Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published February 25, 2022 | Accepted Version + Supplemental Material
Journal Article Open

Mechanism of signal sequence handover from NAC to SRP on ribosomes during ER-protein targeting


The nascent polypeptide–associated complex (NAC) interacts with newly synthesized proteins at the ribosomal tunnel exit and competes with the signal recognition particle (SRP) to prevent mistargeting of cytosolic and mitochondrial polypeptides to the endoplasmic reticulum (ER). How NAC antagonizes SRP and how this is overcome by ER targeting signals are unknown. Here, we found that NAC uses two domains with opposing effects to control SRP access. The core globular domain prevented SRP from binding to signal-less ribosomes, whereas a flexibly attached domain transiently captured SRP to permit scanning of nascent chains. The emergence of an ER-targeting signal destabilized NAC's globular domain and facilitated SRP access to the nascent chain. These findings elucidate how NAC hands over the signal sequence to SRP and imparts specificity of protein localization.

Additional Information

© 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. This is an article distributed under the terms of the Science Journals Default License. Received: 27 July 2021. Accepted: 27 January 2022. Published in print: 25 February 2022. We thank M. Leibundgut, T. Lenarcic, and M. Jaskolowski for discussions; R. Schloemer and E. Coellen for technical assistance; and S. Kreft for help with in vitro cysteine cross-linking experiments. Cryo-EM was collected at ScopeM at the ETH Zurich. We acknowledge the MRC - LMB Electron Microscopy Facility for access and support of electron microscopy sample preparation and data collection for NAC-TTC5-RNC and the Caenorhabditis Genetics Center for strains. This work was supported by the Swiss National Science Foundation (grant no. 310030B_163478); the National Center of Excellence in Research RNA & Disease Program of the SNSF (grant no. 51NF40_141735); a Roessler Prize, Ernst Jung Prize, and Otto Naegeli Prize for Medical Research (to N.B.); the German Science Foundation (grant nos. SFB969/A01 and A07 to E.D. and M.G.); the National Institutes of Health (grant no. R35 GM136321 to S.S.); the National Science Foundation (grant no. MCB-1929452 to S.-o.S); and the UK Medical Research Council (MRC grant MC_UP_A022_1007 to R.S.H.). V.C. was supported by V. Ramakrishnan, whose funding was from the MRC (grant no. MC_U105184332), the Wellcome Trust (grant no. WT096570), the Agouron Institute, and the Louis-Jeantet Foundation. We also acknowledge the support of the NVIDIA Corporation for the Titan Xp GPU through a grant awarded to A.J. Author contributions: A.J., M.G., H.-H.H., R.S.H., S.S., N.B., and E.D. conceived the project. A.J. and A.S. performed cryo-EM data collection for ER-targeting complexes containing NAC and SRP. A.J. determined the cryo-EM structures of NAC-RNC and NAC-SRP-RNC. M.G. and A.W. performed C. elegans in vivo and A.W. cross-linking experiments. H.-H.H. performed FRET titrations and single-molecule experiments. V.C. performed structural analysis of the NAC-TTC5-RNC. Z.U. characterized NAC cysteine variants. A.J., M.G., H.-H.H., S.S., E.D., and N.B. wrote the manuscript. All authors contributed to data analysis and the final version of the manuscript. Data and materials availability: Cryo-EM maps and model coordinates are deposited in the EMDB as EMD-14191, EMD-14192, and EMD-14193 and in the PDB as PDB ID 7QWQ, 7QWR, and 7QWS for the NAC-SRP-RNC_(SS), NAC-RNC_(SS), and NAC-TTC5-RNC_(TUBB), respectively. All other data are available in the main text or the supplementary materials. The authors declare no competing interests.

Attached Files

Accepted Version - EMS142053.pdf

Supplemental Material - science.abl6459_mdar_reproducibility_checklist.pdf

Supplemental Material - science.abl6459_sm.pdf


Files (11.4 MB)
Name Size Download all
8.6 MB Preview Download
2.3 MB Preview Download
475.4 kB Preview Download

Additional details

August 22, 2023
October 23, 2023