Geometrical and Mechanical Properties Control Actin Filament Organization

Abstract

The organization of actin filaments into higher-ordered structures governs eukaryotic cell shape and movement. For all these processes, the collective behavior of the actin filaments is governed by the environmental conditions (associated proteins, geometry, confinement...).We demonstrated recently that nucleation geometry governs ordered actin structure organization (Reymann et al., 2010) but the mechanisms behind this control were not clearly established. To understand how the geometrical parameters governed the actin dynamics, we simulated the filament growth from pattern with the cytoskeleton simulation software Cytosim. The simulation parameters were first calibrated by matching in-vitro and simulated filaments behavior from a simple pattern. The steric interaction between filaments was particularly crucial to obtain a good match between experimental and simulated actin architecture.We then used the simulations to observe the effect of the pattern geometry and filament rigidity on the overall organization of the actin structures. We observed that both the nucleation geometry and the mechanical properties of actin filaments are essential to built from a common pool of actin monomers the diversity of actin organizations observed in vivo (parallel or antiparallel bundles and actin networks). Then we studied the confined behavior of growing filaments, and showed how the relative rigidity of the filament (compared to the confinement size) affects its bending ability and its growing speed. By comparing simulations and experimental results, we determined how elongating actin filaments reaching a nucleated region can trigger new actin assembly.Finally, we combined these findings to show how all the studied parameters (geometry, steric interactions, filament rigidity, nucleation efficiency) were necessary for the formation of in-vivo like structures.