Poster abstracts

Poster number 66 submitted by Lauren Woodward

Translating ribosomes displace Exon Junction Complexes to the 3’UTR before their disassembly

Lauren Woodward (Department of Molecular Genetics), Justin Mabin (Department of Molecular Genetics), Guramrit Singh (Department of Molecular Genetics, Center for RNA Biology)

Abstract:
The EJC is a multiprotein complex that is deposited 24nt upstream of exon junctions during pre-mRNA splicing, and couples splicing to RNA export, localization, translation, and decay. Current models state that the EJC remains bound at the -24nt position until its disassembly during the pioneer round of translation by an elongating ribosome. Following translation termination, if an EJC remains bound to mRNA sufficiently downstream of a stop codon, such mRNAs can be rapidly degraded by nonsense-mediated mRNA decay (NMD). Here, we provide evidence that EJCs are not removed immediately upon contact with the pioneer ribosome but are first displaced before their disassembly. Interestingly, in vivo EJC footprints are not restricted to the -24 positon. Indeed, ~8% of EJC footprints map to the last exon, i.e. downstream of the last expected EJC site. We find that translation inhibition leads to a decrease in the number of reads mapping downstream of stop codons. We also observe a direct relationship between the number of EJC deposition sites in the coding sequence and the amount of EJC signal at the stop codon. Further, we observe a higher 3'UTR accumulation of EJCs that contain a disassembly-deficient EJC core protein Magoh, suggesting that at least some EJC disassembly occurs after its displacement to the 3’UTR. Our preliminary analysis shows that on some messages, EJCs displaced to 3’ UTRs may be active in NMD, as depletion of EJC core proteins increases the abundance of mRNAs that have a high 3'UTR EJC signal. To assess how much of the total EJC footprint signal in 3’UTRs is comprised of displaced EJCs, I am currently comparing the EJC occupancy landscape in newly synthesized versus actively translating mRNPs. I am also investigating the role of 3’UTR EJCs in NMD. Altogether, this study reveals a new step in the EJC lifecycle that may expand its role in influencing mRNA fate.

Keywords: EJC, translation, 3UTR