Shape dynamics of single microtubules under load. (a) Microtubules before (left) and during (right) compression. Traced microtubule shape (green) is overlayed on confocal images of EGFP-tubulin. Scale bar is 10 μm. (b) The ensemble-averaged square amplitude of the Fourier modes of microtubule shape is plotted as a function of the wavenumber. Yellow (blue) color indicates before (under) a compression. Shaded area indicates the difference in the mode amplitude between 0.7 and 3 μm−1. More than 100 microtubules from >10 cells were analyzed. Inset: enlarged plot in semi-log scale. (c–e) Non-Euler buckling of microtubules. (c) Time-lapse images of an axially compressed microtubule. A traced microtubule shape (green) is overlayed on confocal images of EGFP-tubulin. t = 0 sec corresponds to the onset of the compression. Scale bar is 2 μm. (d) The time evolution of mode amplitudes of the microtubule is shown in c. Shaded area indicates the period of compression. The mode amplitude is also color coded. (e) The most unstable wavenumber of axially compressed microtubules in control (n = 12 microtubules) and latrunculin B (n = 6 microtubules) conditions. (f) Effective spring constant of the bulk cytoplasm measured with particle-based magnetic tweezers. Beads coated either with phalloidin (n = 11 for control, n = 10 for latrunculin B) or PEG (n = 9 for control, n = 11 for latrunculin B) were used. Error bars indicate the standard deviation.
