3'-(p-Azidobenzamido)taxol photolabels the N-terminal 31 amino acids of β-tubulin

Taxol possesses an unusual chemical structure, a unique mechanism of action, and demonstrated activity in human malignancies. It is the only antitumor agent that has a binding site on the microtubule polymer. The interaction of Taxol with the microtubule polymer results in the formation of stable bundles of cellular microtubules that are resistant to depolymerization. Although it has become evident that the microtubule, specifically β-tubulin, is the target for Taxol, no information is available on the binding site for the drug. In this report, we demonstrate that 3'-(p- azidobenzamido)taxol, an analogue with similar biological activities as Taxol, covalently binds to the N-terminal domain of β-tubulin after irradiation of the microtubule-drug complex. Taxol competes with [³H]3'-(p- azidobenzamido)-taxol binding, suggesting that the photoaffinity analog and Taxol are binding at the same or overlapping sites. Formic acid cleavage of [³H]3'-(p-azidobenzamido)-taxol-photolabeled β-tubulin and subsequent protein sequence and mass analyses have identified the N-terminal 31 amino acids as the major site for [³H]3'-(p-azidobenzamido)taxol photoincorporation.