Barttin is an accessory subunit of ClC-K channels, a subclass of ClC-type chloride channels that contribute to transepithelial NaCl- absorption in the kidney and to K+ secretion in the inner ear. It promotes channel insertion into the surface membrane and also modifies ion conduction and gating (Scholl et al. (2006), PNAS

103, 11411-11416). To study the effects on channel gating in more detail, we examined gating of WT and of V166E rClC-K1 channels. V166E rClC-K1 exhibits a glutamate at the homologous position of a glutamate that is presumed to act as fast gate of ClC- 0, -1 and 2. When expressed in tsA 201 cells and studied through whole-cell patch clamping, WT and mutant rClC-K1 both form functional channels in the presence as well as in the absence of barttin. Both channels displayed voltage-dependent gating that could be described assuming two independent gating processes. Without barttin, WT channels showed fast deactivation followed by slower activation upon membrane depolarization. The voltage dependence of mutant channel gating was inverted with fast deactivation and slow activation upon hyperpolarization. WT and mutant channels differed in the contribution of the two gating processes to the channel open probability. Co-expression of barttin resulted in a monoexponential activation and deactivation of WT and mutant channels without substantial contribution of the slower gating process. Barttin modifies the voltage dependence of the fast gate and locks the slow gate in its open state.