Microtubule curvatures under perpendicular electric forces reveal a low persistence length.

The mechanics of microtubules, cylindrical protein filaments that constitute the cytoskeleton, have been well characterized on long length scales. Here, we investigate the persistence length of short (approximately 0.1 microm) ends of microtubules by measuring the trajectories of kinesin-propelled microtubules under perpendicular electric forces. We relate the measured trajectory curvatures to the biased thermal fluctuations of the leading microtubule end, and upon including all electrohydrodynamic forces, we find that the persistence length of the microtubule ends is only 0.08 +/- 0.02 mm. This is significantly shorter than the well established value of approximately 4-8 mm that is measured for long microtubules. Our data are in good agreement with recent theoretical predictions that microtubules mechanically behave as a loose assembly of independent protofilaments on these short length scales.