Participation of guanine nucleotides in nucleation and elongation steps of microtubule assembly.

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Abstract
Critical concentrations for formation of [bovine brain] microtubules from subunits with GTP and its .beta.,.gamma.-imido and .beta.,.gamma.-methylene analogs are similar when adequate time is given for equilibration. Dilution of microtubules into GTP and GDP yielded values of 0.1 and 0.19 mg/ml for the critical concentration, results similar to those reported previously. GDP is capable of supporting elongation of preformed microtubules, but it efficiently poisons the nucleation events. Reported experiments also demonstrate that the critical tubulin concentration of the tubulin-GDP complex can be accurately measured in both the assembly and disassembly directions. Evidence is presented that GTP is involved in early nucleation events but that microtubules are stabilized in the presence of either GTP or GDP. These results are discussed in terms of a condensation-equilibrium model in which tubulin subunits equilibrate rapidly with microtubule ends, and their affinity for the ends is governed by the nucleotide ligand at the exchangeable site.