Figure 2. Microtubule softening upon external constraint.

a, Sequence of bending cycles. Images show the overlay of microtubule deformations during each successive bending cycle. Bending flows were applied during 10 seconds. A pause of 10 second was respected between bending steps. Deformations appeared larger cycle after cycle. Scale bar is 3 μm.

b, Overlay of microtubule maximal deformation during each bending cycle. Distinct colors have been attributed to each bending cycle. Scale bar is 3 μm.

c, Measurements of microtubule persistent length evolution over the successive bending cycles. Delay between bending cycles was 10 s. Microtubule persistent lengths were normalized to their initial value. Error bars correspond to the standard deviation calculated from 5 consecutive frames for each bending cycle. As a test of tendency, Spearman correlation tests for persistent length values over the successive cycles were performed. Green curves show microtubules for which the persistent length was not significantly affected over the bending cycles, whereas red curves show those that have been soften during the cyclic stress.

d, Comparison of initial microtubule persistent length for microtubules assembled in the presence of 14 μM or 26 μM of free tubulin dimers. Values were compared with an unpaired t test (two-tailed).

e, Overlay of microtubule maximal deformation during bending cycles for microtubules assembled in the presence of 14 μM or 26 μM of free tubulin dimers.

f, Measurements of microtubule persistent length evolution over the successive bending cycles for microtubules assembled in the presence of various concentration of free tubulin dimers (26 μM: n=9; 20 μM: n=15, 18 μM: n=15; 14 μM: n=13). Microtubule persistent lengths were normalized to their initial values. Values correspond to average persistent length of individual measurements shown in Supplementary Figure 1. Error bars show standard deviation between distinct microtubule bending experiments. All curves were considered significantly different using a two-way ANOVA test, except 14 and 18 μM. A T test on the average persistent length showed all these differences became significant at the fourth bending cycle.