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Title: Synergism Between Halide Binding and Proton Transport in a CLC-type Exchanger

Abstract

The Cl{sup -}/H{sup +} exchange-transporter CLC-ec1 mediates stoichiometric transmembrane exchange of two Cl{sup -} ions for one proton. A conserved tyrosine residue, Y445, coordinates one of the bound Cl{sup -} ions visible in the structure of this protein and is located near the intersection of the Cl{sup -} and H{sup +} pathways. Mutants of this tyrosine were scrutinized for effects on the coupled transport of Cl{sup -} and H{sup +} 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{sup -} transport but greatly impaired movement of H{sup +}+. The obligatory 2 Cl{sup -}/1 H{sup +} 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{sup -} binding region correlates with functional H{sup +} coupling. In particular, as determined by anomalous diffraction in crystals grown in Br{sup -}, an electrophysiologically competent Cl{sup -} analogue, the well-coupled transporters show strong Br{sup -} electron density at the 'inner' and 'central' Cl{sup -} binding sites.more » However, in the uncoupled mutants, Br{sup -} 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{sup +} for Cl{sup -}, but at a reduced H{sup +}-to-Cl{sup -} ratio; likewise, both the central and inner sites are occupied by Br{sup -}, but the central site shows lower Br{sup -} 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{sup +}, a synergism that is not readily understood in terms of alternating-site antiport schemes.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
930095
Report Number(s):
BNL-80733-2008-JA
Journal ID: ISSN 0022-2836; JMOBAK; TRN: US0806703
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Molecular Biology; Journal Volume: 362; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ANIONS; COORDINATES; CORRELATIONS; COUPLING; CRYSTALS; DENSITY; DIFFRACTION; ELECTRON DENSITY; FUNCTIONALS; HALIDES; IONS; MUTANTS; PROTEIN STRUCTURE; PROTEINS; PROTON TRANSPORT; PROTONS; STOICHIOMETRY; SYNERGISM; TRANSPORT; TYROSINE; national synchrotron light source

Citation Formats

Accardi,A., Lobet, S., Williams, C., Miller, C., and Dutzler, R.. Synergism Between Halide Binding and Proton Transport in a CLC-type Exchanger. United States: N. p., 2006. Web. doi:10.1016/j.jmb.2006.07.081.
Accardi,A., Lobet, S., Williams, C., Miller, C., & Dutzler, R.. Synergism Between Halide Binding and Proton Transport in a CLC-type Exchanger. United States. doi:10.1016/j.jmb.2006.07.081.
Accardi,A., Lobet, S., Williams, C., Miller, C., and Dutzler, R.. Sun . "Synergism Between Halide Binding and Proton Transport in a CLC-type Exchanger". United States. doi:10.1016/j.jmb.2006.07.081.
@article{osti_930095,
title = {Synergism Between Halide Binding and Proton Transport in a CLC-type Exchanger},
author = {Accardi,A. and Lobet, S. and Williams, C. and Miller, C. and Dutzler, R.},
abstractNote = {The Cl{sup -}/H{sup +} exchange-transporter CLC-ec1 mediates stoichiometric transmembrane exchange of two Cl{sup -} ions for one proton. A conserved tyrosine residue, Y445, coordinates one of the bound Cl{sup -} ions visible in the structure of this protein and is located near the intersection of the Cl{sup -} and H{sup +} pathways. Mutants of this tyrosine were scrutinized for effects on the coupled transport of Cl{sup -} and H{sup +} 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{sup -} transport but greatly impaired movement of H{sup +}+. The obligatory 2 Cl{sup -}/1 H{sup +} 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{sup -} binding region correlates with functional H{sup +} coupling. In particular, as determined by anomalous diffraction in crystals grown in Br{sup -}, an electrophysiologically competent Cl{sup -} analogue, the well-coupled transporters show strong Br{sup -} electron density at the 'inner' and 'central' Cl{sup -} binding sites. However, in the uncoupled mutants, Br{sup -} 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{sup +} for Cl{sup -}, but at a reduced H{sup +}-to-Cl{sup -} ratio; likewise, both the central and inner sites are occupied by Br{sup -}, but the central site shows lower Br{sup -} 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{sup +}, a synergism that is not readily understood in terms of alternating-site antiport schemes.},
doi = {10.1016/j.jmb.2006.07.081},
journal = {Journal of Molecular Biology},
number = 4,
volume = 362,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • CLC-ec1, a bacterial homologue of the CLC family's transporter subclass, catalyzes transmembrane exchange of Cl- and H+. Mutational analysis based on the known structure reveals several key residues required for coupling H+ to the stoichiometric countermovement of Cl-. E148 (Gluex) transfers protons between extracellular water and the protein interior, and E203 (Gluin) is thought to function analogously on the intracellular face of the protein. Mutation of either residue eliminates H+ transport while preserving Cl- transport. We tested the role of Gluin by examining structural and functional properties of mutants at this position. Certain dissociable side chains (E, D, H, K,more » R, but not C and Y) retain H+/Cl- exchanger activity to varying degrees, while other mutations (V, I, or C) abolish H+ coupling and severely inhibit Cl- flux. Transporters substituted with other nonprotonatable side chains (Q, S, and A) show highly impaired H+ transport with substantial Cl- transport. Influence on H+ transport of side chain length and acidity was assessed using a single-cysteine mutant to introduce non-natural side chains. Crystal structures of both coupled (E203H) and uncoupled (E203V) mutants are similar to wild type. The results support the idea that Gluin is the internal proton-transfer residue that delivers protons from intracellular solution to the protein interior, where they couple to Cl- movements to bring about Cl-/H+ exchange.« less
  • Cited by 6
  • Cited by 6
  • The CLC family of Cl--transporting proteins includes both Cl- channels and Cl-/H+ exchange transporters. CLC-ec1, a structurally known bacterial homolog of the transporter subclass, exchanges two Cl- ions per proton with strict, obligatory stoichiometry. Point mutations at two residues, Glu148 and Tyr445, are known to impair H+ movement while preserving Cl- transport. In the x-ray crystal structure of CLC-ec1, these residues form putative 'gates' flanking an ion-binding region. In mutants with both of the gate-forming side chains reduced in size, H+ transport is abolished, and unitary Cl- transport rates are greatly increased, well above values expected for transporter mechanisms. Cl-more » transport rates increase as side-chain volume at these positions is decreased. The crystal structure of a doubly ungated mutant shows a narrow conduit traversing the entire protein transmembrane width. These characteristics suggest that Cl- flux through uncoupled, ungated CLC-ec1 occurs via a channel-like electrodiffusion mechanism rather than an alternating-exposure conformational cycle that has been rendered proton-independent by the gate mutations.« less