Figure 4.

The mean number of RNA molecules per granule $\langle S\rangle$ as a function of A2 concentration. (A) Mix of 0.005 μM of non-A2RE and A2RE RNA (100 molecules each) of effective valency of 10, and 0.1 μM TOG (2000 molecules). For dissociation constant between TOG sites and RNA sites K_D = 0.15 μM (solid triangles), increasing A2 concentrations at >1 μM leads to formation of small granules. For lower K_D (1 μM, solid circles; 50 μM, solid squares) small granules are always more prevalent than large aggregates, but there is an A2 concentration that maximizes $\langle S\rangle$ for (0.1 and 0.8 μM, respectively). (B) Simulations for RNA valency = 100. For K_D = 0.15 μM (solid triangles) and K_D = 1 μM (solid circles), A2 concentration has to be above a certain threshold to enable formation of small granules only. For K_D = 50 μM (solid squares), the average granule size increases with A2 concentration on the order of 0.1 μM, reaching sequestration of all RNA into a single aggregate for A2 concentrations on the order of 1 μM. Further increase of A2 concentration on the order of 10 μM leads to disruption of the large aggregate into smaller granules. Variations are shown for averages of 100 simulations. (Large arrows) Beginning of the A2 range facilitating formation of granules (for K_D = 50 μM); (short arrows) beginning of the A2 range disrupting formation of granules (for K_D between 0.15μM and 50 μM). To see this figure in color, go online.