Figure 1: Substoichiometric concentrations of PAR are sufficient to promote condensation of FUS.

(A) Schematic of purified FUS added to Cy5-labeled PARs or Cy3-labeled poly-uracil RNAs of varying lengths. (B) Representative wide-field fluorescence microscopy images of 1 μM wild-type FUS incubated with decreasing concentrations of PARs or poly-uracil RNAs for 4 h. Asterisks denote images with increased contrast to better visualize weakly fluorescent droplets. Scale bar, 5 μm. (C) Quantification and heatmap of the area density (the fraction of area covered by condensates) of the data from (B). The density of the heatmap was normalized from 0 to 1 based on the condition with the highest coverage (1 μM U₄₀). (D) Intradroplet concentration of varying lengths of PAR from the FUS + 100 nM PAR conditions. Concentrations were interpolated from a free dye standard curve (see Figure S1G). Error bars are standard error of the mean (n > 30). (E) Top, brightfield still images from C-Trap fusions of FUS-U₄₀ or FUS–PAR₁₆ condensates. The scale bar is 3 μm. Bottom left, aspect ratio over time plot of one FUS–PAR₁₆ fusion event. Bottom right, average fusion time (τ) of FUS-U₄₀ and FUS–PAR₁₆ condensates. Error bars represent standard error of the mean (n > 5). Statistics were calculated using Welch’s t-test (d.f. = 4) where ns = not significant. (F) Average fluorescence recovery after photobleaching of 1 μM PAR₁₆ with 10 nM Cy5-PAR₁₆ (magenta) or 1 μM U₄₀ with 10 nM Cy3-U₄₀ (green) incubated with 1 μM FUS. Error bars are standard deviation (n>8).