Fig 2: Molecular Dissection of FUS Toxicity(A) Scheme illustrating different domain deletions tested.(B) Deletion RGG2 and NLS domains rescues FUS toxicity on both eclosion rates and the appearance of the immature phenotype, whereas deletion of QGSY only rescues eclosion rates. Kruskal-Wallis, Dunn’s multiple comparison.∗∗∗∗p < 0.0001.

Fig 3: N- and C-Terminal LCDs Are Sufficient to Replicate Full-Length Toxicity(A) Comparison between caz and FUS. Both proteins share a similar domain architecture, protein disorder, and amino acid composition, but caz lacks the QGSY domain. Amino acid composition was calculated for each domain.(B) Humanizing caz shows that addition of the FUS QGSY domain to the benign caz NLS mutant makes the chimeric protein toxic again.(C) Drosophilizing FUS shows that deleting QGSY generates a protein with similar properties to the benign caz NLS mutant.(D and E) Schematic representation of the constructs containing different regions of FUS that were used to create transgenic flies (D) and the effect on toxicity (E). Only when fused together do the N- and C-terminal LCDs cause toxicity.For (B)–(E), Kruskal-Wallis, Dunn’s multiple comparison. ∗∗p < 0.01, ∗∗∗∗p < 0.0001.

Fig 4: RGG Domains Are Required for Efficient Binding of the FUS C-Terminal Domain to Phase-Separated QGSY Hydrogels and Liquid Droplets(A) Scheme illustrating the C-terminal deletion constructs tested for QGSY droplet and hydrogel binding.(B) The RGG domains are the main domains affecting binding of GFP fusion constructs to mCherry-QGSY hydrogels or QGSY droplets. Asterisk indicates higher exposure setting to show that there is still some residual binding of single ΔRGG mutants to the hydrogel.(C) Partitioning of the C-terminal deletion constructs in the QGSY droplets and hydrogels in vitro correlates with the toxicity of the corresponding full-length deletion construct in fly. The red fit takes into account all values; the blue fit considers ΔRGG1 as an outlier.

Fig 5: Arginines in the RGG2 Domain Are Involved in the SG Targeting and Maturation of FUS(A) Comparison between charge plots of QGSY and RGG2 domains. Mutated arginines of RGG2 are highlighted in green.(B) RGG2 mutant full-length FUS shows reduced liquid droplet formation. Note that droplets over time fuse and wet the glass surface.(C) Quantification of FUS phase separation; SEM.(D) Deletion of RGG2 or mutating RGG2 arginines have a similar effect on the SG enrichment of FUS. Whiskers indicate 95% confidence interval; Kruskal-Wallis, Dunn’s multiple comparison.(E) Examples of arsenite-induced and spontaneous SGs positive for FUS and the general SG marker G3BP1.(F and G) Mutation of RGG2 arginines decreases the immobile fraction (1-plateau) and half-life of FUS in SGs induced by arsenite treatment (F), as well as SGs induced by FUS overexpression (G). FRAP graphs: two-way ANOVA; plateau and half-life comparisons: unpaired t test. Error bars, mean ± SEM.(H) Mutating RGG2 arginines partially rescues FUS toxicity in fly. Kruskal-Wallis, Dunn’s multiple comparison.∗∗p < 0.01, ∗∗∗p < 0.001, and ∗∗∗∗p < 0.0001.