不同因子与核糖体相关联，来帮助蛋白链的启动、延伸和终止。教科书上对蛋白合成的解释只描述了两个普遍保守的“翻译延伸因子”，即EF-Tu/eEF1A 和 EF-G/eEF2。
现在，对酿酒酵母的蛋白合成所做的一项研究，重新将一个以前被认为与启动过程相关的因子（eIF5A）定位为延伸过程中的一个核心因子。eIF5A的不寻常之处是，它含有一个罕见的氨基酸，即Hypusine，eIF5A必须有这种氨基酸才能够刺激延伸。根据在缺少eIF5A时所观察到的缺陷，研究人员提出，这个因子可能与eEF2一起在转位过程中发挥功能。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 459, 118-121 (7 May 2009) | doi:10.1038/nature08034
Hypusine-containing protein eIF5A promotes translation elongation
Preeti Saini1, Daniel E. Eyler2, Rachel Green2 & Thomas E. Dever1
1 Laboratory of Gene Regulation and Development, NICHD, National Institutes of Health, Bethesda, Maryland 20892, USA
2 Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
Translation elongation factors facilitate protein synthesis by the ribosome. Previous studies identified two universally conserved translation elongation factors, EF-Tu in bacteria (known as eEF1A in eukaryotes) and EF-G (eEF2), which deliver aminoacyl-tRNAs to the ribosome and promote ribosomal translocation, respectively1. The factor eIF5A (encoded by HYP2 and ANB1 in Saccharomyces cerevisiae), the sole protein in eukaryotes and archaea to contain the unusual amino acid hypusine (N -(4-amino-2-hydroxybutyl)lysine)2, was originally identified based on its ability to stimulate the yield (endpoint) of methionyl-puromycin synthesis—a model assay for first peptide bond synthesis thought to report on certain aspects of translation initiation3, 4. Hypusine is required for eIF5A to associate with ribosomes5, 6 and to stimulate methionyl-puromycin synthesis7. Because eIF5A did not stimulate earlier steps of translation initiation8, and depletion of eIF5A in yeast only modestly impaired protein synthesis9, it was proposed that eIF5A function was limited to stimulating synthesis of the first peptide bond or that eIF5A functioned on only a subset of cellular messenger RNAs. However, the precise cellular role of eIF5A is unknown, and the protein has also been linked to mRNA decay, including the nonsense-mediated mRNA decay pathway10, 11, and to nucleocytoplasmic transport12, 13. Here we use molecular genetic and biochemical studies to show that eIF5A promotes translation elongation. Depletion or inactivation of eIF5A in the yeast S. cerevisiae resulted in the accumulation of polysomes and an increase in ribosomal transit times. Addition of recombinant eIF5A from yeast, but not a derivative lacking hypusine, enhanced the rate of tripeptide synthesis in vitro. Moreover, inactivation of eIF5A mimicked the effects of the eEF2 inhibitor sordarin, indicating that eIF5A might function together with eEF2 to promote ribosomal translocation. Because eIF5A is a structural homologue of the bacterial protein EF-P14, 15, we propose that eIF5A/EF-P is a universally conserved translation elongation factor.
