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. Author manuscript; available in PMC: 2014 Oct 10.
Published in final edited form as: Cell. 2008 Apr 18;133(2):314–327. doi: 10.1016/j.cell.2008.02.030

Figure 7. Model for Phospho-Upf1-Mediated Repression of Translation Initiation.

Figure 7

Upf1, which is primarily hypophosphorylated, forms the SURF complex together with SMG-1, eRF1, and eRF3 when translation terminates during a pioneer round, which involves CBP80/20-bound mRNA. If termination occurs sufficiently upstream of a post-splicing EJC, e.g., at the specified premature termination codon (PTC), then Upf1 and SMG-1 associate with the EJC. Upon association, SMG-1 phosphorylates Upf1. Phospho-Upf1 then represses translation by binding to eIF3 subunits and inhibiting the eIF3-mediated joining of 60S ribosomal subunits to 40S/Met-tRNAiMet/mRNA. Phospho-Upf1 also recruits mRNA decay factors, including Dcp1a, Xrn1, and Rrp4.