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. Author manuscript; available in PMC: 2016 Apr 20.
Published in final edited form as: Annu Rev Genet. 2015 Oct 2;49:339–366. doi: 10.1146/annurev-genet-112414-054639

Figure 3.

Figure 3

Major decay events in yeast and mammalian nonsense-mediated mRNA decay (NMD). (a) Major decay events in yeast NMD. At the end of the premature termination process, Upf1 is still associated with the 40S subunit, which remains attached to the mRNA. Upf1 on the 40S subunit then interacts with the Dcp1/Dcp2 decapping enzyme to trigger decapping of the mRNA. After decapping, the mRNA is digested by the Xrn1 5′-to-3′ exonuclease. (b) Major decay events in mammalian NMD. At the end of the premature termination process, phosphorylated Upf1 is still associated with the 40S subunit, which remains attached to the mRNA. Phosphorylated Upf1 on the 40S subunit then interacts with the Smg6 endonuclease and the effector Smg5-Smg7 heterodimer to trigger either endonucleolytic cleavage in the vicinity of the premature termination codon or deadenylation from the 3′ end of the mRNA. In the endonucleolytic cleavage pathway, the 5′ cleavage product is digested by the cytoplasmic exosome and the 3′ cleavage product is digested by Xrn1. In the deadenylation pathway, the Ccr4-Not deadenylase is recruited to the mRNA through an interaction between Smg7 and Pop2. After deadenylation, the mRNA is decapped by the Dcp1/Dcp2/Edc4 complex and then digested by Xrn1.