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, 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.
Jayaram
Hariharan
H
Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454, USA.
Accardi
Alessio
A
Wu
Fang
F
Williams
Carole
C
Miller
Christopher
C
D019943:Amino Acid Substitution; D000818:Animals; D018118:Chloride Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002712:Chlorides / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D006801:Humans; D017136:Ion Transport / Q000502:physiology; D017354:Point Mutation; D017434:Protein Structure, Tertiary / Q000235:genetics; D011522:Protons
D018118:Chloride Channels; D002712:Chlorides; D011522:Protons