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. Author manuscript; available in PMC: 2016 Jul 17.
Published in final edited form as: J Mol Biol. 2015 May 14;427(14):2348–2359. doi: 10.1016/j.jmb.2015.05.003

Figure 5. Model for SecA-mediated protein translocation.

Figure 5

SecA starts out in the cytosol with an open clamp. Upon binding to the SecY channel, PPXD moves and closes the clamp. The substrate polypeptide is embraced by the clamp and forms a short β-strand that interacts with the two-stranded β-sheet at the back of the clamp. During translocation, the two-helix finger of SecA moves towards the channel and drags the polypeptide chain with it. It then resets to grab the next polypeptide segment. The polypeptide chain can also slide back and forth through the SecA-SecY complex and only occasionally gets a push from the two-helix finger (“push-and-slide” model)18.