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

RanBP2/Nup358 is one of the main components of the cytoplasmic filaments of the nuclear pore complex. It has been speculated that RanBP2, which has an E3 SUMO-ligase domain, may alter the composition of messenger ribonucleoprotein (mRNP) complexes as they emerge from the nuclear pore and thus regulate the ultimate fate of the mRNA in the cytoplasm. 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 influenza and parainfluenza. 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. Our data indicates that soon after its production, the IL6 mRNP recruits the RNA-induced silencing complex (RISC) bound to Let7 miRNA. After this mRNP is exported, RanBP2 sumoylates the RISC-component AGO1, thereby stabilizing it and enforcing mRNA silencing. Collectively, these results support a model whereby RanBP2 promotes an mRNP remodelling event that is critical for the miRNA-mediated suppression of clinically relevant mRNAs, such as IL6.