Microtubule Mechanics

Microtubules are the most rigid of all filaments composing the cytoskeleton. This rigidity is important to their function of supporting the cellular architecture - especially in long, extended structures such as the cilium or the axon of neurons. There are a variety of methods to measure the rigidity of microtubules. Further, microtubule-associated proteins and other stabilizing factors have been shown to alter the rigidity of microtubules. For a review, please see: T. Hawkins, M. Mirigian, M. Selcuk Yasar, J.L. Ross, “Mechanics of Microtubules,” Journal of Biomechanics, 43, 23-30 (2010) web Hawkins-JBiomech-2010.pdf

By directly imaging single microtubules fluctuating under thermal forces, we can measure the flexibility of microtubules. The rigidity is backed out of the variance in the shape fluctuations. With our collaborator, David Sept (University of Michigan), we have determined a method to use Bootstrapping to resample the data to determine the error in our variance to perform a weighted fit to the data. We found that the distribution in the persistence length measurement is not normal, but is typically log-normal. By log-transforming, we can compare distributions that are normal to get correct statistics to compare data sets. T.L. Hawkins, M. Mirigian, J. Li, M.S. Yasar, D.L. Sackett, D. Sept, J.L. Ross, “Perturbations in Microtubule Mechanics from Tubulin Preparation,” Cellular and Molecular Bioengineering, 5, 227-238 (2012). web Hawkins-CAMB-2012.pdf

We recently used our new technique to measure the persistence length of microtubules that are doubly stabilized with Taxol and another stabilizer such as GMPCPP, GTPgS, MAP4, or Tau. We found that GMPCPP, a non-hydrolyzable analog of GTP, stiffens microtubules and dominates the effects of Taxol, which is known to make microtubules more flexible. Unlike GMPCPP, a different non-hydrolyzable analog of GTP called GTPgS, makes microtubules more flexible. MAP4 does not stiffen microtubules, but alters the distribution of measured persistence lengths. Tau, a microtubule-associated protein known to bind microtubules and stabilize the filaments in axons of nerve cells, had the largest effect on the microtubule stiffness. We found that when tau was added after polymerization had no effect on the stiffness of microtubules. Yet, when tau was present during polymerization, we saw a 6-fold increase in the rigidity of the microtubules. T.L. Hawkins, D. Sept, B. Moogessie, A. Straube, J.L. Ross, “Mechanics of Doubly Stabilized Microtubules,” Biophysical Journal, 104, 1517-1528 (2013). web Hawkins-BPJ-2013.pdf