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Microtubule Structure

Using small-agnle X-ray scattering over the past years, we have determined the structure of microtubule (MT) at increasing level of resolution. Initially we started by modeling the MT as a uniform hollow cylinder (PNAS 2004, Phys. Rev. Lett. 2004, or PNAS 2005). We have then included finer structureal features like the three-start helical character  of the MT and the fact it is built from subunits that we represented as small spheres (J. Appl. Crys, 2010).

MT model
Illustration of the gradually increasing resolution of microtubule models. At low resolutions, the microtubules can be modeled as a hollow cylinder. As resolution is increased, ridges appear, turning it into a three-start helix, which in turn can be further discretized to be made of spheres.

We have then developped experimental protocols to isolate the MT from the other coexisting tubulin assemblies (confirmed by both solution X-ray scattering and cryo-TEM, done by Dr. Yael Levi-Kalisman). Using the new data, we have developped algorithms for docking of the atomic model of tubulin dimers into the three-start left handed helical lattice of MT (Phys. Chem. B. 2017). In this analysis we have determined the radii, the pitch, and the distribution of protofilament number (between 12 and 15 protofilaments per MT) that best fit the scattering data from dynamic MT or MT that was stabilized by either paclitaxol or GMPPCP (a nonhydrolyzable GTP analog).

 

MT protofilament number
Atomic model of MTs with different protofilament numbers (13, 14, and 15).

More recently we have been also able to take into account the contribution of the hydration layer of the entire atomic MT model (J Appl. Crys. 2019).