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Bending Dynamics of Fluctuating Biopolymers Probed by Automated High-Resolution Filament Tracking

Clifford P. Brangwynne ^*, Gijsje H. Koenderink ^*, Ed Barry , Zvonimir Dogic , Frederick C. MacKintosh  and David A. Weitz ^* 

^* Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts; Rowland Institute at Harvard, Cambridge, Massachusetts; Department of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands; and Department of Physics, Harvard University, Cambridge, Massachusetts

Correspondence: Address reprint requests to David A. Weitz, Gordon Mckay Professor of Applied Physics and Professor of Physics, Harvard University, Pierce Hall, Rm. 321, 29 Oxford St., Cambridge, MA 02138. Tel.: 617-496-2842; Fax: 617-495-2875; E-mail: weitz{at}seas.harvard.edu.

Microscope images of fluctuating biopolymers contain a wealth of information about their underlying mechanics and dynamics. However, successful extraction of this information requires precise localization of filament position and shape from thousands of noisy images. Here, we present careful measurements of the bending dynamics of filamentous (F-)actin and microtubules at thermal equilibrium with high spatial and temporal resolution using a new, simple but robust, automated image analysis algorithm with subpixel accuracy. We find that slender actin filaments have a persistence length of 17 µm, and display a q^–4-dependent relaxation spectrum, as expected from viscous drag. Microtubules have a persistence length of several millimeters; interestingly, there is a small correlation between total microtubule length and rigidity, with shorter filaments appearing softer. However, we show that this correlation can arise, in principle, from intrinsic measurement noise that must be carefully considered. The dynamic behavior of the bending of microtubules also appears more complex than that of F-actin, reflecting their higher-order structure. These results emphasize both the power and limitations of light microscopy techniques for studying the mechanics and dynamics of biopolymers.

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