Microtubule-stabilizing activity of microtubule-associated proteins (MAPs) is due to increase in frequency of rescue in dynamic instability: shortening length decreases with binding of MAPs onto microtubules.

The role of microtubule associated proteins (MAPs) on the dynamic instability of microtubules was examined under a dark-field microscope using bovine brain tubulin purified by DEAE-Sepharose column chromatography. In the absence of MAPs, the transition from the shortening phase to the growing phase (the rescue) occurred rarely both in self-assembled microtubules and seeded ones, especially at the plus end. Even under the conditions unfavorable to stabilize microtubule, the addition of a small amount of crude MAPs or purified microtubule associated protein 2 (MAPs) to the microtubules allowed them to undergo the rescue. At increased concentrations of MAPs or MAP2, both the length change required for a rescue during shortening phase ("shortening length") and for a catastrophe (transition from the growing to the shortening phase) ("growth length") decreased. Under these conditions, the rescue often occurred at the same site where previous rescues occurred. Distribution of immunofluorescent MAP2 antibodies along individual microtubules showed that MAP2 molecules bound onto microtubules by forming discrete clusters. The number of MAP2 molecules per cluster was estimated to be between 25 and 60. Because both the "shortening length" and the distance between MAP2 clusters in a microtubule decreased with increased MAPs concentration, we suggest that the MAP2 clusters may form the specific site at which the shortening of the microtubule readily stops. MAP2 possibly regulates the dynamic instability by stopping the shortening, which is a prerequisite for the rescue.