Activated GTPase movement on an RNA scaffold drives cotranslational protein targeting

Roughly one third of the proteome is initially destined for the eukaryotic endoplasmic reticulum or the bacterial plasma membrane(1). 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(2–4) and, via interactions with the SRP receptor(5,6), delivers them to the protein translocation machinery on the target membrane(7). The SRP is an ancient ribonucleoprotein particle containing an essential, elongated SRP RNA whose precise functions have remained elusive. Here, we used single molecule fluorescence microscopy to demonstrate that the SRP-receptor GTPase complex, after initial assembly at the tetraloop end of SRP RNA, travels over 100 Å 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 to ensure the productive exchange of the targeting and translocation machineries at the ribosome exit site with exquisite spatial and temporal accuracy. Our results show that large RNAs can act as molecular scaffolds that enable the facile 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 ribonucleoprotein complexes.