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Three-dimensional structure of the bacterial protein-translocation complex SecYEG

Abstract

Transport and membrane integration of polypeptides is carried out by specific protein complexes in the membranes of all living cells. The Sec transport path provides an essential and ubiquitous route for protein translocation1. In the bacterial cytoplasmic membrane, the channel is formed by oligomers of a heterotrimeric membrane protein complex consisting of subunits SecY, SecE and SecG2,3. In the endoplasmic reticulum membrane, the channel is formed from the related Sec61 complex4. Here we report the structure of the Escherichia coli SecYEG assembly at an in-plane resolution of 8 Å. The three-dimensional map, calculated from two-dimensional SecYEG crystals, reveals a sandwich of two membranes interacting through the extensive cytoplasmic domains. Each membrane is composed of dimers of SecYEG. The monomeric complex contains 15 transmembrane helices. In the centre of the dimer we observe a 16 × 25 Å cavity closed on the periplasmic side by two highly tilted transmembrane helices. This may represent the closed state of the protein-conducting channel.

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Figure 1: Double membrane crystal of SecYEG.
Figure 2: Immunogold labelling with antibodies directed against C-terminal fragments of SecG (bd) or of SecY (a and e).
Figure 3: Dimeric structure of the SecYEG complex.

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Acknowledgements

We thank E. Or for the production of the polyclonal antibodies, F. Joos for immunostaining of thin sections, and F. Duong for critically reading the manuscript. C.B. is grateful to D. Mills for help with the JEOL3000SFF electron microscope, V. Unger for advice on image processing, J. Vonck for discussions on noncrystallographic symmetry, and D. Picot for discussions and advice on crystallographic programs. I.C. was a fellow of the Human Frontiers Science Program at Harvard University. C.B. acknowledges support from Jean-Luc Popot and the CNRS-UMR7099, where she carried out the final part of the analysis.

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Correspondence to Cécile Breyton or Ian Collinson.

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Breyton, C., Haase, W., Rapoport, T. et al. Three-dimensional structure of the bacterial protein-translocation complex SecYEG. Nature 418, 662–665 (2002). https://doi.org/10.1038/nature00827

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