Amyotrophic lateral sclerosis (ALS) is an uncommon neurodegenerative disease 
caused by degeneration of upper and lower motor neurons. Several genes, 
including SOD1, TDP-43, FUS, Ubiquilin 2, C9orf72 and Profilin 1, have been 
linked with the sporadic and familiar forms of ALS. FUS is a DNA/RNA-binding 
protein (RBP) that forms cytoplasmic inclusions in ALS and frontotemporal 
lobular degeneration (FTLD) patients' brains and spinal cords. However, it is 
unknown whether the RNA-binding ability of FUS is required for causing ALS 
pathogenesis. Here, we exploited a Drosophila model of ALS and neuronal cell 
lines to elucidate the role of the RNA-binding ability of FUS in regulating 
FUS-mediated toxicity, cytoplasmic mislocalization and incorporation into stress 
granules (SGs). To determine the role of the RNA-binding ability of FUS in ALS, 
we mutated FUS RNA-binding sites (F305L, F341L, F359L, F368L) and generated 
RNA-binding-incompetent FUS mutants with and without ALS-causing mutations 
(R518K or R521C). We found that mutating the aforementioned four phenylalanine 
(F) amino acids to leucines (L) (4F-L) eliminates FUS RNA binding. We observed 
that these RNA-binding mutations block neurodegenerative phenotypes seen in the 
fly brains, eyes and motor neurons compared with the expression of 
RNA-binding-competent FUS carrying ALS-causing mutations. Interestingly, 
RNA-binding-deficient FUS strongly localized to the nucleus of Drosophila motor 
neurons and mammalian neuronal cells, whereas FUS carrying ALS-linked mutations 
was distributed to the nucleus and cytoplasm. Importantly, we determined that 
incorporation of mutant FUS into the SG compartment is dependent on the 
RNA-binding ability of FUS. In summary, we demonstrate that the RNA-binding 
ability of FUS is essential for the neurodegenerative phenotype in vivo of 
mutant FUS (either through direct contact with RNA or through interactions with 
other RBPs).