Cheng Shijun, Sultana Shemaila, Goss Dixie J, Gallie Daniel R
Department of Biochemistry, University of California, Riverside, California 92521-0129, USA.
J Biol Chem. 2008 Dec 26;283(52):36140-53. doi: 10.1074/jbc.M807716200. Epub 2008 Oct 31.
The eukaryotic translation initiation factor (eIF) 4B promotes the RNA-dependent ATP hydrolysis activity and ATP-dependent RNA helicase activity of eIF4A and eIF4F during translation initiation. eIF4B also helps to organize the assembly of the translational machinery through its interactions with eIF4A, eIF4G, eIF3, the poly(A)-binding protein (PABP), and RNA. Although the function of eIF4B is conserved among plants, animals, and yeast, eIF4B is one of the least conserved of initiation factors at the sequence level. Mammalian eIF4B is a constitutive dimer; however, conflicting reports have suggested that plant eIF4B may exist as a monomer or a dimer. In this study, we show that eIF4B from wheat can form a dimer and we identify the region responsible for its dimerization. Zinc stimulated homodimerization of eIF4B and bound eIF4B with a Kd of 19.7 nM. Zinc increased the activity of the eIF4B C-terminal RNA-binding domain specifically. Zinc promoted the interaction between eIF4B and PABP but not the interaction between eIF4B and eIF4A or eIFiso4G, demonstrating that the effect of zinc was highly specific. The interaction between PABP and eIFiso4G was also stimulated by zinc but required significantly higher levels of zinc. Interestingly zinc abolished the ability of eIFiso4G to compete with eIF4B in binding to their overlapping binding sites in PABP by preferentially promoting the interaction between eIF4B and PABP. Our observations suggest that wheat eIF4B can dimerize but requires zinc. Moreover zinc controls the partner protein selection of PABP such that the interaction with eIF4B is preferred over eIFiso4G.
真核生物翻译起始因子（eIF）4B在翻译起始过程中促进eIF4A和eIF4F的RNA依赖性ATP水解活性以及ATP依赖性RNA解旋酶活性。eIF4B还通过与eIF4A、eIF4G、eIF3、聚腺苷酸结合蛋白（PABP）和RNA相互作用，帮助组织翻译机器的组装。尽管eIF4B的功能在植物、动物和酵母中是保守的，但在序列水平上，eIF4B是最不保守的起始因子之一。哺乳动物eIF4B是一种组成型二聚体；然而，相互矛盾的报道表明植物eIF4B可能以单体或二聚体形式存在。在本研究中，我们表明小麦的eIF4B可以形成二聚体，并确定了其负责二聚化的区域。锌刺激eIF4B的同源二聚化，并以19.7 nM的解离常数（Kd）结合eIF4B。锌特异性地增加了eIF4B C端RNA结合结构域的活性。锌促进了eIF4B与PABP之间的相互作用，但不促进eIF4B与eIF4A或eIFiso4G之间的相互作用，表明锌的作用具有高度特异性。锌也刺激了PABP与eIFiso4G之间的相互作用，但所需的锌水平要高得多。有趣的是，锌通过优先促进eIF4B与PABP之间的相互作用，消除了eIFiso4G在与PABP中重叠结合位点结合时与eIF4B竞争的能力。我们的观察结果表明，小麦eIF4B可以二聚化，但需要锌。此外，锌控制PABP的伴侣蛋白选择，使得与eIF4B的相互作用优于与eIFiso4G的相互作用。
