Monte Carlo study of the GTP cap in a five-start helix model of a microtubule.

Earlier Monte Carlo studies on a single-helix model of the GTP cap at the end of a microtubule are extended here to a more realistic five-start helix model of the microtubule end. As in the earlier work, phase changes occur at the microtubule end: the end is either capped with GTP and growing slowly or uncapped and shortening rapidly, and these two regimes alternate (at a given tubulin concentration) at steady state. Macroscopic rate constants for the two-phase model are deduced from the Monte Carlo results. The macroscopic rate constants lead to properties that are in semiquantitative agreement with related experiments of Mitchison and Kirschner.