Structure of the CLC-1 chloride channel from .

CLC channels mediate passive Cl conduction, while CLC transporters mediate active Cl transport coupled to H transport in the opposite direction. The distinction between CLC-0/1/2 channels and CLC transporters seems undetectable by amino acid sequence. To understand why they are different functionally we determined the structure of the human CLC-1 channel. Its 'glutamate gate' residue, known to mediate proton transfer in CLC transporters, adopts a location in the structure that appears to preclude it from its transport function. Furthermore, smaller side chains produce a wider pore near the intracellular surface, potentially reducing a kinetic barrier for Cl conduction. When the corresponding residues are mutated in a transporter, it is converted to a channel. Finally, Cl at key sites in the pore appear to interact with reduced affinity compared to transporters. Thus, subtle differences in glutamate gate conformation, internal pore diameter and Cl affinity distinguish CLC channels and transporters.