Mutations in the gene encoding Fused in Sarcoma (FUS) cause amyotrophic lateral 
sclerosis (ALS), a fatal neurodegenerative disorder. FUS is a predominantly 
nuclear DNA- and RNA-binding protein that is involved in RNA processing. Large 
FUS-immunoreactive inclusions fill the perikaryon of surviving motor neurons of 
ALS patients carrying mutations at post-mortem. This sequestration of FUS is 
predicted to disrupt RNA processing and initiate neurodegeneration. Here, we 
demonstrate that C-terminal ALS mutations disrupt the nuclear localizing signal 
(NLS) of FUS resulting in cytoplasmic accumulation in transfected cells and 
patient fibroblasts. FUS mislocalization is rescued by the addition of the 
wild-type FUS NLS to mutant proteins. We also show that oxidative stress 
recruits mutant FUS to cytoplasmic stress granules where it is able to bind and 
sequester wild-type FUS. While FUS interacts with itself directly by 
protein-protein interaction, the recruitment of FUS to stress granules and 
interaction with PABP are RNA dependent. These findings support a two-hit 
hypothesis, whereby cytoplasmic mislocalization of FUS protein, followed by 
cellular stress, contributes to the formation of cytoplasmic aggregates that may 
sequester FUS, disrupt RNA processing and initiate motor neuron degeneration.