Competition between severing and tubulin induced repair of microtubules

Cytoskeletal structures undergo dynamic changes in size to aid in cell polarization, motility, and intracellular transport. Such changes require a rapid turnover of cytoskeleton proteins but how various cytoskeleton-associated proteins coordinate to achieve that is not well understood. Recent experiments have addressed this question by adding key cytoskeleton-associated proteins to cytoskeletal filaments in-vitro and have reported novel synergistic effects. Free pool of tubulin had previously been thought to primarily influence assembly of microtubules, however new experiments indicate a novel effect on their disassembly – free tubulin pool can repair nanoscale damages created by severing proteins. Based on this observation, we propose a model for microtubule severing as a competition between the processes of damage spreading and repair. Using theory and simulations, we demonstrate that this model is in quantitative agreement with recent in vitro experiments, and predict the existence of a critical tubulin concentration above which severing becomes rare but fast. We find that length - control via this new model of severing is more sensitive to changes in the concentration of tubulin and severing protein, and leads to a dramatically increased dynamic range. Our work describes how the concerted action of multiple microtubule associated proteins produces novel dynamical properties of microtubules, which are not seen when studying the action of proteins individually.