Free diffusion of Dad1 in the ER (red) is slowed by association with translocons (green) and polysomes (blue).

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 times less than the diffusion constant of a freely diffusing control protein. When translation initiation is inhibited or nascent polypeptide chains are terminated, the GFP-Dad1 diffusion constant increases significantly, but remains two- to threefold less than that of the control protein.

The results suggest that as a polysome associates with multiple translocon complexes, tethering them together, diffusion of the resulting array is severely restricted. The extension of multiple polypeptide chains from this array into the viscous lumen of the ER may further restrict diffusion. The diffusion rate of GFP-Dad1 in cells where translation is inhibited suggests that the translocon complex remains assembled even when it is not associated with a polysome. These free translocon complexes are able to diffuse in the ER membrane considerably faster than complexes associated with polysomes.

According to the authors' model, polysome arrays, once formed, may remain relatively immobile and define the location of the rough ER. Translocon complexes released from these arrays after translation termination could diffuse relatively freely through the ER before reassociating with the polysomes to initiate a new round of translation. ▪