Ion permeation through a Cl[superscript -]-selective channel designed from a CLC Cl[superscript -]/H[superscript +] exchanger
- Brandeis
The CLC family of Cl{sup -}-transporting proteins includes both Cl{sup -} channels and Cl{sup -}/H{sup +} exchange transporters. CLC-ec1, a structurally known bacterial homolog of the transporter subclass, exchanges two Cl{sup -} ions per proton with strict, obligatory stoichiometry. Point mutations at two residues, Glu{sup 148} and Tyr{sup 445}, are known to impair H{sup +} movement while preserving Cl{sup -} 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{sup +} transport is abolished, and unitary Cl{sup -} transport rates are greatly increased, well above values expected for transporter mechanisms. Cl{sup -} 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{sup -} 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.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1006786
- Journal Information:
- Proc. Natl. Acad. Sci. USA, Vol. 105, Issue (32) ; 08, 2008; ISSN 0027-8424
- Country of Publication:
- United States
- Language:
- ENGLISH
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