Simulations with islands of varying binding affinity.

<p>(A) A simulation set-up showing the rear (leftmost) island binds on average 18.75% of all Rac1, and each of the other three bind 6.25% on average. Unbound Rac1 diffuses with D = 10μm<sup>2</sup>/s. If bound, Rac1 diffuses with D = 1μm<sup>2</sup>/s. (B) The resulting intensity carpet shows the highest accumulation at the rear island. (C) The pCF carpet (yellow curve) reveals four arc features, which indicate regions of slow molecular flow, across each island. The time needed to flow 0.5μm to the right (pCF(5)) is longer near the island with the highest affinity (0.6s) than the other islands (0.2s). (D) A simulation wherein the islands form a gradient of binding affinities. The affinities range from 37.5–6.25% from the rear to the front of the cell. (E) The resulting intensity carpet shows a gradient of accumulation of Rac1. (F) The pCF carpet reveals a gradient of arc features whose position corresponds to the position of the islands and whose length correlates with the affinity of the islands. The time scale for molecular flow near the islands ranges from 1s, 0.8s, 0.5s and 0.2s (back to front). (G) Simulation of a cell with actin islands forming a gradient of binding affinities. The affinities range from 6.25–37.5% from the back to the front of the cell. (H) The resulting intensity carpet shows a gradient of accumulation of Rac1. (I) The pCF carpet reveals a gradient of arc features whose position corresponds to the position of the islands and whose length correlates with the affinity of the islands. The pair correlation pattern is opposite of the previous simulation (F). The time scale for molecular flow near the islands ranges from 0.2s, 0.5s, 0.8s and 1s (back to front). This gradient is analogous to the gradient calculated for 3min after EGF stimulation (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0143753#pone.0143753.g002" target="_blank">Fig 2F</a>).</p>