Abstract
An important step in the biosynthesis of many proteins is their partial or complete translocation across the plasma membrane in prokaryotes or the endoplasmic reticulum membrane in eukaryotes1. In bacteria, secretory proteins are generally translocated after completion of their synthesis by the interaction of the cytoplasmic ATPase SecA and a protein-conducting channel formed by the SecY complex2. How SecA moves substrates through the SecY channel is unclear. However, a recent structure of a SecA–SecY complex raises the possibility that the polypeptide chain is moved by a two-helix finger domain of SecA that is inserted into the cytoplasmic opening of the SecY channel3. Here we have used disulphide-bridge crosslinking to show that the loop at the tip of the two-helix finger of Escherichia coli SecA interacts with a polypeptide chain right at the entrance into the SecY pore. Mutagenesis demonstrates that a tyrosine in the loop is particularly important for translocation, but can be replaced by some other bulky, hydrophobic residues. We propose that the two-helix finger of SecA moves a polypeptide chain into the SecY channel with the tyrosine providing the major contact with the substrate, a mechanism analogous to that suggested for hexameric, protein-translocating ATPases.
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Acknowledgements
We thank B. DeLaBarre and B. Burton for critical reading of the manuscript, and R. Sauer, T. Baker and A. Horwich for discussion. The work was supported by an NIH grant. T.A.R. is a HHMI investigator. Y.N. was supported by the Damon Runyon Cancer Research Foundation (DRG 1953-07).
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Erlandson, K., Miller, S., Nam, Y. et al. A role for the two-helix finger of the SecA ATPase in protein translocation. Nature 455, 984–987 (2008). https://doi.org/10.1038/nature07439
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DOI: https://doi.org/10.1038/nature07439
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