Activated GTPase movement on an RNA scaffold drives co-translational protein targeting

摘要

信号识别粒子是一种核糖核蛋白，它是蛋白的正确定位所必需的。它识别新翻译出的蛋白，并与核糖体一新生链复合物结合。这项研究利rn用单分子方法来阐释从信号识别粒子到SecYEG通道(该通道处理蛋白向内质网中的转位过程)的“交货”机制。所取得的结果为这样一个模型提供了证据在该模型中，大型功能性RNA起分子脚手架的作用，来协调大规模的蛋白运动。%Approximately one-third of the proteome is initially destined for the eukaryotic endoplasmic reticulum or the bacterial plasma membrane. The proper localization of these proteins is mediated by a universally conserved protein-targeting machinery, the signal recognition particle (SRP), which recognizes ribosomes carrying signal sequences and, through interactions with the SRP receptor, delivers them to the protein-translocation machinery on the target membrane. The SRP is an ancient ribonudeoprotein particle containing an essential, elongated SRP RNA for which precise functions have remained elusive. Here we used single-molecule fluorescence microscopy to show that the Escherichia coli SRP-SRP receptor GTPase complex, after initial assembly at the tetra-loop end of SRP RNA, travels over 100 A to the distal end of this RNA, where rapid GTP hydrolysis occurs. This movement is negatively regulated by the translating ribosome and, at a later stage, positively regulated by the SecYEG translocon, providing an attractive mechanism for ensuring the productive exchange of the targeting and translocation machineries at the ribosome exit site with high spatial and temporal accuracy. Our results show that large RNAs can act as molecular scaffolds that enable the easy exchange of distinct factors and precise timing of molecular events in a complex cellular process; this concept may be extended to similar phenomena in other ribonudeoprotein complexes.