Phase Separation by Low Complexity Domains Promotes Stress Granule Assembly and Drives Pathological Fibrillization

Stress granules are membrane-less organelles composed of RNA-binding proteins (RBPs) and RNA. Functional impairment of stress granules has been implicated in amyotrophic lateral sclerosis, frontotemporal dementia, and multisystem proteinopathy—diseases that are characterized by fibrillar inclusions of RBPs. Genetic evidence suggests a link between persistent stress granules and the accumulation of pathological inclusions. Here, we demonstrate that the disease-related RBP hnRNPA1 undergoes liquid-liquid phase separation (LLPS) into protein-rich droplets mediated by a low complexity sequence domain (LCD). While the LCD of hnRNPA1 is sufficient to mediate LLPS, the RNA recognition motifs contribute to LLPS in the presence of RNA, giving rise to several mechanisms for regulating assembly. Importantly, while not required for LLPS, fibrillization is enhanced in protein-rich droplets. We suggest that LCD-mediated LLPS contributes to the assembly of stress granules and their liquid properties and provides a mechanistic link between persistent stress granules and fibrillar protein pathology in disease. [Display omitted] •hnRNPA1 undergoes spontaneous concentration-dependent liquid-liquid phase separation•Liquid-liquid phase separation is mediated by a low complexity sequence domain•Stress granules assemble in a RNA-binding protein concentration-dependent manner•Pathological fibrillization of hnRNPA1 is driven by liquid-liquid phase separation Liquid-liquid phase separation by RNA-binding proteins harboring low complexity sequence domains is the molecular basis for stress granule assembly, and persistent stress granules promote pathological protein fibrillization.