Promotion of MAP/MAP interaction by taxol.

The effects of taxol on microtubule-associated proteins of high molecular weight (MAPs) were studied in vitro. After negative staining, microtubules reconstituted in the presence of taxol from preparations of partially purified tubulin and MAPs, besides being bundled, displayed prominent elongated or globular extensions without apparent regularity. These extensions, but not the tubulin polymer, were heavily decorated after immuno-gold-labeling using antibodies to MAP-1 and MAP-2. Microtubules reconsituted in the absence of taxol showed a much more regular, and apparently helical, arrangement of MAPs along their surfaces. The formation of polymeric structures was also observed when preparation of MAPs free of tubulin were incubated with taxol. In this case in addition to large network-type aggregates with little apparent substructure, more regular structures seemingly consisting of approximately 5-nm-thick filaments arrayed in parallel were observed. Taxol-induced MAP aggregation occurred rapidly and was directly proportional to the concentration of protein, as revealed by optical density measurements. It is concluded that taxol, aside from promoting the assembly of tubulin and stabilizing microtubules, promotes MAP/MAP interaction.