TAXOL DIFFERENTIALLY MODULATES THE DYNAMICS OF MICROTUBULES ASSEMBLEDFROM UNFRACTIONATED AND PURIFIED BETA-TUBULIN ISOTYPES

Substoichiometric binding of taxol to tubulin in microtubules potently suppresses microtubule dynamics, which appears to be the most sensiti ve antiproliferative mechanism of taxol. To determine whether the beta -tubulin isotype composition of a microtubule can modulate sensitivity to taxol, we measured the effects of substoichiometric ratios of taxo l bound to tubulin in microtubules on the dynamics of microtubules com posed of purified alpha beta(II)-, alpha beta(III)-, or alpha beta(IV) -tubulin isotypes and compared the results with the effects of taxol o n microtubules assembled from unfractionated tubulin. Substoichiometri c ratios of bound taxol in microtubules assembled from purified beta-t ubulin isotypes or unfractionated tubulin potently suppressed the shor tening rates and the lengths shortened per shortening event. Correlati on of the suppression of the shortening rate with the stoichiometry of bound taxol revealed that microtubules composed of purified alpha bet a(II)-, alpha beta(III)-, and alpha beta(IV)-tubulin were, respectivel y, 1.6-, 7.4-, and 7.2-fold less sensitive to the effects of bound tax ol than microtubules assembled from unfractionated tubulin, These resu lts indicate that taxol differentially modulates microtubule dynamics depending upon the beta-tubulin isotype composition. The results are c onsistent with recent studies correlating taxol resistance in tumor ce lls with increased levels of beta(III)- and beta(IV)-tubulin expressio n and suggest that altered cellular expression of beta-tubulin isotype s can be an important mechanism by which tumor cells develop resistanc e to taxol.