Energetics and Dynamics of Constrained Actin Filament Bundling

¹ Physics Department, Washington University
² Biomedical Engineering, Washington University
³ Physics Department, Washinton University

^* To whom correspondence should be addressed. E-mail: aec{at}physics.wustl.edu.

Submitted on November 8, 2005
Revised on December 19, 2005
Accepted on 24 February 2006

	Abstract

The formation of filopodia-like bundles from a dendritic actin network has been observed to occur in vitro as a result of branching induced by Arp2/3 complex. We study both the energetics and dynamics of actin filament bundling in such a network in order to evaluate their relative importance in bundle formation processes. Our model considers two semflexible actin filaments fixed at one end and free at the other, described using a normal-mode approximation. This model is studied by both Brownian dynamics and Monte Carlo energetics methods. Remarkably, even short filaments can bundle at separations comparable to their lengths. In the dynamic simulations, we evaluate the time required for the filaments to interact and bind, and examine the dependence of this bundling time on the filament length, the distance between the filament bases, and the crosslinking energy. In most cases, bundling occurs in a second or less. Beyond a certain critical distance, we find that the bundling time increases very rapidly with increasing inter-filament separation and/or decreasing filament length. For most of the cases we have studied, the energetics results for this critical distance are similar to those obtained from dynamics simulations run for 10 s, suggesting that beyond this time scale energetics, rather than kinetic constraints, decide whether or not bundling occurs. Over a broad range of conditions, we find that the times required for bundling from a network are compatible with experimental observations.

Key Words: Brownian dynamics, Green's function, Monte Carlo, crosslinking, fascin, filopodia