Department of Biochemistry, University of California, Riverside, California 92521-0129, USA.
Biochemistry. 2010 Sep 28;49(38):8251-65. doi: 10.1021/bi1008529.
Eukaryotic translation initiation factor 4G (eIF4G) functions to organize the assembly of initiation factors required for the recruitment of a 40S ribosomal subunit to an mRNA and for interacting with the poly(A) binding protein (PABP). Many eukaryotes express two highly similar eIF4G isoforms. eIFiso4G, one of two isoforms in plants, is highly divergent and unusually small in size. Unlike animal and yeast eIF4G, the domain organization of plant eIF4G proteins is largely unknown. Consequently, little is known about the conservation of plant eIF4G with those in other eukaryotes. In this study, we show that eIFiso4G is similar to other eIF4G proteins in that there are interaction domains for eIF4A and PABP and we identify, for the first time, the interaction domain for eIF4B. In contrast to previous reports, two eIF4A binding domains in eIFiso4G were identified, similar in number and organization to those of animal eIF4G. The eIFiso4G domain organization does differ, however, in that the N-terminal eIF4A binding domain overlaps with the eIF4B and PABP binding domains. Moreover, the eIF4B and PABP binding domains overlap. PABP and eIF4B compete with eIF4A for binding eIFiso4G in the absence of the C-terminal eIF4A binding domain but not when both eIF4A binding domains are present, suggesting that the C-terminal eIF4A interaction domain functions to stabilize the association of eIF4A with eIFiso4G in the presence of eIF4B or PABP. Competitive binding to eIFiso4G was also observed between eIF4B and PABP. These observations reveal an important function of the C-terminal eIF4A binding domain in maintaining the interaction of multiple partner proteins with eIFiso4G despite the substantial divergence in its size and domain organization.
真核翻译起始因子 4G（eIF4G）的功能是组织参与招募 40S 核糖体亚基到 mRNA 以及与多聚（A）结合蛋白（PABP）相互作用的起始因子的组装。许多真核生物表达两种高度相似的 eIF4G 同工型。eIFiso4G 是植物中两种同工型之一，高度分化且大小异常小。与动物和酵母 eIF4G 不同，植物 eIF4G 蛋白的结构域组织在很大程度上是未知的。因此，人们对植物 eIF4G 与其他真核生物的同源性知之甚少。在这项研究中，我们表明 eIFiso4G 与其他 eIF4G 蛋白相似，因为它具有 eIF4A 和 PABP 的相互作用结构域，并且我们首次鉴定出 eIF4B 的相互作用结构域。与之前的报道相反，在 eIFiso4G 中鉴定出两个 eIF4A 结合结构域，其数量和组织与动物 eIF4G 相似。然而，eIFiso4G 的结构域组织确实不同，因为 N 端 eIF4A 结合结构域与 eIF4B 和 PABP 结合结构域重叠。此外，eIF4B 和 PABP 结合结构域重叠。在不存在 C 端 eIF4A 结合结构域的情况下，PABP 和 eIF4B 与 eIF4A 竞争与 eIFiso4G 结合，但当两个 eIF4A 结合结构域都存在时，它们不竞争，这表明 C 端 eIF4A 相互作用结构域的功能是在存在 eIF4B 或 PABP 的情况下稳定 eIF4A 与 eIFiso4G 的结合。还观察到 eIF4B 和 PABP 之间对 eIFiso4G 的竞争性结合。这些观察结果揭示了 C 端 eIF4A 结合结构域的重要功能，即使在其大小和结构域组织存在显著差异的情况下，它也能维持多种伴侣蛋白与 eIFiso4G 的相互作用。
