'''Structure of the Escherichia coli ClC chloride channel Y445W mutant and Fab complex'''
===Structure of the Escherichia coli ClC chloride channel Y445W mutant and Fab complex===
==Overview==
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The Cl-/H+ exchange-transporter CLC-ec1 mediates stoichiometric transmembrane exchange of two Cl- ions for one proton. A conserved tyrosine residue, Y445, coordinates one of the bound Cl- ions visible in the structure of this protein and is located near the intersection of the Cl- and H+ pathways. Mutants of this tyrosine were scrutinized for effects on the coupled transport of Cl- and H+ determined electrophysiologically and on protein structure determined crystallographically. Despite the strong conservation of Y445 in the CLC family, substitution of F or W at this position preserves wild-type transport behavior. Substitution by A, E, or H, however, produces uncoupled proteins with robust Cl- transport but greatly impaired movement of H+. The obligatory 2 Cl-/1 H+ stoichiometry is thus lost in these mutants. The structures of all the mutants are essentially identical to wild-type, but apparent anion occupancy in the Cl- binding region correlates with functional H+ coupling. In particular, as determined by anomalous diffraction in crystals grown in Br-, an electrophysiologically competent Cl- analogue, the well-coupled transporters show strong Br- electron density at the "inner" and "central" Cl- binding sites. However, in the uncoupled mutants, Br- density is absent at the central site, while still present at the inner site. An additional mutant, Y445L, is intermediate in both functional and structural features. This mutant clearly exchanges H+ for Cl-, but at a reduced H+-to-Cl- ratio; likewise, both the central and inner sites are occupied by Br-, but the central site shows lower Br- density than in wild-type (or in Y445F,W). The correlation between proton coupling and central-site occupancy argues that halide binding to the central transport site somehow facilitates movement of H+, a synergism that is not readily understood in terms of alternating-site antiport schemes.
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==About this Structure==
==About this Structure==
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[[Category: H+/cl- antiporter]]
[[Category: H+/cl- antiporter]]
[[Category: Membrane protein]]
[[Category: Membrane protein]]
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Revision as of 00:12, 28 July 2008
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Structure of the Escherichia coli ClC chloride channel Y445W mutant and Fab complexStructure of the Escherichia coli ClC chloride channel Y445W mutant and Fab complex
Synergism between halide binding and proton transport in a CLC-type exchanger., Accardi A, Lobet S, Williams C, Miller C, Dutzler R, J Mol Biol. 2006 Sep 29;362(4):691-9. Epub 2006 Aug 2. PMID:16949616
