title: X-ray structure of a protein-conducting channel.
abstract: 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.
authors.affiliation: Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Bert
authors.initials: B
authors.lastname: Van den Berg
authors.forename: William M
authors.initials: WM
authors.lastname: Clemons
authors.forename: Ian
authors.initials: I
authors.lastname: Collinson
authors.forename: Yorgo
authors.initials: Y
authors.lastname: Modis
authors.forename: Enno
authors.initials: E
authors.lastname: Hartmann
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Harrison
authors.forename: Tom A
authors.initials: TA
authors.lastname: Rapoport
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000648:ultrastructure; D046911:Macromolecular Substances; D017017:Methanococcus / Q000737:chemistry*; D008958:Models, Molecular; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D021381:Protein Transport; D000069816:SEC Translocation Channels; D055672:Static Electricity; D013489:Suppression, Genetic
chemical_list: D019843:Archaeal Proteins; D029968:Escherichia coli Proteins; D046911:Macromolecular Substances; D021122:Protein Subunits; D000069816:SEC Translocation Channels; C057841:SecY protein, E coli