A quantitative description of microtubule formation in the presence of tubulin-colchicine.

The overall polymerization of microtubule protein in the presence of tubulin-colchicine is described by competition between an intrinsically unaltered nucleation process and the process of propagation inhibited by the binding of tubulin-colchicine to the microtubule ends. The inhibition of propagation can be quantified with the binding constant previously determined [Lambeir and Engelborghs (1980) Eur. J. Biochem. 109, 619-624]. A quantitative description of the competition between nucleation and propagation follows from the kinetic theory of Oosawa. Comparison of several subsequent cycles of polymerization/depolymerization shows that a fraction of cold-stable complexes are formed. An equilibrium derivation is presented which shows the enhanced nucleation upon binding of inhibiting proteins, by the increase of the nucleation parameter A [Oosawa and Asakura (1975) Thermodynamics of the Polymerization of Protein, Academic Press, London, New York]. The kinetic and equilibrium derivations presented here are generally applicable to all capping factors, e.g. some of the actin-binding proteins.