Is there a twist in the Escherichia coli signal recognition particle pathway?

Integral membrane proteins (IMPs) are usually synthesized by membrane-bound ribosomes, and this process requires proper localization of ribosomes and IMP-encoding transcripts. However, the underlying molecular mechanism of the pathway has not yet been fully established in vivo. The prevailing hypothesis is that ribosomes and transcripts are delivered to the membrane together during IMP translation by the signal recognition particle (SRP) and its receptor. Here, I discuss an alternative hypothesis that posits that ribosomes and transcripts are targeted separately. Ribosome targeting to the membrane might be mediated by the SRP receptor, rather than by SRP, and IMP-encoding transcripts might be targeted to the membrane in a translation-independent manner. According to this scenario, the SRP executes its essential function on the membrane at a later stage of the targeting pathway.