DDX6 interacts with DDX3X to repress translation in microRNA-mediated silencing.

DDX6 is known to repress messenger RNA (mRNA) translation and promote mRNA decay in microRNA-mediated silencing. In embryonic stem cells (ESCs), DDX6 primarily functions at the translation level, independent of mRNA destabilization; however, the precise molecular mechanism of how DDX6 represses translation remains unclear. Here, we identify DDX3X as a key downstream target of DDX6-mediated translational repression in ESCs. Conditional knockout of DDX3X demonstrates its essential role in microRNA (miRNA) silencing. Biochemical analyses reveal that DDX6 directly binds to DDX3X, with the C-terminal region of DDX6 being necessary for this interaction. ESCs lacking DDX6 and rescued with a DDX6 mutant that is defective in DDX3X interaction continue to exhibit miRNA silencing defects. Furthermore, the mutant DDX6 is unable to inhibit 48S preinitiation complex formation in vitro. These findings uncover a novel mechanism in which DDX6 represses target mRNA translation via its interaction with DDX3X.