In eukaryotes, shortening of the 3′-poly(A) tail is the rate-limiting step in the degradation of most mRNAs, and two major mRNA deadenylase complexes—Caf1–Ccr4 and Pan2–Pan3—play central roles in this process, referred to as deadenylation. However, the molecular mechanism triggering deadenylation remains elusive. Previously, we demonstrated that eukaryotic releasing factor eRF3 mediates deadenylation and decay of mRNA in a manner coupled to translation termination. Here, we report the mechanism of mRNA deadenylation. The eRF3-mediated deadenylation is catalyzed by both Caf1–Ccr4 and Pan2–Pan3. Interestingly, translation termination complexes eRF1–eRF3, Pan2–Pan3, and Caf1–Ccr4 competitively interact with polyadenylate-binding protein PABPC1. In each complex, eRF3, Pan3, and Tob, respectively, mediate PABPC1 binding, and a combination of a PAM2 motif and a PABC domain is commonly utilized for their contacts. A translation-dependent exchange of eRF1–eRF3 for the deadenylase occurs on PABPC1. Consequently, PABPC1 binding leads to the activation of Pan2–Pan3 and Caf1–Ccr4. From these results, we suggest a mechanism of mRNA deadenylation by Pan2–Pan3 and Caf1–Ccr4 in cooperation with eRF3 and PABPC1.

In yeast, aberrant mRNAs lacking in-frame termination codons are recognized and degraded by the non-stop decay (NSD) pathway. The recognition of non-stop mRNAs involves a member of the eRF3 family of GTP-binding proteins, Ski7. Ski7 is thought to bind the ribosome stalled at the 3'-end of the mRNA poly(A) tail and recruit the exosome to degrade the aberrant message. However, Ski7 is not found in mammalian cells, and even the presence of the NSD mechanism itself has remained enigmatic. Here, we show that unstable non-stop mRNA is degraded in a translation-dependent manner in mammalian cells. The decay requires another eRF3 family member (Hbs1), its binding partner Dom34, and components of the exosome-Ski complex (Ski2/Mtr4 and Dis3). Hbs1-Dom34 binds to form a complex with the exosome-Ski complex. Also, the elimination of aberrant proteins produced from non-stop transcripts requires the RING finger protein listerin. These findings demonstrate that the NSD mechanism exists in mammalian cells and involves Hbs1, Dom34, and the exosome-Ski complex.

Tob is a member of the anti‐proliferative protein family, which functions in transcription and mRNA decay. We have previously demonstrated that Tob is involved in the general mechanism of mRNA decay by mediating mRNA deadenylation through interaction with Caf1 and a general RNA‐binding protein, PABPC1. Here, we focus on the role of Tob in the regulation of specific mRNA. We show that Tob binds directly to a sequence‐specific RNA‐binding protein, cytoplasmic polyadenylation element‐binding protein 3 (CPEB3). CPEB3 negatively regulates the expression of a target by accelerating deadenylation and decay of its mRNA, which it achieves by tethering to the mRNA. The carboxyl‐terminal RNA‐binding domain of CPEB3 binds to the carboxyl‐terminal unstructured region of Tob. Tob then binds Caf1 deadenylase and recruits it to CPEB3 to form a ternary complex. The CPEB3‐accelerated deadenylation was abrogated by a dominant‐negative mutant of either Caf1 or Tob. Together, these results indicate that Tob mediates the recruitment of Caf1 to the target of CPEB3 and elicits deadenylation and decay of the mRNA. Our results provide an explanation of how Tob regulates specific biological processes.

The RNA-binding protein Ataxin-2 binds to and stabilizes a number of mRNA sequences, including that of the transactive response DNA-binding protein of 43 kDa (TDP-43). Ataxin-2 is additionally involved in several processes requiring translation, such as germline formation, long-term habituation, and circadian rhythm formation. However, it has yet to be unambiguously demonstrated that Ataxin-2 is actually involved in activating the translation of its target mRNAs. Here we provide direct evidence from a polysome profile analysis showing that Ataxin-2 enhances translation of target mRNAs. Our recently established method for transcriptional pulse-chase analysis under conditions of suppressing deadenylation revealed that Ataxin-2 promotes post-transcriptional polyadenylation of the target mRNAs. Furthermore, Ataxin-2 binds to a poly(A)-binding protein PABPC1 and a noncanonical poly(A) polymerase PAPD4 via its intrinsically disordered region (amino acids 906–1095) to recruit PAPD4 to the targets. Post-transcriptional polyadenylation by Ataxin-2 explains not only how it activates translation but also how it stabilizes target mRNAs, including TDP-43 mRNA. Ataxin-2 is known to be a potent modifier of TDP-43 proteinopathies and to play a causative role in the neurodegenerative disease spinocerebellar ataxia type 2, so these findings suggest that Ataxin-2–induced cytoplasmic polyadenylation and activation of translation might impact neurodegeneration (i.e. TDP-43 proteinopathies), and this process could be a therapeutic target for Ataxin-2–related neurodegenerative disorders.

Emerging evidence has demonstrated that regulating the length of the poly(A) tail on an mRNA is an efficient means of controlling gene expression at the post-transcriptional level. In early development, transcription is silenced and gene expression is primarily regulated by cytoplasmic polyadenylation. In somatic cells, considerable progress has been made toward understanding the mechanisms of negative regulation by deadenylation. However, positive regulation through elongation of the poly(A) tail has not been widely studied due to the difficulty in distinguishing whether any observed increase in length is due to the synthesis of new mRNA, reduced deadenylation or cytoplasmic polyadenylation. Here, we overcame this barrier by developing a method for transcriptional pulse-chase analysis under conditions where deadenylases are suppressed. This strategy was used to show that a member of the Star family of RNA binding proteins, QKI, promotes polyadenylation when tethered to a reporter mRNA. Although multiple RNA binding proteins have been implicated in cytoplasmic polyadenylation during early development, previously only CPEB was known to function in this capacity in somatic cells. Importantly, we show that only the cytoplasmic isoform QKI-7 promotes poly(A) tail extension, and that it does so by recruiting the non-canonical poly(A) polymerase PAPD4 through its unique carboxyl-terminal region. We further show that QKI-7 specifically promotes polyadenylation and translation of three natural target mRNAs (hnRNPA1, p27^kip1 and β-catenin) in a manner that is dependent on the QKI response element. An anti-mitogenic signal that induces cell cycle arrest at G1 phase elicits polyadenylation and translation of p27^kip1 mRNA via QKI and PAPD4. Taken together, our findings provide significant new insight into a general mechanism for positive regulation of gene expression by post-transcriptional polyadenylation in somatic cells.

The 2′-5′-oligoadenylate synthetase (OAS)/RNase L pathway is an innate immune system that protects hosts against pathogenic viruses and bacteria through cleavage of exogenous single-stranded RNA; however, this system's selective targeting mechanism remains unclear. Here, we identified an mRNA quality control factor Dom34 as a novel restriction factor for a positive-sense single-stranded RNA virus. Downregulation of Dom34 and RNase L increases viral replication, as well as half-life of the viral RNA. Dom34 directly binds RNase L to form a surveillance complex to recognize and eliminate the exogenous RNA in a manner dependent on translation. Interestingly, the feature detected by the surveillance complex is not the specific sequence of the viral RNA but the ‘exogenous nature’ of the RNA. We propose the following model for the selective targeting of exogenous RNA; OAS3 activated by the exogenous RNA releases 2′-5′-oligoadenylates (2–5A), which in turn converts latent RNase L to an active dimer. This accelerates formation of the Dom34-RNase L surveillance complex, and its selective localization to the ribosome on the exogenous RNA, thereby promoting degradation of the RNA. Our findings reveal that the selective targeting of exogenous RNA in antiviral defense occurs via a mechanism similar to that in the degradation of aberrant transcripts in RNA quality control.