GIGYF2 mediates post-transcriptional mRNA repression through recruitment of the CCR4/NOT complex
Initially identified as a factor involved in tyrosine kinase receptor signalling, GRB10-interacting GYF protein 2 (GIGYF2) has later been shown to interact with the 5 cap-binding protein m4EHP as part of a translation repression complex, and to mediate post-transcriptional repression of tethered reporter mRNAs. We recently observed that GIGYF2 also interacts with the miRNA-induced silencing complex and modulates its translation repression activity. Here we have further investigated how GIGYF2 represses mRNA function. In RNA tethering reporter assays we show that GIGYF2 exerts its action through a combination of translational repression and stimulated mRNA decay. Using truncation variants we identify two distinct effector domains within GIGYF2. In this assay GIGYF2-mediated repression is independent of m4EHP but dependent on the deadenylation activity of the CCR4/NOT complex. We further show that GIGYF2 interacts with multiple subunits of the CCR4/NOT complex and interestingly depletion of the CNOT1 scaffold subunit does not affect GIGYF2-mediated repression. Finally, we identify endogenous mRNA targets of GIGYF2 that recapitulate m4EHP - independent repression. Altogether, we propose that GIGYF2 has two distinct mechanisms of repression: one depends on m4EHP binding and affects translation, the other is m4EHP-independent and relies on the deadenylation activity of the CCR4/NOT complex.