Ribosome exchange revisited: a mechanism for translation-coupled ribosome detachment from the ER membrane

In current models, ribosome release from the endoplasmic reticulum (ER) is coupled to the termination of protein translation. Thus, coincident with te rmination, membrane-bound ribosomes dissociate into their component subunit s and are released into the cytosol. Here, we review past and current data and propose that the affinity of the ribosome for the ER membrane is decrea sed during translation, with ribosome release occurring when a membrane-bou nd ribosome is engaged in the synthesis of a protein lacking a signal seque nce. Our model emphasizes a role for the conformation of the large ribosoma l subunit in the regulation of membrane affinity and provides a mechanism f or translation-coupled ribosome release.