Department of Cell and Molecular Biology, John A. Burn School of Medicine, University of Hawaii at Manoa, Honolulu, Hawaii 96813, USA.
RNA. 2014 Aug;20(8):1248-56. doi: 10.1261/rna.043463.113. Epub 2014 Jun 19.
Selenoproteins contain the unique amino acid selenocysteine (Sec), which is encoded by the triplet UGA. Since UGA also serves as a stop codon, it has been postulated that selenoprotein mRNAs are targeted for degradation by the nonsense-mediated mRNA decay pathway (NMD). Several reports have observed a hierarchy of selenoprotein mRNA expression when selenium (Se) is limiting, whereby the abundance of certain transcripts decline while others do not. We sought to investigate the role of NMD in this hierarchical response that selenoprotein mRNAs exhibit to environmental Se status. Selenoprotein mRNAs were categorized as being predicted sensitive or resistant to NMD based on the requirements held by the current model. About half of the selenoprotein transcriptome was predicted to be sensitive to NMD and showed significant changes in mRNA abundance in response to cellular Se status. The other half that was predicted to be resistant to NMD did not respond to Se status. RNA immunoprecipitation with essential NMD factor UPF1 revealed that the mRNAs that were the most sensitive to Se status were also the most enriched on UPF1 during Se deficiency. Furthermore, depletion of SMG1, the kinase responsible for UPF1 phosphorylation and NMD activation, abrogated the decline in transcript abundance of Se-responsive transcripts. Lastly, mRNA decay rates of Se-responsive transcripts were altered upon the addition of Se to resemble the slower decay rates of nonresponsive transcripts. Taken together, these results present novel evidence in support of a crucial role for the NMD pathway in regulating selenoprotein mRNA levels when Se is limiting.
硒蛋白含有独特的氨基酸硒代半胱氨酸（Sec），其由 UGA 三联体编码。由于 UGA 也充当终止密码子，因此有人假设硒蛋白 mRNA 会被无意义介导的 mRNA 降解途径（NMD）靶向降解。几项研究报告在硒（Se）受到限制时观察到硒蛋白 mRNA 表达的层次结构，其中某些转录本的丰度下降，而其他转录本则没有。我们试图研究 NMD 在硒蛋白 mRNA 对环境 Se 状态的这种层次响应中的作用。根据当前模型的要求，硒蛋白 mRNA 被归类为预测对 NMD 敏感或耐受。大约一半的硒蛋白转录组被预测对 NMD 敏感，并且对细胞 Se 状态的 mRNA 丰度有明显变化。被预测对 NMD 耐受的另一半则没有响应 Se 状态。用必需的 NMD 因子 UPF1 进行 RNA 免疫沉淀表明，对 Se 状态最敏感的 mRNA 在 Se 缺乏时在 UPF1 上的富集程度也最高。此外，耗尽负责 UPF1 磷酸化和 NMD 激活的激酶 SMG1，可消除 Se 响应转录本的转录本丰度下降。最后，加入 Se 后，Se 响应转录本的 mRNA 衰减率发生变化，类似于非响应转录本的较慢衰减率。总之，这些结果提供了新的证据，支持 NMD 途径在 Se 限制时调节硒蛋白 mRNA 水平的关键作用。
