Mechanics of severing for large microtubule complexes revealed by coarse-grained simulations.

We investigate the mechanical behavior of microtubule (MT) protofilaments under the action of bending forces, ramped up linearly in time, to provide insight into the severing of MTs by microtubule associated proteins (MAPs). We used the self-organized polymer model which employs a coarse-grained description of the protein chain and ran Brownian dynamics simulations accelerated on graphics processing units that allow us to follow the dynamics of a MT system on experimental timescales. Our study focused on the role played in the MT depolymerization dynamics by the inter-tubulin contacts a protofilament experiences when embedded in the MT lattice, and the number of binding sites of MAPs on MTs. We found that proteins inducing breaking of MTs must have at least three attachment points on any tubulin dimer from an isolated protofilament. In contrast, two points of contact would suffice when dimers are located in an intact MT lattice, in accord with experimental findings on MT severing proteins. Our results show that confinement of a protofilament in the MT lattice leads to a drastic reduction in the energy required for the removal of tubulin dimers, due to the drastic reduction in entropy. We further showed that there are differences in the energetic requirements based on the location of the dimer to be removed by severing. Comparing the energy of tubulin dimers removal revealed by our simulations with the amount of energy resulting from one ATP hydrolysis, which is the source of energy for all MAPs, we provided strong evidence for the experimental finding that severing proteins do not bind uniformly along the MT wall.