Structural analysis of SecA-SecYE complex from Thermus thermophilus

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Evolutionarily conserved membrane protein complex, the Sec translocon, functions as a protein-conducting channel to facilitate secretory protein export and membrane protein integration. In this project, we determined the first atomic resolution crystal structure of the SecYE translocon from a SecA (bacterial translocation motor)-containing organism, Thermus thermophilus, in complex with an anti-SecY Fab fragment. The structure has revealed a significant conformational change from the previously determined archaeal SecYEβ structure, in that several SecY transmembrane helices are shifted to create a hydrophobic crevasse open to the cytoplasm. Disulfide mapping indicate that this "pre-open" state of the bacterial SecYE complex was induced by the Fab fragment and SecA, which in common bind to SecY residues at the tip of the cytoplasmic domain. Our disulfide crosslinking experiments revealed that some of these SecY residues contact specific residues of SecA that are otherwise buried and unavailable in the isolated SecA molecule. These results suggest that the motor components of the Sec machinery undergo cooperative conformational changes upon their interaction, presumably as important steps for the entrance of the preprotein-SecA complex and enhancement of the ATPase activity of SecA.