Microtubule dynamics depart from wormlike chain model
Microtubules are cytoskeletal protein filaments that play an essential role in a multitude of cell functions in all eucaryotes. While specialized structures such as cilia, flagellae and axons require microtubule lengths of up to several hundred micrometers, fundamental tasks like cell division and intracellular transport involve microtubules with lengths that are comparable to or smaller than typical eucaryotic cell sizes (10-20 micrometer).
The standard model for semiflexible polymers is the wormlike chain which envisions a homogeneous, isotropic, continuously flexible rod characterized by its bending stiffness. We have shown that this standard model fails for microtubules, mainly due to the highly anisotropic molecular architecture where protofilaments are arranged in parallel to form a hollow tube of 25 nanometer in diameter. This protofilament architecture makes microtubules a model system for the generalized theory of wormlike bundles [arXiv:q-bio/0607040] [Phys. Rev. Lett. 99, 048101 (2007)], which describes a broad range of bundle-like structures including nanotubes.
We have analyzed thermal shape fluctuations of grafted microtubules using high resolution particle tracking of attached fluorescent beads. First mode relaxation times were extracted from the mean-square displacement in the transverse bending mode. For microtubules with length L shorter than 10 micrometer, 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 explained in the framework of the wormlike bundle model. For microtubules shorter than 5 micrometers, high drag coefficients indicate contributions from internal friction to the fluctuation dynamics.
These findings not only emphasize the importance of molecular architecture to be taken into account for models of semiflexible polymer mechanics, but also suggest a wealth of possibilities for natural modulation of biopolymer properties in cells.