Cohesin resolves to let go

or microtubule asters, just as with life in general, sometimes you get stuck in a corner. And, say Cendrine Faivre-Moskalenko and Marileen Dogterom (FOM Institute for Atomic and Molecular Physics, Amsterdam, Netherlands), it can take a catastrophe to get things moving again. Dogterom has been trying to reproduce aster behavior in the minimal environment of a microfabricated chamber. Her beads, which are coated with cross-linked tubulin seeds to direct the growth of microtubules, are nudged to the chamber’s center as microtubules push on the chamber walls. But Dogterom realized that, in live cells, asters move and reposition constantly during mitosis. She mimicked these events by grabbing the beads with a laser trap. Beads placed in a corner stayed put, because the established, long microtubules bent around the chamber walls. But if Dogterom added Op18 protein to induce catastrophic microtubule shortening, the number of lengthy microtubules was reduced. Growing microtubules once again encountered the chamber walls head-on, allowing them to push the aster around. In the smaller cells of fission yeast, a microtubule that hits the cell surface might induce its own catastrophe. The force upon contact slows growth rate, and thus increases the likelihood that the microtubule will lose its protective cap. But in larger cells, the longer microtubules will buckle more readily. Here, peripheral motors may help pull asters to a central position. Dogterom hopes to test such a model by tethering motors to the walls of her chambers.