Dynamic Structure of the Translocon SecYEG in Membrane

Background: Numerous proteins are exported across membranes by the translocon SecYEG, a highly conserved complex. Results: Multiple structural conformations and oligomeric states of SecYEG observed in lipid bilayers. Conclusion: Cytoplasmic membrane-external segments of SecYEG that orchestrate translocon function are highly dynamic. Significance: Direct visualization of disordered, flexible structures and oligomeric states in lipid bilayers provides a near-native vista of the translocon. Purified SecYEG was reconstituted into liposomes and studied in near-native conditions using atomic force microscopy. These SecYEG proteoliposomes were active in translocation assays. Changes in the structure of SecYEG as a function of time were directly visualized. The dynamics observed were significant in magnitude (∼1–10 Å) and were attributed to the two large loops of SecY linking transmembrane helices 6–7 and 8–9. In addition, we identified a distribution between monomers and dimers of SecYEG as well as a smaller population of higher order oligomers. This work provides a new vista of the flexible and dynamic structure of SecYEG, an intricate and vital membrane protein.

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