Division of Gene Function in Animals, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan.
Nucleic Acids Res. 2012 Aug;40(14):6887-97. doi: 10.1093/nar/gks344. Epub 2012 May 2.
Upf1 is a highly conserved RNA helicase essential for nonsense-mediated mRNA decay (NMD), an mRNA quality-control mechanism that degrades aberrant mRNAs harboring premature termination codons (PTCs). For the activation of NMD, UPF1 interacts first with a translation-terminating ribosome and then with a downstream exon-junction complex (EJC), which is deposited at exon-exon junctions during splicing. Although the helicase activity of Upf1 is indispensable for NMD, its roles and substrates have yet to be fully elucidated. Here we show that stable RNA secondary structures between a PTC and a downstream exon-exon junction increase the levels of potential NMD substrates. We also demonstrate that a stable secondary structure within the 3'-untranslated region (UTR) induces the binding of Upf1 to mRNA in a translation-dependent manner and that the Upf1-related molecules are accumulated at the 5'-side of such a structure. Furthermore, we present evidence that the helicase activity of Upf1 is used to bridge the spatial gap between a translation-termination codon and a downstream exon-exon junction for the activation of NMD. Based on these findings, we propose a model that the Upf1-related molecular motor scans the 3'-UTR in the 5'-to-3' direction for the mRNA-binding factors including EJCs to ensure mRNA integrity.
UPF1 是一种高度保守的 RNA 解旋酶，对无意义介导的 mRNA 降解（NMD）至关重要，NMD 是一种 mRNA 质量控制机制，可降解含有提前终止密码子（PTC）的异常 mRNA。为了激活 NMD，UPF1 首先与翻译终止核糖体相互作用，然后与下游外显子连接复合物（EJC）相互作用，EJC 在剪接过程中外显子-外显子连接处被沉积。虽然 UPF1 的解旋酶活性对于 NMD 是必不可少的，但它的作用和底物尚未完全阐明。在这里，我们表明 PTC 和下游外显子-外显子连接之间稳定的 RNA 二级结构增加了潜在 NMD 底物的水平。我们还证明，3'-非翻译区（UTR）内的稳定二级结构以翻译依赖的方式诱导 UPF1 与 mRNA 的结合，并且这种结构的 5'-侧积累了 Upf1 相关分子。此外，我们提供了证据表明，Upf1 的解旋酶活性用于桥接翻译终止密码子和下游外显子-外显子连接之间的空间间隙，以激活 NMD。基于这些发现，我们提出了一个模型，即 Upf1 相关的分子马达在 5'-3'方向上扫描 3'-UTR，以寻找包括 EJCs 在内的 mRNA 结合因子，以确保 mRNA 的完整性。
