Tracer Studies on F-Actin Fluctuations

We present a study on the fluctuations of semiflexible actin filaments using fluorescence videomicroscopy, focusing on the end-to-end fluctuations of single filaments. In order to specifically measure the position of the polymer’s ends, we developed a novel noninvasive method that consists of annealing short end tags to the filaments. This allows us to probe polymer fluctuations to a very high accuracy. We compared the distribution of the end-to-end distance with recent theoretical results, and found excellent agreement. We also studied the dynamics of the mean-square end-to-end distance $\delta R^2(t)$ and orientation of the ends, $\delta {\Theta }^2(t)$, finding power laws $t^{3/4}$ and $t^{1/4}$, respectively. Scaling behavior for $\delta R^2(t)$ is observed over several decades in relaxation time in agreement with theoretical results.

Figure 1

Fluorescence microscopy image of an actin filament. As shown on the sketch, it is made of one long filament that annealed by the ends with two bright fragments. The scale bar is $5\mu \mathrm{m}$. Monomers are not at scale on the sketch. The end tags are constituted of $\sim 50–100$ monomers.

Figure 2

Measured radial distribution function (bars), and theoretical expression for 2D RDF (line) with ${\ell }_p=16.1\pm 0.2\mu \mathrm{m}$ and $L_c=13.40\pm 0.01\mu \mathrm{m}$. Inset: Measured tangent correlation function of the filament ($-+-$), and single exponential decay adjustment $\mathbf{t}(s+x)·\mathbf{t}(s)=e^{-x/2l_p}$ (thick line), yielding a persistence length $l_p=16.2\pm 0.7\mu \mathrm{m}$.

Figure 3

Rescaled MSD $F=(90{\ell }_p^2)\delta R^2(L,t)/L^4$ as a function of rescaled time $t/{\tau }_c=t(4.73/L{)}^4(\kappa /\zeta )$; the solid line is the prediction for a weakly bending rod in two dimensions. Inset: Raw data before rescaling for filaments with lengths $L[\mu \mathrm{m}]=\{5.9$ (bottom), 10.4, 10.7, 11.8, 11.8, 11.9, 12.9, 25.6 (top)$\}$. The curve results from the analysis of over 60 000 images.

Figure 4

Measured MSD for the angle of a tag, giving local orientational dynamics. The plot is logarithmic; a $\propto t^{1/4}$ guideline is represented.