Mechanical stress induced mechanism of microtubule catastrophes.

Microtubules assembled in vitro from pure tubulin can switch occasionally from growing to shrinking states or resume assembly, an unusual behavior termed "dynamic instability of microtubule growth". Its origin remains unclear and several models have been proposed, including occasional switching of the microtubules into energetically unfavorable configurations during assembly. In this study, we have asked whether the excess energy accumulated in these configurations would be of sufficient magnitude to destabilize the capping region that must exist at the end of growing microtubules. For this purpose, we have analyzed the frequency distribution of microtubules assembled in vitro from pure tubulin, and modeled the different mechanical constraints accumulated in their wall. We find that the maximal excess energy that the microtubule lattice can store is in the order of 11 kBT per dimer. Configurations that require distortions up to approximately 20 kBT are allowed at the expense of a slight conformational change, and larger distortions are not observed. Modeling of the different elastic deformations suggests that the excess energy is essentially induced by protofilament skewing, microtubule radial curvature change and inter-subunit shearing, distortions that must destabilize further the tubulin subunits interactions. These results are consistent with the hypothesis that unfavorable closure events may trigger the catastrophes observed at low tubulin concentration in vitro. In addition, we propose a novel type of representation that describes the stability of microtubule assembly systems, and which might be of considerable interest to study the effects of stabilizing and destabilizing factors on microtubule structure and dynamics.