In the article, Janson et al. use an in vitro assay to show that force on microtubules leads to microtubule shortening. The group measured the time required for microtubules to reach catastrophe, the point at which growing polymers start to shorten. Microtubules that contacted a synthetic barrier experienced forces that hastened catastrophe. Stronger forces had increasingly larger effects on growth velocity, by lowering the rate of tubulin addition. The relationship between growth velocity and the time until catastrophe was the same in the presence or absence of force. Thus, force's only contribution to shortening is to decrease growth velocity. A delay before addition of the next tubulin dimer could provide more time for structural changes (perhaps altered connections between protofilaments, for example) that might lead to catastrophe.
By sensing force, microtubules can respond to changes in cell shape. For instance, in fission yeast, the nucleus sits in the middle of the cell. Instability as microtubules contact the edges of cells may create the space necessary for nuclear repositioning; otherwise, the nucleus might get stuck somewhere in a corner of the cell. Forces exerted by centrosomes, kinetochores, or molecular motors may similarly affect microtubule dynamics during cell division. That microtubules themselves sense and respond to forces means no localized catastrophe-promoting factors are required. However, microtubules that persist for long periods of time, such as those at the kinetochore, may require stabilizing factors on their growing ends to resist force-induced catastrophes. ▪