Fahlke C, Rhodes T H, Desai R R, George A L
Department of Medicine (Nephrology), and The Center for Molecular Neuroscience, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6600, USA.
Nature. 1998 Aug 13;394(6694):687-90. doi: 10.1038/29319.
Ion channels allow ions to pass through cell membranes by forming aqueous permeation pathways (pores). In contrast to most known ion channels, which have single pores, a chloride channel belonging to the CIC family (Torpedo CIC-0) has functional features that suggest that it has a unique 'double-barrelled' architecture in which each of two subunits forms an independent pore. This model is based on single-channel recordings of CIC-0 that has two equally spaced and independently gated conductance states. Other CIC isoforms do not behave in this way, raising doubts about the applicability of the model to all CIC channels. Here we determine the pore stoichiometry of another CIC isoform, human CIC-1, by chemically modifying cysteines that have been substituted for other amino acids located within the CIC ion-selectivity filter. The CIC-1 channel can be rendered completely susceptible to block by methanethiosulphonate reagents when only one of the two subunits contains substituted cysteines. Thiol side chains placed at corresponding positions in both subunits can form intersubunit disulphide bridges and coordinate Cd2+, indicating that the pore-forming regions from each subunit line the same conduction pathway. We conclude that human CIC-1 has a single functional pore.
离子通道通过形成水相渗透途径（孔道）使离子穿过细胞膜。与大多数已知的具有单个孔道的离子通道不同，一种属于CIC家族的氯离子通道（电鳐CIC-0）具有一些功能特性，这表明它具有独特的“双筒”结构，其中两个亚基各自形成一个独立的孔道。该模型基于对CIC-0的单通道记录，CIC-0具有两个等间距且独立门控的电导状态。其他CIC同工型并非如此表现，这使得该模型对所有CIC通道的适用性受到质疑。在这里，我们通过化学修饰被替换为位于CIC离子选择性过滤器内其他氨基酸的半胱氨酸，来确定另一种CIC同工型人CIC-1的孔道化学计量。当两个亚基中只有一个包含被替换的半胱氨酸时，CIC-1通道可被甲硫基磺酸盐试剂完全阻断。置于两个亚基相应位置的硫醇侧链可形成亚基间二硫键并配位Cd2+，这表明每个亚基的孔道形成区域排列在同一条传导途径上。我们得出结论，人CIC-1具有单个功能性孔道。
