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Published May 19, 2017 | Published + Supplemental Material
Journal Article Open

Structure of the quaternary complex between SRP, SR, and translocon bound to the translating ribosome


During co-translational protein targeting, the signal recognition particle (SRP) binds to the translating ribosome displaying the signal sequence to deliver it to the SRP receptor (SR) on the membrane, where the signal peptide is transferred to the translocon. Using electron cryo-microscopy, we have determined the structure of a quaternary complex of the translating Escherichia coli ribosome, the SRP–SR in the 'activated' state and the translocon. Our structure, supported by biochemical experiments, reveals that the SRP RNA adopts a kinked and untwisted conformation to allow repositioning of the 'activated' SRP–SR complex on the ribosome. In addition, we observe the translocon positioned through interactions with the SR in the vicinity of the ribosome exit tunnel where the signal sequence is extending beyond its hydrophobic binding groove of the SRP M domain towards the translocon. Our study provides new insights into the mechanism of signal sequence transfer from the SRP to the translocon.

Additional Information

© 2017 The Author(s). This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Received: 25 January 2017; Accepted: 29 March 2017; Published online: 19 May 2017. We would like to thank David Ramrath, Philipp Bieri and members of the Ban lab for discussions. Cryo-EM data was collected at the Scientific Center for Optical and Electron Microscopy at the ETH Zurich (ScopeM). We thank Shuai Wang for preparation of translation extracts and SecYEG used in the GTPase assays. We are thankful to Peter Tittmann and members of the ScopeM facility for technical support. We acknowledge the use of computing infrastructure provided by the Central Information Technology Services of the ETH Zurich. This work was supported by the Swiss National Science Foundation (SNSF), the National Center of Excellence in Research (NCCR) Structural Biology and RNA & Disease programs of the SNSF, and European Research Council grant 250071 under the European Community's Seventh Framework Programme (to N.B.) and by National Institutes of Health (NIH) Grant GM078024 (to S.-o.S). Author Contributions: N.B. and A.J. conceived the study. N.B., S.-o.S. and A.J. designed experiments. A.J., D.B. and M.L. performed cryo-EM data collection, image processing and modelling. Y.-H.H.F. performed GTPase stimulation assays. A.J., N.B. and S.-o.S. wrote the manuscript. All authors discussed the results, commented on the data and contributed to the final version of the manuscript. The authors declare no competing financial interests.

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August 19, 2023
October 25, 2023