BPS2026 – DDX6 regulates the assembly, size, and fluidity of FUS condensates

FUS is a highly disordered RNA binding protein that readily undergoes phase separation. Mutations in FUS have been implicated in neurodegenerative diseases where mislocalized FUS facilitates the formation of aberrant RNP granules that aggregate in the cytoplasm. Still, the molecular interactions which effect the assembly, size, and fluidity of FUS-containing granules remain unclear. Through an RNAi-based knockdown screen, we identified an RNA helicase, DDX6, which tunes FUS-positive stress granule (FUS-SG) formation in neuroblastoma cells. In vitro, DDX6 plays a dual role in regulating FUS condensation by promoting the assembly of nano and meso-scale FUS clusters while limiting the size of larger micron-scale FUS droplets as a function of concentration. Remarkably, we show that a single DDX6 molecule is sufficient to recruit and assemble FUS into higher order complexes. Upon FUS phase separation into liquid droplets, DDX6 increases interphase molecular exchange, thereby promoting their fluidity. Notably, DDX6 also forms a non-contiguous ring around FUS condensates, preventing coarsening and reducing the interfacial tension on the droplets. We propose that the regulatory activity of DDX6 we observe may contribute to the biogenesis and regulation of FUS containing RNP granules in cells.