Tubulin Taxol binding

Another drug is taxol, which is extracted from the bark of the Pacific yew tree, Taxus brevijolia. Unlike colchicine and the vinca alkaloids, taxol binds tightly to microtubules and stabilizes them against depolymerization by Ca. It also enhances the rate and yield of microtubule assembly, thereby decreasing the amount of soluble tubulin in the cytosol pool. Again, the overall effect of taxol is to arrest dividing cells in mitosis. Taxol is used in cancer chemotherapy. 

Fig. 5. 3D EM shows how kinesin and tau bind to microtubules. (A) Reconstruction of a microtubule decorated with kinesin heads (ochre). One kinesin head binds per afi-tubulin heterodimer (grey) and, due to its asymmetric form, can be used to distinguish between the subunits. (B) Inside view of a microtubule that was coassembled with gold-labeled tau and decorated with kinesin heads. The kinesin heads can be seen on the outside through the holes between the protofilaments. The labeled repeat motif of tau binds to the inside face of microtubule. The averaged nanogold density (yellow), which is attached to a repeat motif of tau through a linker, can only be seen near the Taxol binding site of -tubulin, but not on the a subunit (Kar et al, 2003a). The ribbon diagram of the refined zinc-sheet structure is also shown for reference (see Figure 3).

Taxol binds specifically to the [3-tubulin subunit of microtubules and appears to antagonize the disassembly of this key cytoskeletal protein, with the result that bundles of microtubules and... 

Discodermolide (DDM, Fig. 16) is a marine natural product that promotes MT assembly more potently than PTX and is active against some PTX-resistant cell lines [118-120], The photoaffmity analogue C19-BPC-DDM labels (3-tubulin in close proximity to the taxol binding site [121] and DDM itself is a competitive inhibitor of PTX binding to MT [120], suggesting that DDM also binds to the PTX binding site on P-tubulin. 

Fig. 30 Taxoid site on (3-tubulin and predicted peloruside site on a-tubulin. a Surface representation (view from the inner side of the microtubule) of a tubulin dimer with FIX (red) bound to P-tubulin (green) and peloruside A (orange) bound to the predicted site in a-tubulin (blue), b View of the peloruside binding site. Hydrogen bonds are represented as yellow dashed lines, and the residues involved in these bonds are labeled. Some secondary structure elements are also labeled, c View of the taxol binding site. Some secondary structure elements are labeled. In panels b and c, H7 is colored in orange, and the N-terminal and intermediate domains are colored in green and blue, respectively. (Reprinted with permission from [17]. Copyright 2006 American Chemical Society)...

Based on analysis of electronic crystallography and NMR data for the bindings of Taxol and epothilone A to tubulin subunits, it was proposed that they did not share a common pharmacophore (similar binding mode and sites) as hypothesized for a long time, because they bind to their receptors uniquely and independently. Also, the T-shape conformation of Taxol binding to tubulin was supported from this study. 

The binding of paclitaxel to microtubules has been studied extensively. The polymeric and noncrystalline nature of the tubulin-taxol complex prevents a direct approach by X-ray crystallography, but Lowe et al. (35) could determine the structure of tubulin at 3.5 A resolution by electron diffraction. Using this structure, various possible binding orientations of paclitaxel on tubulin have been proposed, but recent REDOR NMR... 

Rao, S., He, L., Chakravarty, S., et al. (1999) Characterization of the taxol binding site on the microtubule. Identification of Arg282 in P-Tubulin as the site of photoincorporation of a 7-benzophenone analogue of taxol. Journal of Biological Chemistry, 274, 37990-37994. 

Other drugs bind to different sites on tubulin dimers or to microtubules and therefore affect microtubule stability through different mechanisms. For example, at low concentrations, taxol binds to microtubules and stabilizes them by inhibiting their shortening. 

Known antimitotic agents (see Fig. 2) bind to a number of characterized sites on the tubulin protein. The most well characterized sites include the taxol binding site, the vinca alkaloid domain and the colchicine site. 

Rao S, Orr GA, Chaudhary AG et al. Characterization of the taxol binding site on the microtubule. 2-(m-Azido benzoyljtaxol photolabels a pepdde (amino acids 217-231) of tubulin. J Biol Chem 1995 270 20235-20238. 

These include rhazinilam (15.4.6) 494), which inhibits the disassembly of microtubules but has a different mechanism of action than taxol, laulimalide (15.4.7) and isolaulimalide (15.4.8) 496), WS9885B (15.4.9) 497), and polyisoprenylated benzophenones such as guttiferone E (15.4.10) 497). The naturally occurring 3(2H)-furanone derivative geiparvin has been found to counteract the microtubule-assembly effects of taxol, suggesting that it is a competitive inhibitor at the taxol-binding site of tubulin 498). 

A third approach to the taxol pharmacophore was developed by Giannakakou et al 503), who compared the effects of taxol and various epothilone analogs on the polymerization of native tubulin and of modified tubulins carrying / -tubulin mutations near the taxol-binding... 

Hrmg, D.T., Chen, J., and Schreiber, S.L. (1996) (-l-)-Discodermolide binds to microtubules in stoichiometric ratio to tubulin dimers, blocks taxol binding and results in mitotic arrest. Chem. Biol., 3, 287-293. 

As indicated above, the antitumor activity of Epo B is based on its ability to bind to microtubules and to alter their intrinsic stability and dynamic properties. (For excellent reviews on microtubule structure and function see ref. 6 and 47-49.) Epothilones prevent the Ca or cold-induced depolymerization of pre-existing microtubule polymers in cell-free systems at the same time, they promote the polymerization of soluble tubulin into microtubule-like polymers under conditions that would normally destabilize microtubules.As demonstrated by kinetic experiments, epothilones inhibit the binding of taxol to microtubules in a competitive manner and they bind to the taxol binding site on p-tubulin with affinities that exceed (Epo B) or are comparable (Epo A) to taxol affinity likewise Epo B is a more potent tubulin-polymerizing agent than taxol or Epo Structural studies on... 

Paclitaxel (Taxol) is a highly complex, organic compound isolated from the bark of the Pacific yew tree. It binds to tubulin dimers and microtubulin filaments, promoting the assembly of filaments and preventing their depolymerization. This increase in the stability of microfilaments results in disruption of mitosis and cytotoxicity and disrupts other normal microtubular functions, such as axonal transport in nerve fibers. 

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