MICROTUBULE DYNAMICS IN-VITRO ARE REGULATED BY THE TUBULIN ISOTYPE COMPOSITION

The growing and shortening dynamics of individual bovine brain microtu bules at their plus ends at steady state in vitro, assembled from isot ypically pure alpha beta(II), alpha beta(III), or alpha beta(IV) tubul in dimers, were determined by differential interference contrast video microscopy. Microtubules assembled from the purified alpha beta(III) isotype were considerably more dynamic than microtubules made from the alpha beta(II) or alpha beta(IV) isotypes or from unfractionated phos phocellulose-purified tubulin. Furthermore, increasing the proportion of the alpha beta(II) isotype in a mixture of the alpha beta(II) and a lpha beta(III) isotypes suppressed microtubule dynamics, demonstrating that microtubule dynamics can be influenced by the tubulin isotype co mposition. The data support the hypothesis that cells might determine the dynamic properties and functions of its microtubules in part by al tering the relative amounts of the different tubulin isotypes.