The spliceosome component U1-70k loads nascent transcripts onto FVE/SGS3 via co-condensation to embark on RNA silencing

Despite biochemical framework of RNA silencing has been studied well, how nascent transcripts enter RNA silencing remains poorly understood. Here we recovered U1-70k, a canonical spliceosome factor, as a novel component in posttranscriptional gene silencing (PTGS). We show that U1-70k interacts with SGS3 and FVE, two key components that supply RNA substrate for RNA-dependent RNA polymerase 6 (RDR6) to generate dsRNA. U1-70k promotes the loading of nascent RNA into the SGS3/FVE complex in the nucleus, which is subsequently translocated onto RDR6 in the cytosol. Importantly, U1-70k contains C-terminal low complexity (LC) domain and displays liquid-liquid phase separation (LLPS) pattern. U1-70k forms co-condensate with SGS3, another intrinsically disordered protein (IDP). The co-condensation is critical for PTGS as U1-70k LC variants display reduced SGS3 co-condensation, loss of RNA loading onto SGS3, and impaired RNAi function. Taken together, U1-70k channels nascent RNA to SGS3/FVE via its LLPS action to enter PTGS pathway.