When it's not busy making endocytic vesicles, the clathrin heavy chain stabilizes the mitotic spindle. Lin et al. reveal that the heavy chain performs this job by recruiting another key spindle-supporting protein.

The spindle has important work to do during mitosis, helping to arrange and separate the chromosomes, and cells can't afford a premature microtubule collapse. Loss of the clathrin heavy chain causes kinetochore fibers to break down, prevents the chromosomes from lining up properly, and slows mitosis. But how the protein reinforces the spindle has eluded researchers.

Lin et al. discovered that the heavy chain interacts with the spindle-stabilizing protein TACC3 after the latter is phosphorylated by the aurora A kinase. TACC3 attracts the protein ch-TOG, which spurs microtubules to assemble. Without the clathrin heavy chain, little TACC3 reached the spindle, which became jumbled.

The next step, the researchers say, is determining how the clathrin heavy chain maneuvers TACC3 into position. Another question is whether the heavy chain's partner in endocytosis, the clathrin light chain, also affects spindle dynamics.

et al
J. Cell Biol.