The effect of branching on the critical concentration and average filament length of actin

The dependences of the steady-state critical concentration and average. lament length of actin solutions, on the. lament branching and capping rates, are calculated using a rate methodology based on the total number of actin. laments. The methodology generalizes calculations of the "treadmilling'' actin concentration at which an average. lament has net zero growth rate. The predictions of the rate methodology are validated by comparison with stochastic-growth simulations that track the positions of all. lament 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. lament length drops with increasing branching, because the critical concentration drops. Even small rates of. lament uncapping have a large impact on the average. lament length in vitro. The potential significance of these phenomena for cell behavior is evaluated.