Paclitaxel Suppresses Tau-mediated Microtubule Bundling in a Concentration-dependent Manner

Background: Microtubules (MTs) are protein nanotubes comprised of straight protofilaments (PFs), head to tail assemblies of αβ-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> observed for increasing Tau/tubulin-dimer molar ratio Φ at paclitaxel/tubulin-dimer molar ratio Λ=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≤Λ≤1/4) and Tau (human isoform 3RS, 0≤Φ≤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 Λ=1/1 where microtubule bundles are absent, in the lower paclitaxel concentration regime (Λ≤1/4), we observe both microtubule doublets and triplets with increasing Tau. Furthermore, increasing paclitaxel concentration (up to Λ=1/1) slightly decreased the average microtubule diameter (by ~1 PF) while increasing Tau concentration (up to Φ=1/2) significantly increased the diameter (by ~2-3 PFs).

Conclusions: The suppression of Tau-mediated microtubule bundling with increasing paclitaxel is consistent with paclitaxel 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.