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Originally published as MBC in Press, 10.1091/mbc.E06-05-0439 on July 5, 2006

Vol. 17, Issue 9, 3860-3869, September 2006

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Ribosome Binding to and Dissociation from Translocation Sites of the Endoplasmic Reticulum MembraneFormula

Julia Schaletzky, and Tom A. Rapoport

Department of Cell Biology and Howard Hughes Medical Institute, Harvard Medical School, Boston, MA 02115

Submitted May 22, 2006; Accepted June 26, 2006
Monitoring Editor: Peter Walter

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.

Formula The online version of this contains supplemental material at MBC Online (http://www.molbiolcell.org).

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06-05-0439) on July 5, 2006.

Address correspondence to: Tom A. Rapoport (tom_rapoport{at}hms.harvard.edu)

Abbreviations used: ER, endoplasmic reticulum; luc, firefly luciferase; ppl, preprolactin; RNC, ribosome-nascent chain complex; SRP, signal recognition particle; SR, signal recognition particle receptor.




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