mRNA targeting eliminates the need for the signal recognition particle during membrane protein insertion in bacteria

Signal-sequence-dependent protein targeting is essential for the spatiotemporal organization of eukaryotic and prokaryotic cells and is facilitated by dedicated protein targeting factors such as the signal recognition particle (SRP). However, targeting signals are not exclusively contained within proteins but can also be present within mRNAs. By in vivo and in vitro assays, we show that mRNA targeting is controlled by the nucle-otide content and by secondary structures within mRNAs. mRNA binding to bacterial membranes occurs independently of soluble targeting factors but is dependent on the SecYEG translocon and YidC. Importantly, membrane insertion of proteins translated from membrane-bound mRNAs occurs independently of the SRP pathway, while the latter is strictly required for proteins translated from cytosolic mRNAs. In summary, our data indicate that mRNA targeting acts in parallel to the canonical SRP-dependent protein targeting and serves as an alternative strategy for safeguarding membrane protein insertion when the SRP pathway is compromised.

Automated summary

Signal-sequence-dependent protein targeting is crucial for the organization of eukaryotic and prokaryotic cells, facilitated by factors like the signal recognition particle. However, targeting signals can also be present within mRNAs. In this study, the authors demonstrated that mRNA targeting is regulated by nucleotide content and secondary structures within mRNAs. Importantly, membrane insertion of proteins translated from membrane-bound mRNAs occurs independently of the SRP pathway, providing an alternative strategy for protein targeting when the SRP pathway is compromised.