LOCAL TRANSLATION. Comment on "Principles of ER cotranslational translocation revealed by proximity-specific ribosome profiling"

Abstract

Jan et al. (Research Articles, 7 November 2014, p. 716) propose that ribosomes translating secretome messenger RNAs (mRNAs) traffic from the cytosol to the endoplasmic reticulum (ER) upon emergence of the signal peptide and return to the cytosol after termination. An accounting of controls demonstrates that mRNAs initiate translation on ER-bound ribosomes and that ribosomes are retained on the ER through many cycles of translation.

Figure 1. ER-bound ribosomes prior to signal sequence emergence

The enrichment of ribosomes in the biotin-labeled pulldown relative to input was calculated at each codon for mRNAs that encode signal sequences. Plots were generated for (A) BirA-Ssh1 and (B) BirA-Ubc6_TA. Shaded area represents SD across all analyzed genes. Each sample was analyzed after a 7 min biotin pulse without cycloheximide.

Figure 2. Stable ribosome binding to the ER

(A) The fraction of BirA-Sec63 labeled ribosomes engaged in the translation of cytosolic or ER proteins was calculated for various labeling times. (B) Stacked bar plot indicating the proportion of ER-associated mRNAs that encode cytosolic proteins from various studies.