent that binds preferentially to beta-tubulin. Tubulins are guanosine triphosphate (GTP)-binding proteins. Beta-tubulin is a GTPase, whereas alpha-tubulin has no enzyme activity. We reasoned that polymerase chain reaction (PCR) and DNA sequencing of the beta-tubulin gene could reveal more information regarding the connection between beta-tubulin mutations and primary paclitaxel resistance.

PATIENTS AND METHODS

Constitutional genomic DNA and paired tumor DNA were isolated from 49 biopsies from 43 Spanish and six American stage IIIB and IV NSCLC patients who had been treated with a 3-hour, 210 mg/m(2) paclitaxel infusion and a 24-hour, 200 mg/m(2) infusion, respectively. Oligonucleotides specific to beta-tubulin were designed for PCR amplification and sequencing of GTP- and paclitaxel-binding beta-tubulin domains.

RESULTS

Of 49 patients with NSCLC, 16 (33%; 95% confidence interval [CI], 20.7% to 45.3%) had beta-tubulin mutations in exons 1 (one patient) or 4 (15 patients). None of the patients with beta-tubulin mutations had an objective response, whereas 13 of 33 (39.4%; 95% CI, 22.8% to 56%; P = 0.01) patients without beta-tubulin mutations had complete or partial responses. Median survival was 3 months for the 16 patients with beta-tubulin mutations and 10 months for the 33 patients without beta-tubulin mutations (P =.0001).

CONCLUSION

We have identified beta-tubulin gene mutations as a strong predictor of response to the antitubulin drug paclitaxel; these mutations may represent a novel mechanism of resistance and should be examined prospectively in future trials of taxane-based therapy in NSCLC.

Direct photoaffinity labeling of cysteine 211 or a nearby amino acid residue of beta-tubulin by guanosine 5'-diphosphate bound in the exchangeable site

Tubulin with [8-14C]GDP bound in the exchangeable site was exposed to ultraviolet light, and radiolabel was cross-linked to two peptide regions of the beta-subunit. Following enrichment for peptides cross-linked to guanosine by boronate chromatography, we confirmed that the cysteine 12 residue was the major site of cross-linking. However, significant radiolabel was also incorporated into a peptide containing amino acid residues 206 through 224. Although every amino acid in this peptide except cysteine 211 was identified by sequential Edman degradation, implying that this was the amino acid residue cross-linked to guanosine, radiolabel at C-8 was usually lost during peptide processing (probably during chromatography at pH 10). Consequently, the radiolabeled amino acid could not be unambiguously identified.

The effects of various GTP analogues on microtubule assembly

We synthesized 27 GTP analogues with modification or substitution at positions C2, C6, C8 and ribose moiety to investigate their effect on microtubule (Mt) assembly. It was found that C2 and C6 are both functional for the analogues supporting Mt assembly. It was surprising to find that 2-amino- ATP (n2ATP) substantially supports assembly, and that the appearance of the assembled Mts was indistinguishable from those assembled in the standard GTP assembly buffer solution. Furthermore, 2-amino dATP and dGTP are even more potent than GTP in supporting assembly. The substitution of oxo group at C6 with reactive thiol largely reduced the activity of the analogue to support assembly. When free rotation of the glycosidic linkage of GTP was blocked by the introduction of sulfur atom between C8 and C2' of ribose moiety, it resulted in total suppression of assembly. Purine nucleoside triphosphate was found to support assembly better than GTP, and even more efficient was 2-amino purine nucleoside triphosphate. Interestingly, their deoxy-type analogues were totally inhibitory. Although 2-amino 8-hydroxy ATP and other analogues supported assembly much better than did GTP, their diphosphate analogues were totally incapable of supporting assembly. Finally, bulky fluorescent probes were introduced at C3' of ribose moiety (Mant-8-Br-GTP or Mant-GTP) to visualize the fluorescent signal in assembled Mts. Even in this case, the number of most protofilaments was found to be 14, consistent with that found in Mts assembled in GTP standard buffer solution.

Distances between the paclitaxel, colchicine, and exchangeable GTP binding sites on tubulin

Distances between the paclitaxel, colchicine, and exchangeable GTP binding sites on tubulin polymers have been probed using fluorescence spectroscopy. Techniques for measuring fluorescence resonance energy transfer (FRET) between fluorescent or chromophoric ligands for each binding site were employed. 2-Debenzoyl-2-(m-aminobenzoyl)paclitaxel (2-AB-PT) was the fluorophore ligand for the paclitaxel binding site; thiocolchicine, allocolchicine, and MDL 27048 were probes for the colchicine site, and 2'(or 3')-O-(trinitrophenyl)guanosine 5'-triphosphate (TNP-GTP) was the fluorophore ligand for the exchangeable GTP site. The distance between the colchicine and paclitaxel binding sites was determined with two different acceptor ligands in the colchicine site. An average distance distribution of 17 A was found in both cases. Energy transfer between 2-AB-PT bound to the paclitaxel site and TNP-GTP (acceptor) bound to the exchangeable GTP site was observed in the polymer. The average distance distribution between the fluorophores was 16.0 A, but the half-width of the distribution was large (17.9 A), which indicates that energy transfer between more than one donor-acceptor pair occurred in the system. One interpretation of this result is that 2-AB-PT serves as an energy transfer donor for two GTP sites, one contained on the same subunit and one on an adjacent protofilament. No FRET was observed between ligands bound to the colchicine and exchangeable GTP sites, indicating that the result of colchicine binding on the GTP region of beta-tubulin is a long range, allosteric effect. The results from these experiments are interpreted in terms of known structural features of microtubules.

Inhibition of acetyl CoA carboxylase by GTP gamma S

The effect of nonhydrolyzable guanine nucleotides on mammalian acetyl CoA carboxylase (ACC) activity was examined. Using porous rat adipocytes and crude fat cell homogenates to study metabolic pathway flux, GMPPNP and/or GTP gamma S inhibited [14C]fatty acid formation by up to 95% when either [6-14C]glucose-6-phosphate or [1-14C]acetyl CoA was used as substrate. If [2-14C]malonyl CoA initiated flux, however, no inhibition was apparent. These pathway flux studies suggested that ACC was the locus of inhibition, and that the mechanism might involve a disruption of guanine nucleotide hydrolysis by the nonhydrolyzable analogues. Using partially and avidin-sepharose-purified ACC preparations from rat fat, liver and mammary tissue, citrate-stimulated ACC activity was inhibited by 25-75% with 50 microM GTP gamma S. Related compounds and nucleotides had absent-to-minimal effects on ACC. ATP gamma S was inhibitory (10-30% at 5-15 microM), but always to a lesser degree than equimolar GTP gamma S. Filter binding assays with [alpha-32P]GTP or [35S]GTP gamma S were negative, but low-level GTPase activity was detected. Using photoaffinity labelling techniques, [alpha-32P]GTP was found to bind ACC and not pyruvate carboxylase. The hypothesis that citrate-responsive ACC activity may be modulated by an intrinsic or associated GTP binding site is explored. Since ACC forms polymers, as does the cytoskeletal protein beta-tubulin, amino acid sequence comparisons between ACC and atypical GTP binding domain of beta tubulin are presented.

Direct photoaffinity labeling of cysteine-295 of alpha-tubulin by guanosine 5'-triphosphate bound in the nonexchangeable site

The alphabeta-tubulin heterodimer has two high affinity guanosine 5'-triphosphate binding sites, so that purified tubulin usually contains two molecules of bound guanosine nucleotide. Half this nucleotide is freely exchangeable with exogenous guanine nucleotide, and its binding site has been readily localized to the beta-subunit. The remaining nonexchangeable guanosine 5'-triphosphate can only be released from tubulin by denaturing the protein. We replaced the exchangeable site nucleotide of tubulin with 2'-deoxyguanosine 5'-diphosphate, exposed the resulting tubulin to ultraviolet light, degraded the protein, and isolated ribose-containing peptide derived from the nonexchangeable site. A large cyanogen bromide peptide was recovered, and its further degradation with endoproteinase Glu-C established that cysteine-295 of alpha-tubulin was the major reactive amino acid cross-linked to guanosine by ultraviolet irradiation.

Structure of the alpha beta tubulin dimer by electron crystallography

The alphabeta tubulin heterodimer is the structural subunit of microtubules, which are cytoskeletal elements that are essential for intracellular transport and cell division in all eukaryotes. Each tubulin monomer binds a guanine nucleotide, which is nonexchangeable when it is bound in the alpha subunit, or N site, and exchangeable when bound in the beta subunit, or E site. The alpha- and beta-tubulins share 40% amino-acid sequence identity, both exist in several isotype forms, and both undergo a variety of posttranslational modifications. Limited sequence homology has been found with the proteins FtsZ and Misato, which are involved in cell division in bacteria and Drosophila, respectively. Here we present an atomic model of the alphabeta tubulin dimer fitted to a 3.7-A density map obtained by electron crystallography of zinc-induced tubulin sheets. The structures of alpha- and beta-tubulin are basically identical: each monomer is formed by a core of two beta-sheets surrounded by alpha-helices. The monomer structure is very compact, but can be divided into three functional domains: the amino-terminal domain containing the nucleotide-binding region, an intermediate domain containing the Taxol-binding site, and the carboxy-terminal domain, which probably constitutes the binding surface for motor proteins.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

A putative beta-tubulin phosphate-binding motif is involved in lateral microtubule protofilament interactions

We have investigated the role of a putative GTP-binding beta-tubulin motif in microtubule polymerization. A peptide containing residues 126-142 of the beta-tubulin subunit (peptide G) was synthesised and an antibody against it raised. Peptide G prevents the binding of GTP to tubulin and also microtubule polymerization but not the formation of vinblastine-induced tubulin spirals, suggesting that it may prevent lateral but not longitudinal tubulin-tubulin interactions. The antibody to peptide G shows little reaction with the interphase microtubule network, mitotic spindles or midbody of cultured cells, whereas it clearly reacts with vinblastine-induced paracrystals. These results suggest that this putative phosphate-binding site present in beta-tubulin could be involved in the lateral tubulin-tubulin interactions along the microtubule structure.

Analysis of the interaction of FtsZ with itself, GTP, and FtsA

The interaction of FtsZ with itself, GTP, and FtsA was examined by analyzing the sensitivity of FtsZ to proteolysis and by using the yeast two-hybrid system. The N-terminal conserved domain consisting of 320 amino acids bound GTP, and a central region of FtsZ, encompassing slightly more than half of the protein, was cross-linked to GTP. Site-directed mutagenesis revealed that none of six highly conserved aspartic acid and asparagine residues were required for GTP binding. These results indicate that the specificity determinants for GTP binding are different than those for the GTPase superfamily. The N-terminal conserved domain of FtsZ contained a site for self-interaction that is conserved between FtsZ proteins from distantly related bacterial species. FtsZ320, which was truncated at the end of the conserved domain, was a potent inhibitor of division although it expressed normal GTPase activity and could polymerize. FtsZ was also found to interact directly with FtsA, and this interaction could also be observed between these proteins from distantly related bacterial species.

Paclitaxel-resistant human ovarian cancer cells have mutant beta-tubulins that exhibit impaired paclitaxel-driven polymerization

Acquired resistance to paclitaxel can be mediated by P-glycoprotein or by alterations involving tubulin. We report two paclitaxel-resistant sublines derived from 1A9 human ovarian carcinoma cells. Single-step paclitaxel selection with verapamil yielded two clones that are resistant to paclitaxel and collaterally sensitive to vinblastine. The resistant sublines are not paclitaxel-dependent, and resistance remained stable after 3 years of drug-free culture. All cell lines accumulate [3H]paclitaxel equally, and no MDR-1 mRNA was detected by polymerase chain reaction following reverse transcription. Total tubulin content is similar, but the polymerized fraction increased in parental but not in resistant cells following the paclitaxel addition. Purified tubulin from parental cells demonstrated paclitaxel-driven increased polymerization, in contrast to resistant cell tubulin, which did not polymerize under identical conditions. In contrast, epothilone B, an agent to which the resistant cells retained sensitivity, increased assembly. Comparable expression of beta-tubulin isotypes was found in parental and resistant cells, with predominant expression of the M40 and beta2 isotypes. Sequence analysis demonstrated acquired mutations in the M40 isotype at nucleotide 810 (T --> G; Phe270 --> Val) in 1A9PTX10 cells and nucleotide 1092 (G --> A; Ala364 --> Thr) in 1A9PTX22 cells. These results identify residues beta270 and beta364 as important modulators of paclitaxel's interaction with tubulin.

Newly-synthesized beta-tubulin demonstrates domain-specific interactions with the cytosolic chaperonin

Tubulin folding requires two chaperone systems, i.e., the 900 kDa cytosolic chaperonin referred to as the TCP-1 complex or TRiC which facilitates folding of the alpha- and beta-tubulin subunits and a ca. 180 kDa complex which facilitates further assembly into heterodimer. beta-Tubulin mutants were expressed in rabbit reticulocyte lysates, and the effect of C-terminal, N-terminal, and internal deletions on the binding of beta-tubulin polypeptides to the 900 and 180 kDa complexes was ascertained. Proteolytic studies of chaperonin-bound beta-tubulin were also implemented. These studies support the concept of quasi-native chaperonin-bound intermediates [Tian et al. J. Biol. Chem. (1995) 270, 1-4]. Three "domains" similar in size to the domains in the native protein were implicated in facilitated folding: i.e., an internal or "M-domain" composed of residues approximately 140-260 which binds to TRiC; a "C-domain" composed of residues approximately 300-445 which interacts less strongly with TRiC and may contain regulatory sequences for tubulin release from the chaperonin; and an "N-domain" composed of residues approximately 1-140 which apparently does not interact with TRiC but does interact with the 180 kDa complex. The major TRiC-interacting region, residues approximately 150-350 (the "interactive core"), overlapped portions of the M- and C-domains and included a putative hydrophobic-rich interdomain segment which may be a preferential site of interaction with TRiC. This segment may also be important for microtubule assembly and/or tubulin dimer formation. Removal of two residues from the N-terminal end or ca. 27 residues from the C-terminal and caused the polypeptide to arrest on TRiC. It is proposed that N- and C-terminal regions of beta-tubulin structurally interact with TRiC-binding region approximately 150-350 to inhibit binding to TRiC.

Tubulin secondary structure analysis, limited proteolysis sites, and homology to FtsZ

The far-ultraviolet circular dichroism spectrum of the alpha beta-tubulin dimer analyzed by six different methods indicates an average content of approximately 33% alpha helix, 21% beta sheet, and 45% other secondary structure. Deconvolution of Fourier transform infrared spectra indicates 24% sheet, 37% (maximum) helix, and 38% (minimum) other structure. Separate alignments of 75 alpha-tubulin, 106 beta-tubulin, and 14 gamma-tubulin sequences and 12 sequences of the bacterial cell division protein FtsZ have been employed to predict their secondary structures with the multiple-sequence method PHD [Rost, B., & Sander, C. (1993a) J. Mol. Biol. 232, 584-599]. The predicted secondary structures average of 33% alpha helix, 24% beta sheet, and 43% loop for the alpha beta dimer. The predictions have been compared with sites of limited proteolysis by 12 proteases at the surfaces of the heterodimer and taxol-induced microtubules [de Pereda, J. M., & Andreu, J. M. (1996) Biochemistry 35, 14184-14202]. From 24 experimentally determined nicking sites, 18 are at predicted loops or at the extremes of secondary structure elements. Proteolysis zone A (including acetylable Lys40 and probably Lys60 in alpha-tubulin and Gly93 in beta-tubulin) and proteolysis zone B (extending between residues 167 and 183 in both chains) are accessible in microtubules. Proteolysis zone C, between residues 278 and 295, becomes partially occluded in microtubules. The alpha-tubulin nicking site Arg339-Ser340 is at a loop following a predicted alpha helix in proteolysis zone D. This site is protected in taxol microtubules; however, a new tryptic site appears which is probably located at the N-terminal end of the same helix. Zone D also contains beta-tubulin Cys354, which is accessible in microtubules. Proteolysis zone E includes the C-terminal hypervariable loops (10-20 residues) of each tubulin chain. These follow the two larger predicted helical zones (residues 372-395 and 405-432 in beta-tubulin), which also are the longer conserved part of the alpha- and beta-tubulin sequences. Through combination of this with other biochemical information, a set of surface and distance constraints is proposed for the folding of beta-tubulin. The FtsZ sequences are only 10-18% identical to the tubulin sequences. However, the predicted secondary structures show two clearly similar (85-87 and 51-78%) regions, at tubulin positions 95-175 and 305-350, corresponding to FtsZ 65-135 and 255-300, respectively. The first region is flanked by tubulin proteolysis zones A and B. It consists of a predicted loop1-helix-loop2-sheet-loop3-helix-loop4-sheet fold, which contains the motif (KR)GXXXXG (loop1), and the tubulin-FtsZ signature G-box motif (SAG)GGTG(SAT)G (loop3). A simple working model envisages loop1 and loop3 together at the nucleotide binding site, while loops 2 and 4 are at the surface of the protein, in agreement with proteolytic and antigenic accessibility results in tubulin. The model is compatible with studies of tubulin and FtsZ mutants. It is proposed that this region constitutes a common structural and evolutionary nucleus of tubulins and FtsZ which is different from typical GTPases.

Release of both native and non-native proteins from a cis-only GroEL ternary complex

Protein folding by the double-ring chaperonin GroEL is initiated in cis ternary complexes, in which polypeptide is sequestered in the central channel of a GroEL ring, capped by the co-chaperonin GroES. The cis ternary complex is dissociated (half-life of approximately 15 s) by trans-sided ATP hydrolysis, which triggers release of GroES. For the substrate protein rhodanese, only approximately 15% of cis-localized molecules attain their native form before hydrolysis. A major question concerning the GroEL mechanism is whether both native and non-native forms are released from the cis complex. Here we address this question using a 'cis-only' mixed-ring GroEL complex that binds polypeptide and GroES on only one of its two rings. This complex mediates refolding of rhodanese but, as with wild-type GroEL, renaturation is quenched by addition of mutant GroEL 'traps', which bind but do not release polypeptide substrate. This indicates that non-native forms are released from the cis complex. Quenching of refolding by traps was also observed under physiological conditions, both in undiluted Xenopus oocyte extract and in intact oocytes. We conclude that release of non-native forms from GroEL in vivo allows a kinetic partitioning among various chaperones and proteolytic components, which determines both the conformation and lifetime of a protein.

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.

Identification of cysteine 354 of beta-tubulin as part of the binding site for the A ring of colchicine

The colchicine analog 3-chloroacetyl-3-demthylthio-colchicine (3CTC) is a competitive inhibitor of colchicine binding to tubulin, binds to tubulin at 37 degrees C, but not at 0 degree C, and covalently reacts with beta-tubulin at 37 degree C, but not at 0 degree C, in a reaction inhibited by colchicine site drugs. The approximate intramolecular distance between the oxygen at position C-3 in 3CTC and the chlorine atom of the 3-chloroacetyl group is 3 A. using decylagarose chromatography, we purified beta-tubulin that had reacted with 3-(chloromethyl-[14C] Carbonyl)-3- demethylthiocolchicine ([14C]3CTC). This beta-tubulin that had reacted with 3-(chloromethyl-[14C]carbonyl)- 3-demethythiocolchicine ([14C]3CTC). This beta-tubulin was digested with formic acid, cyanogen bromide, endoproteinase Glu-C, or endoproteinase Lys-C, and the radio-labeled peptide(s) were isolated. The sequences of these peptides indicated that as much as 90% of the covalent reaction between the [14C]3CTC and beta-tubulin occurred at cysteine 354. This finding indicates that the C-3 oxygen atom of colchicinoids is within 3 A of the sulfur atom of the Cys-354 residue, suggests that the colchicine A ring lies between Cys-354 and Cys-239, based on the known 9 A distance between these residues, and may indicate that the tropolone C ring lies between the peptide region containing Cys-239 and the amino-terminal beta-tubulin sequence, based on the labeling pattern observed following direct photoactivation of tubulin-bound colchicine.

Site-directed mutagenesis of Saccharomyces cerevisiae beta-tubulin: interaction between residue 167 and benzimidazole compounds

Benzimidazoles are widely used as anthelmintic agents and systemic fungicides. In susceptible organisms, benzimidazoles bind to beta-tubulin and block microtubule polymerization. To further characterize this interaction, site-directed mutagenesis followed by gene replacement was used to change Saccharomyces cerevisiae beta-tubulin residue Phe-167 to Tyr. Consistent with previous studies, this mutation resulted in at least 3-4-fold decreased sensitivity to the benzimidazole derivatives carbendazim and nocodazole. The Tyr-167 mutant was cold sensitive, implying a direct effect on benzimidazole binding rather than a nonspecific increase in microtubule stability. Surprisingly, the mutant had 8-fold increased sensitivity to the derivative benomyl, which is structurally identical to carbendazim except at position 1. This suggests that residue 167 interacts with benzimidazoles in the vicinity of the 1-position.

Characterization of nucleoside transport activity in rabbit cortical synaptosomes

Rabbit central nervous system (CNS) preparations have been used to study the central effects of adenosine, but little is known about the specific uptake mechanisms in rabbit brain involved in the regulation of extracellular adenosine concentrations. The present study assessed the kinetic and pharmacological characteristics of the uptake of [3H]uridine (a poorly metabolized substrate for adenosine transporters) by rabbit cortical synaptosomes, to define the transporter subtypes involved and to evaluate species variability in transporter characteristics. [3H]Uridine transport into rabbit cortical synaptosomes was mediated by two saturable, facilitated diffusion systems with characteristics compatible with the es and ei transporter subtypes identified in other mammalian species. About 65% of the total transport was mediated by the es system, and Km estimates of 320 and 94 microM were determined for [3H]uridine uptake by the es and ei transporter, respectively. These results differ significantly from the subtype ratio and kinetic characteristics reported for rat and guinea pig cortical synaptosomes, where most of the transport was mediated by an ei subtype. Dipyridamole, dilazep, nitrobenzylthioinosine, R75231, soluflazine, and mioflazine were relatively more effective as inhibitors of es-mediated uptake (compared with ei), while the substrates adenosine, cytidine, and guanosine did not distinguish between the es and ei transporters in rabbit cortical synaptosomes. These results highlight the significant species-tissue variability in nucleoside transporter characteristics and subtype expression, and emphasize the need to characterize the transporters in human CNS tissue to allow the rational development of CNS-active therapeutics based on inhibition of nucleoside transport.

The spongistatins, potently cytotoxic inhibitors of tubulin polymerization, bind in a distinct region of the vinca domain

The highly cytotoxic, sponge-derived, antimitotic macrolide polyether spongistatin 1 has been previously shown to inhibit microtubule assembly, the binding of vinblastine and GTP to tubulin, and displacement of GDP bound in the exchangeable site of tubulin. We have now examined in detail inhibition by spongistatin 1 of both [3H]vinblastine and [3H]dolastatin 10 binding to tubulin. We found spongistatin 1 to be a noncompetitive inhibitor of the binding of both radiolabeled drugs to tubulin, in contrast to competitive patterns obtained with vincristine versus [3H]vinblastine and with a chiral isomer of dolastatin 10 versus [3H]dolastatin 10. Since dolastatin 10 is itself a noncompetitive inhibitor of vinca alkaloid binding to tubulin, this implies at least three distinct binding sites for the structurally complex and diverse natural products that interfere with each others binding to tubulin and with nucleotide exchange. Spongistatin 1, in contrast to both vinca alkaloids and peptide antimitotic agents like dolastatin 10, does not induce formation of a GTP-independent, morphologically distinctive polymer ("aggregate"). We also examined eight compounds closely related structurally to spongistatin 1 (spongistatins 2-9). The most distinctive in their properties were spongistatins 6 and 8. These two compounds, despite activity comparable to spongistatin 1 as inhibitors of tubulin polymerization and [3H]vinblastine binding, had much reduced activity as inhibitors of nucleotide exchange and [3H]dolastatin 10 binding. Spongistatins 1 and 6 were compared for effects on dolastatin 10-induced aggregate formation in conjunction with effects on [3H]dolastatin 10 binding. Spongistatin 6 was about 4-fold less active than spongistatin 1 as an inhibitor of aggregation and over 20-fold less active as an inhibitor of dolastatin 10 binding.

Colchicine binding by the "isolated" beta-monomer of tubulin

At mole ratios of lactoperoxidase to tubulin monomers of 3-4, bovine lactoperoxidase forms 1:1 adducts with both alpha- and beta-tubulin from rat brain, thereby separating the tubulin heterodimer into its monomers. This mixture binds colchicine normally, and we show here by direct photoaffinity labeling that the bulk of the [3H]colchicine becomes attached to beta-tubulin under these conditions. When the alpha-tubulin has been displaced by lactoperoxidase, the ratio of label in beta-tubulin to alpha-tubulin is increased. The amount of label in alpha-tubulin decreases with a corresponding appearance of label in lactoperoxidase. The rate of labeling of beta-tubulin remains slow. We conclude that alpha-tubulin is not necessary for colchicine binding and propose a model wherein the A and C rings of colchicine bind to beta-tubulin, while the B ring faces alpha-tubulin in the dimer.

Structure of tubulin at 6.5 A and location of the taxol-binding site

Tubulin, the major component of microtubules, is a heterodimer of two chains, alpha and beta, both of relative molecular mass 50,000 (Mr50K) and with 40-50% identity. The isotypic variety and conformational flexibility of tubulin have so far made it impossible to obtain crystals for X-ray work. Structural knowledge of tubulin has been limited to about 20 A from X-ray diffraction of oriented microtubules, and from electron microscopy of microtubules and zinc-induced crystalline sheets in negative stain. The sheets consist of protofilaments similar to those in microtubules but associated in an antiparallel arrangement, and their two-dimensional character is ideal for high-resolution electron microscopy. Here we present a three-dimensional reconstruction of tubulin to 6.5 A resolution, obtained by electron crystallography of zinc-induced two-dimensional crystals of the protein. The alpha- and beta-subunits appear topologically similar, in agreement with their sequence homology. Several features can be defined in terms of secondary structure. An apparent alpha-helical portion, adjacent to both interdimer and inter-protofilament contacts, is tentatively attributed to a segment near the carboxy terminus of the protein. We can assign the alpha- and beta-subunits on the basis of projection studies of the binding of taxol, which show one taxol site per tubulin heterodimer, in agreement with the known stoichiometry of taxol in microtubules. These studies indicate that taxol affects the interaction between protofilaments; to our knowledge, this is the first time that a ligand-binding site has been visualized in the tubulin molecule.