Microtubule-stabilizing agents (MSAs) are efficacious chemotherapeutic drugs widely used for the treatment of cancer. Despite the importance of MSAs for medical applications and basic research, their molecular mechanisms of action on tubulin and microtubules remain elusive. Here we determined high-resolution crystal structures of aß-tubulin in complex with two unrelated MSAs, zampanolide and epothilone A. Both compounds were bound to the taxane-pocket of ß-tubulin and used their respective side chain to induce structuring of the M-loop into a short helix. Because the M-loop establishes lateral tubulin contacts in microtubules, these findings explain how taxane-site MSAs promote microtubule assembly and stability. They further offer fundamental structural insights into the control mechanisms of microtubule dynamics. Here we determined high-resolution crystal structures of -tubulin in complex with two unrelated MSAs, zampanolide and epothilone A. Both compounds were bound to the taxanepocket of -tubulin and used their respective side chain to induce structuring of the M-loop into a short helix. Because the M-loop establishes lateral tubulin contacts in microtubules, these findings explain how taxane-site MSAs promote microtubule assembly and stability. They further offer fundamental structural insights into the control mechanisms of microtubule dynamics.One sentence summary:Microtubule-stabilizing agents use a common mechanism to stabilize a major loop in tubulin that controls microtubule assembly and stability. suggesting that binding of MSAs and TTL does not induce significant structural changes in the T 2 R complex. Both Zampa and EpoA were deeply buried in a pocket formed by predominantly hydrophobic residues of helix H7, -strand S7, and the loops H6-H7, S7-H9 (designated the Mloop (7)) and S9-S10 of -tubulin; this pocket is commonly known as the 'taxane-pocket' (8, 9)In the T 2 R-TTL-Zampa complex, the C9 atom of Zampa was covalently bound to the NE2 atom of His229 of -tubulin (Fig. S1B), which is consistent with mass spectrometry data (10). In addition, two hydrogen bonds were formed between the OH20 group and the O1' atom of Zampa, and the main chain carbonyl oxygen and the NH group of Thr276, respectively. In the T 2 R-TTL-EpoA complex, the O1, OH3, OH7 and N20 groups of EpoA were hydrogen bonded to atoms of residues Thr276 (main chain NH), Gln281 (side chain amide nitrogen), Asp226 (side chain oxygen) and Thr276 (side chain hydroxyl group) of -tubulin, respectively. The binding 4 mode of EpoA in the tubulin-EpoA structure is fundamentally different from the one proposed based on electron crystallography data of zinc-stabilized tubulin sheets (Fig. S2A); however, the orientation of the ligand in the taxane-pocket was ambiguous in the electron crystallography structure because the density of the ligand in experimental omit maps was discontinuous and limited in quality (9, 11). In contrast, the density of EpoA in our tubulin-EpoA X-ray crystal structure is very well defined and allowed the o...

Summary Zampanolide and its less active analog dactylolide compete with paclitaxel for binding to microtubules and represent a new class of microtubule-stabilizing agent (MSA). Mass spectrometry demonstrated that the mechanism of action of both compounds involved covalent binding to β-tubulin at residues N228 and H229 in the taxane site of the microtubule. Alkylation of N228 and H229 was also detected in α,β-tubulin dimers. However, unlike cyclostreptin, the other known MSA that alkylates β-tubulin, zampanolide was a strong MSA. Modeling the structure of the adducts, using the NMR-derived dactylolide conformation, indicated that the stabilizing activity of zampanolide is likely due to interactions with the M-loop. Our results strongly support the existence of the luminal taxane site of microtubules in tubulin dimers and that microtubule nucleation induction by MSAs may proceed through an allosteric mechanism.

A new total synthesis of the marine macrolide (-)-zampanolide (1) and the structurally and stereochemically related non-natural levorotatory enantiomer of (+)-dactylolide (2), that is, ent-2, has been developed. The synthesis features a high-yielding, selective intramolecular Horner-Wadsworth-Emmons (HWE) reaction to close the 20-membered macrolactone ring of 1 and ent-2. The β-keto phosphonate/aldehyde precursor for the ring-closure reaction was obtained by esterification of a ω-diethylphosphono carboxylic acid fragment and a secondary alcohol fragment incorporating the THP ring that is embedded in the macrocyclic core structure of 1 and ent-2. THP ring formation was accomplished through a segment coupling Prins-type cyclization. Employing the same overall strategy, 13-desmethylene-ent-2 as well as the monocyclic desTHP derivatives of 1 and ent-2 were prepared. Synthetic 1 inhibited human cancer cell growth in vitro with nM IC(50) values, while ent-2, which lacks the diene-containing hemiaminal-linked side chain of 1, is 25- to 260-fold less active. 13-Desmethylene-ent-2 as well as the reduced versions of ent-2 and 13-desmethylene-ent-2 all showed similar cellular activity as ent-2 itself. The same activity level was attained by the monocyclic desTHP derivative of 1. Oxidation of the aldehyde functionality of ent-2 gave a carboxylic acid that was converted into the corresponding N-hexyl amide. The latter showed only μM antiproliferative activity, thus being several hundred-fold less potent than 1.

The novel strigolactam derivatives described here compare favourably with the corresponding GR-28 strigolactones in terms of biological activity and physicochemical properties. However, we believe strigolactone and strigolactam derivatives require further structural optimisation to improve their soil persistence to demonstrate a potential for agronomical applications. © 2016 Society of Chemical Industry.

An efficient new synthesis has been elaborated for non-natural (-)-dactylolide ((-)-2) and its 13-desmethylene analogue 4, employing a HWE-based macrocyclization approach with beta-keto-phosphonate/aldehyde 19 and the respective 13-desmethylene derivative as the key intermediates. Both (-)-2 and 4 as well as the corresponding C20 alcohols inhibit human cancer cell proliferation with IC(50) values in the sub-micromolar range and induce the polymerization of tubulin in vitro.