It is well established that mRNAs encoding secretory or membrane-bound proteins are translated on the surface of the endoplasmic reticulum (ER). The extent to which mRNAs that encode cytosolic proteins associate with the ER, however, remains controversial. To address this question, we quantified the number of cytosolic protein-encoding mRNAs that co-localize with the ER using single-molecule RNA imaging in fixed and living cells. We found that a small but significant number of mRNAs that encode cytosolic proteins associate with the ER and show that this interaction is translation dependent. Furthermore, we demonstrate that cytosolic protein-encoding transcripts can remain on the ER with dwell times consistent with multiple rounds of translation and have higher ribosome occupancies than transcripts translated in the cytosol. These results advance our understanding of the diversity and dynamics of localized translation on the ER.
