title: The structure of BtuB with bound colicin E3 R-domain implies a translocon.
abstract: Cellular import of colicin E3 is initiated by the Escherichia coli outer membrane cobalamin transporter, BtuB. The 135-residue 100-A coiled-coil receptor-binding domain (R135) of colicin E3 forms a 1:1 complex with BtuB whose structure at a resolution of 2.75 A is reported. Binding of R135 to the BtuB extracellular surface (DeltaG(o) = -12 kcal mol(-1)) is mediated by 27 residues of R135 near the coiled-coil apex. Formation of the R135-BtuB complex results in unfolding of R135 N- and C-terminal ends, inferred to be important for unfolding of the colicin T-domain. Small conformational changes occur in the BtuB cork and barrel domains but are insufficient to form a translocation channel. The absence of a channel and the peripheral binding of R135 imply that BtuB serves to bind the colicin, and that the coiled-coil delivers the colicin to a neighboring outer membrane protein for translocation, thus forming a colicin translocon. The translocator was concluded to be OmpF from the occlusion of OmpF channels by colicin E3.
authors.affiliation: Department of Biological Sciences, Purdue University, Lilly Hall of Life Sciences, 915 W. State St., West Lafayette, Indiana 47907-1392, USA.
authors.forename: Genji
authors.initials: G
authors.lastname: Kurisu
authors.forename: Stanislav D
authors.initials: SD
authors.lastname: Zakharov
authors.forename: Mariya V
authors.initials: MV
authors.lastname: Zhalnina
authors.forename: Sufiya
authors.initials: S
authors.lastname: Bano
authors.forename: Veronika Y
authors.initials: VY
authors.lastname: Eroukova
authors.forename: Tatiana I
authors.initials: TI
authors.lastname: Rokitskaya
authors.forename: Yuri N
authors.initials: YN
authors.lastname: Antonenko
authors.forename: Michael C
authors.initials: MC
authors.lastname: Wiener
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D001425:Bacterial Outer Membrane Proteins; D003087:Colicins / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D018272:Porins / Q000378:metabolism; D017434:Protein Structure, Tertiary; D018000:Receptors, Peptide / Q000378:metabolism*
chemical_list: D001425:Bacterial Outer Membrane Proteins; C044592:BtuB protein, E coli; D003087:Colicins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; C109237:OmpF protein; D018272:Porins; D018000:Receptors, Peptide