SUBSTOICHIOMETRIC BINDING OF TAXOL SUPPRESSES MICROTUBULE DYNAMICS

We have measured the effects of taxol (10 nM to 1 mu M) on the growing and shortening dynamics at the ends of individual bovine brain microt ubules in vitro and have correlated the effects both with the stoichio metry of taxol binding to tubulin in microtubules and with the changes in the microtubule polymer mass. The results indicate that taxol supp resses microtubule dynamic instability differently depending upon the stoichiometry of taxol binding to the microtubules. At the lowest effe ctive concentrations (less than or equal to 100 nM), substoichiometric binding of taxol to tubulin in microtubules (between 0.001 and 0.01 m ol of bound taxol/mol of tubulin in microtubules) potently and selecti vely suppresses the rate and extent of shortening at plus ends in asso ciation with some increase (28% to 60%) in the mass of microtubule pol ymer. At intermediate taxol concentrations (between 100 nM and 1 mu M) , the binding of additional taxol molecules to the microtubules (betwe en 0.01 and 0.1 mol of taxol bound/mol of tubulin in microtubules) inh ibits both growing and shortening events at both microtubule ends with no additional increase in microtubule polymer mass, At high taxol con centrations and high taxol binding stoichiometries (greater than or eq ual to 1 mu M taxol and greater than or equal to 0.1 mol of taxol boun d/mol of tubulin in microtubules), microtubule mass increases sharply and dynamics is almost completely suppressed. The data support the hyp othesis that binding of a molecule of taxol to a tubulin subunit in mi crotubules induces a conformational change in that subunit that strong ly reduces its ability to dissociate when the subunit becomes exposed at the microtubule end.