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 lowstrain 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.