Long protein chains in the form of microtubules and actin filaments give cells their shape and rigidity, and also perform chores such as yanking apart copies of chromosomes. Two mathematical models explain how the length of these chains is kept in check by molecular motor-proteins that 'walk' along the chains.

Karsten Kruse and his team at Saarland University in Saarbrücken, Germany, modelled the growth and shrinkage of actin filaments, which occurs at both ends of the chain. Erwin Frey and his group at the Ludwig Maximilian University in Munich, Germany, focused on microtubules, which are trimmed by molecular motors at a single tip.

The teams found that molecular motors control chain length by forming 'traffic jams' at specific places along the chain. At these spots, protein building blocks are continuously gained and shed so that the overall length remains stable. The location of the traffic jam determines the length of the chain, which terminates at the jam.

Phys. Rev. Lett. 108, 258103; (2012) , Phys. Rev. Lett. 108, 258104; (2012)