title: Structure and function of a membrane component SecDF that enhances protein export.
abstract: Protein translocation across the bacterial membrane, mediated by the secretory translocon SecYEG and the SecA ATPase, is enhanced by proton motive force and membrane-integrated SecDF, which associates with SecYEG. The role of SecDF has remained unclear, although it is proposed to function in later stages of translocation as well as in membrane protein biogenesis. Here, we determined the crystal structure of Thermus thermophilus SecDF at 3.3 Å resolution, revealing a pseudo-symmetrical, 12-helix transmembrane domain belonging to the RND superfamily and two major periplasmic domains, P1 and P4. Higher-resolution analysis of the periplasmic domains suggested that P1, which binds an unfolded protein, undergoes functionally important conformational changes. In vitro analyses identified an ATP-independent step of protein translocation that requires both SecDF and proton motive force. Electrophysiological analyses revealed that SecDF conducts protons in a manner dependent on pH and the presence of an unfolded protein, with conserved Asp and Arg residues at the transmembrane interface between SecD and SecF playing essential roles in the movements of protons and preproteins. Therefore, we propose that SecDF functions as a membrane-integrated chaperone, powered by proton motive force, to achieve ATP-independent protein translocation.
authors.affiliation: Department of Biophysics and Biochemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan.
authors.forename: Tomoya
authors.initials: T
authors.lastname: Tsukazaki
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Mori
authors.forename: Yuka
authors.initials: Y
authors.lastname: Echizen
authors.forename: Ryuichiro
authors.initials: R
authors.lastname: Ishitani
authors.forename: Shuya
authors.initials: S
authors.lastname: Fukai
authors.forename: Takeshi
authors.initials: T
authors.lastname: Tanaka
authors.forename: Anna
authors.initials: A
authors.lastname: Perederina
authors.forename: Dmitry G
authors.initials: DG
authors.lastname: Vassylyev
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Kohno
authors.forename: Andrés D
authors.initials: AD
authors.lastname: Maturana
authors.forename: Koreaki
authors.initials: K
authors.lastname: Ito
authors.forename: Osamu
authors.initials: O
authors.lastname: Nureki
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001120:Arginine / Q000378:metabolism; D001216:Asparagine / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D008954:Models, Biological; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D019897:Periplasm / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D058767:Protein Unfolding; D018892:Proton-Motive Force; D055672:Static Electricity; D013329:Structure-Activity Relationship; D016963:Thermus thermophilus / Q000737:chemistry* / Q000166:cytology
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C075767:secD protein, Bacteria; C075768:secF protein, Bacteria; D001216:Asparagine; D000255:Adenosine Triphosphate; D001120:Arginine