Gating of human ClC‐2 chloride channels and regulation by carboxy‐terminal domains

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Abstract
Eukaryotic ClC channels are dimeric proteins with each subunit forming an individual protopore. Single protopores are gated by a fast gate, whereas the slow gate is assumed to control both protopores through a cooperative movement of the two carboxy‐terminal domains. We here study the role of the carboxy‐terminal domain in modulating fast and slow gating of human ClC‐2 channels, a ubiquitously expressed ClC‐type chloride channel involved in transepithelial solute transport and in neuronal chloride homeostasis. Partial truncation of the carboxy‐terminus abolishes function of ClC‐2 by locking the channel in a closed position. However, unlike other isoforms, its complete removal preserves function of ClC‐2. ClC‐2 channels without the carboxy‐terminus exhibit fast and slow gates that activate and deactivate significantly faster than in WT channels. In contrast to the prevalent view, a single carboxy‐terminus suffices for normal slow gating, whereas both domains regulate fast gating of individual protopores. Our findings demonstrate that the carboxy‐terminus is not strictly required for slow gating and that the cooperative gating resides in other regions of the channel protein. ClC‐2 is expressed in neurons and believed to open at negative potentials and increased internal chloride concentrations after intense synaptic activity. We propose that the function of the ClC‐2 carboxy‐terminus is to slow down the time course of channel activation in order to stabilize neuronal excitability