vandneBerg - X-ray structure of a protein-conducting...

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Unformatted text preview: X-ray structure of a protein-conducting channel Bert van den Berg 1 * , William M. Clemons Jr 1 * , Ian Collinson 2 , Yorgo Modis 3 , Enno Hartmann 4 , Stephen C. Harrison 3 & Tom A. Rapoport 1 1 Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA 2 Max Planck Institute of Biophysics, Marie-Curie-Strasse 13-15, D-60439 Frankfurt am Main, Germany 3 Howard Hughes Medical Institute, Children’s Hospital and Harvard Medical School, 320 Longwood Avenue, Boston, Massachusetts 02115, USA 4 University Luebeck, Institute for Biology, Ratzeburger Allee 160, Luebeck, D-23538, Germany * These authors contributed equally to this work ........................................................................................................................................................................................................................... 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 a-subunit has two linked halves, transmembrane segments 1–5 and 6–10, clamped together by the g-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. A decisive step in the biosynthesis of many secretory and plasma- membrane proteins is their transport across the endoplasmic reticulum (ER) membrane in eukaryotes or across the cytoplasmic membrane in prokaryotes (for a review, see ref. 1). These polypep- tides are first targeted to the membrane by hydrophobic amino-acid sequences, which are either cleavable signal sequences or transmem- brane segments (TM) of membrane proteins. Soluble proteins, such as those destined for secretion, are subsequently transported across the membrane through a protein-conducting channel with a hydro- philic interior 2,3 . In the case of membrane proteins, when a hydrophobic TM arrives in the channel, it is released through an opening in the channel wall into the surrounding lipid phase. The capacity of the channel to open laterally towards the lipid and the wide variety of substrates that it must transport distinguish it from many other channels....
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vandneBerg - X-ray structure of a protein-conducting...

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