Figure 4.

(A) Schematic representation of the likely processes during ERES formation on the ER network. Starting from initially dispersed constituents, ERESs (green dots) self-assemble by demixing as protein-rich domains on individual ER tubules (red lines). ERESs are free to move on their host tubule but are eventually confined in their long-range motion by ER junctions (blue circles), prohibiting the fusion of domains on neighboring tubules. (B) Schematic representation of the model simulations (see also main text and Materials and methods for details). Starting from dispersed ERES constituents (green dots) on a square surface (edge length L), domains of different sizes form because of demixing. For confined diffusion, i.e., if the surface is dissected into panels of size λ × λ with diffusion-based material exchange being allowed only via gaps of size g < λ in panel boundaries, a lattice-like arrangement of spots with fairly uniform size emerges. Removing all diffusion barriers (g = λ), the spatial arrangement is more random, and domain sizes become less uniform. (C) Representative microscopy-like images of simulation results for both cases illustrate the obtained patterns (L = 6 and λ = 9 μm with g = λ/10 and g = λ for confined and free diffusion, respectively). (D) The PDF of spot areas, p(A), as determined from microscopy-like images of simulation data show a fairly narrow peak when diffusional exchange between panels is constrained (g = λ/10, gray histogram). Almost no changes are observed when diffusional exchange is totally abolished (g = 0, blue circles). Upon lifting all diffusion barriers, i.e., for g = λ, the PDF is broadened (red squares), featuring a marked tail for large spot areas (for better visibility, values of p(A) have been scaled up by a factor of 2). Albeit a quantitative comparison with experimental data is not meaningful because of the reduced complexity of the simulation model, these results are in very good qualitative agreement with experimental observations on ERES patterns (cf. Figs. 2 and 3). To see this figure in color, go online.