K⁺channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K⁺channels, pH-regulated K_2Pchannels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K_2Pand Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K⁺homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K_2Pchannels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K_2Pchannels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K_2Pand Kir K⁺-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

The Cl– channel ClC-2 is expressed in transporting epithelia and has been proposed as an alternative route for Cl– efflux that might compensate for the malfunction of CFTR in cystic fibrosis. There is controversy concerning the cellular and membrane location of ClC-2, particularly in intestinal tissue. The aim of this paper is to resolve this controversy by immunolocalization studies using tissues from ClC-2 knockout animals as control, ascertaining the sorting of ClC-2 in model epithelial cells and exploring the possible molecular signals involved in ClC-2 targeting. ClC-2 was exclusively localized at the basolateral membranes of surface colonic cells or villus duodenal enterocytes. ClC-2 was sorted to the basolateral membranes in MDCK, Caco-2 and LLC-PK1-μ1B, but not in LLC-PK1-μ1A cells. Mutating a di-leucine motif (L812L813) to a di-alanine changed the basolateral targeting of ClC-2 to an apical location. The basolateral membrane localization of ClC-2 in absorptive cells of the duodenum and the colon is compatible with an absorptive function for this Cl– channel. Basolateral targeting information is contained in a di-leucine motif (L812L813) within CBS-2 domain at the C-terminus of ClC-2. It is speculated that ClC-2 also contains an apical sorting signal masked by L812L813. The proposal that CBS domains in ClC channels might behave as regulatory sites sensing intracellular signals opens an opportunity for pharmacological modulation of ClC-2 targeting.

Functional and structural studies demonstrate that Cl− channels of the ClC family have a dimeric double-barrelled structure, with each monomer contributing an identical pore. Single protopore gating ...

ClC-2 is a ubiquitously expressed, two-pore homodimeric Cl⁻ channel opened by hyperpolarisation. Little is known about its gating mechanisms. Crystallographic and functional studies in other ClC channels suggest that a conserved glutamate residue carboxylate side-chain can close protopores by interacting with a Cl⁻-binding site in the pore. Competition for this site is thought to provide the molecular basis for gating by extracellular Cl⁻. We now show that ClC-2 gating depends upon intra- but not extracellular Cl⁻ and that neutralisation of E217, the homologous pore glutamate, leads to loss of sensitivity to intracellular Cl⁻ and voltage. Experiments testing for transient activation by extracellular protons demonstrate that E217 is not available for protonation in the closed channel state but becomes so after opening by hyperpolarisation. The results suggest that E217 is a hyperpolarisation-dependent protopore gate in ClC-2 and that access of intracellular Cl⁻ to a site normally occupied by its side-chain in the pore stabilises the open state. A remaining hyperpolarisation-dependent gate might correspond to that closing both pores simultaneously in other ClC channels.