On the stall force for growing microtubules

The assembly of microtubules generates forces that play a role in cellular motility processes such as the motion of chromosomes during mitosis. Recent ly, Mogilner and Oster proposed a model for the growth of microtubules that agrees quantitatively with the force-velocity relation measured for indivi dual microtubules. In addition, the authors predicted that the stall force for any polymer consisting of N independently growing protofilaments should increase as the square root of N. We simulated this model and found that t he stall force increases linearly with N, and is in fact consistent with th e maximum force predicted by thermodynamic arguments. We show that this dis crepancy can be explained by a more careful treatment of the "off-term" in the Mogilner-Oster model.