The polymerization ratchet model explains the force-velocity relation for growing microtubules

The polymerization of filamentous proteins generates mechanical forces whic h drive many cellular processes. Dogterom and Yurke measured the force-velo city relation generated by a single microtubule. They found that the force is generally in the range predicted by the "polymerization ratchet" mechani sm, but the force-velocity relationship decreased faster than that theory p redicted. Here we generalize the polymerization ratchet model to take into account the "subsidy effect" that arises because a microtubule consists of 13 protofilaments. With this generalization the model fits the experimental data well. The biological implications of the polymerization ratchet model are discussed.