Microtubules are dynamically unstable polymers of tubulin heterodimers that form a central part of the cytoskeleton in eukaryotic cells. Microtubule dynamics are driven by the GTPase activity of tubulin, whereby specific nucleotide states facilitate either polymerization (i.e., GTP) or depolymerization (i.e., GDP). Recent work has highlighted the large degree of microtubule lattice structural changes caused by the binding of motor proteins, microtubule-associated proteins, and cancer therapeutics, such as Taxol. The binding of Taxol to microtubules prevents microtubule depolymerization, thereby inhibiting cell division. Despite decades of structural studies, conflicting models exist as to how Taxol stabilizes the microtubule lattice due to microtubule “decoration” by associated proteins (e.g., kinesin) or technical issues that limit the resolution of cryo-EM reconstructions. Here, we set out to determine high-resolution (< 2.5Å) structures of undecorated human microtubules to understand the microtubule stabilization mechanism of Taxol. Through the development of a new cryo-EM analysis pipeline, we determined reconstructions at 2.2–2.3Å for GMPCPP (GTP analog), GDP, and GDP-Taxol-bound microtubules. Our structures are the highest-resolution microtubule structures to date, showing over 400 water molecules across two heterodimers. Surprisingly, we find that GDP-Taxol-bound microtubules have nearly identical high-resolution details to GMPCPP-stabilized microtubules (avg. RMSD = ∼0.2Å), showing that Taxol reverts the GDP tubulin lattice to a pre-hydrolysis state despite different water networks and nucleotide states. Inspection of the asymmetric microtubule “seam” reveals a staggered longitudinal compaction and striking lateral kinking upon GTP hydrolysis that is reversed upon Taxol binding. Our high-resolution structures support a seam-centric model of Taxol stabilization: weak seam interactions in the GDP microtubule lattice are over-ridden via Taxol-mediated lattice expansion, allowing Taxol to promote polymerization and prevent depolymerization.
Vangos et al. (Sun,) studied this question.