Probing the Pore Drug Binding Site ofMicrotubules with Fluorescent Taxanes:Evidence of Two Binding Poses

The pore site in microtubules has been studied with the use of Hexaflutax, a fluorescent probe derived from paclitaxel. The compound is active in cells with similar effects to paclitaxel, indicating that the pore may be a target to microtubule stabilizing agents. While other taxanes bind microtubules in a monophasic way, thus indicating a single type of sites, Hexaflutax association is biphasic. Analysis of the phases indicates that two different binding sites are detected, reflecting two different modes of binding, which could arise from different arrangements of the taxane or fluorescein moieties in the pore. Association of the 4-4-20 antifluorescein monoclonal antibody-Hexaflutax complex to microtubules remains biphasic, thus indicating that the two phases observed arise from two different poses of the taxane moiety.