Multistep process of FUS aggregation in the cell cytoplasm involves RNA-dependent and RNA-independent mechanisms

Fused in sarcoma (FUS) is an RNA-binding protein involved in pathogenesis of several neurodegenerative dis-eases.AggregationofmislocalizedFUS intonon-amyloid inclusions isbelieved tobepivotal in thedevelopment of cell dysfunction, but themechanismof their formation is unclear. Using transient expression of a panel of de-letion and chimeric FUS variants in various cultured cells, we demonstrated that FUS accumulating in the cyto-plasmnucleates a novel type of RNAgranules, FUSgranules (FGs), that are structurally similar but not identical to physiological RNA transport granules. Formation of FGs requires FUS N-terminal prion-like domain and the ability to bind specific RNAs. Clustering of FGs coupled with further recruitment of RNA and proteins produce larger structures, FUS aggregates (FAs), that resemble but are clearly distinct from stress granules. In condi-tions of attenuated transcription, FAs lose RNA and dissociate into RNA-free FUS complexes that become precursors of large aggresome-like structures. We propose a model of multistep FUS aggregation involving RNA-dependent and RNA-independent stages. This model can be extrapolated to formation of pathological inclusions in human FUSopathies