title: Upf1 phosphorylation triggers translational repression during nonsense-mediated mRNA decay.
abstract: 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.
authors.affiliation: Department of Biochemistry and Biophysics, School of Medicine and Dentistry, 601 Elmwood Avenue, Box 712, University of Rochester, Rochester, NY 14642, USA.
authors.forename: Olaf
authors.initials: O
authors.lastname: Isken
authors.forename: Yoon Ki
authors.initials: YK
authors.lastname: Kim
authors.forename: Nao
authors.initials: N
authors.lastname: Hosoda
authors.forename: Greg L
authors.initials: GL
authors.lastname: Mayeur
authors.forename: John W B
authors.initials: JW
authors.lastname: Hershey
authors.forename: Lynne E
authors.initials: LE
authors.lastname: Maquat
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D018389:Codon, Nonsense; D006367:HeLa Cells; D016174:Hepacivirus / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation; D014176:Protein Biosynthesis; D020365:RNA Helicases; D020871:RNA Stability; D012333:RNA, Messenger / Q000378:metabolism*; D012261:Ribonucleoproteins / Q000378:metabolism; D015534:Trans-Activators / Q000378:metabolism*
chemical_list: D018389:Codon, Nonsense; D012333:RNA, Messenger; D012261:Ribonucleoproteins; D015534:Trans-Activators; C026452:messenger ribonucleoprotein; D020365:RNA Helicases; C101828:UPF1 protein, human