Binding affinity of fluorochromes and fluorescent proteins to Taxol™ crystals

The ability of Taxol (paclitaxel) to bind and stabilize microtubules is the basis for its use as an anti-mitotic drug as well as an additive for in vivo and in vitro studies of microtubules. The low solubility of Taxol in aqueous solutions, however, facilitates the formation of Taxol crystals that can be decorated with fluorescent tubulin. In cells treated with Taxol, these decorated Taxol crystals may be mistaken for self-assembled tubulin asters when observed with a fluorescent microscope. We confirmed via fluorescent and differential interference contrast microscopy that Taxol crystals can be decorated not only with fluorescent tubulin but also with other fluorescent proteins and fluorochromes without perturbing their morphology. We used theoretical calculations to further investigate Taxol-fluorescent agent interactions. Using computational docking studies we identified a new, potential Taxol binding site within the tubulin dimmer, allowing the interaction between crystalline Taxol and tubulin. Our calculations, however, show that fluorescent tubulin binding to Taxol crystals is more favorable via the fluorochromes covalently linked to the tubulin dimmers, rather than via the new Taxol-binding site, what is in accordance with our experimental data.