Interaction of phomopsin A with porcine brain tubulin. Inhibition of tubulin polymerization and binding at a rhizoxin binding site

Biosynthetic Studies of Phomopsins Unveil Posttranslational Installation of Dehydroamino Acids by UstYa Family Proteins

UstYa family proteins (DUF3328) are widely and specifically distributed in fungi. They are known to be involved in the biosynthesis of ribosomally synthesized and posttranslationally modified peptides (RiPPs) and nonribosomal peptides, and possibly catalyze various reactions, including oxidative cyclization and chlorination. In this study, we focused on phomopsin A, a fungal RiPP consisting of unique nonproteinogenic amino acids. Gene knockout experiments demonstrated that three UstYa homologues, phomYc, phomYd, and phomYe, are essential for the desaturation of amino acid moieties, showing unprecedented function among UstYa family proteins. Sequence similarity network analysis indicated that their amino acid sequences are highly diverged and that most remain uncharacterized, paving the way for genome mining of fungal metabolites with unique modifications.

Characterization of an acid rock drainage microbiome and transcriptome at the Ely Copper Mine Superfund site

The microbial oxidation of metal sulfides plays a major role in the formation of acid rock drainage (ARD). We aimed to broadly characterize the ARD at Ely Brook, which drains the Ely Copper Mine Superfund site in Vermont, USA, using metagenomics and metatranscriptomics to assess the metabolic potential and seasonal ecological roles of microorganisms in water and sediment. Using Centrifuge against the NCBI "nt" database, ~25% of reads in sediment and water samples were classified as acid-tolerant Proteobacteria (61 ± 4%) belonging to the genera Pseudomonas (2.6-3.3%), Bradyrhizobium (1.7-4.1%), and Streptomyces (2.9-5.0%). Numerous genes (12%) were differentially expressed between seasons and played significant roles in iron, sulfur, carbon, and nitrogen cycling. The most abundant RNA transcript encoded the multidrug resistance protein Stp, and most expressed KEGG-annotated transcripts were involved in amino acid metabolism. Biosynthetic gene clusters involved in secondary metabolism (BGCs, 449) as well as metal- (133) and antibiotic-resistance (8501) genes were identified across the entire dataset. Several antibiotic and metal resistance genes were colocalized and coexpressed with putative BGCs, providing insight into the protective roles of the molecules BGCs produce. Our study shows that ecological stimuli, such as metal concentrations and seasonal variations, can drive ARD taxa to produce novel bioactive metabolites.

Natural antitubulin agents: importance of 3,4,5-trimethoxyphenyl fragment

Microtubules are polar cytoskeletal filaments assembled from head-to-tail and comprised of lateral associations of α/β-tubulin heterodimers that play key role in various cellular processes. Because of their vital role in mitosis and various other cellular processes, microtubules have been attractive targets for several disease conditions and especially for cancer. Antitubulin is the most successful class of antimitotic agents in cancer chemotherapeutics. The target recognition of antimitotic agents as a ligand is not much explored so far. However, 3,4,5-trimethoxyphenyl fragment has been much highlighted and discussed in such type of interactions. In this review, some of the most important naturally occurring antimitotic agents and their interactions with microtubules are discussed with a special emphasis on the role of 3,4,5-trimethoxyphenyl unit. At last, some emerging naturally occurring antimitotic agents have also been tabulated.

Expedient synthesis of threo-beta-hydroxy-alpha-amino acid derivatives: phenylalanine, tyrosine, histidine, and tryptophan

An expedient synthesis of enantiomerically pure threo-beta-hydroxy-alpha-amino acid derivatives of phenylalanine, tyrosine, histidine, and tryptophan is described. The NBS-mediated radical bromination of the N,N-di-tert-butoxycarbonyl protected alpha-amino acids and subsequent treatment with silver nitrate in acetone provided the trans-oxazolidinones predominantly. Cesium carbonate catalyzed hydrolysis then generated the beta-hydroxy amino acid derivatives in excellent overall yield.

Drug target interaction of tubulin-binding drugs in cancer therapy

Microtubules and their component protein, tubulin, constitute a popular target for the treatment of cancer. Many drugs that are presently used in clinics or in clinical trials and drugs that show promise as anticancer drugs bind to tubulin and microtubules. There are three conventional binding sites on beta-tubulin where many of these drugs bind. The binding properties, conformational changes upon binding, association constants and thermodynamic parameters for the drug-tubulin interaction on these three sites are discussed. The antiproliferative activities of these drugs and the possible correlation with the binding properties are also described.

Interactions of antimitotic peptides and depsipeptides with tubulin

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

Characterization of the interaction of TZT-1027, a potent antitumor agent, with tubulin

TZT-1027, a derivative of dolastatin 10 isolated from the Indian Ocean sea hare Dolabella auricularia in 1987 by Pettit et al., is a potent antimicrotubule agent. We have compared the activity of TZT-1027 with that of dolastatin 10 as well as the vinca alkaloids vinblastine (VLB), vincristine (VCR) and vindesine (VDS). TZT-1027 and dolastatin 10 inhibited microtubule polymerization concentration-dependently at 1 - 100 microM with IC50 values of 2.2 +/- 0.6 and 2.3 +/- 0.7 microM, respectively. VLB, VCR and VDS inhibited microtubule polymerization at 1 - 3 microM with IC50 values of 2.7 +/- 0.6, 1.6 +/- 0.4 and 1.6 +/- 0.2 microM, respectively, but showed a slight decrease in inhibitory effect at concentrations of 10 microM or more. TZT-1027 also inhibited monosodium glutamate-induced tubulin polymerization concentration-dependently at 0.3 - 10 microM, with an IC50 of 1.2 microM, whereas VLB was only effective at 0.3 - 3 microM, with an IC50 of 0.6 microM, and caused so-called "aggregation" of tubulin at 10 microM. Scatchard analysis of the binding data for [(3)H]VLB suggested one binding site (Kd 0.2 +/- 0.04 microM and Bmax 6.0 +/- 0.26 nM / mg protein), while that for [(3)H]TZT-1027 suggested two binding sites, one of high affinity (Kd 0.2 +/- 0.01 microM and Bmax 1.7 +/- 0.012 nM / mg protein) and the other of low affinity (Kd 10. 3 +/- 1.46 microM and Bmax 11.6 +/- 0.83 nM / mg protein). [(3)H]TZT-1027 was completely displaced by dolastatin 10 but only incompletely by VLB. [(3)H]VLB was completely displaced by dolastatin 10 and TZT-1027. Furthermore, TZT-1027 prevented [(3)H]VLB from binding to tubulin in a non-competitive manner according to Lineweaver-Burk analysis. TZT-1027 concentration-dependently inhibited both [(3)H]guanosine 5'-triphosphate (GTP) binding to and GTP hydrolysis on tubulin. VLB inhibited the hydrolysis of GTP on tubulin concentration-dependently to a lesser extent than TZT-1027, but no inhibitory effect of VLB on [(3)H]GTP binding to tubulin was evident even at 100 microM. Thus, TZT-1027 affected the binding of VLB to tubulin, but its binding site was not completely identical to that of VLB. TZT-1027 had a potent inhibitory effect on tubulin polymerization and differed from vinca alkaloids in its mode of action against tubulin polymerization.

Biosynthesis of radiolabeled curacin A and its rapid and apparently irreversible binding to the colchicine site of tubulin

Curacin A is a potent competitive inhibitor of colchicine binding to tubulin, and it inhibits the growth of tumor cells. We prepared [(14)C]curacin A biosynthetically to investigate its interaction with tubulin. Binding was rapid, even at 0 degrees C, with a minimum k(f) of 4.4 x 10(3) M(-1) s(-1). We were unable to demonstrate any dissociation of the [(14)C]curacin A from tubulin. Consistent with these observations, the K(a) value was so high that an accurate determination by Scatchard analysis was not possible. The [(14)C]curacin A was released from tubulin following urea treatment, indicating that covalent bond formation does not occur. We concluded that curacin A binds more tightly to tubulin than does colchicine. Besides high-affinity binding to the colchicine site, we observed significant superstoichiometric amounts of the [(14)C]curacin A bound to tubulin, and Scatchard analysis confirmed the presence of two binding sites of relatively low affinity with a K(a) of 3.2 x 10(-5) M(-1).

Effects of arenastatin A and its synthetic analogs on microtubule assembly

Inhibition of microtubule assembly by arenastatin A (1) and five synthetic analogs (3-7) was examined. Arenastatin A and the triamide 6 showed potent and moderately strong inhibitory activities, respectively (IC50; 2.3 microM for 1, 7.8 microM for 6) and also depolymerized preformed microtubules. The other analogs tested showed no activity.

Ustiloxins, new antimitotic cyclic peptides: interaction with porcine brain tubulin

Biochemical and electron microscopic studies demonstrated that ustiloxins A-D, which are antimitotic 13-membered cyclic peptides produced by the rice plant pathogen Ustilaginoidea virens, strongly inhibited the polymerization of porcine brain tubulin in vitro and depolymerized pre-formed microtubules. The IC50 values of polymerization inhibited by ustiloxins A-D were determined to be 0.7, 2.8, 4.4 and 6.6 microM, respectively, under the experimental conditions used, indicating that ustiloxin A is the most potent inhibitor of tubulin polymerization currently known. Ustiloxins A-C were found to inhibit the binding of radiolabelled rhizoxin to tubulin with inhibition constants (Ki) of 0.08, 0.13 and 0.23 microM, respectively, and also inhibited the binding of radiolabelled phomopsin A as strongly as rhizoxin. These results suggest that the binding site of ustiloxins is identical with that of rhizoxin.

Interaction of marine toxin dolastatin 10 with porcine brain tubulin: competitive inhibition of rhizoxin and phomopsin A binding

Dolastatin 10, a cytostatic peptide containing several unique amino acid subunits, was isolated from the marine shell-less mollusk Dolabella auricularia. It inhibits microtubule assembly at concentrations below 5.0 microM (IC50, 3.0 microM) and causes formation of tubulin aggregates at higher (> 10 microM) concentrations in a somewhat different manner from that caused by vinblastine. Electron microscopical analysis showed irregular aggregates of microtubule proteins in the presence of 10 microM dolastatin 10. Dolastatin 10 inhibited the binding of both radiolabeled rhizoxin and phomopsin A to tubulin with inhibition constants (Ki) of 7 x 10(-8) M and 1 x 10(-7) M, respectively. The results suggest that at least one of the binding sites of dolastatin 10 on tubulin is the rhizoxin binding site.

Identification of the fragment photoaffinity-labeled with azidodansyl-rhizoxin as Met-363-Lys-379 on beta-tubulin

The rhizoxin (RZX)-binding site on porcine brain tubulin was investigated by photoaffinity labeling with the 5-azido-1-naphthalene sulfonyl (azidodansyl) derivative of RZX, nor-rhizoxin-22-al-5'-azidonaphthalene-1'-sulfonylhydrazo ne (azidodansylrhizoxin: Adan-RZX). Upon ultraviolet irradiation, Adan-RZX generates a highly reactive nitrene, which irreversibly binds to an amino acid residue(s) near the RZX-binding site. The label was found to be on beta-tubulin. Enzymatic digestion of the labeled tubulin generated only one major fluorescent peak on C18 reverse phase HPLC analysis. The labeled site(s) was mapped by using various combinations of highly specific peptidases in succession. That is, the labeled fragment generated by the first peptidase was purified by HPLC and exposed to a second peptidase; if the retention time in HPLC changed after the second digestion, the fragment generated in the first digestion must have contained the recognition site(s) of the second enzyme. From the results of these successive digestions and the known polypeptide sequences, we could identify the labeled fragment as Met-363-Lys-379 of beta-tubulin. This peptidase combination technique should be widely applicable.

Differential effects of active isomers, segments, and analogs of dolastatin 10 on ligand interactions with tubulin. Correlation with cytotoxicity

Dolastatin 10 is a potent antimitotic peptide isolated from the marine mollusk Dolabella auricularia. Four of its five residues are modified amino acids (in sequence, dolavaline, valine, dolaisoleuine, dolaproine, dolaphenine). Besides inhibiting tubulin polymerization, dolastatin 10 non-competitively inhibits vinca alkaloid binding to tubulin, inhibits nucleotide exchange and formation of the beta s cross-link, and stabilizes the colchicine binding activity of tubulin. To examine the mechanism of action of dolastatin 10 we prepared six chiral isomers, one tri- and one tetrapeptide segment, and one pentapeptide analog of dolastatin 10, all of which differ little from dolastatin 10 as inhibitors of tubulin polymerization. However, only two of the chiral isomers were similar to dolastatin 10 in their cytotoxicity for L1210 murine leukemia cells and in their effects on vinblastine binding, nucleotide exchange, beta s cross-link formation, and colchicine binding. These were isomer 2, with reversal of configuration at position C(19a) in the dolaisoleuine moiety, and isomer 19, with reversal of configuration at position C(6) in the dolaphenine moiety. The pentapeptides with reduced cytotoxicity and reduced effects on tubulin interactions with other ligands were all modified in the dolaproine moiety at positions C(9) and/or C(10). The tripeptide and tetrapeptide segments which inhibited polymerization but not ligand interactions were the amino terminal tripeptide (lacking dolaproine and dolaphenine) and the carboxyl terminal tetrapeptide (lacking dolavaline). We speculate that strong inhibition of other ligand interactions with tubulin requires stable peptide binding to tubulin (i.e. slow dissociation), but that inhibition of polymerization requires only rapid binding to tubulin.

Binding selectivity of rhizoxin, phomopsin A, vinblastine, and ansamitocin P-3 to fungal tubulins: differential interactions of these antimitotic agents with brain and fungal tubulins

The binding of four potent antimitotic agents, rhizoxin (RZX), phomopsin A (PMS-A), ansamitocin P-3 (ASMP-3), and vinblastine (VLB), to tubulins from RZX-sensitive and -resistant strains of Aspergillus nidulans, Schizosaccharomyces pombe, and Saccharomyces cerevisiae was investigated. Mycelial extracts to which RZX could bind contained beta-tubulin with Asn as the 100th amino acid residue (Asn-100) in all cases, and those without affinity for RZX contained beta-tubulins with either Ile-100 or Val-100. Though PMS-A shares the same binding site as RZX and ASMP-3 on porcine brain tubulin (Asn-100), only ASMP-3 bound Asn-100 fungal tubulins in a competitive manner with respect to RZX. PMS-A and VLB, which strongly bind to porcine brain tubulin, did not bind to any of the fungal mycelial extracts examined. The results indicate differential interactions of these antimitotic agents with brain and fungal tubulins.

Natural products which interact with tubulin in the vinca domain: maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B

This paper summarizes published data on the interactions of tubulin with antimitotic compounds that inhibit the binding of vinca alkaloids to the protein. These are all relatively complex natural products isolated from higher plants, fungi and marine invertebrate animals. These agents are maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B and their congeners. Effects on tubulin polymerization, ligand binding interactions and structure-activity relationships are emphasized.