Trailing membrane protein mRNAs on their way to the membrane in Escherichia coli: the effect of CspE

Previous studies have suggested that mRNAs encoding integral membrane proteins (MPRs) are delivered to membrane-bound ribosomes, but how they actually target the membrane remains unknown. To investigate this hypothesis, we initially performed a high-throughput analysis of the cellular distribution of MPRs in Escherichia coli. The results confirmed that MPRs are overrepresented on the membrane, as expected. Surprisingly, however, the results also showed that MPRs are relatively abundant in the cytosolic, ribosomal-free fraction. We propose that the “free” form of MPRs represents a stage during their targeting to the membrane in a translation-independent manner. Remarkably, we demonstrate that cold shock proteins, which were shown to interact with MPRs, play a role in linking the intriguing subcellular localization of MPRs with their translation into integral membrane proteins.