Ion permeation through a Cl⁻-selective channel designed from a CLC Cl⁻/H⁺ exchanger

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 T...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2008-08, Vol.105 (32), p.11194-11199
Main Authors: Jayaram, Hariharan, Accardi, Alessio, Wu, Fang, Williams, Carole, Miller, Christopher
Format: Article
Language:English
Subjects:
60 APPLIED LIFE SCIENCES
Amino Acid Substitution
Animals
BASIC BIOLOGICAL SCIENCES
Biochemistry
Biological Sciences
Chloride channels
Chloride Channels - chemistry
Chloride Channels - genetics
Chloride Channels - metabolism
Chlorides - chemistry
Chlorides - metabolism
CRYSTAL STRUCTURE
Crystallography, X-Ray
GENE MUTATIONS
Humans
Ion channels
Ion Transport - physiology
Ion transporters
Ions
Liposomes
MUTANTS
MUTATIONS
PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
Point Mutation
Protein Structure, Tertiary - genetics
PROTEINS
PROTONS
RESIDUES
Solutes
STOICHIOMETRY
TRANSPORT
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Summary: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⁻ 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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0804503105