Microtubule Dynamics Depart from the Wormlike Chain Model

Thermal shape fluctuations of grafted microtubules were studied using high resolution particle tracking of attached fluorescent beads. First mode relaxation times were extracted from the mean square displacement in the transverse coordinate. For microtubules shorter than ∼10  μm, the relaxation times were found to follow an L² dependence instead of L⁴ as expected from the standard wormlike chain model. This length dependence is shown to result from a complex length dependence of the bending stiffness which can be understood as a result of the molecular architecture of microtubules. For microtubules shorter than ∼5  μm, high drag coefficients indicate contributions from internal friction to the fluctuation dynamics.