Background: Microtubules (MTs) are protein nanotubes comprised of straight protofilaments (PFs), head to tail assemblies of alpha beta-tubulin heterodimers. Previously, it was shown that Tau, a microtubule-associated protein (MAP) localized to neuronal axons, regulates the average number of PFs in microtubules with increasing inner radius <R-in(MT)> observed for increasing Tau/tubulin-dimer molar ratio Phi(Tau) at paclitaxel/tubulin-dimer molar ratio Lambda(ptxl) = 1/1.
Methods: We report a synchrotron SAXS and TEM study of the phase behavior of microtubules as a function of varying concentrations of paclitaxel (1/32 <= Lambda(ptxl) <= 1/4) and Tau (human isoform 3RS, 0 <= Phi(3Rs) <= 1/2) at room temperature.
Results: Tau and paclitaxel have opposing regulatory effects on microtubule bundling architectures and microtubule diameter. Surprisingly and in contrast to previous results at Air = 1/1 where microtubule bundles are absent, in the lower paclitaxel concentration regime (Lambda(ptxl) <= 1/4), we observe both microtubule doublets and triplets with increasing Tau. Furthermore, increasing paditaxel concentration (up to Lambda(ptxl) = 1/1) slightly decreased the average microtubule diameter (by similar to 1 PF) while increasing Tau concentration (up to Phi(3RS) = 1/2) significantly increased the diameter (by similar to 2-3 PFs).
Conclusions: The suppression of Tau-mediated microtubule bundling with increasing paclitaxel is consistent with paditaxel seeding more, but shorter, microtubules by rapidly exhausting tubulin available for polymerization. Microtubule bundles require the aggregate Tau-Tau attractions along the microtubule length to overcome individual microtubule thermal energies disrupting bundles.
General significance: Investigating MAP-mediated interactions between microtubules (as it relates to in vivo behavior) requires the elimination or minimization of paclitaxel. (C) 2016 Elsevier B.V. All rights reserved.