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 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.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 959472
- Report Number(s):
- BNL-82458-2009-JA; PNASA6; TRN: US201016%%616
- Journal Information:
- Proceedings of the National Academy of Sciences of the USA, Vol. 105; ISSN 0027-8424
- Country of Publication:
- United States
- Language:
- English
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