Fig. 7
From: Network heterogeneity regulates steering in actin-based motility
Mechanochemical model for growing branched actin network. a Network growth rate (green box) can be summarized by three terms; (i, white box) V 0, free polymerization rate of barbed ends; (ii, yellow boxes) the monomer depletion factor, which is inversely proportional to (χ × η × Φ), where χ represents monomer consumption due to polymerization corrected by diffusion, η is the network density, and Φ is the network geometry factor of 0.7 calculated in Fig. 1; and, (iii, blue boxes). As depicted in yellow boxes, because soluble proteins were maintained constant throughout the experiments, higher nucleation density (pink to red), translates into higher filament density and yields higher local monomer depletion, thus lower growth rate. Blue boxes summarize the influence of the geometry factor according to which filaments push more efficiently against the load when they are in contact with the NPF area. Accordingly, actin filaments not in contact with the nucleation area do not produce as much force and are more prone to bending and/or capping. b Rationale for the control of speed and steering during actin network growth according to two scenarios based on actin filaments density and network architecture. The first is, from (1) to (2), where actin networks evolved by a geometrical reorganization of the network at a constant network density, or from (1) to (3) where actin networks evolved by maintaining the geometry of the network constant but increasing its density. As depicted in cartoons, for the compacting networks—the (1) to (2) case—as the network changes from low (1) to high (2) organization, the density is constant but the efficiency of the network pushing against the load is higher. Therefore, the actin network in (2) is growing faster than the network in (1). For concentrating networks—the (1) to (3) case—individual networks generated by nucleation surface unit (pink to red dots) become denser, without changing their organization. Therefore, the actin network in (3) is growing slower than the network in (1). According to these simple rules, heterogeneous networks will steer (or turn) towards the less growth-efficient actin network overall
