Beyond the Identification of Transcribed Sequences:
Functional, Evolutionary and Expression Analysis
12th International Workshop
October 25-28, 2002
Washington, DC


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Nonsense-Mediated mRNA Decay in Mammalian Cells: Evidence for a "Pioneer" Round of mRNA Translation

Lynne Maquat
University of Rochester Medical Center, Rochester, NY 14610 USA
Telephone: 585-273-5640
Fax: 585-172-2683
Email: lynne_maquat@urmc.rochester.edu

Nonsense-mediated mRNA decay (NMD) is a quality control mechanism used by cells to eliminate mRNAs that prematurely terminate translation (Maquat LE and Carmichael GG, 2001, Cell, 104: 173-176).  As a general rule, nonsense codons elicit NMD when located within mRNA more than 50-55 nucleotides upstream of an exon-exon junction.  Therefore, intron position within pre-mRNA is an important determinant of NMD. 

Newly synthesized mRNA in mammalian cells is generally characterized by a splicing-dependent complex of proteins located approximately 20-24 nucleotides upstream of exon-exon junctions.  Components of this complex are thought to recruit Upf3, a nucleocytoplasmic shuttling factor that functions in NMD.  Upf3 and Upf2, another NMD factor, are detected on mRNA bound by the major nuclear cap binding proteins CBP80 and CBP20 but not mRNA bound by the major cytoplasmic cap binding protein eIF4E.  These and other data indicate that NMD targets primarily CBP80-bound mRNA during a “pioneer” round of translation (Ishigaki Y, Li X, Serin G and Maquat LE, 2001, Cell 106: 607-17). 

Data indicate that nuclear CBP80 but not nuclear eIF4E is readily detected in association with intron-containing RNA as well as the C-terminal domain of RNA polymerase II (Lejeune F, Ishigaki Y, Li X and Maquat, LE, 2002, EMBO J., 21: 3536-3545).  Consistent with this, components of the exon-exon junction complex, including RNPS1, Y14, SRm160, REF/Aly and TAP, as well as Upf3 and Upf2, are detected in the nuclear fraction on CBP80-bound but not eIF4E-bound mRNA.  Each of these components is also detected on CBP80-bound mRNA in the cytoplasmic fraction, indicating a presence on mRNA after export.  Data suggest that a pioneer round of translation generally elicits the NMD of CBP80-bound mRNA harboring a nonsense codon located more than 50-55 nucleotides upstream of an exon-exon junction.  Otherwise, mRNP is remodeled and becomes immune to NMD.  The dynamics of mRNP remodeling and additional factors involved in NMD will be discussed.



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