Principles of ER Co-Translational Translocation Revealed by Proximity-Specific Ribosome Profiling

Localized protein synthesis is a fundamental mechanism for creating distinct subcellular environments. Here we developed a generalizable proximity-specific ribosome profiling strategy that enables global interrogation of translation in defined subcellular locations. We applied this approach to the endoplasmic reticulum (ER) in yeast and mammals. We observed the vast majority of secretory proteins to be co-translationally translocated, including substrates capable of post-translational insertion in vitro. Distinct translocon complexes engaged nascent chains at different points during synthesis. Whereas most proteins engaged the ER immediately following or even before signal sequence (SS) emergence, a class of Sec66-dependent proteins entered with a looped SS conformation. Finally, we observed rapid ribosome exchange into the cytosol following translation termination. These data provide insights into how distinct translocation mechanisms act in concert to promote efficient co-translational recruitment. Ribosome profiling of whole cell or streptavidin-purified ribosomes biotinylated by cytosolic, ER or mitochondrially localized biotin ligase in yeast and mammalian cells