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.