Tail-anchored proteins have a single C-terminal transmembrane domain (TMD) and are inserted into ER membrane post-translationally through the TMD recognition complex (TRC). Perplexingly, even though some tail-anchored proteins are crucial to cellular homeostasis, depletion of TRC pathway (GET in yeast) components has little effect on cell viability. Thus, Alexander Palazzo and colleagues followed up on recent suggestions that some mRNAs can localise to the ER independently of ribosomes and, on page , present evidence that there is indeed another pathway for ER-targeting of tail-anchored proteins. By performing fluorescent in situ hybridisation in human osteosarcoma (U2OS) cells after digitonin extraction, they showed that mRNAs encoding, among others, the tail-anchored protein Sec61β associated with the ER independently of ribosomes or active translation. Importantly, the open reading frame of Sec61β was necessary for its ribosome-independent targeting to the ER, whereas neither p180, an mRNA receptor known to promote association of some mRNAs with the ER, nor the TRC pathway components TRC40 or BAT3 were required. Finally, overexpression of Sec61β-encoding mRNA interfered with the localisation of other mRNAs to the ER, likely by displacing them from translocon-bound ribosomes, indicating that Sec61β can be synthesised directly on the surface of the ER to facilitate its membrane insertion. Taken together, these results identify Sec61β as the first tail-anchored protein whose mRNA is targeted to the ER by a new pathway for its membrane insertion.
