Normally, eIF3 drives a key change in a complex (40S/Met-tRNAiMet/mRNA) that consists of mRNA and part of a functional ribosome. The binding of phosphorylated UPF1 to eIF3 prevents this complex from going on to form a complex (80S/Met-tRNAiMet/mRNA) that is capable of driving translation and consists of mRNA and the completed functional ribosome.
The team corroborated the importance of eIF3 as a target for translational repression during NMD using an experiment with an mRNA sequence from cricket paralysis virus. Where human cells use eIF3 to initiate translation, the cricket virus mRNA sequence does not. Researchers found that the non-eIF3 translation initiation directed by the cricket virus sequence in mammalian cells was resistant to NMD, and thus that eIF3 is a must for the translational repression that makes NMD possible.
In Maquats model of NMD, phospho-UPF1 not only inhibits the pioneer round of translation so that the translational machinery "falls away" from the flawed mRNA at hand, but also recruits degradative enzymes to that mRNA.
Along with Maquat, the study was authored by post-doctoral associates Olaf Isken, Yoon Ki Kim and Nao Hosoda under the auspices of the Medical Center. Greg L. Mayeur and John W.B. Hershey from the Department of Biological Chemistry at the University of California at Davis provided important reagents and advice. This work was supported by the National Institutes of Health.
Our study provides the first evidence that translational repression does indeed occur during NMD in mammalian cells, Maquat said. One implication of these results
|Contact: Greg Williams|
University of Rochester Medical Center