Getting trapped in a rough neighborhood

The ER is differentiated into rough ER, where membrane-bound polysomes translate proteins for insertion into or translocation across the ER membrane, and smooth ER, which lacks polysomes. But how is this differentiation established? Nikonov et al. (page 497) analyzed the lateral diffusion of translocon complexes in the ER membrane. The results suggest that association of translocons with polysomes provides the basis for ER differentiation, and that translocons remain assembled even when they are not translocating nascent polypeptides.

The authors were able to measure the diffusion of translocon complexes in the ER membrane by transfecting a temperature-sensitive mutant cell line with a GFP-tagged version of the translocon-associated oligosaccharyltransferase component Dad1. In cells that are actively carrying out translation, the diffusion constant of GFP-Dad1 is about seven...