Molecular Investigations into the Unfoldase Action of Severing Enzymes on Microtubules

Microtubules associated proteins regulate the dynamic behavior of microtubules during cellular processes. Microtubule severing enzymes are the associated proteins which destabilize microtubules by removing subunits from the lattice. One model for how severing enzymes remove tubulin dimers from the microtubule lattice is by unfolding its subunits through pulling on the carboxy-terminal tails of tubulin dimers. This model stems from the fact that severing enzymes are AAA+ unfoldases. To test this mechanism, we apply pulling forces on the carboxy-terminal regions of microtubule subunits using coarse grained molecular simulations. In our simulations we used different microtubule lattices and concentrations of severing enzymes. We compare our simulation results with data from in-vitro severing assays and find that the experimental data is best fit by a model of cooperative removal of protofilament fragments by severing enzymes which depends on the severing enzyme concentration and placement on the microtubule lattice.