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KcsA potassium channel
The potassium crystallographically-sited activation channel (KcsA) is a prokaryotic potassium ion channel from the soil bacteria ''Streptomyces lividans'' activated by changes in pH. Roderick MacKinnon and his colleagues were the first to crystallize a KcsA potassium channel.
KcsA has become the most extensively studied potassium channel, and is widely used as the template in potassium channel research. KcsA potassium channels have only 2 transmembrane segments, instead of the 6 transmembrane segments observed in many vertebrate potassium channels.〔
== KcsA Channel ==
KcsA is a tetramer composed of four identical subunits of two transmembrane helices (the outer-helix M1 and the inner-helix M2) linked by a reentrant loop, dispersed symmetrically around a common axis corresponding to the central pore. The pore can be divided into three parts: a selectivity filter near the extracellular side, a dilated water-filled cavity at the center, and a closed gate near the cytoplasmic side formed by four packed M2 helices. This architecture is found to be highly conserved in the potassium channel family including both the eukaryotic and prokaryotic ones.
The KcsA channel is proton-activated, and it opens at acidic pH. It has two pH-sensing regions: (1) the charge cluster region at the boundary between the membrane and cytoplasm, and (2) the cytoplasmic domain. There is evidence to suggest that the main pH sensor is in the cytoplasmic domain. For example, Hirano et al. showed that exchanging negatively charged amino acids for neutral ones made the KcsA channel insensitive to pH even though there were no amino-acid changes at the transmembrane region. The KcsA channel is blocked by Cs+ ions and gating requires the presence of Mg2+ ions.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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