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 similar to 10 mu m, the relaxation times were found to
follow an L(2) dependence instead of L(4) 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 similar to 5 mu m,
high drag coefficients indicate contributions from internal
friction to the fluctuation dynamics.