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      Structural and Mechanistic Insights into Protein Translocation

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      Annual Review of Cell and Developmental Biology

      Annual Reviews

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          Identification of prokaryotic and eukaryotic signal peptides and prediction of their cleavage sites

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            X-ray structure of a protein-conducting channel.

            A conserved heterotrimeric membrane protein complex, the Sec61 or SecY complex, forms a protein-conducting channel, allowing polypeptides to be transferred across or integrated into membranes. We report the crystal structure of the complex from Methanococcus jannaschii at a resolution of 3.2 A. The structure suggests that one copy of the heterotrimer serves as a functional translocation channel. The alpha-subunit has two linked halves, transmembrane segments 1-5 and 6-10, clamped together by the gamma-subunit. A cytoplasmic funnel leading into the channel is plugged by a short helix. Plug displacement can open the channel into an 'hourglass' with a ring of hydrophobic residues at its constriction. This ring may form a seal around the translocating polypeptide, hindering the permeation of other molecules. The structure also suggests mechanisms for signal-sequence recognition and for the lateral exit of transmembrane segments of nascent membrane proteins into lipid, and indicates binding sites for partners that provide the driving force for translocation.
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              Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes.

               Tom Rapoport (2007)
              A decisive step in the biosynthesis of many proteins is their partial or complete translocation across the eukaryotic endoplasmic reticulum membrane or the prokaryotic plasma membrane. Most of these proteins are translocated through a protein-conducting channel that is formed by a conserved, heterotrimeric membrane-protein complex, the Sec61 or SecY complex. Depending on channel binding partners, polypeptides are moved by different mechanisms: the polypeptide chain is transferred directly into the channel by the translating ribosome, a ratcheting mechanism is used by the endoplasmic reticulum chaperone BiP, and a pushing mechanism is used by the bacterial ATPase SecA. Structural, genetic and biochemical data show how the channel opens across the membrane, releases hydrophobic segments of membrane proteins laterally into lipid, and maintains the membrane barrier for small molecules.
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                Author and article information

                Journal
                Annual Review of Cell and Developmental Biology
                Annu. Rev. Cell Dev. Biol.
                Annual Reviews
                1081-0706
                1530-8995
                October 06 2017
                October 06 2017
                : 33
                : 1
                : 369-390
                10.1146/annurev-cellbio-100616-060439
                © 2017

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