title: Ribosome binding to and dissociation from translocation sites of the endoplasmic reticulum membrane.
abstract: We have addressed how ribosome-nascent chain complexes (RNCs), associated with the signal recognition particle (SRP), can be targeted to Sec61 translocation channels of the endoplasmic reticulum (ER) membrane when all binding sites are occupied by nontranslating ribosomes. These competing ribosomes are known to be bound with high affinity to tetramers of the Sec61 complex. We found that the membrane binding of RNC-SRP complexes does not require or cause the dissociation of prebound nontranslating ribosomes, a process that is extremely slow. SRP and its receptor target RNCs to a free population of Sec61 complex, which associates with nontranslating ribosomes only weakly and is conformationally different from the population of ribosome-bound Sec61 complex. Taking into account recent structural data, we propose a model in which SRP and its receptor target RNCs to a Sec61 subpopulation of monomeric or dimeric state. This could explain how RNC-SRP complexes can overcome the competition by nontranslating ribosomes.
authors.affiliation: Department of Cell Biology and Howard Hughes Medical Institute, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Julia
authors.initials: J
authors.lastname: Schaletzky
authors.forename: Tom A
authors.initials: TA
authors.lastname: Rapoport
mesh_terms: D000818:Animals; D000906:Antibodies / Q000276:immunology; D002417:Cattle; D004285:Dogs; D018870:Endoplasmic Reticulum, Rough / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D008954:Models, Biological; D014176:Protein Biosynthesis; D012270:Ribosomes / Q000378:metabolism*; D000069816:SEC Translocation Channels; D018271:Signal Recognition Particle / Q000378:metabolism
chemical_list: D000906:Antibodies; D008565:Membrane Proteins; D000069816:SEC Translocation Channels; D018271:Signal Recognition Particle