Nousch Marco, Reed Victoria, Bryson-Richardson Robert J, Currie Peter D, Preiss Thomas
Molecular Genetics Program, Victor Chang Cardiac Research Institute, Darlinghurst, Sydney, NSW, Australia.
RNA. 2007 Mar;13(3):374-84. doi: 10.1261/rna.372307. Epub 2007 Jan 19.
The eukaryotic initiation factor (eIF) 4G family plays a central role during translation initiation, bridging between the 5' and 3' ends of the mRNA via its N-terminal third while recruiting other factors and ribosomes through its central and C-terminal third. The protein p97/NAT1/DAP5 is homologous to the central and C-terminal thirds of eIF4G. p97 has long been considered to be a translational repressor under normal cellular conditions. Further, caspase cleavage liberates a p86 fragment that is thought to mediate cap-independent translation in apoptotic cells. We report here that, surprisingly, human p97 is polysome associated in proliferating cells and moves to stress granules in stressed, nonapoptotic cells. Tethered-function studies in living cells show that human p97 and p86 both can activate translation; however, we were unable to detect polysome association of p86 in apoptotic cells. We further characterized the zebrafish orthologs of p97, and found both to be expressed throughout embryonic development. Their simultaneous knockdown by morpholino injection led to impaired mesoderm formation and early embryonic lethality, indicating conservation of embryonic p97 function from fish to mammals. These data indicate that full-length p97 is a translational activator with essential role(s) in unstressed cells, suggesting a reassessment of current models of p97 function.
真核生物起始因子（eIF）4G家族在翻译起始过程中起核心作用，通过其N端三分之一区域连接mRNA的5'和3'末端，同时通过其中心和C端三分之一区域招募其他因子和核糖体。蛋白质p97/NAT1/DAP5与eIF4G的中心和C端三分之一区域同源。长期以来，p97在正常细胞条件下一直被认为是一种翻译抑制因子。此外，半胱天冬酶切割可释放出一个p86片段，该片段被认为可介导凋亡细胞中的非帽依赖性翻译。我们在此报告，令人惊讶的是，人p97在增殖细胞中与多核糖体相关，并在应激的非凋亡细胞中转移至应激颗粒。活细胞中的系留功能研究表明，人p97和p86都能激活翻译；然而，我们在凋亡细胞中未能检测到p86与多核糖体的关联。我们进一步对p97的斑马鱼直系同源物进行了表征，发现它们在整个胚胎发育过程中均有表达。通过吗啉代注射同时敲低它们会导致中胚层形成受损和早期胚胎致死，这表明从鱼类到哺乳动物，胚胎p97功能具有保守性。这些数据表明全长p97是一种翻译激活因子，在未受应激的细胞中起重要作用，这提示对当前p97功能模型进行重新评估。
