Bradshaw Niels, Walter Peter
Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, CA 94158, USA.
Mol Biol Cell. 2007 Jul;18(7):2728-34. doi: 10.1091/mbc.e07-02-0117. Epub 2007 May 16.
The RNA component of the signal recognition particle (SRP) is universally required for cotranslational protein targeting. Biochemical studies have shown that SRP RNA participates in the central step of protein targeting by catalyzing the interaction of the SRP with the SRP receptor (SR). SRP RNA also accelerates GTP hydrolysis in the SRP.SR complex once formed. Using a reverse-genetic and biochemical analysis, we identified mutations in the E. coli SRP protein, Ffh, that abrogate the activity of the SRP RNA and cause corresponding targeting defects in vivo. The mutations in Ffh that disrupt SRP RNA activity map to regions that undergo dramatic conformational changes during the targeting reaction, suggesting that the activity of the SRP RNA is linked to the major conformational changes in the signal sequence-binding subunit of the SRP. In this way, the SRP RNA may coordinate the interaction of the SRP and the SR with ribosome recruitment and transfer to the translocon, explaining why the SRP RNA is an indispensable component of the protein targeting machinery.
信号识别颗粒（SRP）的RNA组分是共翻译蛋白质靶向所普遍必需的。生化研究表明，SRP RNA通过催化SRP与SRP受体（SR）的相互作用参与蛋白质靶向的核心步骤。一旦形成SRP.SR复合物，SRP RNA还会加速其中的GTP水解。通过反向遗传学和生化分析，我们在大肠杆菌SRP蛋白Ffh中鉴定出了一些突变，这些突变消除了SRP RNA的活性，并在体内导致相应的靶向缺陷。Ffh中破坏SRP RNA活性的突变定位到在靶向反应过程中经历显著构象变化的区域，这表明SRP RNA的活性与SRP信号序列结合亚基的主要构象变化相关联。通过这种方式，SRP RNA可能协调SRP和SR与核糖体募集以及向转运体转移的相互作用，解释了为什么SRP RNA是蛋白质靶向机制中不可或缺的组分。
