Direct photoaffinity labeling of tubulin with taxol.

Taxol is a potent inhibitor of the replication of eukaryotic cells and has significant antitumor activity in human malignancies. The drug induces the formation of bundles of stable microtubules and blocks cells in the mitotic phase of the cell cycle. In vitro, taxol enhances the polymerization of tubulin to microtubules that are resistant to depolymerization. Although it is evident that taxol interacts with the tubulin-microtubule system, no information has been available on the binding site for the drug on the microtubule. Our purpose was to determine if taxol binds to one or both of the tubulin subunits. In the absence of a photoaffinity-labeled analogue of taxol, [3H]taxol was used directly to photolabel tubulin. A complex of microtubule protein and [3H]taxol was irradiated by ultraviolet light and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The radiolabeled drug preferentially binds covalently to the beta-subunit of tubulin, and the binding can be competed with unlabeled taxol. This observation is the first step in a study to determine the binding site for taxol on the microtubule.