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PDBsum entry 2fee
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Proton transport,membrane protein
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PDB id
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2fee
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Contents |
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444 a.a.
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221 a.a.
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211 a.a.
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References listed in PDB file
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Key reference
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Title
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Separate ion pathways in a cl-/H+ exchanger.
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Authors
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A.Accardi,
M.Walden,
W.Nguitragool,
H.Jayaram,
C.Williams,
C.Miller.
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Ref.
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J Gen Physiol, 2005,
126,
563-570.
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PubMed id
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Abstract
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CLC-ec1 is a prokaryotic CLC-type Cl(-)/H+ exchange transporter. Little is known
about the mechanism of H+ coupling to Cl-. A critical glutamate residue, E148,
was previously shown to be required for Cl(-)/H+ exchange by mediating proton
transfer between the protein and the extracellular solution. To test whether an
analogous H+ acceptor exists near the intracellular side of the protein, we
performed a mutagenesis scan of inward-facing carboxyl-bearing residues and
identified E203 as the unique residue whose neutralization abolishes H+ coupling
to Cl- transport. Glutamate at this position is strictly conserved in all known
CLCs of the transporter subclass, while valine is always found here in CLC
channels. The x-ray crystal structure of the E203Q mutant is similar to that of
the wild-type protein. Cl- transport rate in E203Q is inhibited at neutral pH,
and the double mutant, E148A/E203Q, shows maximal Cl- transport, independent of
pH, as does the single mutant E148A. The results argue that substrate exchange
by CLC-ec1 involves two separate but partially overlapping permeation pathways,
one for Cl- and one for H+. These pathways are congruent from the protein's
extracellular surface to E148, and they diverge beyond this point toward the
intracellular side. This picture demands a transport mechanism fundamentally
different from familiar alternating-access schemes.
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Secondary reference #1
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Title
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Gating the selectivity filter in clc chloride channels.
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Authors
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R.Dutzler,
E.B.Campbell,
R.Mackinnon.
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Ref.
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Science, 2003,
300,
108-112.
[DOI no: ]
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PubMed id
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Figure 2.
Fig. 2. Structure of the selectivity filter of the wild-type
EcClC Fab complex. (A) Stereo view of electron density in the
selectivity filter at 2.5 Å, contoured at 1  . The view
is from the dimer interface within the membrane. The cytoplasm
is on the bottom, the extracellular side on the top. The map was
calculated from native amplitudes and solvent-flattened two-fold
averaged phases. The refined protein model is shown as sticks.
An (F[Br] - F[Cl]) difference Fourier map at 2.8 Å,
contoured at 4 , is shown
in red. (B) Stereo view of the ion-binding sites. Selected
residues in the vicinity of the bound chloride ions are shown.
Hydrogen bonds between the protein and chloride ions (red
spheres) as well as between the side chain of Glu148 and the
rest of the protein are shown as black dashed lines.
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Figure 5.
Fig. 5. Schematic drawing of the closed and opened conformation
of a ClC chloride channel. In the closed conformation, the
ion-binding sites S[int] and S[cen] are occupied by chloride
ions, and the ion-binding site S[ext] is occupied by the side
chain of Glu148. In the opened conformation, the side chain of
Glu148 has moved out of binding site S[ext] into the
extracellular vestibule. S[ext] is occupied by a third chloride
ion. Chloride ions are shown as red spheres, the Glu148 side
chain is colored red, and hydrogen bonds are drawn as dashed
lines.
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The above figures are
reproduced from the cited reference
with permission from the AAAs
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