Cyclostreptin is the first microtubule-
stabilizing agent whose mechanism of action was discovered to involve formation of a covalent bond with
tubulin. Treatment of cells with
cyclostreptin irreversibly stabilizes their microtubules because
cyclostreptin forms a covalent bond to β-
tubulin at either the T220 or the N228 residue, located at the microtubule pore or
luminal taxoid binding site, respectively. Because of its unique mechanism of action,
cyclostreptin overcomes
P-glycoprotein-mediated multidrug resistance in
tumor cells. We used a series of reactive
cyclostreptin analogues, 6-chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and [(14)C-acetyl]-8-acetyl-
cyclostreptin, to characterize the cellular target of the compound and to map the binding site. The three analogues were cytotoxic and stabilized microtubules in both sensitive and multidrug resistant
tumor cells. In both types of cells, we identified β-
tubulin as the only or the predominantly labeled cellular
protein, indicating that covalent binding to microtubules is sufficient to prevent
drug efflux mediated by
P-glycoprotein. 6-Chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and 8-acetyl-cyclostreptin labeled both microtubules and unassembled
tubulin at a single residue of the same tryptic
peptide of β-
tubulin as was labeled by
cyclostreptin (219-LTTPTYGDLNHLVSATMSGVTTCLR-243), but labeling with the analogues occurred at different positions of the
peptide. 8-Acetyl-cyclostreptin reacted with either T220 or N228, as did the
natural product, while 8-chloroacetyl-cyclostreptin formed a cross-link to C241. Finally, 6-chloroacetyl-cyclostreptin reacted with any of the three residues, thus labeling the pathway for
cyclostreptin-like compounds, leading from the pore where these compounds enter the microtubule to the
luminal binding pocket.