Voltage-dependent charge movement associated with activation of the CLC-5 2Cl-/1H+ exchanger

The family of CLC proteins comprises both Cl(-) channels and Cl(-)/H(+) exchange transporters with varying degrees of voltage dependence. The human CLC-5 is an electrogenic voltage-dependent 2Cl(-)/1H(+) exchanger that gives rise to strongly outwardly rectifying currents when expressed. We conducted whole-cell recordings from HEK293 cells transiently transfected with either wild-type CLC-5 or a permeation-deficient mutant, E268A. With E268A CLC-5 we recorded transient voltage-dependent currents that represent the gating currents associated with CLC-5 activation and had kinetics that could be described by voltage-dependent forward and reverse transition rates. In extracellular solutions rich in Cl(-) or Br(-), CLC-5 exhibited a gating charge of 1.3, but this was reduced to 0.9 in solutions comprising the impermeant anions aspartate, methanesulfonate, sulfate, or HEPES. Extracellular ion depletion by local perfusion with isotonic mannitol failed to reduce the gating charge further. Lowering intracellular pH from 7.4 to 5.4 did not shift the voltage-dependence of the gating currents, but reducing and increasing intracellular Cl(-) shifted the charge-voltage relationship to more negative and positive potentials, respectively. Our data suggest that voltage sensing is an intrinsic property of the CLC-5 protein and that permeant anions, particularly Cl(-), modulate a voltage-dependent transition to an activated state from which Cl(-)/H(+) exchange can occur.