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MBC in Press, published online ahead of print July 5, 2006
Mol. Biol. Cell 10.1091/mbc.E06-05-0439

A more recent version of this article appeared on September 1, 2006
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Submitted on May 22, 2006
Accepted on June 26, 2006

Ribosome Binding to and Dissociation from Translocation Sites of the ER Membrane

Julia Schaletzky and Tom A. Rapoport

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

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 pre-bound 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.

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




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Y. Kida, F. Morimoto, and M. Sakaguchi
Two translocating hydrophilic segments of a nascent chain span the ER membrane during multispanning protein topogenesis
J. Cell Biol., December 31, 2007; 179(7): 1441 - 1452.
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