Abstract. We have isolated a protein factor from Xenopus eggs that promotes microtubule assembly in vitro. Assembly promotion was associated with a 215-kD protein after a 1,000-3,000-fold enrichment of activity. The 215-kD protein, termed Xenopus microtubule assembly protein (XMAP), binds to microtubules with a stoichiometry of 0.06 mol/mol tubulin dimer.

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Abstract. We have used video-enhanced DIC microscopy to examine the effects of XMAP, a Mr 215,000 microtubule-associated protein from Xenopus eggs (Gard, D. L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215, on the dynamic instability of microtubules nucleated from axoneme fragments in vitro. Our results indicate that XMAP substantially alters the parameters of microtubule assembly at plus ends. Specifically, addition of 0.2/~M XMAP resulted in (a) 7-10-fold increase in elongation velocity, (b) approximately threefold increase in shortening velocity, and (c) near elimination of rescue (the switch from rapid shortening to elongation). Thus, addition of XMAP resulted in the assembly of longer, but more dynamic, microtubules from the plus ends of axonemes which upon catastrophe disassembled back to the axoneme nucleation site. In agreement with previous observations (Gard, D. L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215, the effects of XMAP on the minus end were much less dramatic, with only a 1.5-3-fold increase in elongation velocity. These results indicate that XMAP, unlike brain MAPs, promotes both polymer assembly and turnover, and suggests that the interaction of XMAP with tubulin and the function of XMAP in vivo may differ from previously characterized MAPs.

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We have examined the phosphorylation of cellular microtubule proteins during differentiation and neurite outgrowth in N115 mouse neuroblastoma cells.