Paclitaxel suppresses Tau-mediated microtubule bundling in a concentration-dependent manner

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 in^MT> observed for increasing Tau/tubulin-dimer molar ratio Φ_Tau at paclitaxel/tubulin-dimer molar ratio Λ_Ptxl=1/1.

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_Ptxl≤1/4) and Tau (human isoform 3RS, 0≤Φ_3RS≤1/2) at room temperature.

Tau and paclitaxel have opposing regulatory effects on microtubule bundling architectures and microtubule diameter. Surprisingly and in contrast to previous results at Λ_Ptxl=1/1 where microtubule bundles are absent, in the lower paclitaxel concentration regime (Λ_Ptxl≤1/4), we observe both microtubule doublets and triplets with increasing Tau. Furthermore, increasing paclitaxel concentration (up to Λ_Ptxl=1/1) slightly decreased the average microtubule diameter (by ~1 PF) while increasing Tau concentration (up to Φ_3RS=1/2) significantly increased the diameter (by ~2-3 PFs).

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.

Investigating MAP-mediated interactions between microtubules (as it relates to in vivo behavior) requires the elimination or minimization of paclitaxel.