Role of 4.5 S RNA in Assembly of the Bacterial Signal Recognition Particle with Its Receptor

Abstract

The mechanism by which a signal recognition particle (SRP) and its receptor mediate protein targeting to the endoplasmic reticulum or to the bacterial plasma membrane is evolutionarily conserved. In Escherichia coli , this reaction is mediated by the Ffh/4.5 S RNA ribonucleoprotein complex (Ffh/4.5 S RNP; the SRP) and the FtsY protein (the SRP receptor). We have quantified the effects of 4.5 S RNA on Ffh-FtsY complex formation by monitoring changes in tryptophan fluorescence. Surprisingly, 4.5 S RNA facilitates both assembly and disassembly of the Ffh-FtsY complex to a similar extent. These results provide an example of an RNA molecule facilitating protein-protein interactions in a catalytic fashion.