BB41CH02-Rapoport ARI 11 April 2012 8:14 Mechanisms of Sec61/SecY-Mediated Protein Translocation Across Membranes Eunyong Park and Tom A. Rapoport Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115; email:
[email protected] Annu. Rev. Biophys. 2012. 41:21–40 Keywords First published online as a Review in Advance on endoplasmic reticulum, secretory proteins, membrane proteins, membrane December 16, 2011 barrier The Annual Review of Biophysics is online at biophys.annualreviews.org Abstract This article’s doi: The Sec61 or SecY channel, a universally conserved protein-conducting 10.1146/annurev-biophys-050511-102312 Annu. Rev. Biophys. 2012.41:21-40. Downloaded from www.annualreviews.org channel, translocates proteins across and integrates proteins into the eu- Copyright c 2012 by Annual Reviews. Access provided by California Institute of Technology on 03/28/16. For personal use only. karyotic endoplasmic reticulum (ER) membrane and the prokaryotic plasma All rights reserved membrane. Depending on channel-binding partners, polypeptides are 1936-122X/12/0609-0021$20.00 moved by different mechanisms. In cotranslational translocation, the ribo- some feeds the polypeptide chain directly into the channel. In posttrans- lational translocation, a ratcheting mechanism is used by the ER-lumenal chaperone BiP in eukaryotes, and a pushing mechanism is utilized by the SecA ATPase in bacteria. In prokaryotes, posttranslational translocation is facilitated through the function of the SecD/F protein. Recent structural and biochemical data show how the channel opens during translocation, translo- cates soluble proteins, releases hydrophobic segments of membrane proteins into the lipid phase, and maintains the barrier for small molecules.