The endoplasmic reticulum (ER) is a highly dynamic network of membranes. Here, we combine live-cell microscopy with in situ cryo–electron tomography to directly visualize ER dynamics in several secretory cell types including pancreatic β-cells and neurons under near-native conditions. Using these imaging approaches, we identify a novel, mobile form of ER, ribosome-associated vesicles (RAVs), found primarily in the cell periphery, which is conserved across different cell types and species. We show that RAVs exist as distinct, highly dynamic structures separate from the intact ER reticular architecture that interact with mitochondria via direct intermembrane contacts. These findings describe a new ER subcompartment within cells.
The mechanisms by which RAVs emerge from the ER are unknown.
the machinery for local translation of the protein products remains poorly defined
RAVs may therefore represent a new mechanism for local translation, facilitating functional coupling between cell activity and protein synthesis at defined sites in the cell periphery. RAV-driven local translation in dendrites would require less time and energy than the traffic of mRNAs or translated products from conventional ER in the cell body.
This work raises a number of important questions, including whether the ribosomes associated with RAVs translate a unique subset of proteins and whether these structures bear specific targeting machinery different from conventional ER for this translation.
Future work will also define the mechanisms responsible for mRNA trafficking to specific sites of RAV-driven local translation as well for the membrane deformations producing RAV-mitochondrial membrane contact sites.
Common Design 