Krishna V M, Janaki N, Ramaiah K V
Department of Biochemistry, School of Life Sciences, University of Hyderabad, Andhra Pradesh, India.
Arch Biochem Biophys. 1997 Oct 1;346(1):28-36. doi: 10.1006/abbi.1997.0263.
Phosphorylation of serine 51 residue in the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha) impairs the guanine nucleotide exchange (GNE) activity of eIF2B protein and thereby inhibits protein synthesis in mammalian systems, insects and yeast. It is not known if phosphorylation of plant eIF2 can inhibit an eIF2B-like activity. Interestingly purified wheat germ eIF2 (WG x eIF2) can exchange guanine nucleotides in vitro without the addition of any protein factor like eIF2B. It is not clear if this is due to a contaminant eIF2B-like activity associated with WG x eIF2 or because the affinity of WG x eIF2 for GDP and GTP is not markedly different. Our observations here indicate that the GNE activity of WG x eIF2 is not inhibited upon phosphorylation of the p41-42 doublet subunit in WG x eIF2 by reticulocyte eIF2alpha kinases, or in the presence of reticulocyte eIF2(alphaP) in which serine 51 residue is phosphorylated. Further, addition of WG x eIF2 reduces the inhibition in eIF2B activity, protein synthesis, and also the formation of 15S complex that occurs between reticulocyte eIF2(alphaP) and eIF2B protein in heme-deficient or poly(IC)-treated reticulocyte lysates, presumably by a mechanism of competition between wheat germ and reticulocyte eIF2 for phosphorylation. Unlike reticulocyte eIF2(alphaP), phosphorylated WG x eIF2 is unable to interact with reticulocyte eIF2B to form a 15S complex. The ability of WG x eIF2 to exchange guanine nucleotides independent of an eIF2B like protein and the inability of phosphorylated WG x eIF2 to interact with reticulocyte eIF2B suggests that WG x eIF2 is different from mammalian eIF2 and these differences may have occurred in evolution probably due to some changes in the amino acid sequences around the phosphorylation site in eIF2alpha.
真核起始因子2（eIF2）α亚基中丝氨酸51残基的磷酸化会损害eIF2B蛋白的鸟嘌呤核苷酸交换（GNE）活性，从而抑制哺乳动物系统、昆虫和酵母中的蛋白质合成。目前尚不清楚植物eIF2的磷酸化是否会抑制类似eIF2B的活性。有趣的是，纯化的小麦胚eIF2（WG x eIF2）在体外无需添加任何像eIF2B这样的蛋白质因子就能进行鸟嘌呤核苷酸交换。目前尚不清楚这是由于与WG x eIF2相关的一种类似eIF2B的污染活性，还是因为WG x eIF2对GDP和GTP的亲和力没有明显差异。我们在此的观察结果表明，网织红细胞eIF2α激酶对WG x eIF2中p41 - 42双峰亚基进行磷酸化后，或者在存在丝氨酸51残基被磷酸化的网织红细胞eIF2（αP）的情况下，WG x eIF2的GNE活性并未受到抑制。此外，添加WG x eIF2可降低血红素缺乏或经聚肌胞苷酸（poly(IC)）处理的网织红细胞裂解物中网织红细胞eIF2（αP）与eIF2B蛋白之间发生的eIF2B活性抑制、蛋白质合成以及15S复合物的形成，推测这是通过小麦胚和网织红细胞eIF2之间的磷酸化竞争机制实现的。与网织红细胞eIF2（αP）不同，磷酸化的WG x eIF2无法与网织红细胞eIF2B相互作用形成15S复合物。WG x eIF2独立于类似eIF2B的蛋白进行鸟嘌呤核苷酸交换的能力以及磷酸化的WG x eIF2无法与网织红细胞eIF2B相互作用，这表明WG x eIF2与哺乳动物eIF2不同，这些差异可能在进化过程中出现，可能是由于eIF2α磷酸化位点周围氨基酸序列的某些变化。
