Evidence for a pioneer round of mRNA translation: mRNAs subject to nonsense-mediated decay in mammalian cells are bound by CBP80 and CBP20

Nonsense-mediated decay (NMD) eliminates mRNAs that prematurely terminate t ranslation. We used antibody to the nuclear cap binding protein CBP80 or it s cytoplasmic counterpart eIF4E to immunopurify RNP containing nonsense-fre e or nonsense-containing transcripts. Data indicate that NMD takes place in association with CBP80. We defined other components of NMD-susceptible mRN P as CBP20, PABP2, eIF4G, and the NMD factors Upf2 and Upf3. Consistent wit h the dependence of NMD on translation, the NMD of CBP80-bound mRNA is bloc ked by cycloheximide or suppressor tRNA. These findings provide evidence th at translation can take place in association with CBP80. They also indicate that CBP80-bound mRNA undergoes a "pioneer" round of translation, before C BP80-CBP20 are replaced by eIF4E, and Upf2 and Upf3 proteins dissociate fro m upstream of exon-exon junctions.