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PDBsum entry 1nqf
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Transport protein
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PDB id
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1nqf
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Substrate-Induced transmembrane signaling in the cobalamin transporter btub.
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Authors
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D.P.Chimento,
A.K.Mohanty,
R.J.Kadner,
M.C.Wiener.
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Ref.
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Nat Struct Biol, 2003,
10,
394-401.
[DOI no: ]
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PubMed id
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Abstract
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The outer membranes of Gram-negative bacteria possess transport proteins
essential for uptake of scarce nutrients. In TonB-dependent transporters, a
conserved sequence of seven residues, the Ton box, faces the periplasm and
interacts with the inner membrane TonB protein to energize an active transport
cycle. A critical mechanistic step is the structural change in the Ton box of
the transporter upon substrate binding; this essential transmembrane signaling
event increases the affinity of the transporter for TonB and enables active
transport to proceed. We have solved crystal structures of BtuB, the outer
membrane cobalamin transporter from Escherichia coli, in the absence and
presence of cyanocobalamin (vitamin B(12)). In these structures, the Ton box is
ordered and undergoes a conformational change in the presence of bound
substrate. Calcium has been implicated as a necessary factor for the
high-affinity binding (K(d) approximately 0.3 nM) of cyanocobalamin to BtuB. We
observe two bound calcium ions that order three extracellular loops of BtuB,
thus providing a direct (and unusual) structural role for calcium.
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Figure 2.
Figure 2. Crystallographic structures of BtuB. a, apo BtuB;
b, Ca^2+ -BtuB; and c, Ca^2+ -B[12] -BtuB.  -barrel
domains are shown in green; hatch domains, in purple. Bound
calcium ions are shown in yellow in the Ca^2+ -BtuB and Ca^2+
-B[12] -BtuB structures. The bound cyanocobalamin (vitamin
B[12]) substrate is shown in space-filling representation in the
Ca^2+ -B[12] -BtuB structure. The left column depicts the
structures with extracellular loops pointing upwards and
periplasmic turns downwards; the -strands
of the barrel domain span the outer membrane. The right column
presents views, normal to the surface of the outer membrane,
looking down into the extracellular side of the structures. The
hatch domains extend from residues 6 -132, with the Ton box
located at 6 -12. The hatch domain is formed around a core of
four -strands.
A short linker (133 -136) connects the hatch to the 22-stranded
-barrel
domain (137 -594). Extracellular loops that are disordered in
the apo BtuB structure become partially ordered in the Ca^2+
-BtuB structure and fully ordered in the Ca^2+ -B[12] -BtuB
structure. This ordering occurs in the vicinity of the bound
calcium ions. Waters, detergent molecules and other (weakly)
bound ions are not shown (for clarity).
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Figure 5.
Figure 5. Calcium and cyanocobalamin binding in BtuB a,
Stereo view of the 'aspartate cage' binding the two calcium ions
(yellow) in the Ca^2+ -BtuB and Ca^2+ -B[12] -BtuB structures.
b, Stereo view of cyanocobalamin bound to BtuB. The view is
normal to the surface of the outer membrane, looking down into
the extracellular side. Residues within 4.5 Å of the substrate
are shown, with those of the hatch domain colored purple and
those from the barrel colored green. The calcium ions are
included to indicate the orientation of the molecule (which is
the same as that of Fig. 1).
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The above figures are
reprinted
by permission from Macmillan Publishers Ltd:
Nat Struct Biol
(2003,
10,
394-401)
copyright 2003.
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Secondary reference #1
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Title
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Crystallization and initial X-Ray diffraction of btub, The integral membrane cobalamin transporter of escherichia coli.
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Authors
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D.P.Chimento,
A.K.Mohanty,
R.J.Kadner,
M.C.Wiener.
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Ref.
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Acta Crystallogr D Biol Crystallogr, 2003,
59,
509-511.
[DOI no: ]
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PubMed id
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Figure 3.
Figure 3 (a) Diffraction from a type I BtuB crystal recorded at
CHESS F1 (0.5° oscillation, 20 s exposure,  =
0.900 Å). The region marked with an asterisk is shown magnified
in (b). (b) Diffraction extending beyond 2.0 Å resolution.
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The above figure is
reproduced from the cited reference
with permission from the IUCr
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