A Quantitatiove Desdcription of Microtubule Formadtion in the Presence of Tubulin-Colchicine

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Abstract
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. 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. The kinetic and equilibrium derivations presented here are generally applicable to all capping factors, e.g., some of the actin-binding proteins.