Sen Neelam Dabas, Zhou Fujun, Harris Michael S, Ingolia Nicholas T, Hinnebusch Alan G
Laboratory of Gene Regulation and Development, Eunice K. Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892;
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720; Department of Biology, Johns Hopkins University, Baltimore, MD 21218.
Proc Natl Acad Sci U S A. 2016 Sep 20;113(38):10464-72. doi: 10.1073/pnas.1612398113. Epub 2016 Sep 6.
DEAD-box RNA helicases eukaryotic translation initiation factor 4A (eIF4A) and Ded1 promote translation by resolving mRNA secondary structures that impede preinitiation complex (PIC) attachment to mRNA or scanning. Eukaryotic translation initiation factor 4B (eIF4B) is a cofactor for eIF4A but also might function independently of eIF4A. Ribosome profiling of mutants lacking eIF4B or with impaired eIF4A or Ded1 activity revealed that eliminating eIF4B reduces the relative translational efficiencies of many more genes than does inactivation of eIF4A, despite comparable reductions in bulk translation, and few genes display unusually strong requirements for both factors. However, either eliminating eIF4B or inactivating eIF4A preferentially impacts mRNAs with longer, more structured 5' untranslated regions (UTRs). These findings reveal an eIF4A-independent role for eIF4B in addition to its function as eIF4A cofactor in promoting PIC attachment or scanning on structured mRNAs. eIF4B, eIF4A, and Ded1 mutations also preferentially impair translation of longer mRNAs in a fashion mitigated by the ability to form closed-loop messenger ribonucleoprotein particles (mRNPs) via eIF4F-poly(A)-binding protein 1 (Pab1) association, suggesting cooperation between closed-loop assembly and eIF4B/helicase functions. Remarkably, depleting eukaryotic translation initiation factor 4G (eIF4G), the scaffold subunit of eukaryotic translation initiation factor 4F (eIF4F), preferentially impacts short mRNAs with strong closed-loop potential and unstructured 5' UTRs, exactly the opposite features associated with hyperdependence on the eIF4B/helicases. We propose that short, highly efficient mRNAs preferentially depend on the stimulatory effects of eIF4G-dependent closed-loop assembly.
DEAD盒RNA解旋酶真核翻译起始因子4A（eIF4A）和Ded1通过解开阻碍起始前复合物（PIC）附着于mRNA或扫描的mRNA二级结构来促进翻译。真核翻译起始因子4B（eIF4B）是eIF4A的辅助因子，但也可能独立于eIF4A发挥作用。对缺乏eIF4B或eIF4A或Ded1活性受损的突变体进行核糖体分析发现，尽管整体翻译水平有相当程度的降低，但消除eIF4B比使eIF4A失活会降低更多基因的相对翻译效率，并且很少有基因对这两种因子都有异常强烈的需求。然而，消除eIF4B或使eIF4A失活都优先影响具有更长、结构更复杂的5'非翻译区（UTR）的mRNA。这些发现揭示了eIF4B除了作为eIF4A辅助因子促进PIC附着于结构化mRNA或在其上扫描的功能外，还具有独立于eIF4A的作用。eIF4B、eIF4A和Ded1突变也优先以一种可通过eIF4F - 聚腺苷酸结合蛋白1（Pab1）结合形成闭环信使核糖核蛋白颗粒（mRNP）的能力而减轻的方式损害更长mRNA的翻译，这表明闭环组装与eIF4B/解旋酶功能之间存在协同作用。值得注意的是，耗尽真核翻译起始因子4F（eIF4F）的支架亚基真核翻译起始因子4G（eIF4G）优先影响具有强闭环潜力和无结构5'UTR的短mRNA，这与对eIF4B/解旋酶过度依赖相关的特征完全相反。我们提出，短的、高效的mRNA优先依赖于eIF4G依赖性闭环组装的刺激作用。
