Wang J, Trudeau M C, Zappia A M, Robertson G A
Department of Physiology, University of Wisconsin-Madison Medical School, Madison, Wisconsin 53706, USA.
J Gen Physiol. 1998 Nov;112(5):637-47. doi: 10.1085/jgp.112.5.637.
Abnormalities in repolarization of the cardiac ventricular action potential can lead to life-threatening arrhythmias associated with long QT syndrome. The repolarization process depends upon the gating properties of potassium channels encoded by the human ether-à-go-go-related gene (HERG), especially those governing the rate of recovery from inactivation and the rate of deactivation. Previous studies have demonstrated that deletion of the NH2 terminus increases the deactivation rate, but the mechanism by which the NH2 terminus regulates deactivation in wild-type channels has not been elucidated. We tested the hypothesis that the HERG NH2 terminus slows deactivation by a mechanism similar to N-type inactivation in Shaker channels, where it binds to the internal mouth of the pore and prevents channel closure. We found that the regulation of deactivation by the HERG NH2 terminus bears similarity to Shaker N-type inactivation in three respects: (a) deletion of the NH2 terminus slows C-type inactivation; (b) the action of the NH2 terminus is sensitive to elevated concentrations of external K+, as if its binding along the permeation pathway is disrupted by K+ influx; and (c) N-ethylmaleimide, covalently linked to an aphenotypic cysteine introduced within the S4-S5 linker, mimics the N deletion phenotype, as if the binding of the NH2 terminus to its receptor site were hindered. In contrast to N-type inactivation in Shaker, however, there was no indication that the NH2 terminus blocks the HERG pore. In addition, we discovered that separate domains within the NH2 terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These results suggest that the NH2 terminus stabilizes the open state and, by a separate mechanism, promotes C-type inactivation.
心室动作电位复极化异常可导致与长QT综合征相关的危及生命的心律失常。复极化过程取决于人类去甲肾上腺素相关基因（HERG）编码的钾通道的门控特性，尤其是那些控制失活恢复速率和去激活速率的特性。先前的研究表明，NH2末端的缺失会增加去激活速率，但NH2末端在野生型通道中调节去激活的机制尚未阐明。我们测试了这样一个假设，即HERG NH2末端通过一种类似于Shaker通道中N型失活的机制来减缓去激活，在Shaker通道中，它与孔的内部口结合并阻止通道关闭。我们发现，HERG NH2末端对去激活的调节在三个方面与Shaker N型失活相似：（a）NH2末端的缺失减缓了C型失活；（b）NH2末端的作用对外部K+浓度升高敏感，就好像其沿渗透途径的结合被K+内流破坏；（c）与S4-S5连接区内引入的表型半胱氨酸共价连接的N-乙基马来酰亚胺模拟了N缺失表型，就好像NH2末端与其受体位点的结合受到阻碍。然而，与Shaker中的N型失活不同，没有迹象表明NH2末端会阻塞HERG孔。此外，我们发现NH2末端内的不同结构域介导了去激活的减缓以及C型失活的促进。这些结果表明，NH2末端稳定了开放状态，并通过一种独立的机制促进了C型失活。
