ABSTRACT
Well over half the mass of microtubules is composed of the protein tubulin, and interference with the functions of tubulin in cells can lead to profound disturbances of cellular physiology. Since the mitotic spindle is largely composed of microtubules, one of the most dramatic consequences of microtubule disruption is metaphase arrest. Condensed chromosomes are unable to progress through mitosis because the spindle fails to form. J. Bryan examined the interactions of paracrystals isolated from sea urchin eggs with small ligands. Relative to tubulin they contained one molar equivalent of vinblastine, which would not exchange with free vinblastine, and two molar equivalents of guanine nucleotide, which also was not exchangeable. Finally, the binding of nucleotides to tubulin is profoundly affected by divalent cations; antimitotic drugs have major effects on tubulin-nucleotide interactions. Two guanine-modified analogues, azidoGTP and thioGTP, have high affinities for the exchangeable site and readily support microtubule assembly.
