Barttin Activates CIC-K Channel Function by Modulating Gating

Barttin is an accessory subunit that modifies protein stability, subcellular distribution, and voltage-dependent gating of CIC-K chloride channels expressed in renal and inner ear epithelia CIC-K channels are double-barreled channels with two identical protopores that may be opened by individual or common gating processes. Using heterologous expression in mammalian cells and patch-clamp recordings, we studied the effects of barttin on gating of rat CIC-K1 and human CIC-Ka. In the absence of barttin, rCIC-K1 channels displayed two gating processes with distinct kinetics and voltage dependence. A fast gating process, activated by membrane hyperpolarization, opens and closes individual rCIC-K1 protopores. In addition, slow common gating steps, stimulated by membrane depolarization, act on both protopores together. Coexpression of barttin results in voltage-independent open probabilities of the common gate, causing increased channel activity at physiologic potentials. In contrast to rCIC-K1, human CIC-Ka is functional only when coexpressed with barttin. Single-channel recordings of hCIC-Ka/barttin show double-barreled channels with fast protopore gating without apparent cooperative gating steps. These findings demonstrate that barttin stimulates chloride flux through CIC-K channels by modifying cooperative gating of the double-barreled channels and highlight a physiologic role for gating of epithelial CIC chloride channels.