DIFFERENCES IN THE REGULATION OF MICROTUBULE DYNAMICS BY MICROTUBULE-ASSOCIATED PROTEINS MAP1B AND MAP2

The regulation of microtubule dynamics in vitro by microtubule-associa ted proteins (MAPs) was examined, using purified porcine MAP1B and MAP 2. MAP1B has a significantly smaller effect on the observed critical c oncentration for microtubule assembly than MAP2. Assembly is faster in the presence of either MAP, and the resulting microtubules are shorte r, indicating that nucleation is substantially promoted by the MAPs. B oth MAPs stabilise the microtubule lattice as observed from podophyllo toxin-induced disassembly, but the effect of MAP1B is weaker than the effect of MAP2. At steady-state of assembly MAP1B still allows microtu bule dynamic instability to occur as inferred from microtubule length changes. The comparison of the effects of MAP1B and MAP2 indicates tha t the reduction of the observed critical concentration is attributable to the reduction of the depolymerisation rate and correlates with the extent of suppression of dynamic instability. Numerical simulations i llustrate that microtubule dynamics are strongly influenced by relativ ely small changes in the strength of a limited subset of subunit inter actions in the lattice. The observed characteristic differences betwee n the MAPs may be important for the regulation of distinct populations of microtubules which coexist in the same cell, where differences in stability and dynamics may be essential for their different spatial ro les as, for example, in developing neurons. (C) 1995 Wiley-Liss, Inc.