The role of TNRC6 proteins in gene silencing

Proteins of the GW182 family have recently emerged as key players in miRNAmediated gene silencing. They have been shown to interact with Argonaute proteins, components of the RISC and are assumed to mediate the repression in metazoa. Three paralogues are encoded in the human genome, TNRC6A (GW182), TNRC6B and TNRC6C and only one in fly. Results in Drosophila melanogaster demonstrated that GW182 has the potential to both repress translation and induce mRNA deadenylation and decay.
In this work, we investigate the role of GW182 proteins in miRNA-mediated repression. We demonstrate that the repression mediated by TNRC6C is due to a combination of effects on the mRNA level and mRNA translation. Through deletion analysis, we could identify the C-terminal part of TNRC6C as a key effector domain mediating repression of protein synthesis. Furthermore, we show that two unstructured regions located within the C-terminal part are responsible for the effect. We give evidence for a direct interaction of TNRC6C with PABP and CNOT1. Both interactions are mediated by the C-terminal effector domain, however by different sub-fragments. While repression of protein synthesis is independent of the interaction with PABP, it relies on the interaction with the CCR4–NOT complex. The interaction is mediated by GW-repeats which are located in the two regions flanking the RRM. Finally, we show that the C-terminal effector domain is able to induce repression upon tethering not only of poly(A)+ but also of poly(A)–reporters.
Our results characterize the role of GW182 proteins in gene silencing and clarify some of the recent contradictions about the diverse proposals for the mode of action of miRNAs. The identified effector motifs function as important mediators of miRNA-induced silencing and reveal the recruitment of the CCR4–NOT machinery to the RNA-induced silencing complex. In addition to inducing mRNA decay, the recruitment of GW182 also results in inhibition of mRNA translation independently of deadenylation.