Regulation of Deactivation by an Amino Terminal Domain in Human Ether-à-go-go –related Gene Potassium Channels

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 NH(2) terminus increases the deactivation rate, but the mechanism by which the NH(2) terminus regulates deactivation in wild-type channels has not been elucidated. We tested the hypothesis that the HERG NH(2) 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 NH(2) terminus bears similarity to Shaker N-type inactivation in three respects: (a) deletion of the NH(2) terminus slows C-type inactivation; (b) the action of the NH(2) 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 NH(2) terminus to its receptor site were hindered. In contrast to N-type inactivation in Shaker, however, there was no indication that the NH(2) terminus blocks the HERG pore. In addition, we discovered that separate domains within the NH(2) terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These results suggest that the NH(2) terminus stabilizes the open state and, by a separate mechanism, promotes C-type inactivation.