RanBP2/Nup358 enhances RNAi activity by sumoylating and stabilizing Argonaute 1

RanBP2/Nup358 is one of the main components of the cytoplasmic filaments of the nuclear pore complex. Four separate missense mutations in RanBP2 cause Acute Necrotizing Encephalopathy 1 (ANE1), which manifests as a sharp rise in cytokine production after common viral infections such as infuenza and parainfluenza. Infection in these individuals often leads to seizures, coma and a high rate of mortality. However, how RanBP2 and its ANE1-associated mutations affect cytokine production is not well understood. Here we report that RanBP2 represses the translation of the interleukin-6 (IL6) mRNA, which encodes a cytokine that is aberrantly up-regulated in ANE1. In particular, the SUMO E3-ligase activity of RanBP2 and the Let7 miRNA binding site within the IL6 39untranslated region (UTR) are required for this repression, suggesting that sumoylation promotes efficient miRNA-based silencing. Furthermore, our data indicates that RanBP2-dependent sumoylation of the argonaute protein AGO1 inhibits its ubiquitination and its degradation, and that overexpression of AGO1 partially restores the repression of IL6 in cells that are defective in RanBP2-dependent sumoylation. Collectively, these results support a model whereby RanBP2 promotes the sumoylation of AGO1, which stabilizes it, and ultimately enhances the miRNA-mediated suppression of mRNAs such as IL6.