Pivot-and-bond model explains microtubule bundle formation

During mitosis, bundles of microtubules form a spindle, but the physical mechanism of bundle formation is still not known. Here we show that random angular movement of microtubules around the spindle pole and forces exerted by passive cross-linking proteins are sufficient for the formation of stable microtubule bundles. We test these predictions by experiments in wild-type and ase1{Delta} fission yeast cells. In conclusion, the angular motion drives the alignment of microtubules, which in turn allows the cross-linking proteins to connect the microtubules into a stable bundle.