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. 2019 Jun 28;10:2872. doi: 10.1038/s41467-019-10918-2

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 4 — Polypeptide substrate in SecA. a Top view of the SecA-SecY-substrate complex, showing polypeptide entry into SecA. The SecY channel is underneath SecA and not shown. Substrate density is shown in green. The conserved Arg367 in the PPXD loop is indicated as a red dot. b Close-up view of the β-sheet formed by substrate binding. The connecting β-strands at the back of the clamp are in shown in cyan, the PPXD β-strand in magenta, and the β-strand formed by the substrate in green. c SecA residues surrounding the polypeptide substrate are shown as sticks. Hydrophilic and hydrophobic residues, clustered on different sides, are highlighted with orange and purple dashed curves. d Interaction of the connecting β strands with different peptides. Shown is a superposition of different structures, in which β-strands were induced. The alignment is based on the connecting β-strands. The peptides are derived from (1) the translocating peptide in the present structure (green); (2) a C-terminal SecA segment (orange, PDB 1M74); (3) three amino acids of a synthetic peptide (red, PDB 3JV2); and (4) a segment of NBD2 of a neighboring SecA molecule in the crystal (purple, PDB 2IBM)