The Effect of Branching on the Critical Concentration and Filament Length of Actin

¹ Washington University in St. Louis

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

Submitted on February 21, 2005
Revised on March 26, 2005
Accepted on 20 April 2005

	Abstract

The dependences of the steady-state critical concentration and average filament length of actin solutions, on the filament branching and capping rates, are calculated using a rate methodology based on the total number of actin filaments. The methodology generalizes calculations of the "treadmilling" actin concentration at which an average filament has net zero growth rate. The predictions of the rate methodology are validated by comparison with stochastic-growth simulations which track the positions of all filament subunits over time. For side branching, the critical concentration drops proportionally to the square root of the branching rate; for end branching the drop is linear. The polymerization response to branching has a maximum as a function of the capping-protein concentration. The average filament length drops with increasing branching, because the critical concentration drops. Even small rates of filament uncapping have a large impact on the average filament length in vitro. The potential significance of these phenomena for cell behavior are evaluated.

Key Words: Arp2/3 complex, capping protein, network growth simulation, polymerization

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