In vivo and in vitro studies on the role of HMW-MAPs in taxol-induced microtubule bundling.

The involvement of high molecular weight microtubule-associated proteins (HMW-MAPs) in the process of taxol-induced microtubule bundling has been studied using immunofluorescence and electron microscopy. Immunofluorescence microscopy shows that HMW-MAPs are released from microtubules in granulosa cells which have been extracted in a Triton X-100 microtubule-stabilizing buffer (T-MTSB), unless the cells are pretreated with taxol. 1.0 microM taxol treatment for 48 h results in microtubule bundle formation and the retention of HMW-MAPs in these cells upon extraction with T-MTSB. Electron microscopy demonstrates that microtubules in control cytoskeletons are devoid of surface structures whereas the microtubules in taxol-treated cytoskeletons are decorated by globular particles of a mean diameter of 19.5 nm. The assembly of 3 X cycled whole microtubule protein (tubulin plus associated proteins) in vitro in the presence of 1.0 microM taxol, results in the formation of closely packed microtubules decorated with irregularly spaced globular particles, similar in size to those observed in cytoskeletons of taxol-treated granulosa cells. Microtubules assembled in vitro in the absence of taxol display prominent filamentous extensions from the microtubule surface and center-to-center spacings greater than that observed for microtubules assembled in the presence of taxol. Brain microtubule protein was purified into 6 s and HMW-MAP-enriched fractions, and the effects of taxol on the assembly and morphology of these fractions, separately or in combination, were examined. Microtubules assembled from 6 s tubulin alone or 6 s tubulin plus taxol (without HMW-MAPs) were short, free structures whereas those formed in the presence of taxol from 6 s tubulin and a HMW-MAP-enriched fraction were extensively crosslinked into aggregates. These data suggest that taxol induces microtubule bundling by stabilizing the association of HMW-MAPs with the microtubule surface which promotes lateral aggregation.