N- and C-terminal Upf1 phosphorylations create binding platforms for SMG-6 and SMG-5:SMG-7 during NMD

Department of Molecular Biology, Yokohama City University, School of Medicine, 3-9, Fuku-ura, Kanazawa-ku, Yokohama 236-0004, Japan.
Nucleic Acids Research (Impact Factor: 9.11). 09/2011; 40(3):1251-66. DOI: 10.1093/nar/gkr791
Source: PubMed


Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that detects and degrades mRNAs containing premature termination codons (PTCs). SMG-1-mediated Upf1 phosphorylation takes place in the decay inducing complex (DECID), which contains a ribosome, release factors, Upf1, SMG-1, an exon junction complex (EJC) and a PTC-mRNA. However, the significance and the consequence of Upf1 phosphorylation remain to be clarified. Here, we demonstrate that SMG-6 binds to a newly identified phosphorylation site in Upf1 at N-terminal threonine 28, whereas the SMG-5:SMG-7 complex binds to phosphorylated serine 1096 of Upf1. In addition, the binding of the SMG-5:SMG-7 complex to Upf1 resulted in the dissociation of the ribosome and release factors from the DECID complex. Importantly, the simultaneous binding of both the SMG-5:SMG-7 complex and SMG-6 to phospho-Upf1 are required for both NMD and Upf1 dissociation from mRNA. Thus, the SMG-1-mediated phosphorylation of Upf1 creates a binding platforms for the SMG-5:SMG-7 complex and for SMG-6, and triggers sequential remodeling of the mRNA surveillance complex for NMD induction and recycling of the ribosome, release factors and NMD factors.

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    • "Improper translation termination due to PTCs is thought to recruit UPF1 to stalled ribosomes, which leads to UPF1 phosphorylation (Kashima et al., 2006). Phosphorylated UPF1 facilitates the recruitment of the proteins SMG5, SMG7, and SMG6, which then promote mRNA degradation (Eberle et al., 2009; Okada-Katsuhata et al., 2012; Unterholzner and Izaurralde, 2004). Although UPF1, UPF2, and UPF3 homologs are present in all eukaryotes, homologs of SMG5 and SMG6 appear to be absent from plant genomes (Riehs et al., 2008). "
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    • "The association between SURF and UPF2-UPF3-EJC would induce SMG1- mediated UPF1 phosphorylation (Kashima et al., 2006; Yamashita et al., 2009). Phosphorylated UPF1 recruits phopho-UPF1 recognizing protein complexes that can degrade PTC-mRNA (Glavan et al., 2006; Loh et al., 2013; Okada-Katsuhata et al., 2012). In addition, binding of UPF2 to the CH domain of UPF1 induces a large conformational change that removes the inhibition of the ATPase activity and activates the RNA-dependent 5 0 to 3 "
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    • "Hyperphosphorylation of UPF1 is prerequisite for destruction of PTC-bearing mRNAs. SMG1-induced phosphorylation of UPF1 functions to recruit the additional SMG factors, SMG6 and SMG5–SMG7 (Okada-Katsuhata et al., 2012), as well as pro-line-rich nuclear receptor co-regulatory protein 2 (PNRC2) (Cho et al., 2009). Recruitment of SMG6 to PTC-containing templates causes one or more endonucleolytic cleavages between the PTC and EJC. "
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