Silencing by small RNAs is linked to endosome trafficking

Small RNAs direct RNA induced silencing complexes (RISCs) to regulate the stability and translation of mRNAs1,2. RISCs associated with target mRNAs often accumulate in discrete cytoplasmic foci known as GW-bodies3. However, RISC proteins can associate with membrane compartments such as the Golgi and ER4. Here, we show that GW-bodies are associated with late endosomes or multivesicular bodies (MVBs). Blocking turnover of MVBs into lysosomes by loss of the tethering factor HPS45, enhances siRNA- and miRNA-mediated silencing in Drosophila and humans. It also triggers over-accumulation of GW-bodies. Blocking MVB formation by ESCRT6 depletion results in impaired miRNA silencing and loss of GW-bodies in cells. These results indicate that active RISC is physically and functionally coupled to MVBs. We further show that MVBs promote the competence of RISC to load small RNAs. We suggest that recycling of RISC is promoted by MVBs in order to more effectively engage with small RNA effectors and possibly target RNAs. It may provide a means to enhance the dynamics of RNA silencing in the cytoplasm.