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PDBsum entry 1po3
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Membrane protein
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PDB id
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1po3
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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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Structural evidence for iron-Free citrate and ferric citrate binding to the tonb-Dependent outer membrane transporter feca.
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Authors
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W.W.Yue,
S.Grizot,
S.K.Buchanan.
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Ref.
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J Mol Biol, 2003,
332,
353-368.
[DOI no: ]
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PubMed id
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Abstract
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Escherichia coli possesses a TonB-dependent transport system, which exploits the
iron-binding capacity of citrate and its natural abundance. Here, we describe
three structures of the outer membrane ferric citrate transporter FecA:
unliganded and complexed with iron-free or diferric dicitrate. We show the
structural mechanism for discrimination between the iron-free and ferric
siderophore: the binding of diferric dicitrate, but not iron-free dicitrate
alone, causes major conformational rearrangements in the transporter. The
structure of FecA bound with iron-free dicitrate represents the first structure
of a TonB-dependent transporter bound with an iron-free siderophore. Binding of
diferric dicitrate to FecA results in changes in the orientation of the two
citrate ions relative to each other and in their interactions with FecA,
compared to the binding of iron-free dicitrate. The changes in ligand binding
are accompanied by conformational changes in three areas of FecA: two
extracellular loops, one plug domain loop and the periplasmic TonB-box motif.
The positional and conformational changes in the siderophore and transporter
initiate two independent events: ferric citrate transport into the periplasm and
transcription induction of the fecABCDE transport genes. From these data, we
propose a two-step ligand recognition event: FecA binds iron-free dicitrate in
the non-productive state or first step, followed by siderophore displacement to
form the transport-competent, diferric dicitrate-bound state in the second step.
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Figure 2.
Figure 2. Crystal structures of FecA unliganded (a,
FecA[apo]), bound with iron-free dicitrate (b, FecA[Cit]) and
with diferric dicitrate (c, FecA[FeCit]), viewed perpendicular
to the barrel axis. In all three structures, the overall fold
consists of a 22-stranded b-barrel (yellow) preceded by a plug
domain (magenta) that fits into the barrel lumen. Iron-free
dicitrate and diferric dicitrate (green) are depicted in
space-filling representation (with iron atoms in red). The two
extracellular loops L7 and L8, which are involved in
conformational changes upon binding of diferric dicitrate, are
colored blue. The TonB box motifs in FecA[apo] and FecA[Cit] are
circled. Part of the barrel has been rendered transparent to
reveal the plug domain located in the barrel lumen.
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Figure 6.
Figure 6. C^a superposition of the plug domains in FecA,
FepA and FhuA. The structures of FecA[apo] (gold), FepA
(magenta) and unliganded FhuA (light blue) were used for the
superposition. The TonB box motifs of FecA[apo] and FepA are
circled.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2003,
332,
353-368)
copyright 2003.
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