Effects of the fluorescence dye DAPI on microtubule structure in vitro: formation of novel types of tubulin assembly products.

It has been found that the DNA fluorescence dye 4',6-diamidino-2-phenylindole (DAPI) is able to stain also microtubules. However, electron microscopy revealed that DAPI changed microtubule structure and induced the formation of a broad spectrum of polymorphic tubulin assembly products. Upon addition of DAPI to microtubules assembled from 10 to 15 mumol tubulin (molar DAPI/tubulin ratios of 10 to 40) in the presence of microtubule-associated proteins, most of the microtubules were decorated with additional protofilaments usually running parallel to the protofilaments of the microtubule wall (microtubule-protofilament complexes). When DAPI was already present during assembly, curved C- and S-shaped protofilament ribbons and microtubule-ribbon complexes with 6-shaped profiles were the most prominent products, beside microtubules. Additionally, protofilament bundles, some flat sheets, and hoops occurred. Electrophoresis revealed that DAPI lowered the amount of associated proteins, especially of tau-proteins, bound to the assembly products. Nevertheless, DAPI stimulated the assembly, enabled pure tubulin to assemble even at concentrations as low as 10 mumol, and stabilized the assembly products against cold. The microtubule-protofilament complexes, observed for the first time, are interpreted as the result of DAPI-induced protofilament linking as well as of activation of an additional tubulin-tubulin binding site which is possibly identical to that involved in the formation of microtubule doublets.