Serine 970 of RNA helicase MOV10 is phosphorylated and controls unfolding activity and fate of mRNAs targeted for AGO2-mediated silencing

Abstract

MOV10 is an RNA helicase required for organismal development and is highly expressed in postnatal brain. MOV10 is an AGO2-associated protein that is also necessary for AGO2-mediated silencing. AGO2 is the primary effector of the miRNA pathway. MOV10 has been shown to be ubiquitinated, leading to its degradation and release from bound mRNAs, but no other posttranslational modifications with functional implications have been described. Using mass spectrometry, we show that MOV10 is phosphorylated in cells at the C-terminus, specifically at serine 970 (S970). Substitution of S970 to phospho-mimic aspartic acid (S970D) blocked unfolding of an RNA G-quadruplex, similar to when the helicase domain was mutated (K531A). In contrast, the alanine substitution (S970A) of MOV10 unfolded the model RNA G-quadruplex. To examine its role in cells, our RNA-seq analysis showed that the expression of S970D causes decreased expression of MOV10 enhanced Cross-Linking Immunoprecipitation targets compared to WT. Introduction of S970A had an intermediate effect, suggesting that S970 was protective of mRNAs. In whole-cell extracts, MOV10 and its substitutions bound AGO2 comparably; however, knockdown of AGO2 abrogated the S970D-induced mRNA degradation. Thus, MOV10 activity protects mRNA from AGO2; phosphorylation of S970 restricts this activity resulting in AGO2-mediated mRNA degradation. S970 is positioned C-terminal to the defined MOV10-AGO2 interaction site and is proximal to a disordered region thatlikely modulates AGO2 interaction with target mRNAs upon phosphorylation. In summary, we provide evidence whereby MOV10 phosphorylation facilitates AGO2 association with the 3'UTR of translating mRNAs that leads to their degradation.