ClC-Ka and ClC-Kb are epithelial anion channels necessary for sodium chloride resorption in the loop of Henle and for secretion of potassium by the stria vascularis. They share the dimeric ClC channel structure with two separate conduction pathways that can be either individually or jointly gated. ClC-K function depends critically on the accessory ß-subunit barttin that promotes membrane insertion and switches human ClC-K into an active state. rClC-K1 is unique among ClC-K channels in being active also in the absence of barttin. Barttin modifies rClC-K1 gating by locking common gating in an open state. We here studied the role of the intracellular carboxy-terminus as it has already been demonstrated to participate in common gating of other ClC isoforms. All mammalian ClC carboxy-termini contain two cystathionine-ß-synthase (CBS) domains connected by a variable linker.

rClC-K1 was truncated either before (W534X) or after the first CBS-domain (W610X) and studied using patch clamp measurements and confocal imaging in the presence or absence of barttin. For functional evaluation, we studied V166E rClC-K1, since barttin modifies anion conduction and gating in a pronounced manner.

Both truncations functionally resembled each other. At positive potentials, mutant currents activated on a bi-exponential time course, whereas the conductance at negative potentials was insignificant. Co-expression with barttin had no observable effect on relative open probabilities, time constants and single channel conductances. Homologous truncation mutations in human ClC-Ka resulted in loss-of-function under all conditions.

Our results demonstrate the importance of the intracellular carboxy-terminus for proper common gating and interaction of barttin with ClC-K channels.