Department of Molecular Biology, Yokohama City University, School of Medicine, 3-9, Fuku-ura, Kanazawa-ku, Yokohama 236-0004, Japan.
Nucleic Acids Res. 2012 Feb;40(3):1251-66. doi: 10.1093/nar/gkr791. Epub 2011 Sep 29.
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.
无义介导的 mRNA 降解（NMD）是一种检测和降解含有提前终止密码子（PTC）的 mRNA 的监控机制。SMG-1 介导的 Upf1 磷酸化发生在衰变诱导复合物（DECID）中，该复合物包含核糖体、释放因子、Upf1、SMG-1、外显子连接复合物（EJC）和 PTC-mRNA。然而，Upf1 磷酸化的意义和后果仍有待阐明。在这里，我们证明 SMG-6 结合到 Upf1 的 N 端苏氨酸 28 上新鉴定的磷酸化位点，而 SMG-5:SMG-7 复合物结合到 Upf1 的磷酸化丝氨酸 1096 上。此外，SMG-5:SMG-7 复合物与 Upf1 的结合导致核糖体和释放因子从 DECID 复合物中解离。重要的是，SMG-5:SMG-7 复合物和 SMG-6 同时结合到磷酸化 Upf1 上对于 NMD 和 Upf1 从 mRNA 上的解离都是必需的。因此，SMG-1 介导的 Upf1 磷酸化为 SMG-5:SMG-7 复合物和 SMG-6 创造了结合平台，并触发了 mRNA 监控复合物的顺序重塑，以诱导 NMD 和核糖体、释放因子和 NMD 因子的再循环。
