Translation-dependent displacement of UPF1 from coding sequences causes its enrichment in 3’ UTRs
The RNA helicase UPF1 is best known for its key function in mRNA nonsense-mediated mRNA decay (NMD), but has also been implicated in additional mRNA turnover mechanisms, telomere homeostasis, and DNA replication. In NMD, UPF1 recruitment to target mRNAs is thought to occur through interaction with release factors at terminating ribosomes, but evidence for translation-independent interaction of UPF1 with the 3’ untranslated region (UTR) of mRNAs has also been reported. To map UPF1 binding sites transcriptome-wide, we performed individual-nucleotide resolution UV crosslinking and immunoprecipitation (iCLIP) in human cells, untreated or after inhibiting translation by puromycin. We found a strong association of UPF1 with 3’ UTRs in undisturbed, translationally active cells and a significant increase in UPF1 binding to coding sequence (CDS) after translation inhibition. These results indicate that UPF1 binds RNA before translation and gets displaced from the CDS by translating ribosomes. This evidence for translation-independent UPF1-RNA interaction, which is corroborated by RNA immunoprecipitations experiments and by our observation that UPF1 also crosslinks to long non-coding RNAs, suggests that the decision to trigger NMD occurs after association of UPF1 with the mRNA, presumably through activation of RNA-bound UPF1 by aberrant translation termination. Overall design: Examination of Upf1 binding preferences via iCLIP in untreated HeLa cells and HeLa cells, where translation is blocked by puromycin treatment in vivo crosslinking and immunoprecipitation strategy (iCLIP)