Importance of the C-terminal domain of the human GW182 protein TNRC6C for translational repression

The silencing domain of GW182 interacts with PABPC1 to promote translational repression and degradation of microRNA targets and is required for target release

GW182 family proteins are essential in animal cells for microRNA (miRNA)-mediated gene silencing, yet the molecular mechanism that allows GW182 to promote translational repression and mRNA decay remains largely unknown. Previous studies showed that while the GW182 N-terminal domain interacts with Argonaute proteins, translational repression and degradation of miRNA targets are promoted by a bipartite silencing domain comprising the GW182 middle and C-terminal regions. Here we show that the GW182 C-terminal region is required for GW182 to release silenced mRNPs; moreover, GW182 dissociates from miRNA targets at a step of silencing downstream of deadenylation, indicating that GW182 is required to initiate but not to maintain silencing. In addition, we show that the GW182 bipartite silencing domain competes with eukaryotic initiation factor 4G for binding to PABPC1. The GW182-PABPC1 interaction is also required for miRNA target degradation; accordingly, we observed that PABPC1 associates with components of the CCR4-NOT deadenylase complex. Finally, we show that PABPC1 overexpression suppresses the silencing of miRNA targets. We propose a model in which the GW182 silencing domain promotes translational repression, at least in part, by interfering with mRNA circularization and also recruits the deadenylase complex through the interaction with PABPC1.

Mammalian miRNA RISC recruits CAF1 and PABP to affect PABP-dependent deadenylation

MicroRNAs (miRNAs) inhibit mRNA expression in general by base pairing to the 3'UTR of target mRNAs and consequently inhibiting translation and/or initiating poly(A) tail deadenylation and mRNA destabilization. Here we examine the mechanism and kinetics of miRNA-mediated deadenylation in mouse Krebs-2 ascites extract. We demonstrate that miRNA-mediated mRNA deadenylation occurs subsequent to initial translational inhibition, indicating a two-step mechanism of miRNA action, which serves to consolidate repression. We show that a let-7 miRNA-loaded RNA-induced silencing complex (miRISC) interacts with the poly(A)-binding protein (PABP) and the CAF1 and CCR4 deadenylases. In addition, we demonstrate that miRNA-mediated deadenylation is dependent upon CAF1 activity and PABP, which serves as a bona fide miRNA coactivator. Importantly, we present evidence that GW182, a core component of the miRISC, directly interacts with PABP via its C-terminal region and that this interaction is required for miRNA-mediated deadenylation.

microRNA-mediated messenger RNA deadenylation contributes to translational repression in mammalian cells

Animal microRNAs (miRNAs) typically regulate gene expression by binding to partially complementary target sites in the 3' untranslated region (UTR) of messenger RNA (mRNA) reducing its translation and stability. They also commonly induce shortening of the mRNA 3' poly(A) tail, which contributes to their mRNA decay promoting function. The relationship between miRNA-mediated deadenylation and translational repression has been less clear. Using transfection of reporter constructs carrying three imperfectly matching let-7 target sites in the 3' UTR into mammalian cells we observe rapid target mRNA deadenylation that precedes measureable translational repression by endogenous let-7 miRNA. Depleting cells of the argonaute co-factors RCK or TNRC6A can impair let-7-mediated repression despite ongoing mRNA deadenylation, indicating that deadenylation alone is not sufficient to effect full repression. Nevertheless, the magnitude of translational repression by let-7 is diminished when the target reporter lacks a poly(A) tail. Employing an antisense strategy to block deadenylation of target mRNA with poly(A) tail also partially impairs translational repression. On the one hand, these experiments confirm that tail removal by deadenylation is not strictly required for translational repression. On the other hand they show directly that deadenylation can augment miRNA-mediated translational repression in mammalian cells beyond stimulating mRNA decay. Taken together with published work, these results suggest a dual role of deadenylation in miRNA function: it contributes to translational repression as well as mRNA decay and is thus critically involved in establishing the quantitatively appropriate physiological response to miRNAs.

The GW182 protein family in animal cells: new insights into domains required for miRNA-mediated gene silencing

GW182 family proteins interact directly with Argonaute proteins and are required for miRNA-mediated gene silencing in animal cells. The domains of the GW182 proteins have recently been studied to determine their role in silencing. These studies revealed that the middle and C-terminal regions function as an autonomous domain with a repressive function that is independent of both the interaction with Argonaute proteins and of P-body localization. Such findings reinforce the idea that GW182 proteins are key components of miRNA repressor complexes in metazoa.

Mechanisms of miRNA-mediated post-transcriptional regulation in animal cells

MicroRNAs (miRNAs) are 20-nt-long to 24-nt-long noncoding RNAs acting as post-transcriptional regulators of gene expression in animals and plants. In mammals, more than 50% of mRNAs are predicted to be the subject of miRNA-mediated control but mechanistic aspects of the regulation are not fully understood and different studies have produced often-contradictory results. miRNAs can affect both the translation and stability of mRNAs. In this report, we review current progress in understanding how miRNAs execute these effects in animals and we discuss some of the controversies regarding different modes of miRNA function.

Multiple independent domains of dGW182 function in miRNA-mediated repression in Drosophila

miRNA-mediated repression affects a wide range of biological processes including development and human pathologies. The GW182 protein is a key component of miRNA repression complex, recruited by Argonaute and functioning downstream to repress translation and accelerate mRNA degradation, but little is known about how GW182 proteins act. Using both tethered function and complementation assays, we identify three independent domains of the Drosophila GW182 protein (also termed Gawky) that are sufficient to repress mRNA. Each of these domains also functions independently of poly(A) tails. These results indicate that miRNA-mediated repression is facilitated by multiple domains of GW182.