Bimodal mechanism of mRNA association with the endoplasmic reticulum (ER)

Abstract

mRNA localization is vital for the spatiotemporal control of cellular protein synthesis. Partitioning mRNAs between cytosol and ER is the most prominent mRNA localization process in cells, operating on the entire mRNA transcriptome. mRNAs encoding secretory pathway proteins are recruited to the ER membrane via the signal recognition particle (SRP) pathway, where mRNA binding to ER requires a translating ribosome. Previously, we have shown that mRNAs encoding ER‐resident proteins display ribosome‐independent interactions with ER, suggesting multiple pathways to regulate mRNA localization to ER. Here, we subjected microsomes derived from plasmacytoma cells to high salt/EDTA, which releases membrane‐bound ribosomes. We then analyzed the fraction of mRNA released to assess the interactions of candidate mRNAs encoding either ER resident proteins (mRNARes) or secretory pathway cargo proteins (mRNACargo) to the ER. While mRNARes are resistant to extraction by high salt/EDTA, mRNACargo are readily released. These results indicate that at least two modes of mRNA attachment to the ER are possible, and can be distinguished by the requirement for intact ribosome. As both cohorts of mRNAs encode proteins with N‐terminal signal peptide, these findings suggest that the SRP pathway may serve a primary function in directing mRNAcar to ER, while additional mechanisms could direct ER localization of mRNARes. NIH GM077382.