Nao Hosoda

Upf1 Phosphorylation Triggers Translational Repression during Nonsense-Mediated mRNA Decay

In mammalian cells, nonsense-mediated mRNA decay (NMD) generally requires that translation terminates sufficiently upstream of a post-splicing exon junction complex (EJC) during a pioneer round of translation. The subsequent binding of Upf1 to the EJC triggers Upf1 phosphorylation. We provide evidence that phospho-Upf1 functions after nonsense codon recognition during steps that involve the translation initiation factor eIF3 and mRNA decay factors. Phospho-Upf1 interacts directly with eIF3 and inhibits the eIF3-dependent conversion of 40S/Met-tRNA(i)(Met)/mRNA to translationally competent 80S/Met-tRNA(i)(Met)/mRNA initiation complexes to repress continued translation initiation. Consistent with phospho-Upf1 impairing eIF3 function, NMD fails to detectably target nonsense-containing transcripts that initiate translation independently of eIF3 from the CrPV IRES. There is growing evidence that translational repression is a key transition that precedes mRNA delivery to the degradation machinery. Our results uncover a critical step during NMD that converts a pioneer translation initiation complex to a translationally compromised mRNP.

Translation termination factor eRF3 mediates mRNA decay through the regulation of deadenylation

Messenger RNA decay, which is a regulated process intimately linked to translation, begins with the deadenylation of the poly(A) tail at the 3′ end. However, the precise mechanism triggering the first step of mRNA decay and its relationship to translation have not been elucidated. Here, we show that the translation termination factor eRF3 mediates mRNA deadenylation and decay in the yeast Saccharomyces cerevisiae. The N-domain of eRF3, which is not necessarily required for translation termination, interacts with the poly(A)-binding protein PABP. When this interaction is blocked by means of deletion or overexpression of the N-domain of eRF3, half-lives of all mRNAs are prolonged. The eRF3 mutant lacking the N-domain is deficient in the poly(A) shortening. Furthermore, the eRF3-mediated mRNA decay requires translation to proceed, especially ribosomal transition through the termination codon. These results indicate that the N-domain of eRF3 mediates mRNA decay by regulating deadenylation in a manner coupled to translation.

CBP80 promotes interaction of Upf1 with Upf2 during nonsense-mediated mRNA decay in mammalian cells

In mammalian cells, nonsense-mediated messenger RNA decay (NMD) targets newly synthesized nonsense-containing mRNA bound by the cap-binding-protein heterodimer CBP80-CBP20 and at least one exon-junction complex (EJC). An EJC includes the NMD factors Upf3 or Upf3X and Upf2, and Upf2 recruits Upf1. Once this pioneer translation initiation complex is remodeled so that CBP80-CBP20 is replaced by eukaryotic initiation factor 4E, the mRNA is no longer detectably targeted for NMD. Here, we provide evidence that CBP80 augments the efficiency of NMD but not of Staufen1 (Stau1)-mediated mRNA decay (SMD). SMD depends on the recruitment of Upf1 by the RNA-binding protein Stau1 but does not depend on the other Upf proteins. We find that CBP80 interacts with Upf1 and promotes the interaction of Upf1 with Upf2 but not with Stau1.

Evidence that Poly(A) Binding Protein C1 Binds Nuclear Pre-mRNA Poly(A) Tails

ABSTRACT In mammalian cells, poly(A) binding protein C1 (PABP C1) has well-known roles in mRNA translation and decay in the cytoplasm. However, PABPC1 also shuttles in and out of the nucleus, and its nuclear function is unknown. Here, we show that PABPC1, like the major nuclear poly(A) binding protein PABPN1, associates with nuclear pre-mRNAs that are polyadenylated and intron containing. PABPC1 does not bind nonpolyadenylated histone mRNA, indicating that the interaction of PABPC1 with pre-mRNA requires a poly(A) tail. Consistent with this conclusion, UV cross-linking results obtained using intact cells reveal that PABPC1 binds directly to pre-mRNA poly(A) tails in vivo. We also show that PABPC1 immunopurifies with poly(A) polymerase, suggesting that PABPC1 is acquired by polyadenylated transcripts during poly(A) tail synthesis. Our findings demonstrate that PABPC1 associates with polyadenylated transcripts earlier in mammalian mRNA biogenesis than previously thought and offer insights into the mechanism by which PABPC1 is recruited to newly synthesized poly(A). Our results are discussed in the context of pre-mRNA processing and stability and mRNA trafficking and the pioneer round of translation.

Anti‐proliferative protein Tob negatively regulates CPEB3 target by recruiting Caf1 deadenylase

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.

Failsafe nonsense-mediated mRNA decay does not detectably target eIF4E-bound mRNA

Nonsense-mediated mRNA decay (NMD) generally eliminates messenger RNAs that prematurely terminate translation and occurs in all eukaryotes that have been studied, although with mechanistic variations. In mammals, NMD seems to be restricted to newly synthesized mRNA that is bound by the cap-binding heterodimer CBP80-CBP20 (CBP80/20) and typically has at least one exon junction complex (EJC) situated downstream of the nonsense codon and added post-splicing. However, mammalian NMD can also target spliced mRNA lacking an EJC downstream of the nonsense codon. Here we provide evidence that this additional pathway, known as failsafe NMD, likewise seems to be restricted to CBP80/20-bound mRNA and does not detectably target its subsequently remodeled product, eIF4E-bound mRNA. Our studies, including analyses of factor dependence, reveal important shared features of the two mammalian-cell NMD pathways as well as fundamental differences between NMD in mammals and Saccharomyces cerevisiae.

Efficiency of the Pioneer Round of Translation Affects the Cellular Site of Nonsense-Mediated mRNA Decay

In mammalian cells, nonsense-mediated mRNA decay (NMD) is a consequence of nonsense codon recognition during a pioneer round of translation. This round can occur largely before or largely after the release of newly synthesized mRNA from nuclei, depending on the mRNA, and likely utilizes cytoplasmic ribosomes. We show that increasing the cellular concentration of the splicing factor SF2/ASF augments the efficiency of NMD and ultimately shifts NMD that takes place after mRNA export to the cytoplasm to NMD that occurs before mRNA release from nuclei. These changes are accompanied by an increased association of pioneer translation initiation complexes with SF2/ASF, translationally active ribosomes, and the translational activator TAP. Increased TAP binding correlates with increased SF2/ASF binding, but not increased REF/Aly or Y14 binding. Our results uncover an additional role for SF2/ASF and indicate that the efficiency of the pioneer round of translation influences the efficiency of subsequent rounds of translation.

The Hbs1-Dom34 Protein Complex Functions in Non-stop mRNA Decay in Mammalian Cells

Background: The non-stop decay (NSD) mechanism has remained undetermined in mammalian cells. Results: Degradation of unstable non-stop mRNA requires Hbs1, Dom34, and the exosome-Ski complex in mammalian cells. Conclusion: The NSD mechanism exists in mammalian cells and involves a member of the eRF3 family of G proteins, Hbs1. Significance: Our work provides a foundation for dissecting the detailed molecular basis of the human NSD mechanism. 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.

Antiproliferative protein Tob directly regulates c-myc proto-oncogene expression through cytoplasmic polyadenylation element-binding protein CPEB

The regulation of mRNA deadenylation constitutes a pivotal mechanism of the post-transcriptional control of gene expression. Here we show that the antiproliferative protein Tob, a component of the Caf1–Ccr4 deadenylase complex, is involved in regulating the expression of the proto-oncogene c-myc. The c-myc mRNA contains cis elements (CPEs) in its 3′-untranslated region (3′-UTR), which are recognized by the cytoplasmic polyadenylation element-binding protein (CPEB). CPEB recruits Caf1 deadenylase through interaction with Tob to form a ternary complex, CPEB–Tob–Caf1, and negatively regulates the expression of c-myc by accelerating the deadenylation and decay of its mRNA. In quiescent cells, c-myc mRNA is destabilized by the trans-acting complex (CPEB–Tob–Caf1), while in cells stimulated by the serum, both Tob and Caf1 are released from CPEB, and c-Myc expression is induced early after stimulation by the stabilization of its mRNA as an ‘immediate-early gene’. Collectively, these results indicate that Tob is a key factor in the regulation of c-myc gene expression, which is essential for cell growth. Thus, Tob appears to function in the control of cell growth at least, in part, by regulating the expression of c-myc.

Quantitative characterization of Tob interactions provides the thermodynamic basis for translation termination-coupled deadenylase regulation

Translation termination-coupled deadenylation is the first and often the rate-limiting step of eukaryotic mRNA decay in which two deadenylases, Ccr4-Caf1 and Pan2, play key roles. One of the deadenylases, Caf1, associates with Tob, which recruits Caf1 to the poly(A) tail through interactions with a cytoplasmic poly(A)-binding protein 1 (PABPC1). We previously proposed that the competition between Tob and eRF3 (a translation termination factor that interacts with PABPC1) is responsible for the regulation of deadenylase activity. However, the molecular mechanism of the regulation should be addressed by investigating the binding affinity and the cellular levels of these proteins. In this work, we characterized the human Tob interactions with Caf1 and a C-terminal domain of PABPC1 (PABC). Nuclear magnetic resonance (NMR) and Western blot analyses revealed that Tob consists of a structured N-terminal BTG-Tob domain and an unstructured C-terminal region with two conserved PAM2 (PABPC1-interacting motif 2) motifs. The BTG-TOB domain associates with Caf1, whereas the C-terminal PAM2 motif binds to PABC, with a Kd value of 20 μm. Furthermore, we demonstrated that the levels of eRF3 and Tob in HeLa cells are 4–5 μm and less than 0.2 μm, respectively. On the basis of these results, we propose a thermodynamic mechanism for the translation termination-coupled deadenylation mediated by the Tob-Caf1 complex.