Title | Ion permeation through a Cl--selective channel designed from a CLC Cl-/H+ exchanger. |
Publication Type | Journal Article |
Year of Publication | 2008 |
Authors | Jayaram H, Accardi A, Wu F, Williams C, Miller C |
Journal | Proc Natl Acad Sci U S A |
Volume | 105 |
Issue | 32 |
Pagination | 11194-9 |
Date Published | 2008 Aug 12 |
ISSN | 1091-6490 |
Keywords | Amino Acid Substitution, Animals, Chloride Channels, Chlorides, Crystallography, X-Ray, Humans, Ion Transport, Point Mutation, Protein Structure, Tertiary, Protons |
Abstract | 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, Glu(148) and Tyr(445), 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. |
DOI | 10.1073/pnas.0804503105 |
Alternate Journal | Proc. Natl. Acad. Sci. U.S.A. |
PubMed ID | 18678918 |
PubMed Central ID | PMC2516207 |
Grant List | P30 EB009998 / EB / NIBIB NIH HHS / United States |