The structure and dynamics of microtubule bundles mediated by motor proteins

Mixtures of polar microtubule (MT) filaments and force-generating motor proteins self-organize by cross-linking and sliding MTs into dense networks of polar and apolar bundles. We can measure the geometric structure of these MT-based bundles using small-angle x-ray scattering and determine the polar structure of these materials by taking advantage of their chiral crystal lattice which produces constructive interference of second harmonic signal between polar-aligned MTs. A transition in the bundles' structure to a compressed square lattice corresponds to a change in the microtubules extensile sliding dynamics. These results are useful in understanding the dynamics of filamentous active materials. The quantified geometric packing of microtubule bundles can be incorporated into a framework of highly cross-linked active gel theories.