Mechanisms of Nonsense-Mediated mRNA Decay (NMD)
Dr. Robert Singer Albert Einstein College of Medicine
Model of Nonsense-Mediated mRNA Decay (NMD).
NMD is a splicing- and translation-dependent pathway that targets not only disease-associated transcripts but also naturally occurring transcripts as a way for cells to maintain homeostasis in a changing environmental millieu (reviewed in Maquat et al., 2010, Cell 142:368-74; Hwang and Maquat, 2011, Curr Opin Genet Dev 21:422-30; Popp and Maquat, 2013, Annu Rev Genet 47:139-65; Popp and Maquat, 2014, Mol Cells 37:1-8; Kurosaki and Maquat, 2016, J Cell Sci 129:461-7; Popp and Maquat, 2016, Cell 165:1319-22).
UPF1 association with the cap-binding protein, CBP80, promotes
nonsense-mediated mRNA decay at two distinct steps.
(J. Hwang, H. Sato, Y. Tang, D. Matsuda, &
L.E. Maquat (2010) Mol Cell 39:396-409)
We have made important progress in identifying components of the pioneer translation initiation complex, which consists of the mostly nuclear but shuttling cap-binding proteins CBP80 and CBP20 at the mRNA 5'-end, poly(A) binding proteins PABPN1 and PABPC1 at the mRNA 3'-end, and the exon-junction complex (EJC) of proteins that includes the NMD factors UPF3 or UPF3X, UPF2, and, finally, UPF1 (Chiu et al., 2004, Genes & Dev. 18:645-754; Lejeune et al., 2004, Nat. Struct. Mol. Biol. 11:992-1000; Hosoda et al., 2006, Mol. Cell. Biol. 26:3085-3097). We have found that CBP80 promotes NMD by promoting the interaction between UPF1 and UPF2 (Hosoda et al., 2005, Nat. Struct. Mol. Biol. 12:893-901). More recent data indicate that CBP80 escorts UPF1 and its kinase SMG1 to join the translation termination complex eRF1-eRF3 that forms at a premature termination codon. CBP80 subsequently escorts UPF1 and its kinase SMG1 to join a downstream EJC (Hwang et al., 2010, Mol Cell 39:396-409). Joining the EJC is thought to promote UPF1 phosphorylation and, as a consequence, translational repression: phospho-UPF1 binds eIF3 of the 43S translation initiation complex that is poised at the initiation codon of an NMD target so as to inhibit 60S ribosomal subunit joining and, thus, formation of a translationally active 80S ribosome (Isken et al., 2008, 133:314-327).
Where on mRNA does UPF1 bind? (Kurosaki and Maquat,
2013, Proc. Natl. Acad. Sci. USA 110:3357-3362).
We have found that the pioneer round of translation promotes some but not all steps of mRNP remodeling to form the steady-state translation initiation complex. For example, the pioneer round of translation promotes EJC removal and the replacement of PABPN1 by PABPC1 but, remarkably, not the replacement of CBP80-CBP20 by eIF4E. Instead, the karyopherin importin-β mediates the replacement of cap-bound CBP80-CBP20 by eIF4E by interacting with importin-α, which is a stable constituent of cap-bound CBP80-CBP20 (Sato and Maquat, 2009, Gene & Dev. 23:2537-2550).
Currently, we are interested in further characterizing the pioneer round of translation, during which nonsense codon recognition leads to NMD, how it differs from the bulk of cellular translation and the molecular rearrangements that occur on NMD targets (see, e.g. Kurosaki and Maquat, 2013, Proc Natl Acad Sci U S A 110:3357-62; Kurosaki, Li, Hoque, Popp, Ermolenko, Tian, Maquat, 2014, Genes Dev 28:1900-16; Popp and Maquat, 2015, Nature Commun 6:6632). We are also developing therapeutics for NMD diseases.
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