Permeant anions contribute to voltage dependence of ClC‐2 chloride channel by interacting with the protopore gate

It has been shown that the voltage ( V m ) dependence of ClC Cl − channels is conferred by interaction of the protopore gate with H + ions. However, in this paper we present evidence which indicates that permeant Cl − ions contribute to V m ‐dependent gating of the broadly distributed ClC‐2 Cl − channel. The apparent open probability ( P A ) of ClC‐2 was enhanced either by changing the [Cl − ] i from 10 to 200 m m or by keeping the [Cl − ] i low (10 m m ) and then raising [Cl − ] o from 10 to 140 m m . Additionally, these changes in [Cl − ] slowed down channel closing at positive V m suggesting that high [Cl − ] increased pore occupancy thus hindering closing of the protopore gate. The identity of the permeant anion was also important since the P A ( V m ) curves were nearly identical with Cl − or Br − but shifted to negative voltages in the presence of SCN − ions. In addition, gating, closing rate and reversal potential displayed anomalous mole fraction behaviour in a SCN − /Cl − mixture in agreement with the idea that pore occupancy by different permeant anions modifies the V m dependence ClC‐2 gating. Based on the ec1‐ClC anion pathway, we hypothesized that opening of the protopore gate is facilitated when Cl − ions dwell in the central binding site. In contrast, when Cl − ions dwell in the external binding site they prevent the gate from closing. Finally, this Cl − ‐dependent gating in ClC‐2 channels is of physiological relevance since an increase in [Cl − ] o enhances channel opening when the [Cl − ] i is in the physiological range.