Microtubules soften due to cross-sectional flattening

TL;DR

This study reveals that microtubules soften under high strain due to cross-sectional flattening, challenging classical elastic models and providing a new understanding of their mechanical behavior.

Contribution

It introduces a three-dimensional thin-shell model explaining microtubule softening at high strains, advancing the understanding of their elastic properties.

Findings

Microtubules follow classical elasticity at low strains.

Above a critical strain, microtubules soften due to cross-sectional flattening.

The thin-shell model accurately predicts the softening behavior.

Abstract

We use optical trapping to continuously bend an isolated microtubule while simultaneously measuring the applied force and the resulting filament strain, thus allowing us to determine its elastic properties over a wide range of applied strains. We find that, while in the low-strain regime, microtubules may be quantitatively described in terms of the classical Euler-Bernoulli elastic filament, above a critical strain they deviate from this simple elastic model, showing a softening response with increasingdeformations. A three-dimensional thin-shell model, in which the increased mechanical compliance is caused by flattening and eventual buckling of the filament cross-section, captures this softening effect in the high strain regime and yields quantitative values of the effective mechanical properties of microtubules. Our results demonstrate that properties of microtubules are highly dependent on the magnitude of the applied strain and offer a new interpretation for the large variety in microtubule mechanical data measured by different methods.