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Preserving the membrane barrier for small molecules during bacterial protein translocation

Nature volume 473, pages 239–242 (2011)Cite this article

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Abstract

Many proteins are translocated through the SecY channel in bacteria and archaea and through the related Sec61 channel in eukaryotes1. The channel has an hourglass shape with a narrow constriction approximately halfway across the membrane, formed by a pore ring of amino acids2. While the cytoplasmic cavity of the channel is empty, the extracellular cavity is filled with a short helix called the plug2, which moves out of the way during protein translocation3,4. The mechanism by which the channel transports large polypeptides and yet prevents the passage of small molecules, such as ions or metabolites, has been controversial2,5,6,7,8. Here, we have addressed this issue in intact Escherichia coli cells by testing the permeation of small molecules through wild-type and mutant SecY channels, which are either in the resting state or contain a defined translocating polypeptide chain. We show that in the resting state, the channel is sealed by both the pore ring and the plug domain. During translocation, the pore ring forms a ‘gasket-like’ seal around the polypeptide chain, preventing the permeation of small molecules. The structural conservation of the channel in all organisms indicates that this may be a universal mechanism by which the membrane barrier is maintained during protein translocation.

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Figure 1: Testing the permeability of the resting SecY channel.
Figure 2: Testing the permeability of a translocating SecY channel.
Figure 3: Permeability in pore ring mutants.
Figure 4: Model for the maintenance of the membrane barrier by the SecY channel.

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Acknowledgements

We thank P. Walter, H. Bernstein and G. Phillips for materials, D. Boyd for advice, C. Akey for discussions and C. Akey, A. Osborne and A. Salic for critical reading of the manuscript. The work was supported by a grant from the NIH (GM052586). T.A.R. is a Howard Hughes Medical Institute investigator.

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Authors and Affiliations

  1. Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, 02115, Massachusetts, USA

    Eunyong Park & Tom A. Rapoport

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  1. Eunyong Park
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  2. Tom A. Rapoport
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E.P. performed the experiments and E.P. and T.A.R. wrote the manuscript.

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Correspondence to Tom A. Rapoport.

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The authors declare no competing financial interests.

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This file contains Supplementary Figures 1-16 with legends, Supplementary Tables 1-2, a Supplementary Discussion and additional references. (PDF 5591 kb)

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Park, E., Rapoport, T. Preserving the membrane barrier for small molecules during bacterial protein translocation. Nature 473, 239–242 (2011). https://doi.org/10.1038/nature10014

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